POF Handbook - Optical Short Range Transmission Systems

POF Handbook Second edition Olaf Ziemann · Jürgen Krauser Peter E. Zamzow · Werner Daum POF Handbook Optical Short Ra...

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POF Handbook Second edition

Olaf Ziemann · Jürgen Krauser Peter E. Zamzow · Werner Daum

POF Handbook Optical Short Range Transmission Systems Second edition

123

Professor Dr.-Ing. Olaf Ziemann Georg-Simon-Ohm-Fachhochschule Nürnberg Wassertorstr. 10 90489 Nürnberg Germany [email protected]

Dipl.-Ing. Peter E. Zamzow R & D Cable Systems Erlen-Str. 5b 44795 Bochum Germany [email protected]

Professor Dr. Jürgen Krauser Deutsche Telekom Leipzig FB Optische Nachrichtentechnik Gustav-Freytag-Str. 43–45 04277 Leipzig Germany [email protected]

Professor Dr.-Ing. Werner Daum Bundesanstalt für Materialforschung und -prüfung (BAM) Unter den Eichen 87 12205 Berlin Germany [email protected]

ISBN 978-3-540-76628-5

e-ISBN 978-3-540-76629-2

DOI 10.1007/978-3-540-76629-2 Library of Congress Control Number: 2007943247 © 2008, 2001 Springer-Verlag Berlin Heidelberg This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable for prosecution under the German Copyright Law. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Cover design: deblik, Berlin Printed on acid-free paper 987654321 springer.com

Preface

In almost all areas of daily life the demands on the communications infrastructure increased dramatically. With regard to the volume of data that has to be transmitted, no matter whether we are dealing with public or private networks, industrial fields or automobiles, needs will continue to rise. Consequently, the demands on the bandwidth of communications systems will continue to increase since more and more video data streams with high picture quality (IP TV), in addition to telephone and data circuits, will be transmitted. More data streams are coming about through the connection of an increasing number of Wi-Fi hotspots with high capacities. All these services require a basic infrastructure with high capacities which only optic technologies can offer. The expansion of the DSL network will bring glass fibers closer to the end customer and will generate a demand for easy-toinstall, efficient and favorably priced cabling solutions in buildings. In such a case polymer optical fibers are a veritable alternative. After POF demonstrated its performance capabilities in industrial use and in automotive engineering, nothing more stands in the way for employing these optical solutions within buildings. The use of inexpensive visible LEDs, simple plug connectors and insensitive cables and lines, allows for favorably priced systems which any private user can even install himself whenever required. In contrast to Wi-Fi or Powerline, POF is immune to interference and always guarantees high capacity in point-to-point connections. In combination with new electronic solutions and coding scheems it is possible today to bridge distances of over 100 m at a data rate of 100 Mbit/s using standard step index POF. Thus the technical basis is laid for large-scale use in building cabling. It now remains the task of the component manufacturers to make such systems available to the market as an economical alternative. The main emphasis of polymer fiber applications for data transmission lies in Japan as well as in Germany, Spain and Italy. Germany plays a leading role in many of these applications. The Polymer Optical Fiber Application Center (POF-AC) at the Nuremberg University of Applied Sciences has developed over the last few years into a European Competence Center for POF. The POF AC and Leoni have cooperated closely and successfully for years. For many years, the Leoni AG has been one of the leading manufacturers of POF and fiber glass cables for use in mobile networks (cars, trains, airplanes), in automation technology and in sensor technology.

VI

Preface

Leoni has also recently gone into the manufacture of glass fibers and the requisite preforms. Therein lies a particular main focus on the production of multimode and special fibers as described in this book. The areas of application range from building networks to medical technical applications used in spectrometric systems. Furthermore, Leoni’s developers have presented at conferences held over the last few years new solutions for optical pressure-sensitive sensors based on special plastic fibers. In all these areas of activity Leoni has aimed at combining its experience with optical waveguides from the field of telecommunications with the knowledge and the solutions from the market for special and POF fibers, thus making available new practical solutions to users in the most diverse fields. In 1997, Andreas Weinert, working today with colleagues at Leoni Fiber Optics, published one of the first comprehensive POF books. The book “POF - Optical Polymer Fibers for Data Communication” published in 2001 provided an overview of POF technologies and in the meantime is out of print. This new edition is now a handbook which, in addition to describing POF and thick core glass fibers, can also be used for short-distance communication. This specialized reference book is intended to help users and developers to obtain information themselves quickly and comprehensively about the state-ofthe-art development of POF fibers and to become acquainted with their performance capabilities. In a summarized form it conveys a number of experimental results along with new trends in development and is a valuable adviser for developers of POF systems. The physical fundamentals as well as practical applications are presented in a simple and understandable manner.

Nuremberg, May 2007 Dr. Klaus Probst CEO/Leoni AG

Editor’s Preface

In the past few years polymer optical fibers (POF) and their applications have continued to develop at a dizzying pace. This was the decisive factor in 2005 to decide on completely revising the book, „POF - Optical Polymer Fibers for Data Communication“, which appeared in German in 2001 and in English in 2002. Before you now lie the results of two years of work - almost double the number of pages of the First Edition. One essential reason for the new edition is the diverse results which have been obtained at the Polymer Optical Fiber Application Center at the Nuremberg University of Applied Sciences (POF-AC) since its inception in 2001. The scientific director of the POF-AC has written the majority of the sections in this book. Dr. Christian-Alexander Bunge from the Technical University of Berlin has contributed two sections (Microstructured Polymer Fibers and the Simulation of Optical Fibers). The organization and layout of the new book are essentially based on the following considerations: ¾All parts of the first edition have been taken over in order to provide the many newcomers in the field with the opportunity to completely understand the contents, without having to buy the first volume - which in the meantime is out of print. ¾In addition to the optical polymer fibers many details on other thick-core fibers, e.g. glass fiber bundles and plastic-coated glass fiber, have been added. Many of these fibers not only have the same applications, but also similar characteristics and place similar demands on measurement techniques. ¾Whereas the first edition mostly summarized the results from technical literature, the new edition now presents the POF-AC’s own measurement results on practically all fibers. Consequently, this new book presents and documents the first five years of our institute. ¾The individual chapters correspond to the subject matter of the first edition. The organization and sequence, however, have been adapted to the changed points of emphasis. For example, the waveguides are now dealt with in a separate chapter. The chapters on Fibers (No. 2) and on Systems (No. 6) are among the predominantly new sections. Together these two parts form the core contents of the book and document the progress of technology. Unfortunately, however, these new parts will also become obsolete most quickly since many new solutions will be found in the next few years. ¾The English edition is completely identical with the German one, including all page numbers. No new content has been included.

VIII Editor's Preface

At the time when the first edition of this book appeared POF applications were still exotic. Only in the fields of automation and lighting engineering had this medium already been established. In the meantime, millions of vehicles drive throughout Europe with polymer fiber on-board networks and the next generation is right in front of the door. Many telecommunications companies are working on solutions for transmitting ever higher bit rates via POF within apartments. The Deutsche Telekom, for example, offers its customers a complete set for fast Ethernet. Other large-scale applications for POF in multimedia applications will be introduced into the market in the near future. The authors are therefore very optimistic that this book will accompany the development of polymer fibers from a niche existence to an important basis for data and communications technology. In addition to use in telecommunications, sensor technology and multi parallel data connections above all promise to be wide and interesting fields for their use. A separate section is devoted to each of these areas in Chapter 8. POF are not necessarily a rival of the established technologies such as data transmission on symmetrical copper wires or radio. Different sections of the book show how the diverse technologies can be combined in an optimal fashion in order to achieve the best solutions technically and economically. We have done our best to represent the scientific results and the products available on the market as completely and as impartially as possible. Nevertheless, we are aware of the fact that this goal can only approximately be attained. Should a manufacturer or an institute mentioned in the book not feel being sufficiently represented - this was not intended. The POF-AC gladly offers all interested parties support in gaining access to the growing „POF community“. The POF-AC offers scientific activities such as the ITG Sub Committee 5.4.1 „Polymer Optical Fibers“ as well as technical information such as the „POF-Atlas“ as a German POF product catalog. As editor of the Second Edition I would like to express my thanks to all colleagues at the POF-AC Nuremberg, the Nuremberg University of Applied Sciences and not least my family for their support during the last two years and their giving up so many hours that I actually should have spent with them. A special thank to Prof. Economides (Berlin), who is the translator, and the Prof.’s Poisel and Hartl for their assistance during the correction phase. I wish all readers much pleasure when reading this volume. The aim and intention of this book is to provide you with some help, support, information and food for thought in your work. Please excuse the unavoidable errors and mistakes and do feel free to pass on your reservations, criticisms and ideas to us.

Olaf Ziemann Scientific Director of the POF-AC Nürnberg Chairman of the ITG Sub Committee 5.4.1 „Polymer Optical Fibers“ Member of the „International Cooperative of Polymer Optical Fibers“ as the responsible editor of the second edition, November 2007

Content

1. 1.1 1.1.1 1.1.2 1.1.3 1.1.4 1.1.5 1.1.6 1.1.7 1.2 1.2.1 1.2.1.1 1.2.1.2 1.2.1.3 1.2.1.4 1.2.2 1.2.3 1.2.4 1.2.5 1.3 1.3.1 1.3.2 1.3.3 1.3.3.1 1.3.3.2 1.3.3.3 1.3.3.4 1.3.3.5 1.3.3.6

Abbreviations and Symbols Basics of Optical Data Communication Light Propagation in Optical Fibers and Waveguides Wave and Quantum Nature of Light Electromagnetic Spectrum Refraction and Total Reflection Waveguides and Optical Fibers Singlemode and Multimode Waveguides Overview of Optical Fibers Designations of Optical Fibers Digital and Analog Optical Signal Transmission Digital Optical Signal Transmission Analog and Digital Signals Transmission Quality of Analog and Digital Signals Bit Error Probability and Error Correction Noise in Optical Systems Amplitude, Frequency, and Phase Modulation Modulating a Carrier Frequency Specific Transmission Methods in Optical Communications Modulating a Subcarrier Network Architectures Active and Passive Networks Network Structures Multiple Access Methods Time Division Multiplex Frequency Division Multiplex Code Division Multiplex Wavelength Division Multiplex The Special Features of Optical Multiplexing Bi-directional Transmission

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2. 2.1 2.1.1 2.1.2 2.1.3 2.1.4

Optical Fibers Fundamentals of Optical Fibers Refractive Index Profiles Numerical Aperture Ray Trajectory in Optical Fibers Modes in Optical Fibers

p. p. p. p. p. p.

37 37 37 39 40 42

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2.1.4.1 2.1.4.2 2.1.5 2.1.5.1 2.1.5.2 2.1.5.3 2.1.5.4 2.1.5.5 2.1.5.6 2.1.5.7 2.1.5.8 2.1.5.9 2.2 2.2.1 2.2.2 2.2.3 2.2.4 2.2.5 2.2.6 2.2.7 2.2.8 2.2.9 2.3 2.3.1 2.3.2 2.3.3 2.3.4 2.3.5 2.4 2.4.1 2.4.2 2.4.3 2.4.3.1 2.4.3.2 2.5 2.5.1 2.5.2 2.5.3 2.5.3.1 2.5.3.2 2.5.3.3 2.5.3.4 2.5.3.5 2.5.3.6 2.5.4 2.5.5

The Mode Concept Mode Propagation in Real Fibers Parameters for Describing Real Fibers and Waveguides Attenuation Mode-Dependent Attenuation Mode Coupling Mode Conversion Mode Coupling Lengths Leaky Modes Dispersion in Optical Fibers Mode Dispersion Chromatic Dispersion Index Profiles and Types of Fibers Step Index Profile Fibers (SI) The Step Index Fiber with Reduced NA (low-NA) The Double-Step Index Optical Fiber (DSI) The Multicore Step Index Optical Fiber (MC) The Double Step Index Multicore Fiber (DSI-MC) The Graded Index Optical Fiber (GI) The Multi-Step Index Optical Fiber (MSI) The Semi-Graded Index Profile Fibers (Semi-GI) An Overview of Index Profiles The Development of Polymer Optical Fibers Looking back Step Index Polymer Fibers Double Step Index Profile Polymer Fibers Multi-Core Polymer Fibers Multi-Step Index Profile and Graded Index Profile Fibers Glass Fibers for Short-Range Data Transmission 200 μm Glass Fibers with Polymer Cladding Semi-Graded Index Glass Fibers Glass Fiber Bundles Quartz Glass Fiber Bundles Glass Fiber Bundles Bandwidth of Optical Fibers Definition of Bandwidth Experimental Determination of Bandwidth Experimental Bandwidth Measurements Bandwidth of SI-POF Bandwidth Measurements on SI-POF Bandwidth Measurements on MC- and MSI-POF Bandwidth Measurements with GI-POF Bandwidth Measurements on MC-GOF and PCS Comparison of Bandwidth Measurements and Calculations Chromatic Dispersion in Polymer Optical Fibers Methods for Increasing Bandwidth

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Content

2.5.6 2.6 2.6.1 2.6.2 2.6.3 2.6.4 2.7 2.7.1 2.7.2 2.7.2.1 2.7.2.2 2.7.2.3 2.7.2.4 2.7.2.5 2.7.3 2.7.4 2.7.5 2.7.6 2.8 2.8.1 2.8.2 2.8.2.1 2.8.2.2 2.8.2.3 2.8.2.4 2.8.2.5 2.8.2.6 2.8.2.7 2.8.2.8 2.8.2.9 2.8.3 2.8.3.1 2.8.3.2 2.8.3.3 2.8.3.4 2.8.3.5 2.9 2.9.1 2.9.1.1 2.9.1.2 2.9.1.3 2.9.1.4 2.9.2 2.9.2.1 2.9.2.2 2.9.2.3

Bit Rates and Penalty Bending Properties of Optical fibers Bending Losses in SI-POF Bending Losses in GI Fibers Change of Bandwidth by Bends Bends on PCS, Multicore Fibers and thin POF Materials used for POF PMMA POF for Higher Temperatures Cross-Linked PMMA Polycarbonate POF Elastomer POF Cyclic Polyolefines Comparison of High-Temperature POFs Polystyrene-Polymer Fibers Deuterated Polymers Fluorinated Polymers Overview over Polymers for POF Jackets Fiber and Cable Production Production Processes for POF Production of Graded Index Profiles Interfacial Gel Polymerization Technique Creating the Index Profiles by Centrifuging Combined Diffusion and Rotation Photochemical Generation of the Index Profile Extrusion of Many Layers Production of Semi-GI-PCS Polymerization in a Centrifuge Continuous Production at Chromis Fiberoptics GI-POF with Additional Cladding Cable Manufacturing Cable Construction with SI-POF Elements Non-Stranded SI-POF Cables Stranded SI-POF Cables Principles of Stranding Corrugated Micro Tube Cables Microstructured Fibers Kinds of Wave Guiding Effective Refractive Index Photonic Band Gaps Bragg Fibers Hole-Assisted Fibers Production Methods Microstructured Glass Fibers Microstructured Polymer Fibers (MPOF) End Surface Preparation

XI

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2.9.3 Applications for Microstructured Fibers 2.9.3.1 Dispersion Compensation 2.9.3.2 Endlessly Singlemode 2.9.3.3 Birefringence 2.9.3.4 Highly Nonlinear Fibers 2.9.3.5 Control of the Effective Area 2.9.3.6 Filters 2.9.3.7 Sensor Technology, Tunable Elements 2.9.3.8 Double-Core and Multi-Core Fibers 2.9.3.9 Imaging 2.9.3.10 Multimode Graded Index Fibers

p. 225 p. 225 p. 225 p. 226 p. 227 p. 227 p. 228 p. 228 p. 229 p. 229 p. 230

3. 3.1 3.1.1 3.1.2 3.1.2.1 3.1.2.2 3.1.2.3 3.1.2.4 3.1.3 3.1.3.1 3.1.3.2 3.1.3.3 3.1.3.4 3.1.3.5 3.1.3.6 3.1.3.7 3.1.3.8 3.1.4 3.1.5 3.2 3.2.1 3.2.2 3.2.3 3.2.4 3.2.5 3.2.6 3.2.7 3.2.8 3.2.9 3.3 3.3.1 3.3.2 3.3.2.1 3.3.2.2

p. 233 p. 233 p. 234 p. 235 p. 237 p. 238 p. 238 p. 240 p. 241 p. 241 p. 244 p. 245 p. 246 p. 247 p. 249 p. 250 p. 252 p. 253 p. 257 p. 259 p. 259 p. 259 p. 260 p. 261 p. 262 p. 263 p. 264 p. 265 p. 268 p. 269 p. 269 p. 271 p. 273 p. 274

Passive Components for Optical Fibers Connection Technology for Optical Fibers Connectors for Polymer Optical Fibers Surface Preparation of POF Connectors POF Preparation by Cutting and Polishing Hot Plate Surface Preparation POF Press-Cut Procedure POF Preparation by Milling Overview of Connector Systems The V-Pin Connector System FSMA Connector The DNP System F05 and F07 ST and SC Connectors Connectors for Future In-House Networks Connectors for Vehicle Networks Other Connectors Processing Tools for POF Connectors Connectors for Glass Fibers Basis for Calculating Connector Losses Calculation of Connector Losses with Uniform Mode Distribution Differences in Core Diameter Differences in Numerical Aperture Lateral Offset of the Fibers Losses due to Rough Surfaces Losses through Angles between the Fiber Axis Losses through Fresnel Reflection Losses through Axial Distance of the Fibers Losses due to Different Causes POF Couplers Construction of POF Couplers Commercial Couplers Polished Coupler from DieMount Moulded Couplers from IMM

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3.3.2.3 3.4 3.4.1 3.4.2 3.5 3.6 3.6.1 3.6.2 3.6.3 3.6.4 3.6.5

Waveguide Couplers from the University of Sendai Filters and Attenuators for POF Filters Attenuators Mode Mixers and Converters Optical Slip Rings and Rotary Optical Connectors Rotary Optical Connectors The Micro-rotation Project POF Slip Rings Prism Coupler Slip Ring The Mirror Groove Slip Ring

p. 275 p. 276 p. 276 p. 277 p. 282 p. 285 p. 285 p. 286 p. 288 p. 290 p. 292

4. 4.1 4.1.1 4.1.2 4.1.3 4.1.3.1 4.1.3.2 4.1.3.3 4.1.3.4 4.1.3.5 4.2 4.2.1 4.2.2 4.2.3 4.2.4 4.2.5 4.2.5.1 4.2.5.2 4.2.5.3 4.2.6 4.2.7 4.3 4.4 4.4.1 4.4.2 4.4.3 4.4.4 4.4.5 4.4.6 4.5 4.5.1 4.5.2 4.5.2.1 4.5.2.2

Active Components for Optical Systems Emitters and Receivers The Principle of Light Generation in Semiconductors Structuring Semiconductor Components Structures of Semiconductor Transmitters Luminescence Emitting Diode Laser and Super Luminescence Diodes Surface Emitting Laser Resonant Cavity LED Non Resonant Cavity LED Transmitting Diodes for Data Communication Red LEDs and SLEDs Red Laser Diodes Blue and Green LEDs Green Laser Diodes Vertical Laser Diodes and RC-LED Red RC-LED Red VCSELs VCSEL in the IR Region Non Resonant Cavity LED Pyramid LEDs Wavelengths for POF Sources Receivers Efficiency and Sensitivity Photodiode Structures Junction Capacity and Bandwidth Overview of Receivers Commercial Products Improvement in Sensitivity Transceivers Components before 2000 Fast Ethernet Transceiver POF Solutions from DieMount in Wernigerode Optical Clamps from Ratioplast

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Content

4.5.2.3 4.5.2.4 4.5.2.5 4.5.3 4.5.3.1 4.5.3.2 4.5.3.3 4.5.3.4 4.5.3.5

Transceiver Family from Avago Home Installation by RDM POF Transceivers from Infineon/Siemens Other Systems Comoss IEEE 1394, MOST and Fast Ethernet from Firecomms Japanese Manufacturers Fast Ethernet, Ethernet and Video from Luceat DSL Modem with POF

p. 352 p. 353 p. 353 p. 354 p. 354 p. 355 p. 356 p. 356 p. 357

5. 5.1 5.2 5.3 5.4 5.5 5.5.1 5.5.2 5.5.3 5.5.4 5.6 5.6.1 5.6.2 5.6.3 5.6.4 5.6.5

Planar Waveguides Materials for Waveguide Structures Production of Polymer Waveguides Singlemode Waveguides Multimode Waveguides Functional Components as Waveguides Thermo-Optical Switches Modulators Coupling Components Waveguide Gratings Waveguides as Interconnection Solutions Optical Backplane Systems from DaimlerChrysler Systems from the University of Ulm Electro-optical PCB from the University of Siegen IBM Research Center Zurich /ETH Zurich Results of the NeGIT Project

p. 359 p. 360 p. 361 p. 364 p. 368 p. 371 p. 371 p. 373 p. 373 p. 374 p. 375 p. 375 p. 378 p. 379 p. 380 p. 382

6. 6.1 6.1.1 6.1.2 6.1.3 6.1.3.1 6.1.3.2 6.1.3.3 6.1.3.4 6.1.3.5 6.1.4 6.1.4.1 6.1.4.2 6.1.4.3 6.1.4.4 6.1.4.5 6.1.5 6.1.5.1 6.1.5.2

System Design Link Power Budget of Optical Transmission Systems Changes of the Transmitted Power Sensitivity of the Receiver Attenuation of the Fiber Link Coupling Losses from the Transmitter into the POF Losses in the Fiber Link Connector Losses Passive Component Losses Coupling Losses between POF and Receiver The Link Power Budget of the ATM Forum Specification Loss Analysis by the ATM Forum Changes in the Transmission Power Attenuation of the Polymer Optical Fiber Link Connector Losses Additional Losses through External Influences Choice of Wavelength for POF Systems LED as Transmitters for POF Systems Selection of the Type of Source

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Content

6.1.5.3 6.1.5.4 6.1.5.5 6.1.6 6.2 6.2.1 6.2.2 6.2.3 6.2.4 6.2.5 6.2.5.1 6.2.5.2 6.3 6.3.1 6.3.1.1 6.3.1.2 6.3.1.3 6.3.1.4 6.3.2 6.3.2.1 6.3.2.2 6.3.2.3 6.3.2.4 6.3.3 6.3.3.1 6.3.3.2 6.3.3.3 6.3.4 6.3.4.1 6.3.4.2 6.3.4.3 6.3.5 6.3.5.1 6.3.5.2 6.3.5.3 6.3.6 6.3.6.1 6.3.6.2 6.3.6.3 6.3.7 6.3.7.1 6.3.7.2 6.3.7.3 6.3.7.4 6.3.7.5 6.4

Typical Losses for LED Sources Lasers for POF Systems VCSEL and RC-LED for POF Systems Definition of new LED Parameters Examples of Link Power Budgets ATM Forum Specification IEEE1394b D2B and MOST ISDN over POF Link Power Budget for Bi-Directional Transmission Asymmetrical Couplers Symmetrical Couplers Overview of POF Systems Step Index Profile POF Systems at 650 nm The first SI POF Systems SI POF Systems with over 500 Mbit/s SI-POF Systems with more than 500 Mbit/s SI-POF Systems at the POF-AC Nürnberg Systems with PMMA SI POF at Wavelengths below 600 nm Systems with AIII BV Semiconductor LEDs Systems with GaN LEDs Commercial Developments POF-AC Systems Systems with SI-POF at Wavelengths in the Near Infrared Range PMMA Fiber Systems for Infrared PC Fiber Systems in Infrared System Experiments at the POF-AC Systems with PMMA GI-POF, MSI-POF and MC-POF PMMA GI-POF System Experiments before 2000 Recent PMMA GI-POF Systems System Experiments by Telekom and POF-AC Systems with Fluorinated POF First Systems with PF-GI-POF Experiments at the Technical University of Eindhoven Data Rates over 5 Gbit/s with GI-POF POF Multiplex Wavelength Multiplex Systems with PMMA POF Wavelength Multiplex Systems with PF-GI-POF Bi-Directional Systems with POF Special Systems, for Example, with Analog Signals Video Transmission with POF Transmission of Analog Modulated Digital Signals Radio over Fiber Mode Multiplex Fiber Ribbon Systems Other Optical Transmission Systems with Fibers

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6.4.1 6.4.2 6.4.3 6.4.4 6.5

Data Transmission on High-Temperature POF Multi-Parallel POF Connections Systems with 200 μm PCS and Semi-GI-PCS Systems with Glass Fiber Bundles Overview and Comparison of Multiplex Techniques

p. 546 p. 548 p. 550 p. 555 p. 557

7. 7.1 7.1.1 7.1.2 7.1.3 7.2 7.2.1 7.2.2 7.2.3 7.2.4 7.2.5 7.2.6 7.2.7 7.2.8 7.2.9 7.2.10 7.3 7.3.1

Standards Standards for Polymer and Glass Fibers Polymer Fibers Plastic Clad Glass Fibers Fibers in General Application Standards ATM Forum (Asynchronous Transfer Mode) IEEE 1394b SERCOS (SErial Realtime COmmunication System) Profibus INTERBUS Industrial Ethernet over POF D2B (Domestic Digital Bus) MOST (Media Oriented System Transport) IDB 1394 EN 50173 Standards for Measurement Techniques The VDE / VDI Guideline 5570

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8. 8.1 8.1.1 8.1.1.1 8.1.1.2 8.1.1.3 8.1.1.4 8.1.1.5 8.1.1.6 8.1.1.7 8.1.1.8 8.1.2 8.1.2.1 8.1.2.2 8.1.2.3 8.1.2.4 8.1.2.5 8.1.3 8.1.3.1 8.1.3.2 8.2

Application of Polymer Optical and Glass Fibers Data Transmission with POF POF in the Automotive Field D2B MOST Byteflight IDB 1394 MOST with PCS Outlook for the Automobile Networks Corrugated Micro Tube POF Cable in the Car Optical Camera Links for Trucks Data Networks in Apartments and Buildings Use of POF in LAN Applications Use of POF in Private Networks POF and the Development of Broadband Networks POF and Wireless POF Topologies Interconnection Systems with POF Parallel Date Transmission with Glass Fibers Parallel Data Transmission with POF POF in Lighting Technology

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Content

XVII

8.2.1 8.2.1.1 8.2.1.2 8.2.2 8.3 8.3.1 8.3.2 8.3.2.1 8.3.2.2 8.3.2.3 8.3.3 8.3.3.1 8.3.3.2 8.3.4 8.3.4.1 8.3.4.2 8.3.4.3 8.3.4.4 8.3.5 8.3.5.1 8.3.5.2 8.3.5.3 8.3.5.4 8.3.6

POF for Light Guiding POF for Advertising Pillar Illumination POF Starry Ceiling Lights Side-Lighting Fibers POF in Sensor Technology Remote Powered Sensors Transmission and Reflection Sensors POF as Distance Sensor POF Sensors for Concentration Deformation and Pressure Sensors Sensors with Fibers as Sensitive Elements The POF Scale POF Expansion Sensor Sensors with Surface-Modified Fibers Bending Sensors with Notched Fibers POF Evanescence Field Sensors Fill Level Sensors POF Bragg Grating Sensors Sensors for Chemical Materials Humidity Biosensors Liquids Corrosion Glass Fiber Sensors

p. 634 p. 636 p. 637 p. 639 p. 643 p. 644 p. 645 p. 645 p. 647 p. 647 p. 649 p. 649 p. 650 p. 652 p. 652 p. 654 p. 656 p. 657 p. 658 p. 659 p. 660 p. 661 p. 662 p. 662

9. 9.1 9.2 9.3 9.4 9.4.1 9.4.2 9.4.3 9.4.4 9.4.5 9.4.5.1 9.4.5.2 9.4.5.3 9.4.5.4 9.4.5.5 9.4.6 9.4.6.1 9.4.6.2 9.4.6.3 9.4.6.4 9.4.6.5

Optical Measuring Methods Overview Measuring Power Dependence on the Launch Conditions Measurement of the Optical Parameters Near Field Far Field Inverse Far Field Index Profile Attenuation Insertion and Substitution Methods Cut-Back Method Measuring Attenuation for Discrete Wavelengths Measuring Attenuation over a Larger Spectral Range Results of Measurements Optical Backscattering Method Principle of the ODTR Improvement in the Resolution by Deconvolution Commercial POF OTDR Experimental POF OTDR Measurement of the Connector Attenuation

p. 665 p. 665 p. 666 p. 670 p. 674 p. 675 p. 679 p. 684 p. 687 p. 688 p. 688 p. 690 p. 690 p. 692 p. 698 p. 704 p. 704 p. 708 p. 709 p. 711 p. 713

XVIII

Content

9.4.6.6 9.4.7 9.4.7.1 9.4.7.2 9.5 9.6 9.6.1 9.6.2 9.6.2.1 9.6.2.2 9.6.2.3 9.7 9.7.1 9.7.1.1 9.7.1.2 9.7.1.3 9.7.1.4 9.7.1.5 9.7.1.6 9.7.1.7 9.7.2 9.7.3 9.7.4 9.7.5 9.8

Bandwidth Measurements with OTDR Dispersion Time Based Measurement Frequency Based Measurement Connector Measurements The Reliability of POF Environmental Influences on Polymer Optical Fibers The Effect of Environmental Influences on Optical Transmission Attenuation Factors of Polymer Optical Fibers Detection by Measuring Optical Transmission Detection by Measuring Backscattering Investigation of Reliability under Various Environmental Influences Mechanical Stress Repeated Bending Flexing Torsion Tensile Strength Impact Strength Crushing Strength Vibration Stress due to Change in Climatic Conditions Aging due to the Stress of High Temperature and Humidity Resistance to Chemicals Stress Caused by Ultraviolet and High-Energy Radiation Standards and Specifications

10. Simulation of Optical Waveguides 10.1 Modeling of Polymer Optical Fibers 10.1.1 Types of Fibers 10.1.2 Modeling Approaches 10.1.2.1 Approaches with Wave Theory 10.1.2.2 Ray Tracing Procedure 10.1.3 Wave Theory Description 10.1.3.1 WKB Method 10.1.3.2 Step Index Profile Fiber 10.1.3.3 Graded Index Fibers with Power-Law Profile 10.1.3.4 Multi Step Index Fibers 10.1.3.5 Determining the Mode Power Distribution 10.1.3.6 Calculating the Transmission Function and the Output Signal 10.1.4 Ray-Tracing 10.1.4.1 Step Index Fibers 10.1.4.2 Graded Index Fibers 10.1.4.3 Multi Step Index Fibers 10.1.4.4 Bends 10.1.5 Mode-Dependent Attenuation 10.1.5.1 Additional Path-Dependent Attenuation of Higher Modes

p. 714 p. 716 p. 716 p. 718 p. 719 p. 722 p. 722 p. 724 p. 724 p. 725 p. 727 p. 729 p. 729 p. 729 p. 733 p. 735 p. 738 p. 741 p. 745 p. 746 p. 747 p. 749 p. 756 p. 759 p. 760 p. 763 p. 763 p. 765 p. 766 p. 766 p. 767 p. 768 p. 768 p. 769 p. 770 p. 771 p. 772 p. 772 p. 773 p. 774 p. 774 p. 775 p. 776 p. 776 p. 777

Content

XIX

10.1.5.2 Additional Losses of Higher Modes through Loss-Encumbered Reflections 10.1.5.3 Goos-Hänchen Effect 10.1.6 Mode Mixing 10.1.6.1 Coupled-Mode Theory 10.1.6.2 Diffusion Model 10.1.6.3 Application with the Aid of the Split-Step Algorithm 10.1.6.4 Phenomenological Approach 10.2 Examples for Simulation Results 10.2.1 Calculating the Bandwidth of SI Fibers 10.2.2 A Linear POF Propagation Model 10.3 Measurement and Simulation of Bandwidth of PF-GI-POF 10.4 Simulation of Optical Receivers and Large Area Photodiodes

p. 778 p. 779 p. 780 p. 781 p. 783 p. 784 p. 785 p. 786 p. 786 p. 790 p. 793 p. 797

11. 11.1 11.2 11.3 11.4

p. 803 p. 803 p. 804 p. 807

11.5 11.6 11.7 11.8 11.9 11.10

POF Clubs The Japanese POF Consortium HSPN and PAVNET The French POF Club The Information Technology Society (ITG) sub committee (FG) 5.4.1 “Polymer Optical Fibers” The Polymer Optical Fiber Application Center (POF-AC) at the University of Applied Sciences Nürnberg VDI Working Group „Testing of Polymer Optical Fibers“ Product Directory POF-Atlas The POF-ALL Project The Korean POF Club Worldwide Overview

p. 807 p. 811 p. 815 p. 815 p. 816 p. 820 p. 822

References

p. 823

Translator

P. 874

Index of Key terms

p. 875

List of Advertisers

p. 883

Biographies

p. 885

List of Abbreviations and Symbols

Symbol

Explanation

D

Angle (here in an optical dense medium relative to the axis of incidence) Attenuation (coefficient) in dB/km Maximum propagation angle in the fiber Effective loss Excess loss Loss by cross area mismatch Linear expansion coefficient of HDPE POF attenuation for LED Loss by NA mismatch Linear expansion coefficient of PMMA, PA6 Complete attenuation Attenuation coefficient in km-1 Attenuation coefficient due to Rayleigh scattering Critical angle of total reflection Angle at the grating Propagation constant Non linear refractive index Propagation angle of a ray (see Fig. 2.3) Maximum propagation angle of a ray (see Fig. 2.3) Relative refractive index difference Spectral width Frequency difference Absolute refractive index difference Time difference generally Propagation time difference due to mode dispersion Propagation time difference due to profile dispersion Propagation time difference due to material dispersion Slit width Angle difference of the fiber axis Angle of a ray in the fiber relative to the cladding Non linear parameter Maximum angle of a ray in the fiber relative to the cladding

D Dmax Deff Dexcess DFl DHDPE DLED DNA DPMMA ,DPA6 Ds D‘ D`s DT E E F3 G Gmax ' 'O 'f 'n 't 'tmod 'tprof 'tmat 'x H J J Jmax

XXII

Abbreviations and Symbols

4´ 4max 2 4max Tmax1, Tmax2 Wgr Wm \ Z :1, :2 6z dE/dZ dR/dt

Reciprocal coupling length Coupling efficiency Frequency of the mode group m on the total power Exponent for pulse broadening Wavelength Source wavelength Various wavelengths Blaze wavelength Angle (here in the optical thinner medium, relative to the axis of incident) Angle of a reflected beam Acceptance angle Aperture angle of a fiber Various acceptance angles Group propagation time Propagation time of the mode group m Angle of skew rays relative to tangential plane Circular frequency Various solid angles Summarized number of elements Noise current density Group propagation time Reaction rate

a a a aT aT,L A AN AN Launch AN min, AN max AN1, AN2 A, B A 1, A 2 A/D ADSL AM APD ASK ATM AWG AZ

Fiber core radius Attenuation value Acceleration Acceleration factor Acceleration factor relevant to durability Attenuation Numerical aperture Launch NA Minimum and maximum numerical aperture Various numerical apertures Constants Fiber cross areas Analog/Digital Asymmetrical Digital Subscriber Line Amplitude Modulation Avalange Photo Diode Amplitude Shift Keying Asynchronous Transfer Mode Arrayed Waveguide Grating Active layer

Jf KEl Km N O Osource O1, O2, O3, O4 OB 4


XXIII

B BAM BB BBP BER BK BPSK BR BzMA

Bandwidth (generally) Federal Institute for Material Research and Testing Bromobenzene Benzyl n-Butyl-Phtalate Bit Error Ratio Broadbanf cable (Breitbandkabel) Binary Phase Shift Keying Bit Rate Benzyl Methacrylate

c cm cv C Cmn CPD CAN CCD CCP CD CDC CDM CDMA CMT CNR CSO CTB CYTOP

Velocity of light Velocity of light in a medium Velocity of light in vacuum (2.99792458 · 108 m/s) Constant value (generally) Coupling coefficient between the modes m and n Capacity of the photo diode Controller Area Network Charged Coupled Device Customer Convenience Port Compact Disk Compact Disk Changer Code Division Multiplex Code Division Multiple Access Corrugated Metallic Tube Carrier to Noise Ratio Composite Second Order Composite Triple Beat Cyclic Transparent Optical Polymer (Asahi Glass Comp.)

d d d d 1, d 2 dGM(4) dm dmin, dmax dray D D D D DA DK Dm Drec D2B

Fiber diameter Diameter of the cable lay unit (Chapter 4.2) Reciprocal grating constant Various diameters Field penetration depth, dependent on the angle of incident 4 Cladding thickness Minimum and maximum diameter Beam diameter Wire diameter Distance generally Diffusion constant Dispersion constant Diameter of the stranding basket Insertion loss Average diameter of the cable lay up layer Reciprocal dispersion Digital Domestic Bus (serial bus for automotives)

XXIV


DA DBR DC DEMUX DFB-LD DH DH-MQW DNP DPS DPSK DSI DVB DVD

Wire pair (Doppelader) Distributed Bragg Reflector Direct Current Demultiplexer Distributed Feedback Laser Diode Double Heterostructure Double Heterostructure Multi Quantum Well Dry Non Polish (connector system from AMP) Diphenyl-Sulfide Differential Phase Shift Keying Double Step Index Digital Video Broadcasting Digital Versatile Disk

e/o E EEl ECOC EL ELED EN EMD EOF ETFE EVA

Electro/optical Receiver (Empfänger) Electrical field of the modes European Conference on Optical Communication Effective Laser Launch Edge emitting LED European standard Equilibrium Mode Distribution Elastomer Optical Fiber Tefzel® Ethylen-Vinylacetat-Copolymere

f f f f0 f3dB fa fgr F Fmax FDM FDMA FEC FEP FET FM FOP FP-LD FSK FTTB, FTTH FWHM

Frequency generally (Hz) Extension factor (chapter 4.2) Focal length Reference frequency Bandwidth at 3 dB below maximum Sampling frequency Cut off frequency Force generally Maximum force Frequency Division Multiplex Frequency Division Multiple Access Forward Error Correction Tetrafluoroethylen-Hexafluoropropylene Field Effect Transistor Frequency Modulation French Plastic Optical Fibre Fabry-Perot Laser diode Frequency Shift Keying Fiber To The Building, Fiber To The Home Full Width at Half Maximum


g g(t) GI GI-MPOF GI-PCS GOF GRIN

Index coefficient Pulse response Graded Index Graded Index profile multimode MPOF Graded Index Plastic Clad Silica Glass Optical Fiber Graded Index (continuously index ....)

h h(t) H(f) H0 HAVi HC-MPOF HCS HDMI HDTV HEC HFC HFIP 2-FA HL Homeplanet HPCF HSPN

Planck´s constant (6.629 · 10-34 Js) Impuls response Frequency response Thermal value Home Audio Video Hollow Core MPOF Hard Clad Polymer High Definition Multimedia Interface High Definition Televison Hydroxylethylenzellulose Hybrid Fiber Coax Hexafluoroisopropyle 2-Fluoroacrylate Semiconductor (in German: Halbleiter) Home Plastic Fiber Networks based on HAVi Hard Plastic Clad Fiber High Speed Plastic Network

I Iph IRMS Ith IDB IGPT IR ISDN ISM ITG

Current generally Photo current Noise current (root mean square) Threshold current Intelligent Data Bus Interfacial Gel Polymerization Technique Infrared Integrated Services Digital Network Industrial, Scientific, and Medical Band Informationstechnische Gesellschaft (Information Technology Society)

Jl(u) JIS

Bessel function Japanese Industrial Standard

k Kl (v) kr KF KIST KPCF

Boltzmann´s constant Bessel function Radial component of the propagation vector Correction factor Kwangju Instuitute of Science and Technology Korea POF Communication Forum

XXV

XXVI


l L L1, L2 Lc LAN LD LED Low-NA LWL

Peripheral order Length Lengths of different optical path’s Coupling length Local Area Network Laser Diode Light Emitting Diode Reduced Numerical Aperture Optical waveguide (in German: Lichtwellenleiter)

m M M M ('z) M1, M2 MC MC-GOF MCVD MFC MGDM MIMO MMA MM-GOF MOST MPOF MP3 MP-P MP-MP MPEG MQW MSI MUX

Order of refraction Material dispersion parameter Highest group number Mode coupling matrix Various monomers Multi Core Multi Core Glass Optical Fiber Modified Chemical Vapor Deposition Mode Field Converter Mode Group Division Multiplex Multiple Input - Multiple Output Methylmethacrylat Multimode Glass Optical Fiber Media Oriented System Transport (serial bus in automotives) Microstructured POF Compression method for music Multipoint to Point Multipoint to Multipoint Motion Picture Expert Group (data compression standard) Multi Quantum Well Multi Step Index Multiplexer

n n n0 n1 n2 n1, n2, n3 nair ncladding ncore ncore, max nPMMA N NA

Refractive index Number of layer (chapter 2.8.3) Refractive index of air (approx. 1) Rotational speed of stranding basket (chapter 2.8.3) Rotational direction and speed of the capstan gear (chap. 2.8.3) Refractive index in various media Refractive index of air Refractive index of the cladding Refractive index of the core Maximum refractive index of the core in GI fibers Refractive index of PMMA Number of guided modes Numerical Aperture


NEXT NRC-LED NRZ NTBA NTC

Near End Crosstalk Non Resonant Cavity LED Non Return to Zero (modulation format) Network Termination - Basic Access Negative Temperature Coefficient

o/e OIIC OTDR OVAL

Optical/electrical Optical Interconnected Integrated Circuits Optical Time Domain Reflectometer Optical Video/Audio-Link

p P P P0 P0x, P1x Peff Pelectr, Pel PL PL1, PL2 Popt Pout Pr Preceiv P(f) PA, PA-6 PAM PAVNET PC PC PC(AF) PCS PE PE-FRNC PE HD PE LD; MD PFA PFM PFM PF-POF PhMA pin-PD PLC P-LED PLL PNA

Impulse Profile dispersion Power generally Output power Power for measurements of connector losses Effective power Electrical power Power at the length L Power at fiber outputs Optical power Output power Backscattered power Received power Power at the frequency f Polyamide, Polyamide 6 Phase Amplitude Modulation Plastic Fiber and VCSEL Network Personal Computer Polycarbonate Partially fluorinated polycarbonate Plastic Clad Silica Polyethylene Polyethylene flame-retardant/halogenated Polyethylene (high density) Polyethylene (low density; medium density) Tetrafluoroethylen-Perfluoroalkylvinyl-Ether Preform method Pulse Frequency Modulation Perfluorinated POF Phenyl-Methacrylate Photo diode with p-i-n-semiconductor structure Power Line Communication Polymer LED Phase Locked Loop Phone Network Association

XXVII

XXVIII


PMMA PMMA-d8 PMT POF POF-AC

POFTO PP P-MP P-P PS PRBS PSK PTC PTFE PUR PVC, PVC 90° PVC flame ret.

Polymethylmethacrylate Complete deuterinated PMMA Photo Multiplier Tube Polymer Optical Fiber Polymer Optical Fiber Application Center at the University of Applied Sciences Nürnberg Paving the Optical Future with Affordable Lightning-Fast Links (EU project: www.ist-pof-all.org) POF Trade Organization Polypropylene Point to Multipoint Point to Point Polystyrole Pseudorandom Bit Sequence Phase Shift Keying Positive Temperature Coefficient Polytetrafluoroethylene Polyurethan (thermoplastic) Polyvinylchloride, Polyvinylchloride 90°C Polyvinylchloride flame retardant

QAM QWGext

Quadrature Amplitude Modulation External quantum efficiency

r rk R, R R R RH R RC-LED RIE RML

Radius generally Radius, which is not remained by helical rays (Fig. 2.7) Responsivity Electrical resistance Bending radius Radius vector Relative Humidity Radius of MC fibers Resonant Cavity LED Reactive Ion Etching Restricted Mode Launch

s s sH S S S Sin, Sout SC SCM SDM

Pitch length Axial distance of fibers Produced pitch length Transmitter (German: Sender) Backscattering coefficient Safety coefficient Modulation signal at input and output Strain Compressed Subcarrier Multiplex Space Division Multiplex

POF-ALL


Semi-GI SERCOS SI Si-PD SLED SM-GOF SNR SOA SQW SP1, SP2 ... St.-NA

Semi graded index profile Serial Real-time Communication System Step index Silicon photo diode Super luminescent diode Single Mode Glass Optical Fiber Signal to Noise Ratio Semiconductor Optical Amplifier Single Quantum Well Reference points Standard NA (typ. 0.50)

t t1, t2, t3 ta, tb tf ti tin, tout tr tA tL T T Tmin, Tmax TG TS T-DSL TDM TDMA TTP

Time Different propagation times Efficiency parameter (in Tab. 3.3) Fall time Length of the launched impulse Pulse width at input and output Rise time Aging time Life time (durability) Transmitter Temperature generally Minimum and maximum temperature Glass transition temperature Reference temperature Telekom ADSL Time Division Multiplex Time Division Multiple Access Time Triggered Protocol

u U UI UKW UMTS USB UMD UV UWB

Normalized propagation constant Voltage generally Unit Interval Ultra short wave (Ultrakurzwelle) Universal Mobile Telecommunications System Universal Serial Bus Uniform Mode Distribution Ultra violet Ultra Wide Band

v v vm v, x, y, z V

Group velocity Normalized propagation constant Draw velocity Variables for the calculation of SNR (chapter 1.3.3) Fiber parameter for the calculation of the mode number

XXIX

XXX


V VB VCSEL VDE

VDI VPAc VPE V-pin

Stranding number (chapter 2.3.8) Vinyl Benzoat Vertical Cavity Surface Emitting Laser Association for Electrical, Electronic & Information Technologies (Verband der Elektrotechnik, Elektronik und Informationstechnik) Association of German Engineers (Verein Deutscher Ingenieure) Vinyl-Phenylazetate Crosslinked polyethylene Versatile Link connector from Hewlett Packard

W W WG WG1, WG2 WDM WDMA WigWam WiMax WLF WPAN

Thermal activation energy Photon energy Band gap energy Band gaps of various semiconductors Wavelength Division Multiplex Wavelength Division Multiple Access Wireless Gigabit With Advanced Multimedia Support Worldwide Interoperability for Microwave Access Williams-Landel-Ferry Wireless Personal Area Network

x x, y y, z z z z

Lateral fiber displacement Fractions in quaternary semiconductors Different path lengths Variable value generally Number of layers (chapter 2.8.3) Fiber position

1. Basics of Optical Data Communication

1.1 Light Propagation in Optical Fibers and Waveguides 1.1.1 Wave and Quantum Nature of Light Many of the properties of light such as interference, refraction, and polarization can be explained with the wave model. Others, such as the photo effect, show that light does not always behave like a continuous type of radiation, but is rather made up of very small particles called photons. These photons are elementary quanta and cannot be divided any further. The energy of a photon is expressed by the following equation: W = h · f , where W = the energy in Joule [J], h is Planck's constant = 6.626 · 10-34 Js and f is the frequency of the light in [Hz]. The frequency of the radiation is calculated from c/O, whereby c is the speed of light in vacuum = 2.99792458 · 108 m/s and O is the wavelength of light in [m]. If the energy is expressed in eV, the conversion is 1 eV = 1.602 · 10-19 As · 1 V = 1.602 · 10-19 J, 1 J = 6.25 · 1018 eV. The particle character of light becomes even more prominent, the shorter the wavelength is or the higher the frequency is. 1.1.2 Electromagnetic Spectrum Figure 1.1 shows an overview of the electromagnetic spectrum. The zone of optical waves includes the ultraviolet, visible, and infrared ranges.

frequency [Hz] 1019 1018 1017 1016 1015 1014 1013 1012 1011 1010 109 108 107 106 105 104 103 102 10 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1

1

10

102 103

1

104 105 106 107 108 wavelength [m]

Fig. 1.1: Overview of the electromagnetic spectrum

2

1.1 Light Propagation in Optical Fibers and Waveguides

The interesting ranges as far as optical transmission is concerned are the near infrared range between 850 nm and 1,600 nm for SiO2 fibers (glass optical fiber, GOF) and the visible range between 520 nm and 650 nm for polymer optical fibers (POF) because these show the least attenuation. A detailed overview of the optical range is offered in Fig. 1.2; the white line qualitatively represents the attenuation behavior of PMMA-POF.

10-14

10-12

300

10-10

400

10-8

10-6

10-4

500

10-2

800

1

wavelength [nm] 2000

1000

P O F 5

4

UV range

3

102 104 wavelength [m]

G O F

2

1

visible range

energy [eV]

IR range

Fig. 1.2: UV-, IR- and visible range of the electromagnetic spectrum (POF: Polymer Optical Fiber, GOF: Glass Optical Fiber)

1.1.3 Refraction and Total Reflection When light propagates in a medium such as a polymer, the speed of this light is reduced. The ratio of the vacuum light speed cv to the speed in the medium cm is referred to as the medium's refractive index n, which is expressed as follows: n

cv cm

Apart from the speed, the wavelength O of light passing through a medium is also altered, while the frequency f and thus the energy W remain constant. In Fig. 1.3 we see a light ray entering the optically denser medium at an angle 4 and being refracted at an angle D to the axis of incidence. A part of the light is reflected. Refraction is expressed with: sin 4 sin D

n2 . n1

By reversing the light path (transition from an optically denser medium to an optically thinner one, figure at the right), the ray is refracted away from the axis of incidence. If in this case angle D is continuously increased, the light ray stops


3

being transferred to the other medium when angle D reaches a critical value; instead, it is completely reflected. For the boundary case of total reflection, i.e. when 4 = 90q, the following applies: n1 n2

sin D T axis of incidence n1 4 4‘

axis of incidence optically thin

4

n1 < n2

n2

D

DT

optically dense

D‘ D

Fig. 1.3: Light refraction and total reflection

1.1.4 Waveguides and Optical Fibers An optical fiber consists of a highly transparent core having a refractive index ncore and a surrounding transparent cladding having a refractive index ncladding. To ensure that a light ray that has entered the fiber can be guided along it, the following must hold true: ncore > ncladding (Fig. 1.4), so that below a certain angle 4max total reflection takes place at the interface between the core and the cladding. The surrounding medium is air with the refractive index n0 | 1.

n0

ncladding

cladding

ncore

J Dmax

core

4 max refractive index n Fig. 1.4: Wave guiding within the optical fiber

Rays that strike the end face of the fiber at an angle greater than 4max are no longer completely reflected at the core/cladding interface; instead they are partly refracted into the cladding so that they are no longer completely available for

4


transmitting a signal. The following example illustrates how even small differences here can have great effects: with a core refractive index of 1.56 and a cladding refractive index of 1.49 the critical angle of total reflection is 72.77º. Thus, light rays with a maximum angle of Įmax = 17.23º with respect to the fiber axis, can propagate. Should the propagation angle exceed this value by only 0.001º, then the reflection coefficient is reduced from 100% to 95%. With this angle 310 reflections per meter will result in a fiber with a diameter of 1 mm. The remaining light output power would then be 0.95310 = 1.2 · 10-7 which corresponds to a loss of 69 dB. The shape of the waveguide can vary greatly as the three examples in Fig. 1.5 demonstrate. On the left is a singlemode glass fiber which is almost exclusively used today in the field of telecommunications. A planar waveguide can be seen in the middle and on the right a semiconductor laser in cross-section with which an optical waveguide is also formed.

singlemode fiber

n = 1.466

planar waveguide

n = 1.55

n = 3.55

n = 1.47

Ø: 10 μm

laser diode

n = 1.60

5 u 5 μm

n = 3.60

2 u 0.5 μm

Fig. 1.5: Examples for optical waveguides

Should the waveguide have very small dimensions in the light wave length ranges, then the ray optics description is not sufficient. As can be read in standard works, e.g. [Vog02], the number of possible propagation angles (modes) diminishes with decreasing diameter. An extreme case thereby is the singlemode waveguide which will be introduced in the next section. 1.1.5 Singlemode and Multimode Waveguides The number of modes in an optical waveguide is determined by the so-called V-parameter. The parameters thereby are the core radius a, the light wavelength Ȝ and the numerical aperture AN. V

2Sa AN O

As long as V is smaller than 2.405, only one mode can propagate; otherwise it is a multimode fiber. The number of modes results approximately thus:


N | V2 2 N|V

2

4

5

(step index profile) (graded index profile)

Strictly speaking, there are also two propagation states in a singlemode fiber, namely the two orthogonal polarization directions. As long as the waveguide is exactly rotationally symmetrical, or also square, and the material is completely homogenous, then both polarization directions propagate at the same speed. The number of modes always depends on the wavelength. A fiber is thus a singlemode fiber as of a certain wavelength (cut-off wavelength). The fibers dealt with in this book always have a large number of modes as shown in Table 1.1. (See Chap. 2 for a more detailed description of the fiber types.) Furthermore, you must keep in mind that in spectrally wide sources each wavelength occurring has its own modes. In a POF system with LEDs you not only have to keep the several million fiber modes in mind, but also all emitted wavelengths (a source can be considered ideally monochromatic when the coherence time is large in relation to the differences in propagation time which arise). Table 1.1: Number of modes in optical fibers Fiber type

Profile NA

Radius Otransmitter a [μm] [nm]

V

Number of Modes

Standard-POF

SI

0.50

490

650

2,368 2,804,369

Optimedia-POF

GI

0.37

450

650

1.609

647,592

MC37-POF (single core)

SI

0.50

65

650

314

49,348

PCS

SI

0.37

100

850

274

37,402

MC-GOF (single core)

SI

0.50

27

650

130

8,515

MC613-POF (single core)

SI

0.50

18.5

650

89

3,997

Lucina GI-POF

GI

0.22

60

1,200

69

1,194

GI-GOF (Europe)

GI

0.17

25

850

31

247

®

1.1.6 Overview of Optical Fibers The different types of optical fibers are described in detail in this chapter; an overview of the standards can be found in Chap. 7.2. The two following pictures show an overview of the different refractive index profiles. It can easily be seen that not only the index profiles, but also the refractive index differences - which determine the numerical aperture - and the core diameter vary considerably. The fibers with the greatest core diameter used in data transmission can be seen in Fig. 1.6. Standard SI-POF has an approx. diameter of 1 mm at a NA of 0.50. GI-POF with this diameter, but with a somewhat smaller NA, has also been available for a short time ([Yoo04]).

6


refractive index difference 0.09 0.08 0.07 0.06 0.05

PMMAgraded index fiber AN = 0.39

standard step index polymer fiber AN = 0.50

0.04 0.03 core = 900 μm

0.02 0.01

core = 980 μm

0.00

Fig. 1.6: Index profiles of different polymer optical fibers

The index profiles of different glass and polymer fibers can be seen in the next fig. 1.7. A hybrid is the so-called PCS - polymer clad silica - i.e. a silica glass fiber with a polymer cladding. The singlemode glass fibers have the smallest core diameters. For use in the 1300 nm to 1600 nm range these fibers only have a core diameter of about 10 μm. Special fibers, e.g. for erbium-doped fiber amplifiers or for fibers with non-linear properties, can even lie in the range of only 2 μm for the core diameter. These fibers are not the subject of this book. We can recommend [Vog02] as a work with an excellent overview for this area. refractive index difference 0.040 0.035 0.030 0.025 0.020

PF-GI-POF AN = 0.20

MM-GOF MM-GOF (US) (Europe) AN=0.275 AN=0.20

PCS AN=0.37 semi-GI PCS AN=0.4

0.015 0.010

SM-GOF AN=0.10

0.005 0.000 core=120μm

core=200μm

core=200μm

Fig. 1.7: Index profiles of different glass optical fibers

core= 62μm

core= 50μm

core= 10μm


7

1.1.7 Designations of Optical Fibers There are no general international guidelines for the designation of optical fibers. Due to the enormous variations in the different parameters it is hardly possible to give all fibers clear-cut designations since these would otherwise be much too long. The following list of parameters could be used for providing names. Table 1.2: Parameters in fiber names with possible versions Parameter

Description and Variants

Example

number of modes

mostly it is distinguished between single- and multimode (with V > 2.405)

singlemode fiber (SMF)

core material

General there are the variants glass, SiO2 or polymers

polymer optical fiber (POF)

special core material

primarily special polymers are marked

polycarbonate fibers (PC-POF, PMMA-POF etc.)

cladding material

Special hybrid fibers, for example SiO2-fibers with polymer cladding

polymer clad silica fiber (PCS)

index profile

The index profiles of GOF and POF can have very different variants: ¾Step index profile ¾Double step index profile ¾Graded index profile ¾Multi step index profile ¾Semi-graded index profile

SI-POF DSI-POF GI-POF MSI-POF semi-GI-PCS

number of cores

In GOF and POF there are variants of fibers with many cores

multicore fiber (MC-POF)

polarization behavior

(only in singlemode fibers) special fibers maintain the state of polarization or only one state of polarization can propagate

polarization maintaining fiber (PMF)

chromatic dispersion

(only in singlemode fibers) ¾dispersion shifted fibers ¾fibers with flattened dispersion ¾dispersion compensating fibers

Numerical Aperture

for single and multimode-fibers high NA fiber (HNA) (e.g. high NA for small bending radii)

microstructure

Bragg fibers are built by holes, or the effective refractive index is changed

bending losses fibers optimized for minimal bending losses (by high NA or micro structuring)

DSF DFF DCF

photonic crystal fiber (PCF); microstructured fiber; photonic band gap fiber bend insensitive fibers (BIF)

8

1.2 Digital and Analog Signal Transmission

standard

many fibers are accurately described in ITU-Standards e. g. singlemode fibers in ITU-G.652 G.656

standard-SMF (G.652 B1.1)

application

use in special applications, e.g. mobile networks

MOST-POF

In addition, there are numerous differences in diameter, in the numerical aperture, fiber qualities, kind, thickness, and construction of the primary coating, etc. In order to include all these parameters in fiber names it would be necessary, for example, to say: 500 μm PMMA GI-POF with NA 0.30 and black PE cladding, 1.5 mm. Some of the most common fibers are: ¾standard glass fiber (singlemode, Øcore = 10 μm, NA: 0.10) ¾standard POF (PMMA, Øcore = 980 μm, NA: 0.50) ¾PCS (SiO2 core, Øcore = 200 μm, NA: 0.37) ¾multimode fiber (MMF, GI profile, Øcore = 50 μm, NA: 0.17)

1.2 Digital and Analog Optical Signal Transmission For the communications engineer there is no doubt that, apart from spectral attenuation, the most important parameter of an optical fiber is its bandwidth. In waveguides there are usually different optical paths possible (with the exception of the very thin singlemode waveguides). The different lengths of travel along these different light paths lead to different time delays for an optical pulse, as illustrated schematically in Fig. 1.8. propagation length = fiber length/cos(D)

propagation length = fiber length

D

P

t input pulse Fig. 1.8: Different time delay due to different propagation paths

output pulse


9

For a standard POF it is valid e.g.: core refractive index: cladding refractive index: maximum propagation angle: different flight times:

ncore = 1.49 ncladding = 1.42 Dmax = 17.6° 4.9 %

Only two light paths have been shown in this illustration. However, in a real experiment there are always many rays paths so that the individual pulses overlie each other to form one more or less broadened overall pulse. Figure 1.9 shows the effects of such a pulse broadening on a digital signal.

1 0 0 1 1 1 0 0 1 1 0 1 0 0 1 0 a) fiber length

b) c) d) e)

Fig. 1.9: Influence of mode dispersion to data transmission

An optical signal is launched into the fiber and is switched on and off at the respective bit rate (curve a). With the increase in pulse broadening, the bit edges get more and more fuzzy (signal sequence with downwards increasing transmission length). As long as the amount of broadening is clearly less than the bit time, the signal will remain easily identifiable (curves b and c). If the width of the edges exceeds the bit time, the signal can no longer be detected (curves d and e). The process of pulse broadening is called dispersion. The difference between different light paths described here is called mode dispersion (any possible condition of the propagation of light in a waveguide is called a mode). Apart from mode dispersion there is also the phenomenon of chromatic dispersion (different time delays for different wavelengths) and polarization mode dispersion; however, we will not be dealing with these two phenomena at this stage. The second important quantity which determines signal quality is the signal-tonoise ratio (SNR). In POF systems the optical receiver alone is almost always responsible for the noise. Under certain circumstances you have to pay attention to mode distribution noise within thin multimode glass fibers. In modern singlemode glass fiber systems there are even many more sources of noise, e.g. fiber amplifiers. The following sections provide a short insight into the fundamentals of analog and digital transmission methods, especially in regard to the various sources of interference. The effects which are important for short-range communication are particularly elucidated.

10


1.2.1 Digital Optical Signal Transmission 1.2.1.1 Analog and Digital Signals For readers less familiar with the fundamentals of signal theory, a brief explanation of the various basic terms is provided here for a better understanding of the requirements of the different methods for the components. Generally speaking, transmission systems are classified according to the signal values to be transmitted, i.e. whether they have discrete or random values. In nature, information can be expected to be fully analog (see Fig. 1.10). We will assume that the signal of interest (for example, an acoustic signal) is to be converted into electrical voltage U(t) with the help of an electronic measuring instrument (in this case a microphone).

U(t)

t Fig. 1.10: Analog signal

Here, analog has two meanings. First, the signal is measured at any random time t. Furthermore, U(t) can take on any value. When a signal is digitized, two things usually happen. While the signal is being sampled, the values are not read in continuously but rather at discrete points (Fig. 1.11).

U(t)

sampling points t Fig. 1.11: Sampling an analog signal

11


The second step is that the voltage U must not take any random value but only particular or discrete ones (quantization, Fig. 1.12).

U(t) sampling points quantization steps t Fig. 1.12: Quantization of a signal

Figure 1.12 shows that the values no longer lie exactly on the actual curve but always at the next quantization level. The digitalization of a signal always distorts the original one. Initially, the range of the recorded frequencies is limited by the choice of the sampling rate (sampling points per second). According to the Sampling Theorem, only signals whose upper limit frequency ful is equal or smaller than the half of the sampling rate fs (ful d fs/2) are fully transformed into the digital signal. Figure 1.13 graphically illustrates this problem.

U(t)

U(t)

t

t sufficient sampling rate

sampling rate is too low

Fig. 1.13: Choice of sampling rate

In the left figure, the sampling points are spaced sufficiently close to each other. In the right figure, the signal also changes very quickly between the sampling points (higher frequencies are present). The original signal cannot be reconstructed from the points that are too far away from each other. Quantization also distorts the signal. The difference between the actual value and the quantization level can be interpreted as added noise (Fig. 1.14).

12


U(t)

quantized signal quantization noise (deviation from the original signal)

t Fig. 1.14: Generation of the quantization noise

The signal of a CD player is a good example of a digital signal. The human ear can detect frequencies up to 15 kHz to 18 kHz. Music is stored on a CD with 44,200 values per second. Thus, signal frequencies up to 22.1 kHz can be recorded. Each of these sampling values is divided into 65,536 amplitude steps (216). The original continuous signal is thus broken into 44,200 numbers per second, for example, 23,546; 22,125; 19,714; 13,120 etc.

The errors that occur as a result of this quantization are negligible. If the available levels are equally distributed to positive and negative voltages, for example to the range between +1 V and -1 V, the deviation of the real value to the next quantization level can be at most 15 μV. This is a difference of approximately 96 dB. This is the equivalent of the difference between a whisper and the sound of a loud airplane propeller 5 m away. In the world of digital signal processing, the numbers are represented with the symbols “1” and “0”. 65,536 values can be represented by 16 binary characters (16 Bit). As a binary number, the signal above would then look like this: 0101101111111010,0101011001101101,0100110100000010,0011001101000000

When the signal is transmitted, the commas are, of course, omitted. Both symbols are characterized by various signal states, for example, -1 V for the “0” and +1 V for the “1” or also “light off” for the “0” and “light on” for the “1”. Figure 1.15 illustrates the difference between the original analog signal and the binary signal generated by means of digitalization.

U(t)

analog signal t

U(t)

digital signal t

Fig. 1.15: Analog and digital signal


13

The figure shows that the digital signal changes much faster than the analog signal. This is easier to see if you consider that music at a maximum frequency of approximately 20 kHz must transfer 44,20 × 16 = 707,200 bit/s. Why digital signal processing still has many advantages over analog processing is explained in the next paragraph. 1.2.1.2 Transmission Quality of Analog and Digital Signals The analog signal from Fig. 1.10 will again serve as our starting point. A data cable will be used for data transmission. In reference to the signal, the transmission link represents an obstacle replete with many interference effects. At first, an amplifier is needed at the transmitter to create the required voltage level. He can be limited in his bandwidth and consequently distort the signal through nonlinearity. Similarly, the actual transmission line has a limited bandwidth. Other influences can also distort the signal such as a nearby source of radio interference. The receiver too will only have a limited bandwidth. The signal was attenuated while on the transmission line and thus only has a weak signal level. This is why inherent receiver noise is often the most conspicuous source of interference. Figure 1.16 summarizes the most important sources of interference. U(t) t

transmitter

fg input signal

f

distortion bandwidth limitation

transmission line

fg

f

attenuation external perturbations

receiver

fg

f

noise bandwidth limitation

output signal

Fig. 1.16: Influences on an analog signal transmission

Even though some of the sources of interference have been somewhat exaggerated, they are intended to show the problems involved in transmitting analog signals. Each element involved in the transmission can distort the original signal. These forms of interference can only be eliminated in exceptional cases and are also relevant to digital transmission, as Fig. 1.17 illustrates.

14


U(t) t

transmitter

fg

transmission line

f

fg

receiver

fg

f

f

input distortion attenuation noise signal bandwidth limitation ext. perturbations bandwidth limitation

output signal

Fig. 1.17: Influences on a digital signal transmission

The signal behind the receiver appears to be strongly distorted; but this is where the digital „trick” sets in. The receiver knows that the signal can only have one of two levels and that the signal was transferred with a specific bit rate. This knowledge is then used to reconstruct the signal free of error. The signal is at first filtered to eliminate as much noise as possible. A decision threshold is then defined. For binary signals, this is the border between “0” and “1”. At the sampling points that correspond precisely to the bit raster, the signal is compared with the threshold and then reconstructed. This procedure is shown schematically in Fig. 1.18. U(t) 010011 t received signal

filtering

decide threshold

sampling

reconstructed signal

clock recovery Fig. 1.18: Signal reconstruction in digital transmission systems

Although the signal was clearly distorted, the complete reconstruction of the original bit sequence is possible. This is what users recognize as “CD quality”. The question still remains as to how the analog signal, i.e. the music, is restored. A digital analog converter is used for this purpose. In the example shown, the converter uses 16 bits in the 65,536 intervals between -1 V and +1 V. The signal is then subsequently filtered to eliminate the resulting harmonic waves and then is ready for use (for example, to be fed into a loudspeaker). This step is not needed for communication between the digital devices.

15


1.2.1.3 Bit Error Probability and Error Correction An analog signal can usually be described quite accurately with the Signal to Noise Ratio (SNR). This is done by comparing the average signal power to the average noise power. Since there is noise in every system, the SNR is always finite. As we have seen earlier, this value is not sufficient for digital systems, since the signal is reconstructed at the receiver. To be sure, a complete error-free transmission is also not possible for digital systems. The cause for this is to be found in the statistical character of many noise processes. Only thermal noise will be discussed here. Due to the particle like structure of the electrons, every flow of current is irregular which leads to voltage fluctuations at resistors. This represents a physical bottom limit for the noise power of each system. If you apply the probability of a particular voltage deviation occurrence, this will give you a Gaussian function. In Fig. 1.19, it is assumed that the binary symbols are transmitted in an ideal system by -1 V and +1 V and that both symbols are being distorted through noise.

rel. probability density 1.2 “1” ideal

“0” ideal

1.0 0.8

“1” with noise

“0” with noise

0.6 0.4 0.2 0.0 -2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

voltage [V] Fig. 1.19: Influence of noise on ideal, digital signals

As you can see in Fig. 1.19, the levels +1 and -1 are the most likely ones but there are also other levels. In the case at hand, the decision threshold is 0 V. It appears that it is still possible to clearly differentiate between the two symbols despites the noise. However, the Gaussian curve never drops completely to 0. This is shown in Fig. 1.20 in which the same curve is scaled logarithmically.

16


relative probability density 10 0 10 -1 Symbol “0” with noise

10 -2

Symbol “1” with noise

10 -3 10 -4 10

optimal decision threshold

-5

10 -6 10 -7 10 -8 -2.0

area of bit errors voltage [V] -1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Fig. 1.20: Generation of bit errors in digital transmission systems due to noise

Here you can see that the curves for “0” and “1” overlap. This means that at times a “0” can be distorted to such a degree that it is detected as a “1” or vice versa. The hatched area represents exactly the bit error provided that the decision threshold is actually 0. Integrating this area and putting them into relationship with the integrals of the probabilities for “0” and “1”, yields the bit error probability (Bit Error Ratio BER). In this case the result would be 10-7 . Returning to the CD example, this would mean one error every 10,000,000 bits or, on the average, one error every 14 s. For normal use, this would not pose a major problem. However for data transmission, this would be entirely inadequate. If saved as a file, this book would consist of about 109 bits. At this error rate, 100 bit would be faulty. At best, this would result in incorrect characters or errors in the illustrations. A more likely scenario would be, however, that many of the errors would cause the system to crash and make the file unusable. Considering all the time we authors have invested in this documentation, this would be anything but a welcome course of action. Hence, data connections should be considerably more reliable (for example, BER < 10-15). The characteristics of noise anticipate the conclusion that error-free data transmission is nothing more than an illusion. But this is where we can call on statistics for help. If the voltage levels are increased to ±2 V, the error probability drops to approximately 10-25 for the same noise level. For our CD signal, this would mean one error in 9 billions years, i.e., this book could be transmitted several trillion times without error.


17

However, there are other ways to reduce the error probability. Particular coding schemes make it possible to detect individual errors on the receiver end (FEC: Forward Error Correction). This is achieved by adding so-called „control bits” to the signal flow. The bit rate is usually increased by only a few percent. On the receiver end, practically all errors can be corrected. These procedures are often used in mobile communications where, for example, the less than ideal transmission channels cause an inordinate amount of errors. 1.2.1.4 Noise in Optical Systems As mentioned above, there is a large number of sources of noise in optical systems all of which could hardly be dealt with in a single book. There are entire books, e.g. [Pet88], just on noise in optical transmitters - which are usually laser diodes. In systems for short-range data communication, however, you only have to pay attention to a few noise processes which are dealt with briefly here. Laser Noise: Laser diodes and LEDs are normally very low noise and stable sources. Clean driving is especially important for signal quality. Edge emitting laser diodes in particular can show high noise if light is reflected back from the transmission link. This can hardly be avoided since about 4% of the light is already reflected at the very first frontal area of the fiber. All reflections which appear within the coherence length are disruptive. These can be on occasion many kilometers when using high quality laser diodes. In order to avoid fluctuations in laser performance caused by reflections, anti reflection coatings, optical insulators and special lowreflecting plug-in connections are used. The LEDs used in most POF systems have a coherence length of a few μm and are hardly influenced by reflections. Conventional LDs or VCSELs can be disturbed by reflections. One advantage of using multimode fibers is their greater diameter compared with the emitting surface of the laser. Even if much light is reflected, only a small fraction reaches the active surface of the laser (Fig. 1.21) so that the effect is negligible.

reflected light

VCSEL active = 10 μm

Fig. 1.21: Influence of reflections in POF/PCS-systems

PCS core = 200 μm

18


Quantum Noise of the Light and the Diode Dark Current: Light and current have a basic quantum noise as they are quantized energy flows. Noise in light - called shot noise - is caused by the power and energy of the photons, i.e. by the wavelength. With reference to the photon flow, which is also determined by the responsivity, the following is true: quantum noise current: iQ2

2 e IPH B

2 e R P B

with the quantities: B: e: IPH: P: R:

signal bandwidth (determined by bit rate) elementary charge photodiode current received power spectral sensitivity (mA photocurrent per mW optical power)

As an example, we shall consider a data transmission of 1.25 Gbit/s at a wavelength of 650 nm (R = 0.4 mA/mW). The received power is -24 dBm (4 μW). At a bandwidth of 700 MHz a medium shot noise current of 19 nA results. With reference to the signal photon current of 1.6 μA the result is a SNR of 38.5 dB, consequently, the noise is negligible. In fact, practically all optical transmission systems are limited by electronic receiver noise and not by shot noise. The dark current flowing in most photodiodes also generates quantum noise. The strength of this noise is: iD2

2 e ID B

ID is dark current and lies in the nA range with normal pin-photodiodes so that the additional noise is also negligible. Receiver Noise: The most important source of noise for the systems under consideration in this book is receiver noise. In principle, every optical receiver can be described, at least in a rough comparison, as a combination of a photodiode, an ohmic input resistor and an amplifier (transistor or operational amplifier, Fig. 1.22).

POF

input resistance Fig. 1.22: Optical receivers’ principle

amplifier


19

The photo current is converted into a voltage at the input resistor which then continues to be amplified. The greater the resistance, the higher the signal voltage. On the other hand, however, every resistor generates a thermal noise according to the following equations. 2 uth

with: k: T: B: R:

4 k T B R or:

2 ith

4 k T B R

Boltzmann constant (1.38 10-23 Ws/K) absolute temperature system bandwidth ohmic resistor

In the example above, the input resistor could lie at 500 . The thermal noise current would then be 152 nA, corresponding to a SNR of 20.4 dB. The following amplifier will never work ideally. The noise figure indicates how far the actual noise lies above the theoretical minimum. Depending on the components, values of 1 dB to 3 dB are typical. The SNR is diminished correspondingly. You can easily see that the signal voltage increases proportionally to the resistance R; the thermal noise current, however, only increases with the root of R. In principle, the SNR can be raised whenever you like by increasing R. However, there is a fundamental limit. Every photodiode possesses a junction capacitance which, together with the input resistor of the circuit, forms a RC low pass filter which restricts the speed of the receiver. Therefore, you can only select a resistance as high as allowed by the diode capacitance and the bit rate. With a RC low-pass bandwidth of: f3 dB

1 2 S R C

and a minimum bandwidth of the receiver - which corresponds to one half the bit rate - the maximum input resistance may amount to:

R

1 bit rate S C

With a diode capacitance of 0.5 pF a R of about 500 , as indicated above, results. This calculation is only a very rough approximation, but it does show the principle problem involved. Diodes intended for use in singlemode fiber systems only have to be a little bit larger than the core diameter of the fiber. A typical size for photodiodes is from 30 μm to 50 μm. Their capacitance only lies at some 10 pF, but they do allow large input resistances with high data rates and thus have good sensitivity. For thick polymer fibers and PCS, however, photodiodes with very much larger surface will be needed. Their capacitance lies at some nF or some tenths nF. With high data rates the input resistance must correspondingly be reduced and thus the sensitivity. This is the only indirect influence of the fiber diameter on the possible bit rate with thick optical fibers.

20


Modal Noise: A special kind of noise only appears in multimode fibers: mode distribution noise. In multimode fibers light propagates in different specific modes, whereby each one possesses its own distribution of power over the fiber cross-section. The kind of power distribution between the modes changes with tiny changes in the exterior conditions, e.g. temperature, wavelength of the transmitter or also vibrations of the fiber, but the overall power remains constant. Figure 1.23 shows examples of power distribution of modes (multimode GOF/POF at 650 nm, laser excitation).

Fig. 1.23 Examples for energy distribution of modes in multimode fibers (left: 50 μm GI-GOF, right: 1 mm SI-POF with much higher mode number)

Steadily fluctuating power distributions only become problematical when the overall output power is not transmitted at coupling points. Random exchanges of power occur between the transmitted and the decoupled share which in the end generates additional noise (schematic depiction in Fig. 1.24). opt. power lost power

coupled power in the second fiber

fiber with fluctuating power distribution

cross section of the second fiber

Fig. 1.24: Origin of modal noise

time

21


The intensity of the modal noise depends on the number of the power maxima on the cross-section (in the order of magnitude of the number of modes) and the differences in power. For 50 μm multimode glass fibers the mode distribution noise is typically only a good 20 dB below the overall transmitted level ([Vog02]). This is insignificant for digital transmission, but makes analog transmission impossible. In order to avoid modal noise, transmission must take place either with singlemode or with very many modes since this effect is inversely proportional (statistical effect) to the root of the number of modes. Polymer fibers with several million modes are therefore of interest for analog data transmission and are much better suited than GI glass fibers. 1.2.2 Amplitude, Frequency, and Phase Modulation Whenever digital and analog signals are transmitted, there is virtually always an electrical voltage U(t) present at the beginning and at the end of the transmission path. To cover the distance, other physical parameters can also be used. The changing of these parameters is referred to as modulation. As we will see, when we want to transmit signals with light, there are a number of problems to consider. In general, the desired parameters can be modulated analog, i.e. with varying strength, or with only a small number of discrete steps. Below we will show some examples of binary, digital modulation procedures. We shall start with the electrical transmission procedure depicted in Fig. 1.25:

U(t)

U(t) t

transmitter

t receiver

Fig. 1.25: Most simple electrical signal transmission system

On the transmitter end, the signal is the voltage difference between two conductors (for example, the wires of a twisted data cable or the core and shielding of a coaxial cable). If we neglect such factors as attenuation and band limitation, the signal is tapped again directly at the output end of the cable. This method is, of course, the easiest but, experts know, also the one most susceptible to interference.

22


1.2.3 Modulating a Carrier Frequency A simple but reliable transmission method is to use a carrier frequency that is significantly higher than the cut-off frequency of the signal to be transferred (Fig. 1.26). U(t)

transmitter

Ucarrier(t)

t

t

receiver

source for carrier frequency Fig. 1.26: Signal transmission with carrier frequency

The carrier wave can be described with three parameters: amplitude, frequency, and phase. All three parameters can be used for modulation. For binary signals these are: ASK: FSK: PSK:

Amplitude Shift Keying; Amplitude Modulation Frequency Shift Keying; Frequency Modulation Phase Shift Keying; Phase Modulation

The principle is shown in Fig. 1.27. signal

ASK

FSK

PSK

Fig. 1.27: Signal transmission with ASK, FSK or PSK

In addition, there are a number of other procedures such as DPSK (Differential Phase Shift Keying), in which only the phase difference between 2 sequential bits is of significance or multilevel procedures such as QAM (Quadrature Amplitude Modulation) or combinations of several modulated parameters (PAM, Phase Amplitude Modulation). An in-depth description of these procedures can be found in many standard works on communication technology, for example [Lüke90], [Kreß89] or [Hulz96] or as a very simple introduction, in [Eng86]. Each individual modulation procedure requires special receivers that can recover the original signal. A distinction is made between procedures that measure the power of the carrier and those that operate synchronously to the carrier frequency (for example, PLL tuners for FM radio).

23


1.2.4 Specific Transmission Methods in Optical Communications The advantages of optical communications are undisputed and have been know for some time. The reader will undoubtedly be familiar with many of them. With the low attenuation characteristics of modern singlemode fiber optic cables, many hundreds of kilometers can be bridged with high data rates. If fiber amplifiers are used, transcontinental communication is even possible. Particularly systems with polymer optical fibers are well suited for short distances due to their insensitivity to interference. Light is an electromagnetic wave with a particular frequency. At 500 nm, i.e. green light, this is 6 · 1014 Hz. No electronic component is able to process this frequency. Photodiodes only measure the optical power of a light signal. Furthermore, optical sources cannot maintain their frequencies nearly as accurately as electric oscillators. Directly modulating the parameters frequency or phase (and polarization in particular) of light is only possible with so-called heterodyne receivers. This involves modulating an extremely frequency-stabilized laser on the transmitter end and heterodyning it on the receiver end with a second laser that is just as stabilized. A mixed frequency is produced at the photodiode (the difference between the both laser frequencies), which can be further processed by the subsequent electronic components. Heterodyne systems offer, at least in theory, the best frequency economy of all optical systems. Nevertheless they have not gained acceptance due to the many technical problems involved. For further information, see [Fra88] and [Ziem95]. Therefore, the only parameter left for modulation is amplitude. A photodiode measures the optical power that is converted into a proportional photo current. Since the electrical power, measured at a resistor, is proportional to the square of the current, the following relationship holds true: Pelectr ~ Iph² ~ Popt² Although the electric field of the transmitted light can take on positive and negative values, the actual measured parameter is always positive. This is a significant difference compared to electrical communication systems. Let us take a simple binary signal transmission as an example. In the electrical stage, the bits are switched between -1 V and +1 V at the transmitter. The decision threshold is set to 0 V. The levels 2 mW and 0 mW should be selected for an optical signal. The decision threshold should be at 1 mW (see Fig. 1.28).

Popt(t)

U(t) +1 V 0V -1 V

t

2 mW 1 mW 0 mW

Fig. 1.28: Electrical and optical, digital signal transmission

t

24


Initially, both systems appear similar. In the Fig. 1.29 below, additional attenuation is inserted, for example by increasing the temperature or aging the transmitter. The level drops to 40 %.

U(t)

Popt(t)

+1 V

t

0V -1 V

2 mW 1 mW

t

0 mW

Fig. 1.29: Electrical and optical, digital signal transmission with attenuation

In the electrical system, both symbol levels are equally decreased. If the noise is not too large, it will still function flawlessly. In an optical system, the zero level will, of course, remain unchanged while the “1” level falls below the threshold. The system then no longer functions. This problem, of cause, can be solved. Capacitive couplings or decision threshold controls are used, or the decision level is set initially so low that the threshold is always above the noise level of the “0” symbol (Fig. 1.30 to 1.32). optical signal

photodiode capacitve coupling

+1 V

2 mW 1 mW 0 mW

electrical signal

0V

t

t

-1 V

Fig. 1.30: Receiver with capacitive coupling optical signal

photodiode, threshold control

2 mW 1 mW 0 mW

electrical signal +2 V

t

+1 V 0V

t

Fig. 1.31: Receiver with threshold control optical signal 2 mW 1 mW 0 mW

electrical signal photodiode, threshold optimized +2 V for minimal level t

Fig. 1.32: Receiver with minimal threshold

+1 V 0V

t

25


All of the methods discussed have their advantages and disadvantages which, however, will not be elaborated further here. Various commercial systems for POF use the third method. 1.2.5 Modulating a Subcarrier To be able to use the many possibilities of carrier frequency technology in optical communications, a subcarrier can be used. Hence, light is modulated in its intensity as a sine wave. The signal is then modulated onto this carrier, whereby ASK, FSK, PSK or other methods can again be used. The receiver only needs to process the range around the carrier, i.e., it will always be coupled in a capacitive manner. Figure 1.33 illustrates the method using the subcarrier FSK as an example.

Popt(t) 2 mW 1 mW

t

0 mW Fig. 1.33: Optical signal transmission with a frequency modulated subcarrier

It should be noted that the optical power curve still represents a much higher optical frequency. One advantage of the method shown is that the average optical output power remains unchanged from the succession of the “0” and “1” symbols. Laser diodes are particularly well-suited for this type of modulation.

5 Popt(I) /mW

output signal bias current

4 3 2 1 0 0

10

20

30

40

50 ILD /mA modulation current

Fig. 1.34: Subcarrier modulation of a laser diode

26

1.3 Network Architectures

As described above, lasers are best operated with a bias current. In direct power modulation, this would be slightly below the threshold. In subcarrier modulation, the bias current is set higher than the laser threshold so that the laser is always operated above the threshold current as shown in Fig. 1.34. Thus the tools for transmitting analog and digital signals are now also available in optical communications. Apart from modulation, coding is of great importance. We would like to refer the reader to the relevant technical literature, as this would go beyond the scope of this introduction.

1.3 Network Architectures The next section of the introduction deals with various network architectures. Here too, attention will be drawn to the unique features of optical communications. The first topic is that of Point-to-Point transmission and distributed systems. The Point-To-Point system (P-P) is the simplest form of data communication, involving only one transmitter and one receiver. The second case deals with connecting several devices to each other. In this case, there can be one transmitter and several receivers (Point-to-Multipoint, P-MP), several transmitters and one receiver (MP-P) or also several transmitters and several receivers (MP-MP, see Fig. 1.35). P-P

P - MP

MP - P

MP - MP

Fig. 1.35: Possible network topologies

It should be noted that it is often necessary to send data in both directions (upstream and downstream). For example, a network can be P-MP in one direction and MP-P in the other. When more than two stations are to be connected to each other, several solutions are conceivable. These are discussed in the sections below. First we will briefly discuss active and passive networks. Then a network structure or topology must be selected, for example, a tree-shaped network or bus structure. Finally, a multi-access method must be selected. 1.3.1 Active and Passive Networks A P-MP case will serve as an example of the difference between an active and passive network. Figure 1.36 illustrates both possible solutions for this connection.


P - MP passive

27

P - MP active

Fig. 1.36: Active and passive P-MP connections

Passive means in this case that the receivers are physically connected to the same medium. As a result, every receiver receives the complete signal, even if it is intended for other receivers. This is an ideal situation for distribution services such as broadcast radio. With active networks, there is an interfacing device between transmitter and receivers that has the function of distributing the signal to the proper receivers. In principle, this interface functions as a switch, which is the reason why they are called exactly that: switches. If you take a closer look at this architecture, you can see that it actually consists of several P-P connections. Since the aim of this book is only to describe transmission technology, all other active networks can be regarded as some form of a Point-to-Point solution. You can say that every architecture can be made active by inserting the right number of switches at the branch points. The functionality of the active points can be formed in many different ways. The active points, for instance, could be used solely as an amplifier. This means that the signals are forwarded without regard to their destinations. Address and access control must be performed by other elements. A multiplex function means that the signals are routed according to their destinations and, in a MP-P structure, are combined. Some form of access control must be available also in this case to prevent collisions. A complete switch also takes on the access control functions. This can be made by rejecting non-processable data or by serving as a data buffer. 1.3.2 Network Structures The network structure describes the topology of the data connections. It can refer to the physical structure i.e. the way the cables are arranged as well as to the logical structure, i.e. the flow of the data streams. Figure 1.37 illustrates the most widely known structures. All modern data networks are designed as active star-type structures. The bestknown examples are Switched Ethernet and the ATM network (Asynchronous Transfer Mode). Tree networks play an important role for television distributing networks. Here it is crucial that each receiver receives the complete transmission signals. Data networks for residential areas, such as USB or IEEE 1394 function

28


logically as tree-shaped networks; however, they are physically set up as P-P connections. This means that several other devices can be connected to one device. Each device then forwards the complete data.

star

tree

ring

bus

Fig. 1.37: Typical network structures

In all passive networks, one problem becomes apparent, in particular when you examine the direction to the central element. Several transmitters can access the same medium. To prevent mutual blocking, this form of access must be controlled. This will be the subject of the next section. 1.3.3 Multiple Access Methods The goal of all multiple access methods is the administration of a shared channel (shared medium). The various parameters of the channel which can be used are time, frequency or amplitude. 1.3.3.1 Time Division Multiplex With Time Division Multiplex (TDM), each user is assigned a particular time. In the P-MP direction this is known as multiplexing since the transmitter simply has to split its capacity. The opposite MP-P direction is known as multiple access (TDMA) since each transmitter is only allowed to send data at the proper time. Figure 1.38 illustrates the principle. It is easy to see that the division of time does not have to be steady or continuous. The critical aspect of allotting time slots in TDMA is that data packets sent from two transmitters do not arrive at the receiver simultaneously. In principal, there are two methods for the TDMA. First the central node can be assigned with the task of permitting the transmitters to use the time slots. Obviously, all elements must be synchronized in the network. In addition, each transmitter must at least be given a temporary connection in order to inform the center that further time slots are needed.


P - MP TDM

29

MP - P TDMA

„1“

„1“

„2“

„2“

„3“

„3“

t

t t t

Fig. 1.38: TDM and TDMA

Another possibility is to initially permit every transmitter to randomly access the connection. If a collision takes place, the transmitters must be able to detect it, discontinue the transmission and retry it after a set time. As long as the required transmission capacity is small in relationship to the available capacity, this method works quite well since the probability of collision is relatively low. This is all the more true if delays occurring occasionally due to the wait for free time slots can be accepted. This is the method used for Ethernet. The advantage is that no centralized capacity administration is required. The disadvantage is that transmissions requiring continuous data rates and constant delay times (for example, video) are quite unreliable. This is a good time to address a widely-spread misconception concerning access control and the question of using an active or passive network. Both network types require the same overhead for access control. The only advantage of an active network is that the physical access can be separated from management of the data streams. In the figure above, this means that the identical TDMA method can be selected or that a buffer in the active node can prevent collisions (see Fig. 1.39). In passive networks, the malfunctioning of a single component can lead to a complete failure of the network. If a station sends signals uninterruptedly, all other stations will be affected. In an active network, the incorrect working station can simply be ignored. On the other hand, there are additional components in the active network, i.e., the transmitter and receiver in the distribution point and the central switch. All of these elements can fail and result in a total loss of the network. The question as to what type offers the better reliability cannot be answered conclusively. The particular requirements must always be taken into account.

30


t

t „1“

„1“

„2“

„2“

„3“

„3“

buffer memory

t

t

t

t

t

t

Fig. 1.39: TDMA in active networks

1.3.3.2 Frequency Division Multiplex In frequency multiplex or multiple access (FDM/FDMA), the signals of each station are modulated on a separate carrier. All stations are allowed to transmit simultaneously. The signals are divided using band pass filters, as shown in Fig. 1.40 (Fig. 1.41 illustrates multiple access).

f1

P - MP, FDM band pass 1

f2

band pass 2

f3

band pass 3

Fig. 1.40: FDM in star networks

f1

MP - P, FDMA

band pass 1

f2

band pass 2

f3

band pass 3

Fig. 1.41: FDMA in star networks

The advantage over TDM is that selection of the band pass filters determines the synchronization. If these are permanently built in, there is practically no interference from other stations. But unfortunately it is then no longer possible to dynamically allocate the capacity. This is why the frequency channels are often assigned by the central node temporally on demand.


31

1.3.3.3 Code Division Multiplex For code division multiplex/multi-access (CDM/CDMA), all stations transmit simultaneously in the same frequency range. Instead of individual bits, special sequences (codes) are sent. These must be known to the receiver. Using special receivers that are often very costly, the signals of the various transmitters can be divided. The immense costs for CDMA systems is often worth it for poor quality channels such as in mobile telephone communications since CDMA is highly immune to many sources of interference. In optical communications this method is rather insignificant. 1.3.3.4 Wavelength Division Multiplex A special feature in optical communications is the possibility of using various optical frequencies in a single network. Generally speaking, we are talking about different carrier frequencies. However, they are so great, just like the distances between them, that they cannot be processed by electrical components. For example, processing, combining, separating or filtering the various wavelengths is done exclusively by optical components. In singlemode glass fiber technology, a number of components have been developed over the last few years such as arrayed wave guides (AWG), Fiber-Bragg gratings and wide band tunable lasers. They have made WDM the key technology for optical communications (see, for example, [Hulz96]). Soon many hundred optical channels will be transferred over glass fibers that will be forwarded transparently over hundreds of kilometers in optical nodes. Wavelength division multiplexing also plays an increasing role for polymer optical fibers, as we shall see later. In principle, the access methods introduced here can be combined in any way. A complete time division multiplex channel can, for example, be inserted into a wavelength channel. 1.3.3.5 The Special Features of Optical Multiplexing The special features of optical signal transmission have already been mentioned earlier. Another difference to electrical systems requires some explanation. The level of an electrical signal is defined by the voltage at a resistor. This makes it possible to split a signal into several points, as is done, for example, in bus systems by selecting suitable resistors shown in Fig. 1.42.

U(t) 50 : 15 k: 15 k: 15 k: 15 k: 15 k: 15 k: Fig. 1.42: Distribution of an electrical signal

32


The low-ohmic-transmitter supplies a data line with 50 :of characteristic impedance. The line is also terminated with 50 :. The various stations access the line with high-value resistor connections (these lines should be short). Thus, the signal is only loaded to a small degree. The same level is detected at practically all stations (disregarding the attenuation of the data line). For very broadband signals, for example, for broadcast television, the receivers must all have the same impedance, i.e. 75 :. Splitters are available that can divide up the output with nearly no loss. With two receivers, each receiver detects a level that is 3 dB lower, that is to say, approximately 70% of the transmitting voltage. Unfortunately in optics, a “high-ohmic resistance” access is not possible. Ideally, the optical transmitting power can be distributed to all connected stations. With 2 receivers, the received power is one half of the transmitting power. This results in one half of the photo current at the receiver and consequently 6 dB less electrical power. We encounter the same problem when trying to join optical signals. If you assume uniform mode distribution in optical fibers, then couplers will have losses during distribution as well as when joining that correspond to the number of optical branches. Figure 1.43 shows typical components.

X-splitter 3 dB loss

Y-splitter 3 dB loss

Y-coupler 3 dB loss

1 : 4-splitter 6 dB loss

Fig. 1.43: Losses at different optical couplers

The minimum loss of 3 dB (half of the optical power) is easy to see for the X-coupler as for the Y-divider. Because of the reversibility of the light path, this also applies to the Y-coupler. If the divider at the far right is used as a coupler, its attenuation will also be at least 6 dB. With polymer fibers, additionally the losses that are unavoidable for multimode fibers must be taken into account. There is a „trick”, however, for coupling and splitting light waves without losses for singlemode fibers. If the light segments to be separated or combined differ in wavelength or polarization, the proper WDM couplers (or polarization couplers) can be used. Otherwise WDM systems would not be feasible with over 100 channels. For polymer optical fibers, the appropriate WDM components are relative complex, as indicated below. The special features of optical communications are adverse for particular multiplex methods. If, for example, you want to use time division multiplexing with two transmitters and two receivers that are located at two different sites, they must be arranged as shown in Fig. 1.44.


transmitter 1

33

receiver 1

transmitter 2 coupler

splitter

receiver 2

Fig. 1.44: Example for an optical network

The TDMA method ensures collision-free use of the shared transmission link. The optical components yield a minimal loss of 3 dB + 3 dB = 6 dB. If the signals could be combined before the optical transmitter and split behind the receiver, this loss could be prevented. This would, for example, also increase the range. When designing an optical transmission system, careful consideration should always be given to the question as to which functions can be implemented better optically and which ones better electrically. 1.3.3.6 Bi-directional Transmission Bi-directional transmission on one channel plays a special role in multiplex methods. Only two directions have to access the channel. Classic multiplex methods are applicable particularly in optical signal transmissions. While we have only been discussing ideal systems, at least one interference factor will be explained here that poses restrictions on bi-directional systems. This is near end crosstalk (NEXT). Figure 1.45 illustrates how a receiver can interfere with its own transmitter if there are reflections on the line.

transmitter 1 x dBm

disturbance, depressed by v dB

transmitter 2 y dBm

channel attenuation z dB receiver 1

receiver 2

Fig. 1.45: Near end cross-talk in optical systems

Transmitters 1 and 2 operate simultaneously on both sides of the channel. Since it cannot be assumed that we always have the same power, let us just assume levels x and y. The signal of the remote transmitter 2 therefore arrives attenuated with (y-z) dBm at receiver 1. At points of interference in the channel, for example a plug-in connection, a part of the light emitted from transmitter 1 may directly fall onto its own receiver. We will call the attenuation of transmitter 1 to recei-

34


ver 1 as v. The value of v should be as large as possible. Thus, the interference level through NEXT is x - y. If you assume that a particular SNR is required for error free operation, the following inequality holds true: v > (x-y) + SNR + z Let us look at a practical example: The difference between the transmitting powers is max. 6 dB, i.e. at worst (x - y) = 6 dB. The SNR should be at least 16 dB and the path attenuation is z = 18 dB. The following then applies: v > 6 dB + 16 dB + 18 dB = 40 dB. This value is very difficult to maintain with POF components. Additional measures for NEXT suppression are required. They will be introduced below. First, a system with time division multiplex will be shown (Fig. 1.46). On the fiber medium, the required data transfer rate is more than twice as fast as the individual data streams on both sides since less than half the time is available due to the run times that occur for each direction. It is common to divide up data streams into larger packets that are then transmitted alternately. The larger the blocks are, the less influence the run time has on the line. However, an increasing delay is generated through the required buffering of a complete packet. buffer memory with bit rate adaption transmitter 1

buffer memory with bit rate adaption transmitter 2 in

in fiber line

out

out receiver 1

receiver 2

transceiver 1 transceiver 2 transport:

12

time 21

12

21

Fig. 1.46: Bi-directional transmission with time multiplex

These types of solutions are very good, particularly for systems with low and medium data rates and short distances, for which the POF is also used, and can be implemented at low cost since the entire data processing is performed in integrated circuits. A second possibility is frequency division multiplex, by which the data of both directions are modulated upon different carrier frequencies (Fig. 1.47).

35


source 1

mixer

source 2

mixer in

in transmission link

out filter

receiver 1

out receiver 2

filter

isolation filter transport from 1 to 2

transport from 2 to 1

frequency

Fig. 1.47: Bi-directional transmission with frequency multiplex

The bandwidth requirements of this system are relatively high, because a carrier frequency modulation needs typically twice the bandwidth as a direct NRZ modulation (Non Return to Zero, switching between one and zero at the end of each bit). Furthermore, a certain „guard band” is also required between the bands. On the other hand, subcarrier methods are very immune to interference and allow continuous simultaneous operation in both directions. The NEXT suppression is very good since filtering in the electrical domain of the receiver can function very efficiently. Especially for low data rates, this method is very well suited. Signals can be easily processed with a small number of analog components or else completely with a signal processor. For wavelength division multiplexing, every transmission direction is assigned to a separate wavelength, as can be seen in Fig. 1.48. source O1

source O2 Y-splitter

Y-splitter

in

in out receiver

out

transmission link WDM-filter

WDM-filter

receiver

Fig. 1.48: Bi-directional transmission with wavelength multiplex

This procedure has the big advantage that the full capacity of the fiber medium is available for each direction. Continuous operation without additional delays is possible. The NEXT suppression is performed by optical filters placed before the receivers. Signal processing is not necessary. A disadvantage is that two different transceivers are always required on one link. However, this is a question concerning the system concept. WDM is particularly interesting for fast data transmission, such as IEEE 1394. WDM also provides efficient solutions for systems with asymmetric data rates.

2. Optical Fibers

2.1 Fundamentals of Optical Fibers The term optical fibers indicates special forms of optical waveguides, the most important special features of which are: ¾rotationally symmetrical cross-section ¾flexible ¾can be produced in great lengths The characteristics of optical fibers are determined by a multitude of possible constructive details. For example, the material selected primarily determines the attenuation and the thermal stability. On the other hand, the optical bandwidth, in essence the transmission capacity, is determined by the refractive index profile. This is most likely the reason why most optical fibers are named after their index profile. All current variations will be presented in the following sections. 2.1.1 Refractive Index Profiles The properties of wave guiding through a fiber are governed largely by the profile of the refractive index of the core and cladding. In a step index profile fiber the refractive index is constant across the entire cross section of the core and cladding (Fig. 2.1) while the light rays propagate along straight lines in the core and are completely reflected at the core/cladding interface.

r a refractive index n(r)

-a Fig. 2.1: Refractive index profile in a step index profile fiber

38

2.1 Fundamentals of Optical Fibers

The profile of the refractive index in the core and in the cladding is expressed as follows: n(r )

ncore

for r d a

n(r )

ncladding

for r ! a

where a is the core radius. The individual rays cover different distances, so that there are considerable differences in their respective transit times. Choosing a fiber with a graded-index profile can minimize these differences. Fibers with a graded-index profile are made up of a core having a radius-dependent refractive index and a cladding with a constant refractive index (Fig. 2.2): n(r )² n(r )

where

g § §r· · ncore, max ² ¨1 ' ¨ ¸ ¸ for r d a ¨ © a ¹ ¸¹ © ncladding for r ! a

g is the profile exponent and ' is the relative refractive index difference: '

a

ncore 2 ncladding 2 2 ncore 2

r

-a refractive index n(r) Fig. 2.2: Principle of a fiber with a graded-index profile

Those rays propagating in the center travel a shorter distance, but because of the higher refractive index there, they travel at a lower speed. On the other hand, the smaller refractive index near the cladding causes the rays traveling there to have a higher velocity, but they have a longer distance to travel. By choosing a suitable profile exponent it is possible to compensate for these differences in transit time. For negligible chromatic dispersion the ideal profile exponent is 2. One then speaks of a parabolic index profile.


39

2.1.2 Numerical Aperture When light enters the fiber's input face at an angle 4max, it is refracted at an angle Dmax (Fig. 2.3). Applying the law of refraction we have: n0 sin4max

ncore sinD max

ncore sin(90 - J max )

n0 sin4max

ncore cos J max

n0 sin4max

ncore 1 - sin2 J max , with ncladding ncore

n0 sin4max

ncore 1 - ncladding ncore

sin2 J max

2

ncore 2 ncladding 2 , for n0

n0 sin 4max

2

1 follows

ncore 2 ncladdimg 2

sin 4max

The sine of the maximum incident-ray angle 4max is defined as the numerical aperture AN (Fig. 2.3). The angle 4max is referred to as the acceptance angle, and twice the acceptance angle is referred to as the aperture angle. Using the relative refractive index difference ', the value for AN is obtained as: AN

sin 4max

ncore 2 '

n0 4max

Jmax Dmax

Fig. 2.3: Definition of the acceptance angle

Thus, the value of the numerical aperture (NA) is solely dependent on the difference in the refractive indices of the core/cladding material. Example:

The refractive indices of a standard PMMA fiber are ncore = 1.49 and ncladding = 1.40; we thus obtain AN = 0.50 and 4max = 30q.

Whereas the numerical aperture of the step-index profile fiber remains constant over the entire core, the graded-index profile fiber exhibits a decreasing acceptance angle from the center of the core to the cladding (Fig. 2.4).

40


Fig. 2.4: Acceptance angle of a graded-index profile fiber

Compared with other fiber types (Fig. 2.5), POF has the largest numerical aperture and the largest core diameter. This is one of the most important advantages of POF, since the connection technology that can be used for POF is more economical to apply than that used for glass fibers.

10/ 125 μm

singlemode glass fiber

multimode glass fiber 50/ 125 μm

multimode glass fiber

62.5/ 125 μm

polymer fiber 980/ 1000 μm

(plastic clad) 200/ 230 μm 100/ 140 μm

0 mm

0.5 mm

1.0 mm

Fig. 2.5: Aperture angle and core diameter of glass fibers and polymer fibers

2.1.3 Ray Trajectory in Optical Fibers In the step index profile fiber, light propagates along a zigzag path, being totally reflected at the core/cladding interface; in the graded-index profile fiber, light propagates on a sinusoidal trajectory that is created within the graded-index profile through refraction. If the incident light rays lie within one and the same plane through which the fiber axis runs, meridional rays are formed. In all other cases, skew rays are formed. Figure 2.6 shows the projection onto the fiber's incident face. Step and graded-index profile fibers show the same behavior. The specification of the numerical aperture always refers to the meridional rays.


D’

41

D’ D’ D’

Fig. 2.6: Meridional rays

Skew rays form an angle of \ < 90q with the tangential plane at the core/cladding interface (Fig. 2.7). They never cross the fiber axis and propagate along screw-like paths. For step index profile fibers, the projection onto the crosssectional area resembles a polygonal line so that these rays do not cross a circleshaped area having a radius rk around of the axis.

\ rk

\

Fig. 2.7: Skew rays in step index profile fibers

In graded-index fibers with a parabolic profile, ellipses are formed in the projection (Fig. 2.8 left) that may under certain circumstances form circles; these rays are called helical rays (Fig. 2.8 right). Their distance from the fiber axis is always constant.

Fig. 2.8: Helical rays (left) and skew rays (right) in graded-index profile fibers

42


2.1.4 Modes in Optical Fibers 2.1.4.1 The Mode Concept The phenomena of refraction and reflection discussed so far can be graphically explained with the help of geometrical optics, whereby the size of the wavelength and the diameter of the finite ray are not considered (O and dray= 0). However, to obtain a complete description of the wave guiding phenomenon, the wave properties of light must also be considered. The goal is to calculate the electric field and intensity distribution of the light in the optical fiber. The Eigenvalue equation is derived and solved on the basis of the Maxwell equations. Ref. [Blu98] provides a detailed description. The solutions to the Eigenvalue equations are a finite number of field distributions within the light waveguide. These field distributions are referred to as modes of the waveguide. If we apply this concept to the ray model, this means that apparently not all incidental rays for which 4 < 4max is true can propagate, but rather only those rays that have a particular angle. Figure 2.9 illustrates this situation: in order for light to propagate in a particular direction, a wave must constructively overlap itself with its own reflecting wave in such a way that the phase position is repeated after double reflection. The black lines perpendicular to the direction of propagation identify the planes with the same phase angle. The spacing is O/ncore. transverse direction

O/ncore

d

electrical field Fig. 2.9: Formation of the mode structure within the waveguide

Whereas the zigzag paths would lead to intensity distributions within the rayoptical model that would change depending on the length of the fibers, the wave model provides a constant light-dark distribution that is independent of the length across the waveguide's cross-section. The number N of the guided modes is approximately described by: N|

1 g V2 2 g2

where V = 2S a AN/O, a is the radius and g is the profile exponent (see also Section 1.1.5). For step-index profiles g o f. This results in a value of N | ½ · V² for the number of modes. For parabolic profiles g = 2 and thus N | ¼ · V². A polymer op-


43

tical fiber with AN = 0.5, a core radius of 0.5 mm and a wavelength of O = 650 nm can carry 2.9 million modes. If the angle of total reflection is exceeded, radiation modes are created and the light is radiated into the cladding. If the refractive index of the cladding is higher than the surrounding medium (air, for example), cladding modes may be formed. In the POF, the optical cladding is encased in an absorbing jacket so that no cladding modes can form. In contrast to guided modes, it is not possible to count radiation modes. They do not take part in signal transmission. (Fig. 2.10 special conditions for POF are explained below). Higher modes propagate under a larger angle, lower modes under a smaller one. Under certain circumstances skew rays may turn into so-called leaky waves, which, on the one hand, are guided in the Z-direction and, on the other hand, transfer energy to the cladding. Under certain conditions they can still be detected in POF even after several 10s of meters. Hence, they can influence both the transmission process as well as the measuring techniques used.

radiation mode

higher mode

lower mode cladding mode

Fig. 2.10: Guided, cladding and radiation modes

The following equation describes the relationship between the angles D, \ and G in Fig. 2.11 ([Sny83]): cos D

sin G sin ȥ

D is the angle of the incident and reflected ray relative to the surface normal of the tangential plane in P. \ describes the angle between the reflection plane and the tangent plane, and G is the angle between the projection of the skew ray on the cross-sectional plane and the direction of propagation (parallel to the fiber axis). Figure 2.12 summarizes the various ray types according to the respective angles derived from the above equation ([Bun99a]). For guided rays holds G < Gmax and D > Dmax. The leaky waves are shown in the subsequent rectangle while the ray modes are shown above the line D = Dmax. For meridional rays D = 90q - G because \ = 90q, i.e. they lie on the blue line.

44


D \

D

G

P Fig. 2.11: Designation of the angles of a skew ray; the right diagram shows the angle G, which is obtained by projecting the skew ray on to the cross-sectional plane

0

\ q

inside this triangle there are the radiation rays

D[°]

10 20

\ q

inside this triangle there are the guided rays

30 40

\ q

50 meridional 60 rays 70

\ q

Dmax

80

inside this rectangle there are the leaky modes

Gmax

90 0

10

20

30

40

50

60

70

G[°] 80

90

Fig. 2.12: The different types of rays

2.1.4.2 Mode Propagation in Real Fibers Several chapters of this book discuss the special characteristics of light propagation in POF. Here now, the processes that need to be considered will be looked at as a whole. The function of fibers as a waveguide for passing on light by means of total reflection at the core/cladding interface has already been discussed. If the ray model were applied consistently, then a light ray launched into an ideal fiber would always propagate at the same angle relative to the fiber axis. With a divergent light source, the far field would always remain constant along the length of the fiber. This would not be true for the near field, as Fig. 2.13 illustrates: depending on the course of the ray, different locations along the fiber would


45

generate different near fields in the form of point structures. However, this contradicts the results obtained through experiments: there a continuous distribution of intensity is obtained, and from a certain length onwards the intensity does not change at all. Although the ray model is very illustrative, its practical application is limited as the example above shows. In order to be able to describe experimental results it is thus necessary to move on to the mode concept. In this respect it is important to keep in mind that many optical simulation programs work on the basis of discrete light rays. In order to obtain truly realistic results, a sufficient number of rays has to be simulated. only a few launched discrete modes

nearfield (very schematically) Fig. 2.13: Near fields under conditions of the ray model with only a few discrete light paths (in practice very difficult to measure and visible only on very short lengths)

2.1.5 Parameters for Describing Real Fibers and Waveguides In order to describe the characteristics of real fibers and waveguides different parameters are defined which vary in importance depending on their respective application. All of these parameters are influenced by the propagation conditions of the different modes. In the case of multimode fibers most characteristics depend typically on the mode distribution. This means that a fiber initially allows the propagation of light in different paths (modes). Depending on the light sources at the front end of the fiber, not all these modes are launched, at least not with a uniform power distribution. Since each mode has different characteristics, an altered behavior of the fiber is on average the result. In addition, the problem becomes more complicated since an exchange of energy between the modes can occur over the length. Typical fiber characteristics will be defined and explained in the following sections. Uniform Mode Distribution (UMD) and Equilibrium Mode Distribution (EMD) are the usual standard measuring conditions.

46


2.1.5.1 Attenuation The most important process encountered by light as it passes through a fiber is attenuation. When passing through an optical fiber of the length L, the power of the light decreases (Fig. 2.14). The following equation applies to the optical power: PL

P0 e DcL

where PL and P0 are the power of the light after passage through a fiber of length L in km and at the front end of the fiber, respectively; D´ is the value of the attenuation coefficient in km-1.

PL

P0 L Fig. 2.14: Definition of attenuation

To make it easier to work with the numbers involved here, it is usual to express attenuation logarithmically. Thus, the attenuation coefficient is expressed as D in dB/km. D

10 P log 0 L PL

4,343 Dc

Attenuation value a is the non-dimensional variable (given as a number or in dB) obtained from the product D · L. Figure 1.19 illustrates the relationship between the attenuation value and the change in power as a percentage.

attenuation factor a [dB] 30

0.1

25

20

1

15

10

5

10 power ratio PL/P0 [%]

0

100

Fig. 2.15: Conversion of the power ratio PL/P0 in % into the dB value

Very often there is not a clear differentiation in the technical literature between attenuation per unit length D and attenuation factor a. One often speaks simply of the attenuation of the fiber. The addition “spectral” refers to the wavelength dependence. A mistake is avoided, however, when the unit is indicated. We still have to mention that attenuation and attenuation per unit length are practically always indicated as positive numbers.


Quantity

Symbol

Unit

Formula

attenuation coefficient, lin.

D´

km-1

{ln (P0/PL)}/L

attenuation coefficient, log.

D

dB/km

attenuation

a

dB

47

{10log (P0/PL)}/L 10log (P0/PL)

Especially in the area of optical short-range communication, indicating the fiber attenuations in dB is much more practical than, for example, representing the absolute transmission. POFs are being used more and more in the near infrared range for quite short transmission lengths. Finally, PMMA can also be used for waveguide structures in the mm range. Fig. 2.16 shows the attenuation curve of a PMMA-POF according to [Hess04].

100,000

attenuation [dB/km]

30,000 10,000 3,000 1,000 300

theory measured

100 30 wavelength [nm] 10 500

600

700

800

900

1000

Fig. 2.16: Attenuation spectrum of the PMMA-POF (theory and measured by [Hess04])

Nevertheless, the representation comprises approximately 3 decades, i.e. a factor of 1,000 which cannot be overlooked on a linear scale. 2.1.5.2 Mode-Dependent Attenuation When talking about glass fibers, it is often assumed that the attenuation of all light rays is identical. For practical purposes, this assumption is sufficiently accurate. With POF, the path difference between the rays parallel to the axis and the propagation directions close to the critical angle of total reflection can become quite large. For the standard NA-POF with AN = 0.50 this difference is about 6%. For polycarbonate fibers with AN = 0.90, the difference is even 21%. For this reason alone, there is a considerably greater level of attenuation where large propagation

48


angles are involved. In 100 m of POF, a light ray of this type will travel 6 m farther which results in an additional loss of more than 1 dB when the attenuation level is 200 dB/km. At 1,000 dB/km for polycarbonate fiber, this would result in an additional loss of 4 dB after 20 m of travel (less than 50% of the launched power reach the fiber output). The second, more significant cause for mode-dependent attenuation is the attenuation resulting from the cladding material. Fluorinated polymers are used as optical cladding for PMMA fibers; these claddings may have an attenuation of several 10,000 dB/km [Paar92]. Locking more exactly on the propagation of a plane wave at the interface, we find that, even if total reflection results, the electrical field escapes into the optically thinner medium by a distance in the order of magnitude of the wavelength. This process is also known as the Goos-Hänchen Shift ([Bun99a]) and the model explains this as resulting from a shift of the reflection plane into the optically thinner medium. The reflected ray is hence slightly displaced on the interface surface, as can be seen in Fig. 2.17. In this model, the additional light path would be subjected to the higher attenuation of the cladding material.

cladding core area of higher attenuation Fig. 2.17: Goos-Hänchen shift

Although the light path in the cladding is only in the Pm range for each reflection, it still plays a significant role because of the much higher attenuation encountered there. This effect is particularly striking when the core diameters are reduced in size. Theoretically speaking, attenuation and bandwidth should not be dependent on the core diameter. Nevertheless, thin cores such as those used in multicore fibers have indeed considerably larger bandwidths [Tesh98], a slightly increased attenuation and narrower far-field widths. These effects are explained quite well in [Bun99b] and [Ziem99c]. This effect also occurs in glass fibers. Silica glass fibers with a polymer cladding (PCS) have losses in the core below 10 dB/km (wavelength range from 650 nm to 1,300 nm), whereas the polymer cladding has an attenuation of several 100 to 1,000 dB/km. Attenuation values of 180 dB/km for the core and 9,000 dB/km for the cladding are indicated in [Ebb03] for step index profile glass-glass fibers (used in fiber bundles). Reasonably priced conventional glasses - albeit much purer than in window glass - are used in these fibers and not silica glass. In singlemode and graded-index profile silica fibers there are no mentionable differences in attenuation between the core and the cladding since both consist of


49

Si02. The germanium dopant in the core does not have any great influence. An important consequence of the mode-dependent attenuation is, as will be discussed later on, a significantly narrower far field after greater fiber lengths than one would expect from the fiber NA. 2.1.5.3 Mode Coupling The term mode coupling refers to the process by which energy from one direction of propagation is transferred to several others. This can happen, for example at scattering centers. Since the light scattering in a PMMA-POF makes up a considerable part of the attenuation, this process is always present. Figure 2.18 clarifies the procedure (still in the ray model).

scattering center

Fig. 2.18: Mode coupling at a scattering center

Many experimental results clearly indicate that mode coupling occurs predominately at the core/cladding interface (Fig. 2.19). This can be explained by the fact that is it not possible to create an ideal surface in the sub-nanometer range when very large polymer molecules are involved. Thus, mode coupling is also dependent on the angle of propagation. cladding

core

scattering center

Fig. 2.19: Mode coupling at the core/cladding interface

Mode coupling alters the bandwidth of a fiber. When collimated light is launched, energy is gradually transferred to the higher angle ranges so that mode dispersion increases and bandwidth decreases. If light is introduced in all angle ranges, so that maximum differential delays occur, energy is exchanged between the angles so that the initially slower rays become “faster” and vice versa. Accor-

50


ding to the laws of statistics, the differential delay (or more precisely, the standard deviation) does not increase in a linear relationship to the length but approximately only proportional to the square root of the length. This applies to lengths in excess of a characteristic coupling length, which for PMMA-POF is generally several 10 m. Mode coupling always results in additional attenuation. Whenever there are changes in the light propagation, energy is coupled into those angle ranges in which there is no longer any light guiding. The shorter the coupling length, the larger the additional attenuation will be. If the observed behavior of the POF, namely the filling up of the near field after a few 10 cm of fiber, could be explained exclusively because of the mode coupling, then additional attenuations in the range of 1000 dB/km would result - which indeed does not occur. Figure 2.20 shows an electron microscope picture of the core-cladding interface layer (photo ZWL, 2003). The marked smooth part running from the top left to the bottom right is the surface of the core with the cladding removed. At the top right you can see the cracked core. The step is the 10 μm thick optical cladding. Further theoretical considerations on the problems of scattering can be found in [Kru06a] and [Kru06b].

Fig. 2.20: Photo of the core-cladding interface of SI-POF taken by electron microscope (ZWL Lauf)

2.1.5.4 Mode Conversion The definition of propagation angles or of modes actually applies only to waveguides that are straight. It takes just one bend to make a different approach necessary. The most precise method would be to recalculate the modes for the system of the now bent fiber; however, this is theoretically and practically much too complex a process. It is more appropriate to consider the zone before and after the bend as a straight waveguide and, at the bend, to perform a transformation onto the new reference axis. Formally, light is thus transmitted from one propagation direction to another, as Fig. 2.21 demonstrates.


fiber axis in front of a bend

51

fiber axis behind a bend

new propagation angle

Fig. 2.21: Mode conversion at a bend

Strictly speaking, mode conversion can be described as a special case of mode coupling. The difference is that the number of modes or the propagation directions is not increased. In the POF mode conversion most likely occurs at the core/cladding interface surface, for example at micro bends or at fluctuations in the refractive index difference. The question of the influence of mode conversion and coupling on the additional attenuation depends essentially on the angle dependency of the processes. The more the direction of the light is altered, the more losses occur. A quantitative analysis of these processes for POF is extremely difficult and is yet to be carried out. However, for the physical processes assumed, mode coupling should have a larger angle-independent contribution (scattering on larger inhomogenities).

Fig. 2.22: Far fields of different POF (product A/B at the top/bottom); left/right after 20 m/50 m of fiber, launch with collimated light (AN Launch < 0.016)

52


An impressive experiment that confirmed this statement was shown in [Poi00]. If collimated light is launched into a SI-POF, a ring-shaped far field can be generated at the output even after 50 m of fiber, for which purpose the fiber might be properly bent. This experiment can only be explained under the assumption that mode conversion predominates. However, the different fibers made by different manufacturers show considerable differences in their behavior which do not necessarily have an effect on attenuation. It is easy to see here that the mode field is not completely filled even after 20 m to 50 m (Fig. 2.22). 2.1.5.5 Mode Coupling Lengths The length of a fiber in which a state of equilibrium arises through mode conversion and coupling is described as coupling length whereby different definitions exist. The best known is the description with the aid of a length-dependent bandwidth. Here the coupling length is the point at which the linear decrease in the bandwidth turns to a root dependency (see Fig. 2.36). In practice this point is difficult to measure, but other parameters such as far field width and attenuation, change with fiber length. For example, values for the kilometric attenuation with different launch conditions are shown In Figures 2.23 and 2.24.

400

D [dB/km]

fiber “A”

350

source “I” source “II” source “III” source “IV”

300 250 200 150 100

lPOF [m] 50

1

2

5

10

20

50

100

Fig. 2.23: Attenuation of a SI fiber under different excitation (acc. to [Lub02b])

Both diagrams show very clearly that the different launch conditions (source I emits very widely, source IV nearly collimated) lead to extremely different attenuation values. After some ten meters, however, the differences disappear for the most part through mode coupling. Evidently, there are great differences among the fiber types.


53

D [dB/km]

400

fiber “B” 350

source “I” source “II” source “III” source “IV”

300 250 200 150 100

lPOF [m] 50

1

2

5

10

20

50

100

Fig. 2.24: Attenuation of another SI-fiber at different launch conditions

The next two figures 2.25 and 2.26 show measurements of far field widths for a POF and a PCS each with altered launch conditions. Once again it can clearly be seen how the differences caused by the different coupling conditions are evened out after some 10 to 100 m.

32 30 28 26 24 22 20 18 16 14 12 10

far field width [°]

NALaunch:

0.64 0.48

0.33 0.19

0.09 0.05

PMMA SI-POF POF length [m] 5

20

50

100

Fig. 2.25: Launch dependent far field widths of a PMMA SI-POF

In the 200 μm thick PCS it takes considerably longer to establish the equilibrium mode distribution especially when the length is related to the fiber diameter. The values of the NA (calculated from the 5% far field width) are represented for lengths up to 500 m.

54


0.40

measured NA

0.35 0.30

AN = 0.02 AN = 0.09 AN = 0.17 AN = 0.26 AN = 0.34 AN = 0.48

0.25 0.20 0.15

fiber length [m]

0.10 1

10

100

1000

Fig. 2.26: Excitation dependent far field width of a 200 μm-PCS

In general, the mode coupling length is characterized as the distance in which a parameter has come closer by 1/e to the state of equilibrium. For example, this corresponds to the charging time constants of a capacitor. One cannot therefore say that EMD conditions exist after one coupling length. Depending on how large the tolerated deviations are, several coupling lengths have to be considered. Figure 2.27 shows the theoretical curve of a parameter.

mode dependent fiber parameter [a.u.] 600 500 characteristic at Lc = 100 m

400 parameter deviation for short fibers

300

parameter deviation for 1, 2 und 3 u Lc

200 equilibrium mode value

100 10

20

50

100

200

500 1000 fiber lenght [m]

Fig. 2.27: Approximation of an optical parameter to the equilibrium value by mode coupling (schematically)


55

2.1.5.6 Leaky Modes The significance of leaky modes has already been touched upon earlier. For the sake of completeness, it should be noted here again that light rays that lie above the critical angle of the total reflection do not entirely vanish but still contribute significantly to light propagation even after several 10s of meters. Not until we examine the interaction of attenuation, mode-dependent attenuation, mode coupling and mode conversion and take leak modes into account, can we establish a model for the light propagation of SI polymer fibers that can at least qualitatively describe the experimentally observed behavior. In principle, the same processes take place in GI-POF; however there are basic differences: ¾ ¾ ¾

With GI-POF, there is no core/cladding transition to serve as an essential cause for mode coupling, mode conversion, and mode-dependent attenuation. Fluorinated GI-POF are used in wavelength ranges in which Rayleigh scattering is less significant. To form the index profile, various zones of the fiber, as seen from the axis, are provided with varying concentrations of a dopant or a copolymer so that the attenuation usually gets a gradient. This is probably the most significant cause of mode-dependent attenuation in GI-POF.

Yabre and Zubia made comprehensive observations on mode propagation in GI-POF [Yab00a], [Yab00b], [Arr99], [Arr00]. The problem of mode coupling and mode conversion is sure to be very interesting for multi step index fibers. Bandwidths could result that are larger than what is theoretically expected. Some different theoretical investigations were made in cooperation between the POF-AC and the University of Bilbao (Spain). More details will be given in the fiber simulation chapter. As the example of the multi-core fibers shows, mode-dependent attenuation can be used to exchange attenuation for bandwidth. Less attenuating cladding would reduce the overall attenuation of the POF, but more than likely also reduce the bandwidth (always assuming equilibrium mode distribution). The future will decide which parameter is of greater significance for users. If the transmission budget is sufficiently large, it would be possible to increase the bit rate though multi-level coding or by electrically compensating the dispersion so that a reduction in attenuation is the minimum goal to be targeted in this field. 2.1.5.7 Dispersion in Optical Fibers Dispersion refers initially to all processes that result in a difference in the transit times of various modes. One mode is thereby always a propagation condition of the light that is uniquely defined by the wavelength, polarization, and propagation path. Differential delays between the various light components lead to a reduction in the modulation amplitude of higher frequencies. This makes the fiber a low-pass filter.

56


The bandwidth of a fiber communication transmission system is usually considered to be the frequency for which the optical level of a sine-modulated signal has dropped by 3 dB. Strictly speaking this approach only applies to a Gaussian low-pass filter. This means that a pulse of insignificant width will correspond to the Gauss function after it has traveled the length of the fiber: P( f ) P0 ( f ) e

- f 2 f02

where P(f) is the power of a random frequency f at the end of the measuring path, P0(f) is the launched power and f0 is a constant that describes the bandwidth. Figure 2.28 illustrates the process schematically.

a) P0(f)

b)

pulse response

c)

d)

e) P(f) time t Fig. 2.28: Effect of dispersion on a sine-wave signal

Curve 'a' shows the sine-modulated source optical signal (it must be noted that optical power can only take positive values). Figure 'b' shows how a single pulse approaching a Gaussian function after traveling through the fiber. This is a theoretical borderline case because the Gaussian function extends from -f to +f, but the output pulse cannot begin before the input pulse has started. To measure the shape of the complete output signal, the input signal can be split into a series of pulses, as shown in Fig. 'c'. After traveling through the fiber, every pulse forms a Gaussian function of the respective height (Fig. 'd'). These have to be brought together again to achieve the result in curve 'e' (mathematically speaking, this is a convolution of the input pulse with the so-called pulse response of the transmission link).


57

It is easy to see that the amplitude of the signal has decreased. Attenuation of the light has not been taken into consideration. A short light pulse is briefly broadened when it travels the length of a fiber (Fig. 2.29) and this in turn reduces the transmission bandwidth.

optical input power

optical output power

100 % 100 %

50 %

optical fiber

50 %

time

time

tin

tout

Fig. 2.29: Pulse broadening by passing an optical fiber

If Gaussian-shaped pulses are assumed, the result of the pulse broadening 't is the square root of the difference of the squares of the input and output pulse width (FWHM full width at half maximum): 't

2 2 t out t in

The consequence of this broadening is that the time gap between the bits becomes smaller, that the pulses finally overlap and that the receiver can no longer differentiate between the two. The transmission bandwidth is limited as the light waveguide functions as a low-pass filter. The product of bandwidth and length characterizes the transmission capacity of a fiber. [Gla97] applies to Gaussianshaped pulses: B L |

0.44 L 't

Pulse broadening is caused by mode dispersion and chromatic dispersion. For multimode fibers it is necessary to consider the factors of material, modes and profile dispersion (in graded index fibers). Waveguide dispersion additionally occurs in singlemode fibers, whereas profile dispersion and mode dispersion do not. All the kinds of dispersion appearing in optical fibers are summarized in Fig. 2.30. The mechanisms dependent on the propagation paths are marked in yellow, whereas the wavelength-dependent processes are marked in green.

58


dispersion modal dispersion

chromatic dispersion

(multimode fibers)

(multimode and singlemode fibers)

profile dispersion

material dispersion (multimode and singlemode fibers)

(multimode fibers)

polarization mode dispersion

waveguide dispersion

(singlemode fibers)

(singlemode fibers)

Fig. 2.30: Dispersion mechanisms in optical fibers

In regard to the fibers and applications dealt with in this book only mode and chromatic (material) dispersion play a role so that the following sections deal solely with these two effects. 2.1.5.8 Mode Dispersion Since the light paths have different lengths, the pulses that have started simultaneously arrive at different times at the fiber's output, a fact that leads to pulse broadening. Figure 1.29 shows the 'fastest' (D = 0) and the 'slowest' (D = Dmax) rays.

ncladding

Jmax

L2

a

Dmax

2

1 L1 ncore

Fig. 2.31: Deriving the difference in the transit time


59

The propagation times of the two different propagation paths are determined purely geometrically for: t1

n L1 core c

t2

n L 2 core c

' t mod

t 2 t1

L1 ncore 1 c sin J max

2 L1 ncore c ncladding

§ ncore ncladding · n ¸ L1 core ¨ ¸ ¨ c n cladding ¹ ©

L1 2 L1 ncore AN | ' c 2 c ncladding

Figure 2.32 shows the dependence of the bandwidth on the numerical aperture with which the light is launched. The assumption is that the far field, i.e. the angular distribution of the light in the fiber, will remain constant over the entire length of the sample (no modal coupling or conversion). For a PMMA standard fiber with an AN = 0.50, a differential delay of 't | 25 ns for 100 m is produced. The transit time is proportional to the square of the NA. From the above-mentioned expression B | 0.44/'tmod, a value of 15 MHz results for the bandwidth. theoretical bandwidth [MHz] 1,000 500

200

fiberlength:

100

10 m

50 25 m 20

50 m

75 m 100 m 10 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 numerical aperture Fig. 2.32: Bandwidth calculated as a function of the launch NA

60


The critical angle Jmax of total reflection is determined by the ratio of both refractive indices (example, 1.492 for the core and 1.456 for the cladding): 1.456 = arcsin 0.976 = 77.4q 1.492 (max. angle to axis : Dmax 12.6q) J = arcsin

Thus, the relationship between both paths y and z is: z = y/sin (D) = y 1.0247 The NA of this fiber is determined by: AN = (ncore2 - ncladding2)

0.5

= (1.4922 -1.4562)

0.5

= 0.32

The pulse broadening for a fiber length L is derived as follows: Transit time of the parallel-axis modes: Transit time of the modes with max. angle: Differential delay: For example for 100 m, n = 1.492: With the approximation B · 't = 0.44

t1 = L · n/c0 t2 = L · n/c0 · 1.0247 't = L · n/c0 · 0.0247 't = 12.3 ns B = 33 MHz

Different NA lead to different bandwidths, whereby a doubling of the NA reduces the bandwidth to a quarter: Theoretical bandwidth:

AN = 0.60: AN = 0.50: AN = 0.40: AN = 0.30: AN = 0.25: AN = 0.19:

10 MHz 100 m 14 MHz 100 m 22 MHz 100 m 40 MHz 100 m 57 MHz 100 m 97 MHz 100 m

To correctly calculate the theoretical bandwidth, it is just not sufficient to consider the two possible ray paths selected here. A very comprehensive description of mode propagation in POF is provided in [Bun99a]. In the ray model, each possible propagation direction is described by the two angles D and G (for an explanation of these angles, please refer to Fig. 2.11). As far as transit time is concerned, only angle G is of relevance. Figure 2.33 is an illustration from [Bun99a] of the zone of the guided rays and the leaky rays which themselves are again subdivided. Regardless of the size of \, angle G cannot exceed a particular maximum value so that a maximum possible differential delay is the consequence. Only the marked triangle contains not attenuated rays that are capable of propagation. If one assumes that all possible propagation paths have the same energy (UMD - uniform mode distribution), it can be seen that paths having a larger propagation angle are more probable than rays traveling parallel to the axis.


D>q@

70

61

\ q \ q meridional rays

75

\ q \ q

80

Gmax

85 guided rays

90 0

5

15

10

20

G>q@

Fig. 2.33: Possible rays in an optical fiber

As measurements of the far field (that is the power as a function of the angle to the fiber axis, measured in a sufficient large distance) of a POF shows, this is also reflected in the greater power obtained with larger angles. If the power is expressed in relation to the solid angle element, a constant power density is found because larger angles cover a correspondingly larger arc. This is shown schematically in Fig. 2.34. 1.0

rel. power/solid angle UMD

0.8 0.6 0.4 0.2 0.0 -30

-20

-10

0

10

20

angle to the 30 fiber axis [°]

Fig. 2.34: Power distribution with UMD

The differential delay increases approximately by the square of the angle relative to the fiber axis. If a short pulse having a mode distribution that corresponding to UMD is launched into the fiber input, an approximately rectangular pulse is generated at the output of the length of which corresponds to the approximate values shown above for the maximum differential delay. Figure 2.35 demonstrates the precise results for an assumed attenuation-free standard NA POF for the pulse form obtained after 10 m, 20 m, 50 m, and 100 m of ideal POF (from [Bun99a]).

62


100%

norm. signal

80% 60%

10 m 20 m

50 m

100 m

40% 20% time [ns] 0%

0

5

10

15

20

25

30

35

Fig. 2.35: Output pulses of a POF under UMD conditions ([Bun99a])

Real SI-POF provide considerably higher bandwidths. The main reason for this is the presence of mode-dependent attenuation in conjunction with mode mixing, as will be shown in the next chapter. The differential delay 't increases proportionally to a particular length Lc (coupling length); for longer lengths, the increase is sub-linear (Fig. 2.36). The following holds true: 't v L

for L L c

't v LN

for L ! L c

with

N 1

whereby the exponent N must be determined for each fiber. It is typically between 0.5 and 0.7. The coupling length Lc ranges between 30 m and 40 m for standard SI-POF. 2.5

pulse broadening [a.U.]

W a l

2.0

Wal

1.5

in reality

1.0 0.5

Lc length [m]

0.0 0

20

40

60

80

100 120 140 160 180

200

Fig. 2.36: Schematically representation of the pulse broadening reflecting mode coupling effects


63

The impulse response of a 50 m long standard POF can be seen in Fig. 2.37. The half-value width of the impulse amounts to about 50 ns, i.e. only about 30% of the expected value. Furthermore, it is noticeable that the rear pulse edge drops more slowly. It is in this range that the higher modes lie which are attenuated very greatly by the mode-dependent losses. The dropping off of the rising edge can be explained by the effect of modal mixing.

0.7 U [V]

theoretical pulse shape 'W = 16 ns

0.6 0.5 0.4

'W = 5 ns

0.3 0.2 0.1 0.0 0

5

10

15

20

25

30

35

40 t [ns]

Fig. 2.37: Real pulse shape for 50 m St.-NA-POF

Calculating the bandwidth of graded-index fibers is clearly more complex. Current studies in this field can be found in [Yab00a], [Yab00b] and [Arr99]. Profile dispersion occurs in graded-index profile fibers. It is the remainder of the mode dispersion that can no longer be compensated for and it depends on the relative refractive index difference ', which in turn is wavelength-dependent. An optimization of the profile exponent can be accomplished for a certain wavelength for which d'/dO = 0. A profile exponent of g | 2 causes a temporal broadening of: 't prof

L1 ncore '2 , c 2

in other words, a factor '/2-reduced broadening of the pulse as compared with step index POF; for a typical graded-index POF this means a reduction by approximately 2 orders of magnitude [Blu98]. Mode dispersion or profile dispersion can only be avoided by using singlemode fibers. As explained later on, due to the combination with the chromatic dispersion, certain polymer fibers, have some advantages as opposed to silica glass fibers.

64


2.1.5.9 Chromatic Dispersion Chromatic dispersion describes the influence of the spectral width of a transmitter on a temporal broadening of the input pulse. This includes the material-dispersion and waveguide-dispersion types of dispersion. Both effects also occur in singlemode fibers. Waveguide dispersion is caused by the fact that light waves penetrate into the fiber cladding to various depths, depending on the wavelength of the light wave. Thus, the different speeds of the core and cladding parts result in pulse broadening. Since only a small portion of the light wave in higher modes of large diameter fibers spreads into the cladding, this effect is only considered for singlemode fibers. However, even if only one mode is allowed to propagate, pulse broadening occurs due to material dispersion. Every light source has a spectral width 'O > 0. The following applies for the pulse broadening 'tmax due to material dispersion: ' t mat

where

'O n(O): M(O):

L 'O

O d² nO c d O²

L 'O MO

is the spectral width of the transmitter wavelength-dependent refractive index, material dispersion parameters usually given in ps/kmnm

Figure 2.38 shows the influence of material dispersion on pulse broadening, using polymer fibers as an example. Corresponding to the material dispersion, the longer wavelengths (red) propagate with a greater velocity than the shorter ones (blue). output pulse 't

input pulse

time

length 'O

wavelength

Fig. 2.38: Temporal broadening as a result of material dispersion

The real influence of the chromatic dispersion from different polymer optical fibers to the system bandwidth will be shown in the next chapter which will contain detailed descriptions of the materials and fiber types.

2.2 Index Profiles and Types of Fibers

65

2.2 Index Profiles and Types of Fibers After the theoretical descriptions on the properties of optical fibers in the section on the fundamentals of light propagation and the observations indicated above on mode propagation and the essential characteristics of fibers this following section will deal with concrete, available fibers. First, the different index profiles, as briefly mentioned in 1.1.6, will be introduced using examples. The next section shows the historical development especially in regard to the different POF variants. Thereafter the important characteristics attenuation and bandwidth will be shown in a series of experimental results. Three parameters are basically responsible for the actual properties of optical fibers. The core and cladding materials used determine the attenuation and chromatic dispersion. The refractive index profile determines the mode dispersion and the core diameter is also responsible for the number of modes. Especially the core material and the index profile are at least recognizable from the name of the fiber, a designation method widely used in this book. In the following section the historical development of the different polymer fibers is summarized. The POFs are dealt with in regard to their index profiles. Thereafter, different hybrid and glass fibers for short-range data transmission will also be introduced. The following chapter deals especially with the bandwidth of thick optical fibers since this characteristic is particularly important and also it makes the greatest demands on measurement techniques. 2.2.1 Step Index Profile Fibers (SI) As was the case with silica glass fibers, the first polymer optical fibers were pure step index profile fibers (SI-POF). This means that a simple optical cladding surrounds a homogenous core. For this reason a protective material is always included in the cable. Figure 2.39 schematically represents the refractive index curve. As already shown above, the refractive index step determines the numerical aperture (NA) and thus the acceptance angle. Some typical values are shown in Table 2.1. The refractive index of the core was always taken as 1.5, whereas the cladding has a correspondingly smaller refractive index. The last line is valid for wave guiding against air (n = 1). Here an acceptance angle of 90° is valid since the NA exceeds the value of 1. ncore ncladd jacket optical cladding

core

jacket optical cladding

Fig. 2.39: Structure of a step index profile fiber

66


Table 2.1: Relationship between relative refractive index difference and numerical aperture (core refractive index = 1.50) Relative RefractiveIndex-Difference

Refractive Index of the Cladding

Numerical Aperture

Acceptance Angle of the Fiber

0.22 %

1.497

0.10

6°

0.4 %

1.494

0.13

8°

0.8 %

1.488

0.19

11°

1.0 %

1.485

0.21

12°

1.5 %

1.478

0.26

15°

2.0 %

1.470

0.30

17°

2.7 %

1.460

0.35

20°

4.0 %

1.440

0.42

25°

5.8 %

1.413

0.50

30°

8.0 %

1.380

0.59

36°

12.0 %

1.320

0.71

45°

20.0 %

1.200

0.90

64°

33.3 %

1.000

1.12

90°

A larger acceptance angle of the fiber simplifies the launching of light, e.g. from a semi-conductor source. In addition, a high NA reduces the losses associated with fiber bending, as schematically illustrated in Fig. 2.40.

rays, exceeding the critical angle of total reflection behind the bend

launched light rays

bend radius

Fig. 2.40: Loss at fiber bends

rays, guided behind the bend


67

Due to the effects of bending, the propagation direction of each individual ray is changed relative to the axis of the fiber. In the case of multi-mode fibers, a part of the rays is always extracted because the rays exceed the angle of total reflection at the interface between core and cladding. For fibers with a large NA, the effect of a change in angle for a certain amount of bending is not so significant so that the bending losses diminish. Likewise, when coupling fibers to each other (at connectors) the loss due to angle errors is less significant when there is a large numerical aperture. A disadvantage of fibers with a large NA is the greater difference in time delay between the different light paths, and this in turn leads to a greater level of mode dispersion. This limits the bandwidth. In addition, the loss at coupling points increases if there is a gap between the abutting faces. Some advantages of larger or smaller numerical apertures are listed in Table 2.2. Table 2.2: Influence of higher NA to various fiber parameters Property of the Fiber bending sensitivity fiber coupled power connecting loss for fiber angular mismatch connecting loss for axial fiber gap connecting loss for fiber axis lateral gap bandwidth

Behavior with increasing NA becomes smaller becomes higher becomes smaller becomes higher becomes higher becomes smaller

Silica glass multi-mode fibers usually have an NA of approximately 0.20. Silica glass fibers with polymer cladding have an NA in the range of 0.30 to 0.40 (sometimes 0.65). The large refractive index difference between the materials that are used for the core and the cladding of polymer fibers allows significantly higher NA values. The majority of the initially produced SI-POF had an NA of 0.50 (e.g. [Asa96], [Esk97], [LC95]). SI-POF with an NA around this value are nowadays generally called standard NA-POF or standard POF for short. The bandwidth of such fibers is approximately 40 MHz for a 100 m long link (quoted as the bandwidth-length product 40 MHz · 100 m). For many years this was a completely satisfactory solution for most applications. 2.2.2 The Step Index Fiber with Reduced NA (low-NA) However, when it became necessary to replace copper cables with polymer optical fiber to accomplish the transmission of ATM data rates of 155 Mbit/s (ATM: asynchronous transfer mode) over a distance of 50 m, a higher bandwidth was required for the POF. In the mid-nineties all three important manufacturers developed the so-called low-NA POF. POF with a reduced numerical aperture (low-NA POF) feature a bandwidth increased to approximately 100 MHz · 100 m because the NA has been reduced to approximately 0.30. The first low-NA POF was presented in 1995 by Mitsubishi

68


Rayon ([Koi98]). Figure 2.41 shows that the fiber construction corresponds to the standard POF, the distinction being that the refractive index difference is smaller (approximately 2 %). Usually the same core material is used, but the cladding material has a modified composition.

ncore ncladding jacket optical cladding

core


Fig. 2.41: Structure of a low-NA step index profile fiber

Unfortunately, practical testing showed that although this fiber met the requirements of the ATM forum ([ATM96b]) with respect to bandwidth, it did not meet the requirements with respect to bending sensitivity. These requirements specify that for a 50 m long POF link the losses resulting from a maximum of ten 90° bends having a minimum bending radius of 25 mm should not exceed 0.5 dB. In order to meet both these requirements at the same time it became necessary to find a new structure. 2.2.3 The Double-Step Index Optical Fiber (DSI) The double-step index POF features two claddings around the core, each with a decreasing refractive index (Fig. 2.42). In the case of straight installed links, light guiding is achieved essentially through the total reflection at the interface surface between the core and the inner cladding. This index difference results in an NA of around 0.30, similar to the value of the original low-NA POF.

ncore ncladding1 ncladding2 jacket

core

outer / inner optical cladding Fig. 2.42: Structure of a double step index profile fiber

jacket inner / outer optical cladding


69

When fibers are bent, part of the light will no longer be guided by this inner interface. However, it is possible to reflect back part of the decoupled light in the direction of the core at the second interface between the inner and the outer cladding. At further bends, this light can again be redirected so that it enters the acceptance range of the inner cladding. The inner cladding has a significantly higher attenuation than the core. Light propagating over long distances within the inner cladding will be attenuated so strongly that it will no longer contribute to pulse propagation. Over shorter links the light can propagate through the inner cladding without resulting in too large a dispersion. A schematic illustration is shown in Fig. 2.43. launched light rays

4 rays behind the bend

1 rays, guided only by

3

the inner cladding

1

2 rays, guided by the outer cladding behind the bend

3 rays, guided by the outer cladding over a limited distance

bend radius

2 1 2

4 not guided rays behind the bend Fig. 2.43: Operation of a bent double step index profile fiber

The first generation of DSI-POF primarily served the purpose of increasing the bandwidth of 1 mm fibers from 40 MHz · 100 m to 100 MHz · 100 m with an unchanged minimum bending radius of 25 mm. The respective applications are to be found in LANs and home networks. The fiber producers offer these fibers under the same type names as the original “real” low-NA fibers. It has since become standard procedure to call the fibers low-NA and to indicate DSI as the index profile. Currently, another goal is being pursued: the bandwidth of standard POF is sufficient for applications in vehicle networks, but the bending radius should be reduced. Presently being discussed are POFs, the index steps of which correspond to a NA of 0.50 or 0.65 respectively to the inner and outer cladding. The bending radius can thus almost be halved.

70


2.2.4 The Multi-Core Step Index Optical Fiber (MC) As described above, the requirements of high bandwidth and low sensitivity to bending are difficult to accomplish together within one and the same fiber having a diameter of 1 mm. Fibers with a smaller core diameter can solve this problem since the ratio to the fiber radius is larger for the same absolute bending radius. However, this contradicts the requirements for easy handling and light launching. A PCS with a core diameter of 200 ȝm and an AN = 0.37 permits, for example, a bending radius of 5 mm with very low bending losses. As a compromise, Asahi developed a multi-core fiber (MC-POF, see [Mun94], [Mun96] and [Koi96c]). In this fiber many cores (19 to over 200) are put together in production in such a way that together they fill a round cross-section of 1 mm diameter. First, the individual fibers are all perfectly round and each has its own optical cladding. Only a certain share of the total cross-section of the bundle enters the cores guiding the light, since the cladding areas and the spaces between the fibers have to be accounted for. Figure 2.44 shows the parameters which mark the percentage of the filled-in area. The number N here indicates how many fibers lie next to each other over a diameter while n indicates the entire number of fibers.

R

R dm

dm r N=5

N=1

n = 19

Fig. 2.44: Schematically arrangement of cores in a MC-POF

In the figure, R denotes the radius of the complete fiber (typically 0.5 mm) and d the thickness of the optical cladding (e.g. 5 μm). Let us assume first of all that the individual cores are arranged in a hexagonal shape with N = 2z + 1 cores positioned next to each other. The next Fig. 2.45 shows how the arrangement for fibers is changed for z = 1 to 5. While these sketches can give a clear definition of the number of fibers that can be arranged within a circular shape, for smaller and smaller individual cores the possibilities are more complex. The arrangement at the bottom right shows one possible deviation. For the first five arrangements the number of individual fibers is calculated as follows: n = 3z2 + 3z + 1.


71

It follows that the individual radius r is: r = R/N = R/(2z + 1). N=3

N=5

N=7

N=9

N=11

N´=11

Fig. 2.45: Possible circular arrangements of cores in a MC-POF

In Table 2.3, the degree of coverage of the circle area is calculated for the cases shown. First, the number of individual cores is calculated from z. The radius r results from the overall radius of the fiber (here always 500 μm). Parameter ta indicates what percentage of the total circular area is covered by the individual circles (for the hexagonal arrangement of an infinite number of circles a maximum of 90.69 % of the area can be covered). When calculating parameter tb, the fact that part of the cross-section is lost to the optical claddings (all uniformly 5 μm thick) is taken into account. Table 2.3: Core cross area degree of coverage for MC fibers (ideal) z:

N:

n:

r:

ta:

tb:

0 1 2 3 4 5

1 3 5 7 9 11 11´ 13 15 17 29 -

1 7 19 37 61 91 85 127 169 217 631 -

500 μm 167 μm 100 μm 71.4 μm 55.6 μm 45.5 μm 49.3 μm 38.5 μm 33.3 μm 29.4 μm 17.2 μm -

100.00 % 77.78 % 76.00 % 75.51 % 75.31 % 75.21 % 82.47 % 75.15 % 75.11 % 75.09 % 75.03 % 90.69 %

98.01 % 73.18 % 68.59 % 65.31 % 62.36 % 59.57 % 66.57 % 56.88 % 54.27 % 51.73 % 37.82 % -

6 7 8 14 f

72


Figure 2.46 shows the proportion of core area tb as depending on the number of cores for four different thickness’ of the optical cladding. use of the total cross area tb 100% 80%

dm = 5 μm

dm = 20 μm

dm = 10 μm

dm = 30 μm

60% 40% 20% 0% 1

7

19

37

61

91

127

169

217

number of single cores

Fig. 2.46: Proportion of core area for different cladding thickness

As can be expected, the proportion of the overall covered area decreases with an increasing number of cores because the proportion of cladding area will become larger and larger. A certain minimum thickness of cladding is necessary for it to be able to fulfill its function and still be technologically feasible. The four individual data points show the case of the optimized fiber arrangement with 85 individual cores in accordance with Fig. 2.45. Given a minimum thickness of the optical cladding between 5 μm and 10 μm, these considerations indicate that a maximum number of some 100 single cores should be used, in which case the proportion of useable area will hardly exceed 70 %. It is easy to conclude that a smaller proportion of useable core area would lead to an increase in the losses encountered when connecting transmitters to, and fibers between each other.

Fig. 2.47: 37 core POF with deformed single cores (schematically)


73

Practical experience shows that a better utilization of the area can be achieved. During the manufacturing process the fibers are placed together at higher temperatures which means that they change their shape and thus reduce the gaps between the fibers. Apparently, the resulting deviations from the ideal round shape do not play a significant role in light propagation (the causes for this are not yet completely understood; some points worth discussing can be found in the chapter on light propagation in POF). Figure 2.47 shows a schematic illustration of the cross-section of a fiber with 37 cores, such as e.g. in [Tesh98]. Data of available MC-POF and -GOF are grouped together later. Figure 2.48 shows the refractive index profile of a MC-POF, shown as a crosssection through the diameter of the fiber. The index steps correspond to those of a standard POF.

ncore ncladding jacket

jacket

cores

optical cladding

optical cladding

Fig. 2.48: Structure of a step index multi core fiber

Since the bandwidth only depends on the NA for SI fibers, it should be possible to measure values comparable to the standard POF. However, the fact is that the measured values are actually significantly higher, which has been explained in the chapter 2.1.5.2 discussing mode-selective attenuation mechanisms. Glass fibers are also produced for use in many areas as fiber bundles. In lighting technology fiber glass bundles with a large NA are widely spread. (The lighting of the headlight outer ring at BMW via such a fiber bundle is wellknown.) In the meantime, such fibers are also available for data communication ([Lub04b]). 2.2.5 The Double Step Index Multi-Core Fiber (DSI-MC) In the MC-POF, too, an increase in bandwidth was achieved by reducing the index difference. Due to the smaller core diameters it was still possible to avoid an increase in bending sensitivity. Even better values were achieved with individual cores having a two-step optical cladding such as illustrated in Fig. 2.49. The principle is the same as in the double-step index POF with an individual core. In this case a bundle with single cladding is completely surrounded by a second cladding material (“sea/islands” structure).

74


ncore ncladding1 ncladding2 cores

jacket

jacket

outer / inner optical cladding Fig. 2.49: Structure of a double step index profile multi core POF

2.2.6 The Graded Index Optical Fiber (GI) When using graded index profiles (GI) an even greater bandwidth becomes possible. In these profiles, the refractive index continually decreases (as a gradient), starting from the fiber axis and moving outwards to the cladding. Of particular interest are profiles that follow a power law (remember chapter 1.4.1). refractive index

n=n

fiber axis

g ª § distance to fiber axis · º «1- ' ¨ ¸ » core radius «¬ © ¹ »¼

The parameter g - often also Į - is characterized as the profile exponent. When g = 2 one speaks of a parabolic profile. The borderline case of the step index profile fibers corresponds to g = . The parameter ǻ states the relative refractive index difference between the maximum core and the cladding refractive index. Figure 2.50 shows a parabolic index profile.

ncore ncladding core jacket optical cladding


Fig. 2.50: Structure of a graded index profile fiber

Due to the continually changing refractive index, the light rays in a GI fiber do not propagate in a straight line but are constantly refracted towards the fiber axis. Light rays that are launched at the center of the fiber and do not exceed a certain angle are completely prevented from leaving the core area without any reflections occurring at the interface surface. This behavior is illustrated schematically in Fig. 2.51. The geometric path of the rays running on a parallel to the axis is still significantly smaller than the path of rays that are launched at a greater angle. However, as can be seen, the index is smaller in the regions distant from the core. This means a greater propagation speed. In an ideal combination of parameters the different path lengths and different propagation speeds may cancel each


75

other out completely so that mode dispersion disappears. In reality, this is only possible in approximation. It is possible, however, to increase bandwidths by two to three orders of magnitude compared with the SI fiber. graded index profile fiber

step index profile fiber

n

n

Fig. 2.51: Comparison of step and graded index profile (see also chapter 2.1.1)

When considering not only the pure mode dispersion but also chromatic dispersion, i.e. the dependence of the refractive index on the wavelength and spectral width of the source, an optimum index coefficient 'g' deviating from 2 is achieved. This has been the subject of comprehensive investigations by the research group around Prof. Koike ([Koi96a], [Koi96b], [Ish00], [Koi97a], [Koi96c], [Koi98] and [Ish98]). In [Ish00] and [Koi00] the significance of this effect is particularly pronounced (see also Chapter 2). Due to the smaller chromatic dispersion of fluorinated polymer compared with silica, the bandwidth of GI-POF theoretically achievable is significantly higher than that of multi-mode GI silica glass fibers. In particular, this bandwidth can be realized over a significantly greater range of wavelengths. This makes the PF-GI-POF interesting for wavelength multiplex systems. However, in this case the index profile must be maintained very accurately, a requirement for which no technical solution has as yet been provided. Another factor involved in the bandwidth of GI-POF is the high level of modedependent attenuation ([Yab00a]) compared to silica glass fibers. In this case modes with a large propagation angle are suppressed resulting in a greater bandwidth. An example is the simulation that was carried out in [Yab00a]: the bandwidth of a 200 m long PMMA-GI-POF increases from 1 GHz to over 4 GHz, taking into account the attenuation of higher modes. This is also confirmed in practical trials. Mode coupling is less significant for GI fibers than it is for SI fibers since the reflections at the core-cladding interface do not occur. 2.2.7 The Multi-Step Index Optical Fiber (MSI) Following the many technological problems experienced in the production of graded index fibers having an optimum index profile that remains stable for the duration of its service life, an attempt was made to approach the desired characteristics with the multi-step index profile fiber (MSI-POF). In this case the core consists of many layers (e.g. four to seven) that approach the required parabolic curve in a series of steps. Here a “merging” of these steps during the manufacturing process may even be desirable. A diagram of the structure is shown in Fig. 2.52.

76


ncore ncladding core jacket optical cladding


Fig. 2.52: Structure of a multi step index profile fiber

In this case light rays do not propagate along continually curved paths as in the GI-POF, but on multiple diffracted paths as demonstrated in Fig. 2.53. However, given a sufficient number of steps, the difference to the ideal GI profile is relatively small so that large bandwidths can nevertheless be achieved. MSI-POF were presented in 1999 by a Russian institute (Tver near Moscow [Lev99]) and by Mitsubishi (ESKA-MIU, see [Shi99]). In the meantime, other companies are producing such fibers which are often called GI fibers. These GI and MSI fibers are classified in the same class of standards, e.g. A4e.

n Fig. 2.53: Light propagation in the MSI-POF

2.2.8 The Semi-Graded Index Profile Fibers (Semi-GI) A relatively new version of index profiles are fibers which have a gradient with a slightly varying index above the core cross section, but do have an optical cladding with a great index step as shown in Fig. 2.54 ([Sum00], [Sum03], [Ziem05f] and [Ziem06i]).

ncore ncladding jacket optical cladding

core


Fig. 2.54: Structure of a semi-graded index profile fiber


77

At first sight this variety of fiber has enormous advantages. Light which propagates within the gradient is only subject to very little mode dispersion. If a ray of light has a greater propagation angle, e.g. after being bent, then it continues to be led to the core-cladding interface layer through total reflection. However, these rays do have a very much higher mode dispersion. Figure 2.55 shows how light spreads theoretically and what consequences this has for the pulse response.

input

output „GI“-modes „SI“-modes

t

Fig. 2.55: Light propagation in semi-graded-index profile fibers theoretically

In principle, two different groups of modes can be seen in the picture. The paths designated as GI modes do not touch the cladding and only show a very slight difference in propagation times. The shares designated as SI modes are completely reflected at the core-cladding interface layer. These light paths are also bent in the core, but the light path, now very much longer, can no longer be compensated for in the outer areas by the lower refractive index. With very high data rates the second mode group is drawn out so widely that it is presented solely as a kind of DC offset in the eye diagram. At the POF-AC a data rate of 1 Gbit/s was transmitted over 500 m of a GI PCS fiber with a PRBS signal ([Vin05a]). Data rates up to 3 Gbit/s could be attained with a small surface APD receiver ([Kos95]). In order to do justice to the complex behavior of the semi-GI POF, corresponding modulation formats should be selected. 2.2.9 An Overview of Index Profiles Figures 2.56 through 2.58 again show all index profiles described in an overview. Due to the wide range of possibilities offered in polymer chemistry further developments are certainly to be expected. For example, multi-core graded fibers, fibers with special cladding for a reduction of the losses at the core/cladding interface or to increase the bandwidth or even multi-core fibers with different individual cores are all conceivable. In the following figures POF variants are shown with typical parameters.

78


SI-POF AN = 0.50 40 MHz100 m

Low-NA-POF AN = 0.30 100 MHz100 m

DSI-POF AN = 0.30 100 MHz100 m

Fig. 2.56: POF with single core and step index profile

Single-core fibers with diameters between 125 μm and 3 mm are available from different manufacturers at a reasonable price and in robust quality. Most of the polymer optical fibers used in practical applications are of these types.

MC-SI-POF e.g. 200 cores AN = 0.30 100 MHz100 m

MC-DSI-POF e.g. 37 cores AN = 0.19 400 MHz100 m

Fig. 2.57: POF with multiple cores and step index profile

MC fibers are available from various manufacturers. They are deployed in applications ranging from high data rates transmission systems through to optical image guides. Because of the short lengths produced, the prices are still significantly above expectations. However, further developments in this field can be expected in the future.

GI-POF AN = 0.20 2 GHz100 m

MSI-POF AN = 0.30 500 MHz100 m

Fig. 2.58: Polymer fibers with graded index and multi step index profile

Graded index as well as multi-step index profile POF are commercially available today. Laboratory experiments and a series of practical installations in Japan and Europe, (e.g. [Mös04]) show the great potential in regard to the bit rates possible. Asahi Glass introduced them into the market around 2001. Lucent Technologies, later called OFS and trading under the name of Chromis Fiberoptics as of 2004 ([Whi04], [Park05a]), also announced the possibility of producing large amounts of GI POF in case of demand.

2.3 The Development of POF

79

In Europe, fibers by Nexans are manufactured in Lyon ([Gou04]). All three fibers will consist of the fluorinated polymer material CYTOP®. The core diameter of the LucinaTM Fiber by Asahi Glass is 120 μm with an AN = 0.28. A protective cladding made from PMMA and measuring 500 μm is placed around an area of fluorinated polymer outside the core profile. The duplex cable has external dimensions of approximately 3 by 5 mm. The lowest attenuation achieved to date is approx. 15 dB/km for a wavelength of 1,300 nm. The specified value is < 50 dB/km for 700 nm - 1,300 nm. There has also been significant progress in the manufacture of GI or MSI-POF respectively on a PMMA basis (see Section 2.3.4).

2.3 The Development of Polymer Optical Fibers The following sections will describe the polymer fibers presented so far, whereby particular attention will be paid to the chronological sequence of the developments. Section 2.4 supplements these observations with some types of multimode glass fibers which were not discussed in the first edition. 2.3.1 Looking back The first POF were manufactured by DuPont as early as the late sixties. Due to the incomplete purification of the monomer materials used, attenuation was still in the vicinity of 1,000 dB/km. During the seventies it became possible to reduce losses nearly to the theoretical limit of approximately 125 dB/km at a wavelength of 650 nm. At that point in time glass fibers with losses significantly below 1 dB/km at 1,300 nm/1,550 nm were already available in large quantities and at low prices. Digital transmission systems with a high bit rate were then almost exclusively used in telecommunications for long-range transmissions. The field of local computer networks was dominated by copper cables (either twisted-pair or coaxial) that were completely satisfactory for the typical data rates of up to 10 Mbit/s commonly used then. There was hardly any demand for an optical medium for high data rates and small distances so that the development of the polymer optical fiber was slowed down for many years. A significant indicator for this is the fact that at the beginning of the nineties the company Höchst stopped manufacturing polymer fibers altogether. During the nineties, after data communication for long-haul transmission had become completely digitalized, the development of digital systems for private users was commenced on a massive scale. In many spheres of life we are being increasingly confronted with digital end user equipment. The CD player has largely replaced analog sound carriers (vinyl records and cassettes). The MP3 format is leading to a revolution in music recording and distribution. The DVD (Digital Video Disc) and large hard disk drives could lead to the replacement of the analog video recorder within a few years. Even today more digital television programs

80


are available than analog programs. Decoder boxes have become standardized (MPEG2 format) and will be integrated into television sets in the future. More and more households are using powerful PC and digital telephone connections (ISDN). With offers such as T-DSL (ADSL technology provided by Deutsche Telekom AG) as well as fast internet access via satellite or broadband digital services on the broadband cable network, private users are being offered access to additional digital applications even before the start of the new millennium. Likewise, in the automotive field the step towards digitalization has long been made. CD changers, navigation systems, distance-keeping radar and complex control functions are increasingly part of the standard equipment being provided in all classes of vehicles. The development of electronic outside mirrors, fast network connections even from within an automobile as well as automatic traffic guidance systems will ensure a further increase in the range of digital applications for the motor vehicle. All these examples demonstrate that completely new markets for digital transmission systems are being developed for short-range applications. Polymer optical fibers can meet many of these requirements to an optimum degree and are therefore increasingly of interest. A significant indicator for this development is the history of the International Conference for Polymer Optical Fibers and Applications which has been taking place annually since 1992 and represents the most significant scientific event in this specialized field. Many of the developments described below were presented for the first time at these conferences. 2.3.2 Step Index Polymer Fibers The SI-POF is the oldest variant of all polymer fibers. Its development goes back to the beginning of the 1960’s, i.e. in a period when silica glass fibers were being developed. Today the SI-POF is by far the most common POF variant. In Table 2.4 data from different publications on this fiber type are summarized - without claiming to be complete. Table 2.4: Published data of SI-POF Ref.

Year Producer Product

[Min94] [Koi97a] [Koi96c] [Sai92] [Min94] [Koi95] [Sai92] [Sai92] [Koi95] [Min94]

1963 1964 1968 1976 1978 1982 1983 1983 1983 1984

Du Pont Du Pont Du Pont Mitsubishi Mitsubishi NTT Mitsubishi Mitsubishi Mitsubishi Mitsubishi

CROFON

Eska Super Eska Eska Extra Eska Extra Eska Extra

Øcore Attenuation at O μm dB/km nm 1.000 650 500 650 500 650 300 650 300 650 55 568 124 650 65 570 1000 110 570 150 650

NA Remarks st. st. st. st. st. st. st. st. st. st.

first POF first SI-POF

4 MHzkm


81

Table 2.4: Published data of SI-POF (continued) Øcore Attenuation at O NA Remarks μm dB/km nm Asahi 80 570 st. Mitsubishi Eska Extra 125 650 st. up tp 85°C Mitsubishi Eska Extra 65 570 st. Hoechst 1000 130 650 st. Asahi Luminous-F 175 660 0.50 310 MHz10m AN, LED=0.50, 105°C Asahi X-1 0.37 540 MHz10m AN, LED = 0.50 Asahi X-2 0.28 >1.000 MHz10m AN, LED = 0.50 Höchst EP51 970 190 650 st. 90 MHz100 m with 650 nm LED Mitsubishi Eska Premier 1000 135 650 0.51 up to 85°C Sveton MNCIS 200- 150 650 0.45 up to 70°C Series, Grade U 600 Sveton MFCIS 200- 120 650 0.48 up to 70°C Series, Grade U 1000 Sumitomo n. a. 480 150 650 0.51 200 MHz50m 'n=0.055 Mitsubishi n. a. 1000 110 650 0.47 80 MHz100 m Dig. Optr. n. a. 1000 0.50 2003 announced Luvantix SI type 1000 160 650 0.40 200 MHz bandwidth Nuvitech Nuvilight 1000 250 650 0.38 for illumination Luceat SI-Type 1000 150 650 0.46 30 MHz100 m Nanoptics A-POF 1000 100 650 - conception Huiyuan SI-POF 1000 300 650 - coextrusion Luceat SI-POF 1000 135 650 0.50 from preform 65 520

Ref.

Year Producer Product

[Koi95] [Sai92] [Sai92] [Koi95] [Tesh92]

1985 1991 1991 1991 1992

[Tesh92] 1992 [Tesh92] 1992 [Eng96] 1992 [Kit92] 1992 [Lev93] 1993 [Lev93] 1993 [Non94] 1994 [Koe98] [Mye02] [Luv03] [Nuv04] [Luc05] [Wal05] [Hai05] [Zie06h]

1998 2002 2003 2004 2005 2005 2005 2006

5,000 attenuation [dB/km] 2,000 1,000 500 200 100 50 400

wavelength [nm] 450

500

550

600

650

700

750

800

Fig. 2.59: Attenuation of different standard-NA SI-POF (measurement by POF-AC)

82


It was not until about 1980 that technology made possible the production of POF which came relatively close to the theoretical attenuation minima. Initial problems with the service life and with certain mechanical loads were quickly solved with on-going developments. In Fig. 2.59 the spectral attenuation curves of three SI-POFs are shown (data sheet information). All three fibers from Japanese manufacturers are close together. The visible differences may possibly be due to different methods of measurement. Most manufacturers offer SI-POFs in different diameters. In [Zub01b] and [Nuv04] the properties of these fibers are compared (Table 2.5). Table 2.5: Attenuation of POF with different diameter diameter [μm] Mitsubishi Toray Asahi Chem. BOF Optectron Nuvitech

Attenuation [dB/km] 500 750 < 190 < 180 < 180 < 150 < 180 < 180 < 150 < 150 < 150 < 150 < 250 < 250

250 < 700 < 300 n. a. < 150 < 150 < 350

1.000 < 160 < 150 < 125 < 150 < 150 < 250

For Toray fibers, the losses of fibers with different diameters are listed in the data sheet and are shown in Fig. 2.60.

10000

attenuation [dB/km]

3000 1000 300

core t 750 μm core = 500 μm core = 250 μm

100 30 400

wavelength [nm] 500

600

700

800

900

Fig. 2.60: Attenuation of different PMMA-SI-POF by Toray

With a few exceptions the losses for all fiber diameters are similar. Some reasons for the increase in attenuation with thinner fibers could be that either the high attenuation of the optical cladding plays a greater role or that more stress is exerted on the thin fiber during manufacture. A fiber with a ¼ mm core diameter


83

has only one sixth the thermal capacitance. When the cladding and opaque jacket are applied this fiber is necessarily warmer. The process temperatures during manufacture can indeed lie clearly above the glass transition temperature. The youngest manufacturer of PMMA SI-POF is the Italian company Luceat. Here fibers for diverse applications, mainly in mechanical engineering, are produced. The highest quality is still in the developmental stage. A comparison of the measured values of Luceat fibers (POF-AC 2006, [Ziem06h]) with the values from [Wei98], more or less the POF reference curve up until now, is shown in Fig. 2.61. 500 attenuation [dB/km] 300 200 Luceat

100 80 [Wei98] 60 50 400

450

500

550

600

650 700 wavelength [nm]

Fig. 2.61: Attenuation of SI-POF by Luceat (2006)

In the area of 520 nm this fiber is even somewhat better that the data of the best fibers so far. Thanks to the availability of reasonably priced and fast green LEDs this advantage can be assessed very highly. As part of the European POF project POF-ALL (see www.ist-pof-all.org) the transmission of a 10 Mbit/s data stream was able to be demonstrated over 425 m (see System Chapter). 2.3.3 Double Step Index Profile Polymer Fibers We have already discussed the principle idea of a double step index profile POF. All three important Japanese manufacturers presented such fiber types around 1995. After the expectations that ATM would become the dominating network technology in the home were not fulfilled, these fibers have more or less become niche products today, albeit at relatively high prices. Today in many areas there is a demand for data rates which require the use of these fibers instead of the normal SI-POFs. Technically, DSI-POFs are on a comparable level and would hardly be more expensive than SI-POFs when produced in high volumes.

84


Table 2.6 compares the properties of DSI-POFs of the three manufacturers ([Mit01], [Nich03], [LC00b]). Table 2.6: Overview of DSI-POF

diameter attenuation (650 nm) numerical aperture bandwidth temperature range bend radius

[μm] [dB/km] MHz km [°C] [mm]

Mitsubishi Toray MH4001 PMU-CD1001 980 1000 ± 45 160 170 0.30 0.32 10 >10 -55 .. +75 -20 .. +70 25 -

Asahi AC1000(I) 1000 ± 60 160 0.25 15 -40 .. +70 25

We would like to point out once again that the DSI-POFs are usually offered now as before as low NA POF. In the first few years manufacturers did not provide any information at all about the double cladding structure. In [Eng98b] the double cladding structure was proven quite early on the basis of measurements of the far field and with optical microscopy. In Fig. 2.62 you can see the far field distributions for different fiber lengths measured with the inverse far field method at the FH Gießen/Friedberg. 1.0

Popt

1m 10 m 50 m 90 m

0.8 O= 594 nm

0.6 0.4 0.2 0.0 -30

-20

-10

0

10

20

30

4 [°]

Fig. 2.62: Inverse far field measurement of a DSI-POF

You can clearly see that after short distances much light from the interface layer between inner and outer cladding is still guided. After 50 m these shares have disappeared and the angle distribution corresponds to a true low NA POF. Figure 2.63 shows two microscope photos of DSI-POF (Univ. of Ulm). Both optical claddings can be easily recognized. At the 2003 POF Conference Mitsubishi was the first manufacturer to present the actual structure. The effect of suppressing higher modes by high attenuation of the inner cladding was also confirmed theoretically and experimentally. For example, Asahi gives a value of 6000 dB/km at 650 nm for the losses in the inner cladding.


85

Fig. 2.63: Double cladding structure of a POF

2.3.4 Multi-Core Polymer Fibers Since 1994, polymer fibers as multi-core fibers have been introduced, e.g. in [Tesh98], [Mun94], [Asa97] and [Tesh98]. Table 2.7 shows a few parameters from these publications. Table 2.7: Multi-core POF (Asahi Chemical) Type

Ref.

NMC-1000 PMC-1000 MCS-1000 NMC-1000 PMC-1000 -

POF´94 Data´96 Data´97 POF´98 POF´98 POF´98 Data´98 Data´98 POF´98 POF´98

No. of Structure NA Attenuation Bandwidth Cores at 650 nm 19 SI 0.25 125 dB/km 170 MHz100 m 217 SI 0.15 270 dB/km n. a. 217 SI 320 dB/km n. a. 37 DSI 0.19 155 dB/km 700 MHz50 m 37 DSI 0.25 160 dB/km n. a. 37 DSI 0.33 160 dB/km n. a. 37 DSI 0.25 160 dB/km 500 MHz50 m 37 DSI 0.19 160 dB/km n. a. 217 SI 0.50 160 dB/km n. a. 217 SI 0.33 160 dB/km n. a.

The MC-POF features a noticeably reduced sensitivity to bending and only insignificantly increased attenuation as well as a significantly increased bandwidth compared to single core fibers, this being due to the possibility of smaller numerical apertures. Whether these fibers can be produced at the same price is still an open question. Should this be possible, data rates of 500 Mbit/s up to 1 Gbit/s over 50 m can easily be achieved in commercial applications. At the POF-AC a data rate of over 1 Gbit/s over 100 m MC-POF has already been achieved. At present, only Asahi chemical offers MC-POF for data communication while other manufacturers offer this kind of fiber for lighting purposes or also as image guiding fiber. The following photos show the cross-sections of the three, presently available MC-POFs with 37, 217 and 631 cores (the 19 core variant is no longer available).

86


Fig. 2.64: Photo by microscope of MC-POF, 37, 217 respectively 631 cores

An overview of the technical data of the four different MC-POFs is summarized in the following Table 2.8 (Nichimen data sheets). The PMC 1000 permits the highest data rates on the basis of experiments conducted so far since it possesses a DSI structure. Table 2.8: Data of MC-POF Unit Parameter number of cores μm single core core material cladding material 2nd cladding material NA mm fiber core mm cable jacket attenuation1) dB/km bit rate (50 m) Mbit/s temperature °C bend loss2) dB 1) 2) 3)

MCQ-1000 MCS-1000 NMC-1000 PMC-1000 613 217 37 60 PMMA PMMA fluoro polymer 0.50 0.5 r 0.05 1.0 r 0.06 1.0 r 0.06 2.2 r 0.07 2.2 r 0.10 PE PE (black) 0.65

100

theory 30 fiber length [m]

10 10

20

50

100

Fig. 2.91: Bandwidth measurement according to [Tak91] with different receiver NA

Another measurement of bandwidth on standard NA-POF (1 mm) is presented in [Tak93] (Fig. 2.92). Again, the measurement was carried out using the pulse method at 650 nm. Apart from measuring the bandwidth, the half far field width following the corresponding sample length was also determined. The bandwidth was calculated from the far field width as follows: 't mod

2 A N, FF

2nc

and therefore : B z

C 't mod

(C const.)

whereby tmod is the modal pulse propagation and B · z is the product of bandwidth and length. Parameter C is a free selectable constant which depends on the coupling conditions. The speed of light is c. In the formula AN , FF is not the fiber parameter indicated, but the value measured depending on length. For a sample length of 10 m, the difference between the measured bandwidth for launching the light with AN = 0.10 and AN = 0.65 is more than one order of magnitude. For lengths up to 100 m this factor decreases to 2.

2.5 Bandwidth of Optical Fibers 10,000

109

launching conditions

bandwidth [MHz] 3,000 1,000

AN Launch = 0.10 AN Launch = 0.65

300

theory based on far field width

100 30 10

20

50

fiber length [m] 100

Fig. 2.92: Bandwidth according to [Tak93]

When launching light with a small NA, the bandwidth drops disproportionately, from approximately 80 MHz km to approximately 16 MHz km. This suggests an increasing filling out of modal field. By comparison, when launching with a large NA, the bandwidth is reduced somewhat more slowly than the length, from approx. 4 MHz km to approx. 5 MHz km. This is due to the effect of mode coupling and mode related attenuation. The bandwidth values determined by means of the far field width correlate very well with the results of the bandwidth measurements made by pulse propagation. This suggests that mode dependent attenuation and mode conversion are the determining processes because they affect the bandwidth by changing the mode distribution. In contrast, if mode coupling were more pronounced, the bandwidth would also change without affecting the far field. However, any estimated quantification based on these measured results alone would be questionable. In [Rit93] measured results for the bandwidth of standard NA-POF at launching conditions of AN = 0.10 and AN = 0.65 (Fig. 2.93) are also shown.

5,000

launching conditions

bandwidth [MHz]

2,000

AN Launch = 0.10 AN Launch = 0.65

1,000 500 200 100 50 20 10

30

100

300

fiber length [m] 1,000

Fig. 2.93: Measured bandwidth of a SI-POF according to [Rit93]

110

2.5 Bandwidth of Optical Fibers

Here too, the measured bandwidth for short lengths (20 m) differs by more than an order of magnitude. For large lengths the difference is reduced correspondingly. The authors calculate the bandwidth based on their own theory that follows the concept of the diffusion model. Instead of investigating separate modes, this model investigates modal groups that differ in their 2 angles of propagation (radial and azimuthally). The coupling between the modes is described by a diffusion constant that only takes into account the energy transfer in neighboring mode groups. The model also takes into account mode dependent attenuation. In this work the remaining deviation between theory and measured values is explained by means of the mechanism of mode coupling. In variance to the model, this is a factor that is not independent of the angle. Simulations provide good results if elongated scattering centers of 37 μm length and 2.5 μm diameter are assumed in the fiber with random distribution and orientation along the axis of the fiber (caused by the drawing process), as shown schematically in Fig. 2.94.

scattering centers

Fig. 2.94: Model for scattering centers in POF

An indication of a non-uniform inner structure of the PMMA fiber is the photo (from >Fei00@) of the surface of a cut POF taken by a scanning electron microscope and shown in Fig. 2.95. The fibril-like structures in the sub-Pm range can clearly be seen.

Fig. 2.95: Microscopic structure of a PMMA POF cut ([Fei00])


111

Figure 2.96 shows further experimental results for the bandwidth of polymer optical fibers [Kar92]. In each case collimated light or light with an angle adapted to the fiber's NA (UMD) was launched into the POF. As was the case in the results previously shown, very large differences result for short lengths of fibers. The parameter shown in the figure here is the product of bandwidth and length. 50

bandwidth length product [MHzkm]

launching conditions collimated 1.0 mm 0.5 mm

20 10

UMD 0.5 mm 1.0 mm

5 sample length [m] 2

5

10

20

50

100

200

500

Fig. 2.96: Measured POF bandwidth of a SI-POF according to [Kar92]

Apart from the effect of the launching NA, [Kar92] also investigates whether the bandwidth depends on the size of the launched beam. In fact, for UMD launching, a larger bandwidth was found as well as a smaller light spot, compared with complete illumination of the fiber cross-section; however, the differences are not as pronounced as when the launch angle is changed. For collimated light the relationship is reversed. Because all processes described up to this point are only dependent on the angle, it seems surprising to find that the size of the launching spot has an effect on the measured bandwidth of SI fibers. However, when considering the fact that mode conversion can cause deviations in location and deviations in angle after just a short length of the specimen (see schematic in Fig. 2.97), the result becomes understandable [Kar92].

bending deviation in location deviation in angle

Fig. 2.97: Conversion of spatial and angular distances

112


In [Poi00] the results of [Kar92] are compared with current measurements on 2 standard NA-POF by Toray and Mitsubishi (Fig. 2.98). These measurements qualitatively confirm the previous results. For very short lengths of samples the differences between small and large launching angles are even greater.

30,000 bandwidth [MHz] Toray Mitsubishi [Kar92] Toray Mitsubishi [Kar92]

10,000

3,000

NALaunch: 0.09 0.09 collimated 0.64 0.64 UMD

1,000 theory 300 100 length [m]

30 1

3

10

30

100

300

Fig. 2.98: Measured bandwidth of different SI-POF according to [Poi00]

2.5.3.2 Bandwidth Measurements on SI-POF This section as well as the following four sections deals with bandwidth measurements conducted at the POF-AC Nürnberg. All measurements were carried out under uniform measurement conditions. Semiconductor diodes with a wavelength of 650 nm or 850 nm respectively served as transmitters. Both lasers can be modulated analogously up to 2 GHz. A singlemode glass fiber is mounted firmly to the laser diodes. Using a combination of different microscope lenses and optical apertures the coupling angle in the area AN Launch = 0.01 to 0.64 can be varied. The coupling spot is directly visible through a beam splitter. With the aid of adjustment screws the size as well as the position of the light spot can be changed. Figure 2.99 shows the complete setup of the measurement device. A commercial product on the basis of a 400 μm Si-PIN photodiode with an integrated preamplifier and about 1.5 GHz bandwidth was used as a receiver. In order to attain mode independence the receiver was connected to a 1 mm mixed glass fiber bundle with a large NA.


113

Fig. 2.99: Experimental setup of bandwidth measuring by POF-AC

The lengths and NA-dependent bandwidths were systematically measured for a series of different step index profile fibers. An overview can be found in [Bun02a]. The following Figs. 2.100 to 2.102 show the results for three types of fiber: ¾ 1 mm standard PMMA POF with AN = 0.46 ¾ 1 mm standard POF made of cross-linked PMMA with AN = 0.54 ¾ 1 mm polycarbonate POF with AN = 0.75 B3dB [MHz]

5,000

NAlaunch: 0.64 0.48

2,000 1,000

0.33

500

0.19

200

0.09 0.05

100 50 length [m]

20 5

10

20

50

100

Fig. 2.100: Bandwidth measurement of a 1 mm SI-PMMA-POF

114


For a 1 mm PMMA POF (Toray PFU CD1000, see also [Ziem04a]) 3 dB bandwidths for lengths between 5 m and 100 m were measured. The coupling angle was changed for NA values between 0.05 and 0.65 with the unit described above. For short fiber lengths the bandwidths measured differ by almost a magnitude which demonstrates once again the importance of correct measurement conditions for correctly indicating the bandwidth values. After a 100 m test length there still is a factor of two between the values measured. The curves for under filled launch (small NA) fall more steeply than with length caused by a predominance of mode mixing. For overfilled launch (large NA) the curves run flatter. Here the modedependent attenuation dominates. The next figure shows the results with a 1 mm POF made of modified PMMA (Toray PHKS CD1001). The fiber is specified with a NA of 0.54. NAlaunch:

B3 dB [MHz]

3,000

AN = 0.05 AN = 0.09

1,000

AN = 0.19 AN = 0.33

300

AN = 0.48 AN = 0.64

100 30 5

10

20

100 length [m]

50

Fig. 2.101: Bandwidth measurement of a 1 mm SI-mod. PMMA-POF

Since the losses of this fiber lie at about 300 dB/km at 650 nm, test lengths of only up to 50 m could be measured. Incidentally, the measurement results are similar to a large degree to the results of the PMMA POF. 3.000

NAlaunch:

B3 dB [MHz]

AN = 0,05

1.000

AN = 0,09 AN = 0,19

300

AN = 0,33 AN = 0,48

100

AN = 0,64 length [m]

30 1

2

5

10

Fig. 2.102: Bandwidth measurement of a 1 mm SI-PC-POF

20


115

The third fiber tested is the polycarbonate POF FH4001 from Mitsubishi. The NA of the fiber lies at 0.75, the attenuation amounts to 650 nm at about 800 dB/km, whereby the maximum measurement length remains limited to 20 m. Surprisingly, the bandwidth differences between the three types of fiber are only very slight although there were clear differences in the NA. One explanation for this could be the greater effects for mode mixing and above all for the modedependent attenuation which occurred in the fibers made of modified PMMA and polycarbonate. Figures 2.103 and 2.104 illustrate the far fields of the three fibers in comparison (cf. [Bun02a]). 3.000

B3 dB [MHz] PC PHKS PMMA

1.000 300 NALaunch = 0.33 100

length [m]

30 2

5

10

20

50

100

Fig. 2.103: Comparison of the bandwidths of different SI-POF 1000

power [a.U.] mod. PMMA

800 PC

600 PMMA 400 200

T [°] 0 -40

-30

-20

-10

0

10

20

30

40

Fig. 2.104: Comparison of the farfields of different SI-POF

The fibers of PMMA and PC - each after 10 m - have half-value widths of about 27°. The fibers of modified PMMA have only 17°. Here the share of modedependent attenuation predominates over the nominally larger NA.

116


Fig. 2.105: Comparison of the far fields of different SI-POF (3-d representations)

As part of the European Project POF-ALL (www.ist-pof-all.org) other comprehensive measurements of both length and launch-dependent bandwidths of different fibers were carried out. The following Figures 2.106 and 2.107 show the measurement results for a 1 mm standard POF (Luceat, high quality fiber) and for a 500 μm standard POF. 10,000

B3 dB [MHz]

NALaunch 0.05

5,000

0.10

2,000

0.19 0.34

1,000

0.47

500

0.65

200 100 50 1

2

5

10

20

50 100 fiber length [m]

Fig. 2.106: Bandwidth measurements of a 1 mm SI-POF (Luceat, HQ)

Both fibers essentially show comparable results. Since the fibers also have very similar attenuation values they can be used in almost all the same applications. The advantages of the thinner fibers are primarily the smaller space needed, an important point with multiple cables, and the smaller bending radius. The argument that the fibers with a smaller core diameter would enable higher bit rates or better receiver sensitivity because of the smaller photodiodes has for the most part since been dropped because of technical developments.


10,000

B3 dB [MHz]

117

NALaunch

5,000 0.05 0.10

2,000

0.19 1,000

0.34

500

0.47 0.65

200 100 50 1

2

5

10

20


Fig. 2.107: Bandwidth measurements of a 0.5 mm SI-POF

2.5.3.3 Bandwidth Measurements on MC- and MSI-POF Multicore and multistep index profile POFs allow significantly greater bandwidths than conventional step index profile fibers. In the case of MC-POFs the differences in propagation time between the different individual cores, in addition to mode dispersion, have to be added. The pure length differences, however, should hardly play a role. The pure path differences alone between the modes amount to about 6% at a maximum propagation angle of 20° in the fiber. Since the fibers lie well-ordered in the MC-POF the geometric differences in length lie at the most in the thousandth range. Of greater significance is the fact that the fibers in the MC-POF are deformed in different ways. For example, differences among the fibers in the middle and at the edge of the bundle can already be seen in the attenuation. These differences are also formed for the mode selective processes resulting in different average propagation speeds in the individual cores.

launching with magnified light spot: - medium fibers obtain small angles only - outer fibers obtain large angles

launching with mode field converter: - all fibers obtain around the same optical power and rays of all angles

Fig. 2.108: Optimal launching into multicore fibers

118


In order to register these effects when making a measurement, a so-called mode field converter (MFC) is used. The coupling unit shines into a short piece of SI fiber with a large NA. The far field distribution remains intact as the light is distributed over the fiber cross-section. This ensures that the individual fibers receive approximately identical light intensity and comparable angle distributions. The difference between this arrangement as opposed to a simple widening of the light spot is depicted schematically in Fig. 2.108. As indicated above, there are in the meantime different MC-POFs. Here we wish to present the results of the bandwidth measurements of two 1 mm MC-POFs with 37 cores (Fig. 2.109) and 217 cores (Fig. 2.110, see also [Ziem02a]). 2,000 B3 dB [MHz]

launch NA: AN = 0.09

PMC 1000 37 cores

AN = 0.19 AN = 0.33 AN = 0.48 AN = 0.64

1,000 500

200 length [m] 100 20

30

40

60

80

100

Fig. 2.109: Bandwidth measurements of a 37-cores MC-POF (measured on single samples)

2,000 B3 dB [MHz]

MCS 1000 217 cores

launch NA: AN = 0.09 AN = 0.19 AN = 0.33

1,000

AN = 0.48 AN = 0.64

500

200

100 20

30

40

60

80 100 length [m]

Fig. 2.110: Bandwidth measurements of a 217-core MC-POF (measured on single samples)


119

Both fiber types show considerably greater bandwidth values compared to standard SI-POF. The 37 core fiber above all shows hardly any drop in the bandwidth over great lengths, especially since the dependence on the coupling conditions is very small. The reason is the very great mode dependence of the attenuation. This fiber possesses a double step index structure. Using a laser coupling it has been possible to transmit 1 Gbit/s over this fiber for 100 m. Systematic investigations of the bandwidth have been carried out at the POF-AC on a 37 core POF sample with a relatively small diameter (400 μm). The results for the two different NA are shown in Fig. 2.111. The bandwidth of this fiber is almost independent of the launch conditions. The reason for this is the strong mode-dependent attenuation which, as described above, occurs intensively with very thin fibers and leads to a equilibrium mode distribution after very short lengths.

B3 dB [MHz] 10,000 NALaunch 0.05

5,000

0.65

2,000

1,000

500

200 0.1

0.3

1

3

10


Fig. 2.111: Bandwidth measurements of a MC-POF (measurements on a single fiber sample, cut-back method)

Multi step index fibers have already been introduced by different manufacturers. However, they are not yet ready to go into mass production. The youngest product so far is the ESKA MIU from Mitsubishi-Rayon, a fiber with three different layers. Using a sample length of 100 m of this fiber, a bandwidth of almost 300 MHz was ascertained. Figure 2.112 shows the frequency response.

120 3


rel. level [dB] NA 0.10 NA 0.34 NA 0.64

0 -3 -6 -9 -12 -15 -18 -21 -24 10

20

50

100

200

500 1000 frequency [MHz]

Fig. 2.112: Frequency responses of a 100 m MSI-POF

2.5.3.4 Bandwidth Measurements on GI-POF The bandwidth measurements of graded index profile fibers are associated with a series of particular difficulties. First of all, the attenuation is relatively high with polymer fibers so that the sample lengths can not be very great because of the limited dynamics of the measuring system. For PMMA POF the maximum measuring lengths lie between 50 m to 100 m. For PF GI-POF lengths of some 100 m can be used. On the other hand, POFs have a relatively large core diameter. The detector must be relatively large in order to record most all modes and thus obtain a meaningful bandwidth measurement. Then again the size of the diode limits the bandwidth of the detector. The only commercially available measuring system that can be used for this special task is the optical oscilloscope from Hamamatsu (described in the Chapter on measurement techniques). However, only measurements in the time domain are possible. The bandwidths of PMMA GI-POF and PF-GI-POF have been measured at the POF-AC. The transmission functions for the PMMA GI-PF OM-Giga from Optimedia (see also [Yoo04], [Rich04]) and a PF-GI-POF (Nexans, see [Gou04]) are illustrated in Fig. 2.113 and 2.114. The optical bandwidth of the fiber at 1.504 MHz was ascertained by adapting the measuring curve to a Gaussian function. This value, however, can clearly fluctuate with slightly altered launch conditions. The measuring conditions lie close to the “worst case scenario”. When there is under-launching, e.g. with a VCSEL transmitter, even greater values can be attained.


121

rel. electr. level [dB] 0 -1 -2 -3 50 m OM-Giga OLD = 650 nm ANLaunch = 0.34

-4

f3 dB opt. = 1,504 MHz -5 frequency [MHz] -6 10

20

50

100

200

500

1,000

Fig. 2.113: Frequency response of 50 m OM-Giga (AN = 0.34; 650 nm)

In order also to be able to measure bandwidths of several GHz with thick core fibers, an optical oscilloscope is a practicable device, whereby the widening of a short laser pulse (about 120 ps) is measured. In [Lwin06] the results for the OM-Giga are shown compared with the microstructured POF (with effective graded index profile). 350 pulse broadening [ps] 300 Optimedia 1,000 μm 250 200

MPOF: 500 μm

150 length [m] 100 15

25

35

45

55

65

Abb 2.114: Pulse broadening measurement of a MPOF and GI-POF

A pulse width of about 340 ps corresponds approximately to an optical bandwidth of 1.4 GHz. The value matches pretty much the measurements in the frequency range when taking the various problems of measurement techniques with such large frequencies into account.

122


The next illustration shows the frequency response for a PF-GI-POF at the wavelengths 650 nm and 850 nm together with the fitted Gaussian functions.

rel. electr. level [dB]

+1

measurement 650 nm Gaussian fit 650 nm measurement 850 nm Gaussian fit 850 nm

0 -1 -2 -3 -4

300 m PF-GI-POF OLD = 650 nm/850 nm ANLaunch = 0.10

-5 -6 0

200

400

600

800

1000

1200

frequency [MHz] Fig. 2.115: Frequency response of a PF-GI-POF

The 3 dB bandwidths are around 1,600 MHz for both fibers. The bandwidthlength product is at about 500 MHz · km, somewhat in the range of conventional multimode graded index glass fibers (cf. further results in [Bach01]). 2.5.3.5 Bandwidth Measurements on MC-GOF and PCS

The bandwidth measurement of multimode glass fibers proceeds according to the same principles. First, the glass fiber bundles from the Schott manufacturing company are measured. The bundle consists of about 400 individual fibers each having a diameter of 53 μm. The fibers have been hot pressed into the plug so that the overall diameter is about 1 mm. In Fig. 2.116 the frequency response for various coupling conditions are shown. The NA of the fiber lies at 0.50. The fiber bandwidth does not recognizably change when coupling in at great angles. With full mode launch the measured bandwidth amounts to about 150 MHz · 20 m which is almost exactly the same value as for SI-POF with a comparable NA. Consequently, this fiber type can be used alternatively to the 1 mm St.-NA POF when either high temperatures or very tight bending radii are necessary ([Lub04b]).


2

123


0 -2 20 m fiber at 650 nm

-4 -6

launch NA: -8

-14

AN = 0.10 AN = 0.34 AN = 0.46 AN = 0.60 AN = 0.64

-16 10

20

-10 -12

f [MHz] 50

100

200

500

1000

Fig. 2.116: Bandwidth measurement of 20 m MC-GOF

In another series of measurements the length dependence of the bandwidth of MC-GOF was investigated. Fig. 2.117 shows the results for 3 different launch conditions. The bandwidth decreases almost linearly with the length so that one can assume that the influence of mode mixing is relatively small. B3 dB [MHz] 1 mm MC-GOF 375 cores NAfiber: 0.50 O = 650nm

3,000

1,000 launch NA: AN = 0.64 AN = 0.34

300

AN = 0.10

length [m] 100 2

5

10

20

50

Fig. 2.117: Length- and NA-dependent bandwidth of MC-GOF

Finally, the bandwidth for different lengths was determined using a 650 nm laser. In order to be able to make measurements relatively independently of mode, a 1 m long SI-POF was used as an adapter fiber at both the transmitter and the receiver. Figure 2.118 shows the results.

124


1000

B3 dB, opt. [MHz]

excitation by laser NAlaunch | 0.30 O = 650 nm

500

200 length [m] 100 10

20

30

40

50 60

Fig. 2.118: Bandwidth of a MC-GOF excited by a laser source

This type of fiber is suitable for the transmission of data rates in the Gbit/s range over lengths of 10 m to 20 m. Another glass fiber version which has gained increasing attention is the PCS, i.e. silica glass fibers with a polymer cladding. The typical NA lies around 0.37. However, there are versions available with a NA up to 0.48. Accordingly, the bandwidth of PCS should lie in the range of DSI-POF. At the POF-AC predominantly fibers with a core diameter of 200 μm - the most commonly used value were measured. In Fig. 2.119 the length and launch-dependent results for a typical PCS are represented. The fiber, 200/230 μm with a 500 μm primary coating, was laid out for this measurement as a loose bundle with a diameter of about 30 cm (see also [Ziem04a]). 2000

B3 dB [MHz] launch NA: 0.02 0.09 0.17

1000 500

0.26 0.34 0.46

200 100 50 20 10

200 μm PCS loose bundle 20

length [m] 50

100

200

500

Fig. 2.119: Bandwidth of a 200 μm PCS

This fiber was specified with a bandwidth of 100 MHz · 100 m. This value can be achieved for an under filled launch. For a full launch, however, you can only attain about 60 MHz · 100 m. The differences between the different launch condi-


125

tions hardly decreases with fiber lengths up to 250 m. Mode mixing hardly occurs with this measurement. The measurement was repeated for the same type of fiber, whereby the fiber was wound around a spool. The results are shown in Fig. 2.120.

2000

bandwidth [MHz]

launch NA: 0.02 0.09 0.17

1000 500

0.26 0.34 0.46

200 100 200 μm PCS fiber on a spool

50 20 10

20

50

length [m] 100

200

500

Fig. 2.120: Bandwidth of a 200 μm PCS

The results pretty much agree for short fiber lengths. For longer lengths, however, the differences roughly disappear between the different launch conditions for the rolled up PCS. This can only be explained by a recognizable increase in the mode mixing. The bandwidths dependent on the coupling NA are compared for 250 m long samples in Fig. 2.121. 90

Bopt, 3 dB [MHz]

250 m PCS

80 70 60 50 40

fiber on a spool

30

loose bundle

20 10

launch NA

0 0.00

0.10

0.20

0.30

Fig. 2.121: Bandwidth comparison of 250 m PCS

0.40

0.50

126


Examinations of the different types of 200 μm SI-PCS confirm the measurements mentioned above. The specified bandwidth-length products of 10 to 20 MHz · km could be attained for all fibers examined only with under filled launch. Unfortunately, none of the currently active manufacturers provided any data on measurement conditions for the bandwidths indicated. Even the corresponding standards are completely missing. Should PCS seriously advance into areas of application in which the available bandwidth is to be completely used, then a lot of work still has to be done in this area. A comparison between the launch-dependent bandwidths is represented in Fig. 2.122.

40

BL [MHz km]

30 20 15 10 8 6 4 0.0

0.1

0.2

0.3

0.4

0.5 launch NA

Fig. 2.122: Bandwidth dependence on launch conditions for 5 different PCS types

A number of publications, especially in regard to applications in the GigabitEthernet and 10Gigabit-Ethernet ranges, exist for a bandwidth of 50/125 μm GI glass fibers. Articles providing an overview include [Oeh02] and [Bun03a]. With GI glass fibers, too, keeping the exact parabolic index profile as well as the mode-selective coupling play the most important role in achieving large bandwidths. Originally, two different types of fibers were specified: ¾ customary in the USA: 62.5/125 μm fiber (AN = 0.275 ± 0.015) ¾ customary worldwide: 50/125 μm fiber (AN = 0.200 ± 0.015)

The typical bandwidth-length product is 160 MHz · km to 200 MHz · km (62.5 μm) when using an 850 nm LED as emitter. 500 MHz · km is attained with 1,300 nm laser emitters. The limiting factor is the refractive index dip in the middle of the fiber which is caused by the production technology.


127

For fast Ethernet (125 Mbit/s) the bandwidths entirely suffice to bridge distances of up to 1 km. The first range limitations (maximum of 275 m at 850 nm emitters and 62.5 μm fiber) arise with Gigabit-Ethernet so that a new class of fibers (OM2) has been defined which generally guarantees a transmission range of 550 m. In the worst case a data rate of 10 Gbit/s could be transmitted on OM1 fibers over about only 30 m. OM2 fibers are also limited to about 80 m. In order to be able to transmit high data rates, three different procedures have been suggested: ¾ Splitting the data rate into 4 × 2.5 Gbit/s which are then transmitted by WDM on a fiber. ¾ Emitter with so-called Restricted Mode Launch (RML) or Effective Laser Launch (EL) respectively, whereby the power is coupled if possible within the annulus with a diameter of between 4.5 μm and 19 μm. Moreover, the NA of the emitter may not be too large. ¾ Use of the new OM3 fiber class which has been optimized for the employment of 850 nm VCSEL.

An overview of the specified characteristics of the different GI-GOFs is presented in Table 2.14. Specific products can on occasion clearly surpass these parameters. Table 2.14: Properties of MM-GI glass fibers OM1

OM2

OM3

OM3 550m

Fast Ethernet

Gigabit Ethernet

10Gbit Ethernet

10Gbit Ethernet

[μm]

50/62.5

50/62.5

50

50

D at 850 nm

[dB/km]

3.5

3.5

3.5

3.0

D at 1.300 nm

[dB/km]

1.5

1.5

1.5

1.0

BW 850 nm (OFL)

[MHzkm]

200

500

1,500

3,500

BW 1.300 nm (OFL)

[MHzkm]

500

500

500

500

BW 850 nm (LD)

[MHzkm]

n.d.

n.d.

2,000

4,700

Class

Unit

typical applications core-

(OFL: Overfilled Launch)

Other less customary fiber types are, for example, GI-GOF with a core diameter of 100 μm and a cladding diameter of 140 μm. Fig. 2.123 shows the frequency response of a 500 m long sample with three different launch conditions. At 200 MHz · km the results lie in the range of the fiber specifications. The last fiber presented here is the semi-GI-PCS described above. The measurement conditions become extremely more noticeable here so that the measurement results shown may not be conclusively representative.

128


1


0 -1

500 m fiber at 650 nm

-2 -3 -4

launch NA: AN = 0.10 AN = 0.34 AN = 0.64

-5 -6 -7 -8 1

10

100

f [MHz] 1000

Fig. 2.123: Frequency response of a 100 μm GI-GOF

Fig. 2.124 first shows the frequency response with a 500 m long sample for 6 different launch conditions measured at a wavelength of 650 nm.

rel. opt. power [dB] 0 AN Launch = 0.03 .. 0.64

-2 -4 -6

500 m Semi-GI-PCS

-8 -10 -12

1

3

10

30

100 300 frequency [MHz]

Fig. 2.124: Bandwidth of a Semi-GI-PCS

The bandwidth-length product of the fiber was determined as having values between 24 and 55 MHz · km which clearly lies above the specification of 100 MHz · km. Bandwidths with their length and launch dependence are also determined for this type of fiber. Fig. 2.125 summarizes the results.


129

3000 B3 dB [MHz] launch NA: AN = 0.02 AN = 0.09 AN = 0.17 AN = 0.26 AN = 0.34 AN = 0.46

1000

300

100 length [m] 30 10

20

50

100

200

500

1000

Fig. 2.125: Bandwidth measurement of Semi-GI-PCS

What is striking is the low dependence of the bandwidth on the launch conditions with longer sample lengths. Evidently, there is a significant exchange of energy between the SI and GI modes in the fiber. The specified bandwidth value could only be determined in short fiber lengths with under filled launch. Bandwidth measurements on semi-GI PCS have also been published by [Aiba04] and [Aiba05], whereby a method was used in which a light pulse circulates in a 100 m long ring and passes an acousto-optic modulator after every pass. The numerical aperture of the coupling optics amounts to only 0.25 and SI modes are for the most part suppressed. The results for the frequency response, determined by Fourier transformation, are shown in Fig. 2.126. 0

rel. opt. power [dB] 1st circulation

-2

10th circulation

-4 -6 -8

f [GHz] -10 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Fig. 2.126: Frequency responses of a Semi-GI-PCS according to [Aiba04]

The bandwidths thus determined are shown in Fig. 2.127. The values lie higher by a factor of ten than the values measured with full launch on long fibers. This

130


reveals impressively how important correct specifications of the measurement conditions are when indicating bandwidth values. The succinct statement in the Sumitomo data sheet that “the bandwidth can change under other measurement conditions” is of little help.

2.0

B3 dB, opt. [GHz] semi-GI -PCS

1.5 1.0 0.8 0.6 0.4 0.3 100

length [m] 200

400

600

1000

Fig. 2.127: Bandwidth of a Semi-GI-PCS according to [Aiba04]

2.5.3.6 Comparison of Bandwidth Measurements and Calculations

The diverse measurements of fiber bandwidths show that the same principles are essentially valid for thick glass and polymer fibers. Important effects are: ¾The bandwidth drops with the square of the numerical aperture by increasing the differences in propagation time among the individual modes. ¾The diameter of the fiber does not play any role in regard to the bandwidth. ¾Strong mode-dependent attenuation increases the bandwidth of fibers, but it also leads to a rise in transmission losses. ¾Multicore fibers and fiber bundles permit smaller NAs with the same bending radius and thus greater bandwidths. ¾The bandwidth of fibers greatly depends on the launch and detection conditions. The difference can be » 10 for short fiber lengths. When stating the bandwidth in data sheets, measurements should always be made with UMD (full launch) or EMD (equilibrium mode distribution). ¾Graded index profiles increase the bandwidth up to two magnitudes. However, the index profile must be as ideal as possible - it should be parabolic when the chromatic dispersion is disregarded. ¾In the case of a non-ideal GI profile a large bandwidth can still be attained through a selective launch. ¾In addition, the chromatic dispersion especially with glass GI fibers has to be taken into account (this will be discussed in the next paragraph). ¾It is technically easier to produce a multi-stepped index profile, with which the bandwidth can clearly be increased, than a GI profile.


131

¾Semi-GI fibers have large bandwidths, above all over short lengths and when coupling into small angles. ¾The bandwidth of individual fibers - not yet placed in cables - under laboratory conditions can depend to a great extent on the external conditions, depending on the degree of induced mode coupling.

A comparison between POF and PCS is particularly interesting since both can be used alternatively in many applications. The length-dependent bandwidths of both types of fiber with full launch are illustrated in Fig. 2.128. B3 dB, opt. [MHz] 1.000 PCS, NALaunch = 0.48 POF, NALaunch = 0.64

300

100

30

length [m] 3

10

30

100

Fig. 2.128: Bandwidth comparison of POF (fiber-NA: 0.50) and PCS (NA: 0.37)

Theoretically, the PCS should show about 50% greater bandwidth because of its smaller NA - which has just about been confirmed by measurements. Both measurement curves run approximately parallel which suggests similar magnitudes in mode-dependent processes. The angle-dependent attenuation of a typical PCS fiber is illustrated in Fig. 2.129. 225 excess loss [dB/km] 200 175 150 125 100 75 50 25 0 -25 -20 -15 -10 -5

fiber length 50 m 100 m 200 m

T[°] 0

5

10

15

Fig. 2.129: Mode dependent loss of a PCS (at 650 nm)

20

25

132 400


attenuation [dB/km] 50 m

300

200 100 m

100

11 dB/km 0 -30

-20

-10

0

T [°]

10

20

30

Fig. 2.130: Mode dependent loss of a Semi-GI-PCS (at 650 nm)

PCS does indeed show very large mode-dependent attenuation, the intensity of which is comparable to POF. This explains the similar behavior even if the core material itself has a very much lower attenuation. A schematic comparison of typical bandwidth values for the different multimode fibers described above are illustrated in Fig. 2.131. The values, as already mentioned several times, can clearly deviate for specific products or under different measurement conditions. Ø: 1000 μm Ø: 1000 μm

PC-POF MC-GOF St.-NA-POF

SI-MC-POF

Ø: 1000 μm Ø: 200 μm Ø: 1000 μm Ø: 1000 μm

Semi-GI-PCS

Ø: 200 μm

DSI-MC-POF

Ø: 1000 μm Ø: 750 μm

200 μm PCS DSI-POF

MSI-POF

GI-GOF OM2

Ø: 900 μm Ø: 62.5 μm Ø: 50 μm

PF-GI-POF

Ø: 120 μm

GI-GOF OM3

Ø:

50 μm

OM3 mit LD

Ø:

50 μm

OM-Giga GI-GOF OM1

1

10

100

1,000

10,000

bandwidth [MHz·km] Fig. 2.131: Bandwidth comparison of different optical fibers (typical values)


133

The bandwidths of the fibers presented vary over more than 3 magnitudes. If singlemode fiber is used, however, then nowadays there is practically no longer any bandwidth limit. Mode dispersion no longer arises. Chromatic and polarization mode dispersion can be compensated for as one likes. The significance of chromatic dispersion will be discussed in the next section. 2.5.4 Chromatic Dispersion in Polymer Optical Fibers

In all optical media we can observe the effect that the speed of propagation of light of different wavelengths differs. When we differentiate the propagation constants according to wavelength, we obtain the so-called chromatic dispersion, usually expressed in ps/nm·km. This constant indicates by how much a signal's delay will vary with the wavelength. In the typical application range of optical fibers this value is negative which means that with increasing wavelength the delay becomes smaller (corresponding to greater speed). Figure 2.132 shows the chromatic dispersion for silica glass, PMMA and a typical fluorinated polymer (according to [Koi97a]). 200 dispersion [ps/(nmkm)] 0 -200 -400 -600

PF-Polymer

-800

silica glass PMMA

-1,000 -1,200 400

500

600

700

800 900 1,000 1,200 1,400 1,600 wavelength [nm]

Fig. 2.132: Dispersion of different materials

Typical semiconductor sources feature certain spectral widths that range from some 10 nm for LED up to a few MHz for lasers (corresponding to some 10-5 nm). In addition, there is the fact that when a light source is modulated there is always a spectral broadening that cannot be less than a certain theoretical limit. This effect only plays a role, however, with spectral singlemode lasers and with very high data rates. Figure 2.133 shows a schematic illustration of the effect of chromatic dispersion on a light pulse that has a given spectral width. A pulse with a certain spec-

134


trum of the width 'O is launched into the fiber. After passing through the fiber (length L) and experiencing a certain amount of dispersion D, the pulse has the width 'W = D · L · 'O, whereby the shorter wave components arrive first. (cf. Fig. 2.38 as well). spectral shape of the source 'O

O

't = DL'O

POF

t

length L

input pulse

t output pulse broadening by time

Fig. 2.133: Influence of chromatic dispersion

For silica singlemode fibers, the value for chromatic dispersion at 17 ps/nm·km lies within the range of the smallest fiber attenuation at 1,550 nm wavelength. Today, DFB-laser diodes are predominantly used for long-distance systems, the spectral width of which is a maximum of a few MHz. What matters here essentially is the broadening effect that is brought about by the data itself. In this case, 1 nm corresponds to approximately 125 GHz of spectral width. This means that for a data rate of 10 Gbit/s a spectrum in the range of one-tenth nm is generated. Where the permissible bit broadening is 0.05 ns, the fiber link may have a length of approximately 30 km. For 2.5 Gbit/s this value rapidly increases to approximately 500 km due to the narrower spectrum and the greater pulse broadening permitted. Conventional 2.5 Gbit/s systems can operate without specific actions against dispersion. However, all systems that have many inline fiber amplifiers or higher bit rates require devices to counteract chromatic dispersion. The most common method today is the use of dispersion compensating fibers with strong negative dispersion. Since these fibers utilize waveguide dispersion they can only be produced as singlemode fibers. The situation is significantly different for POF. The chromatic dispersion of PMMA-POF with over 300 ps/nm·km at 650 nm wavelength is over 20 times larger than of silica fibers at 1,550 nm wavelength. For POF it is also usual to use LED with a typical spectral width of 20 nm to 40 nm and not lasers that have just a few tenths of a nanometer of spectral width. On the other hand, there are the typically short distances of POF systems and the moderate bit rates. Table 2.15 lists some examples for the effect of chromatic dispersion in POF systems.


135

Table 2.15: Influence of chromatic dispersion in POF systems Example

SI-POF ATMF DSI-POF ATMF DSI-POF IEEE1394 MC-POF STM16 PF-GI-POF

Bit Rate/ POF-Length 50 Mbit/s / 50 m 155 Mbit/s / 50 m 155 Mbit/s / 100 m 500 Mbit/s / 70 m 2,500 Mbit/s / 200 m

Wavelength/ Spectr. Width 650 nm LED 20 nm FWHM 650 nm LED 40 nm FWHM 525 nm LED 40 nm FWHM 525 nm LED 40 nm FWHM 650 nm LD 2 nm FWHM

Pulse Broadening/ rel. to the Bit Length 0.375 ns 2 % of the bit time 0.75 ns 12 % of the bit time 2.8 ns 43 % of the bit time 1.96 ns 98 % of the bit time 0.05 ns 12 % of the bit time

The first three examples are based on LED for transmitting data rates up to 155 Mbit/s over a maximum length of 100 m. Even in the unfavorable case of using green LED, pulse broadening is less than one ½ the bit length so that there is only a small effect on the system. In the fourth example, the intention is to transmit an IEEE1394 S400 data stream (with 500 Mbit/s physical data rate) over a distance of 70 m using a green LED. Here pulse broadening is nearly in the same range as the bit length. When this deteriorating effect due to mode dispersion is added, one can see that this system can only work with considerable additional efforts. It may, for example, be possible to partially provide electrical compensation, whereby higher optical receiving power is required. When using data rates from ½ Gbit/s to 1 Gbit/s, the use of spectrally narrower sources becomes necessary. These primarily include RC-LED and VCSEL (see Chapter 4), and for even higher requirements DFB laser diodes. In most cases this selection is required anyway due to the limited modulation bandwidth of LED. Fluorinated graded index profile polymer fibers feature significantly reduced chromatic dispersion compared with PMMA-POF. These fibers are designed for use in Gbit/s systems operating at spectral ranges between 800 nm and 1,300 nm. It is for these demands only that laser diodes can be considered, not least due to the smaller core diameters, the spectral width of which is a few nanometers at most. The last row shows that in such a case chromatic dispersion can be neglected even for a transmission length of a few 100 m. 2.5.5 Methods for Increasing Bandwidth

Generally, the theoretical bandwidth of polymer fibers is calculated on the basis of two essential assumptions. One assumption is that the launch of light at the fiber entrance takes place in uniform mode distribution and that the detector will receive all modes. The second assumption is that the attenuation of all modes is nearly constant. However, in practice polymer fibers, and in particular step index fibers, show completely different behavior. In the first place, it is relatively difficult to illuminate all modes uniform at the entrance of the fiber. In many cases

136


laser diodes are used where the emitting angle is significantly smaller than the angle of acceptance of a SI-POF. The use of solid-state lasers or gas lasers, the exact wavelength of which is often required for measuring purposes, is even more problematical. These lasers emit collimated light so that only a small proportion of the POF modes can be excited. When using glow lamps or discharge lamps, optical devices are used to collimate the light to the fiber. For this reason it is difficult to find lenses that actually work with consistent efficiency in the given acceptance range. All this has the effect in a concrete experiment of increasing the deviations of the actual bandwidth in comparison with the theoretical limit value. This is a very undesirable effect when attempting to define characteristics by making measurements of this kind, as shown in Chapter 7. However, for high bit-rate data transmission this situation can in practice also be beneficially exploited as shown by the following examples. Figure 2.134 demonstrates the most important methods for increasing the bandwidth of a POF. launch with small angle IN

fiber without connectors, bends and splices

high pass filter for dispersion precompensation

detection with small angles OUT

high pass filter for dispersion postcompensation

Fig. 2.134: Methods for increasing bandwidth (cf. p. 441 as well)

Launching light at a small angle as well as detecting just a selected angle range has the effect of restricting the modes involved in signal transmission and thus reducing pulse broadening. It is possible to electrically compensate for the resulting low pass behavior, both before as well as after the POF link. To date the most significant increases in bandwidth for a POF system have been described in [Bat92] (see also [Bat96a] and [Yas93]). The following components were utilized: ¾ ¾ ¾ ¾

Launch with a small AN = 0.11, thereby exciting only a few modes with only small differences in delay. Pre-distortion of the LD excitation signal (peaking); high pass (33 pF [[ȱ51 :). Detection with low NA (modes with large delay differences are blanked out). Dispersion compensation behind the receiver; high pass (8 pF [[ȱ200 :).

It was possible to transmit at more than 500 Mbit/s across a distance of 100 m of standard NA-POF (see also chapter 6). However, all these measures are usually at the expense of a reduced power budget, as summarized in Table 2.16.


137

Table 2.16: Consequences of different bandwidth increasing methods Method

Penalty for the Power Budget due to:

peaking low NA launch

lowering of source modulation depth decreasing POF coupled optical power for sources with broad emission angle low NA detection loss of light with high propagation angle at the fiber output post compensation amplified noise at higher frequencies

It follows that the use of such methods is of particular interest in systems that have adequate power reserves. POF attenuation across very short distances is hardly of importance; on the other hand, the use of high data rates is of interest in various applications. Chapter 6 will describe experiments for transmitting Gbit/s over distances of 10 m to 100 m conducted by T-Nova GmbH, the University of Ulm, Daimler Chrysler, the Fraunhofer Institute for Integrated Circuits Nuremberg and the POF-AC Nürnberg. Figure 2.25 shows theoretical considerations with respect to the POF bandwidth at different launching angles (Gaussian shaped far field with 3 dB width calculated relative to fiber NA) according to [Bun99a]. With short lengths and small launch NAs the light remains concentrated in areas with small propagation angles. The small differences in propagation time result in large bandwidths. After approx. 100 m of fiber equilibrium mode distribution is just about reached through mode mixing, and the influence of the launch conditions gradually disappears. This behavior corresponds to a great degree to the measurement results described above. 1,000

B3 dB [MHz]

rel. launch NA (NAfiber = 1) 0.5 1.5 0.7 1.7 1.0 2.0 1.2

500 200 100 50 20 10 10

length [m] 20

50

100

200

500

Fig. 2.135: Theoretical bandwidth with different launching conditions ([Bun99a])

The principle of peaking is demonstrated in Fig. 2.136 and 2.137 ([Zam00b], [Ziem00a] and [Ziem00c]). A high pass filter which dampens lower frequencies and lets high frequencies pass through without losses is switched between the modulation input and the laser. We begin with an illustration of the electrical spectrum of the emitting signal at the laser with and without peaking (1.2 Gbit/s, NRZ, pseudo-random bit sequence).

138 0


rel. power [dB]

-10

without peaking with peaking

-20 -30 -40 -50 -60 -70

¤ Giehmann

frequency [GHz]

-80 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Fig. 2.136: Modulation spectrum with and without peaking

In the experiment, the data rate was 1,200 Mbit/s with NRZ coding. The twinstage pre-distortion filter dampens the signal by 12 dB in the low frequency range so that the higher frequencies can create a stronger modulation. For the pulses this means steeper edges and overshoot at the beginning and end, hence the term peaking, as shown in Fig. 2.137. 18 16 14 12 10 8 6 4 2 0

with peaking without peaking

relative amplitude

time [ns]

¤ Giehmann 2000

0

10

20

30

40

50

60

Fig. 2.137: Laser modulation signal with/without peaking

The disadvantage of peaking can be clearly recognized in the diagram. The peaks at the beginning and the end of the pulse must lie within the admissible operating range of the laser, i.e. between the threshold current and the maximum current. This reduces the actual power per pulse compared with rectangular pulses. Figure 2.138 summarizes the bit rates and transmitted distances of different high-rate transmission systems using SI-POF ([Scha00], [Ziem00a], [Kich99] and [Yas93], [Vin04a], [Vin04b] and [Ziem03g]). Chapter 6 contains detailed expla-


139

nations of the different systems. The diagram also shows the theoretical limits for the bandwidth of standard NA-POF and DSI-POF (assuming NRZ coding and bit rate = 2 u 3-dB bandwidth). system with St.-NA system with DSI-POF system with MC-POF UMD-limit St.-NA UMD-limit DSI-POF

bit rate [Mbit/s] 3,000 POF-AC T-Nova POF-AC

1,000

POF-AC

UNI Ulm

POF-AC

500

Bates ´93

Daimler Chrysler

200

IEEE 1394

Bates ´92

ATM 155 100BaseFX

Kaiser ´92

100 10

20

50

100 200 POF-length [m]

Fig. 2.138: Bit rates of different POF systems (status 2003)

It is easily discernible that a number of systems with standard NA-POF are significantly above the theoretical limits. Particularly for greater lengths the potential for exceeding the limit is considerable. As shown in the next section, the practical application presents some problems such as the bending behavior. It is generally true that extreme dispersion compensation must be adaptive in its execution. That means that above all the limit frequencies of the high passes must be adapted very precisely to the frequency response of the link. If the frequency response changes, the result will be too much or too little compensation so that the pulses become distorted. Such a change may, for example, occur as a result of different lengths of cable; however even a bend in the fiber may have the same effect. In commercial systems it is desirable to avoid having to use automatic adaptations, such as is necessary, for example, in 1000BaseT-systems, or having to provide specific receivers for different cable lengths. One practical solution is to adjust the compensation in such a way that there is just-tolerable over-compensation for short lengths of fiber; based on this level of compensation, the next step is to select the maximum fiber length for which this compensation is still just sufficient. A schematic illustration is shown in Fig. 2.139.

140


increasing transmission distance

frequency response of the POF link (dispersion limited)

f

f

compensation filter (fixed)

f

f

f

f

f

resulting frequency response

f overcompensation

f optimized compensation

undercompensation

Fig. 2.139: Compensation of dispersion for various transmission distances

A proposal for increasing the bandwidth by direct interference with the optical path is described in [Kal99]. By using a mode filter immediately after the transmitter, the light angle range in the fiber is reduced as shown schematically in Fig. 2.140. With this method it was possible to achieve an improvement in bandwidth by 53% and 89% respectively for two standard NA-POF provided by Mitsubishi and Toray. The losses of the mode mixer are approximately 2.5 dB which is perfectly acceptable in many applications.

AN | 0.43

AN | 0.29 POF

source

receiver

mode filter

Fig. 2.140: Increased bandwidth with a mode filter ([Kal99])

Basically, this method is equivalent to the method of light launching using a smaller NA, though probably much easier to implement because no optical components are required and only a simple mechanical clamp needs to be placed on the fiber. If required, this can be repeated in the middle of the link or before the receiver.

141


2.5.6 Bit Rates and Penalty

In general, optical transmission systems are set up in such a way that the system bandwidth amounts to at least 50% of the bit rate with NRZ transmission. Thus, 500 MHz are needed to transmit 1000 Mbit/s. This means that the eye is completely open with ideally adapted filtering. In other words, the transition from the zero symbol to the one symbol and vice versa takes place within the bit duration. If the system bandwidth is smaller than half of the bit rate, then the symbol transition needs more time resulting in a reduction of the vertical eye opening. This effect must either be compensated for through adapted filtering or the reduced eye opening is compensated for by a correspondingly higher receiving level. The deterioration of the signal-to-noise ratio at the receiver through the bandwidth limitation is called penalty (measured in dB). The relationship between signal-tonoise ratio, receiving level and penalty is shown in Fig. 2.141. U

U

U2

U1

t

t

system without noise and with sufficiend bandwidth - the eye open completely

system without noise and with limited bandwidth - the eye in closed partially penalty: 20·log(U2/U1)

U

U

UN US

t

system with noise and with sufficient bandwidth - the eye is open completely SNR = 20·log(US/UN) Fig. 2.141: Definition of penalty

t

system with noise and with limited bandwidth - the eye is more closed SNR is decreased by penalty

142


When describing the sensitivity of a receiver, measurements are always made at the maximum bit rate. A possible penalty is always included. If there is sufficiently large bandwidth, then only the noise should limit the sensitivity. The large diode capacitance produces as a rule a relatively dramatic low-pass effect when using large photodiodes, which are necessary for POF or PCS. The noise rises in proportion to the signal with decreasing receiver impedance, i.e. one will accept some penalty and work with as large an input resistance as possible. Under laboratory conditions data communication can also be carried out with high penalty. Modern bit error analyzers can transmit error free as long as the eye opening amounts to some 10 mV. A typical eye diagram with high penalty is shown in Fig. 2.142. Subsequently, the connection between system bandwidth and penalty for a broad-band receiver at the POF-AC is illustrated.

Fig. 2.142: Data transmission with large penalty

In the example shown 820 Mbit/s were transmitted over 100 m of DSI-POF. Although the eye was almost completely closed, an error free transmission was possible. In a real system, however, certain detection would be relatively difficult since the sampling moment and the decision threshold have to be re-adjusted very exactly. Furthermore, there are no margin whatsoever for fluctuations in the laser power or bending losses.

25 penalty [dB] simulated with fiber

20 15 10 5 0 0.10

0.20

0.30 0.40 0.60 0.80 1.00 bandwidth/bit rate [MHz/Mbit/s]

Fig. 2.143: Effect of system bandwidth on the penalty

2.6 Bending Properties of Optical fibers

143

The simulated values were determined by calculating the penalties with the aid of PSpice analyses. A Gaussian-shaped filter was used as a low-pass system. The measuring points were determined on a 20 m long standard POF with different bit rates. The penalty was estimated from the eye diagram. The measured values tallied greatly with the simulation down to 25% of the system bandwidth, e.g. a transmission of 1 Gbit/s with a system bandwidth of 250 MHz. With higher bit rates the penalty increases more quickly than in the simulation. One main reason is that the frequency response only corresponds to a certain degree to idealized Gaussian behavior. It hardly makes sense to use practical systems with more than a 10 dB penalty. The results show that an exact relationship does not have to necessarily exist between the maximum bit rate and the fiber bandwidth. Furthermore, even with bandwidth-limited systems relatively high data rates can be achieved under laboratory conditions if enough emitting power is available.

2.6 Bending Properties of Optical fibers The sensitivity of optical fibers to bending is of special significance. In practical applications installed links are never completely straight. Often they are fitted around corners where 90º bends are a common occurrence. Even along a link in a straight cable duct there are many small bends, for example, in places where cables are hold with cable ties. When being assembled, the fiber must also be able to withstand mechanically tight bends. In many applications there is continuous bending during operation, for example in drag chains or with a data cable in a car door. That is why one differentiates between different bending loads: ¾Static bending: involves the assessment of how much light is loss in bends. These losses are to be taken into account in the power budget of the system. The bending loss is measured in dependence of the bending radius. ¾Minimum bending radius during assembly: only characterizes what bends the fiber can tolerate for a short time without being mechanically destroyed. ¾Repeated bending: in certain applications fibers must be able to tolerate 105 to some 106 bends without being mechanically destroyed. ¾Reel change bending: arises in particular in drag chains (see also Chapter 9). The following results have either been taken from data sheets or come from measurements made at the Deutsche Telekom and as of 2000 at the POF-AC Nürnberg. Now as before no standards exist for measurements of bending attenuation. We mostly used a long fiber sample stimulated with as large a NA as possible and all modes were detected with the aid of an integrating sphere. On the other hand many manufacturers measure with a small NA whereby much better values automatically come about because the outer modes are more strongly radiated in the bends.

144


Nevertheless, the series of measurements cannot always be compared exactly. In addition to the wavelength the bending attenuation can also depend to a great extent on the primary coating material and on the lateral forces within the bend. The coupling and detection conditions are also always included with short sampling lengths. 2.6.1 Bending Losses in SI-POF The essential parameters which determine the bending sensitivity of a fiber are the diameter and the numerical aperture. The larger the NA, the narrower the permissible bending radii may be in relation to the fiber diameter. Figures 2.144 and 2.145 show the losses for bends of different commercially available fibers according to information in the data sheets ([Tor96a] and [Asa97]). The Fig. 2.144 shows the bending losses of two different SI-POFs with somewhat different NAs. You can clearly see that larger NAs reduce the bending losses. bend losses [dB] 5.0 fiber

4.0

PFU-CD-1001 AN = 0.46

3.0

PGU-CD-1001 AN = 0.50

2.0 1.0 0,0 0

5

10

15

20

25 30 35 40 bend radius [mm]

Fig. 2.144: Loss for 360° bend according to [Tor96a]

Figure 2.145 shows losses resulting from bends in a standard NA-POF, a lowNA-POF and a multi-core fiber (see Chapter 2.3). The low-NA-POF shows significantly larger losses compared to a standard NA-POF. Due to the smaller individual core diameters, the bending sensitivity of the multi-core fiber is comparable with that of the standard NA-POF despite the smaller NA. If many bends directly follow each other, attenuation does not increase proportionally with the number of bends because there is less and less energy present in the higher mode groups. Figure 2.146 shows a measurement of the bending losses for different POF according [Hen99].


6

145

fiber

bend losses [dB]

5

TC 1000

(AN = 0.485)

4

NC 1000

(AN = 0.25)

NCM 1000 (AN = 0.25) 3 2 1 bend radius [mm]

0 0

10

20

30

40

50

60

70

80

Fig. 2.145: Loss for a 360° bend according to [Asa97]

10.0

loss [dB]

NC 1000 (Low-NA) no longer available at the market

5.00 2.00 1.00 AC 1000 (DSI)

0.50

PFU 1000 (St.-NA)

0.20

MH 4000 (DSI-POF)

0.10 0.05 number of turns 0.02

0

1

2

3

4

5

6

7

8

9

10

Fig. 2.146: Bending loss depending on number of turns ([Hen99])

The measurements were taken at 650 nm with LED-launch and a mode mixer. The bending radius was 32 mm and the bends were located at the beginning of a 50 m sample length. PFU 1000 is a standard NA-POF, while MH 4000 and AC 1000 are double-step index POF. Their losses are approximately identical and up to 10 windings are significantly below 1.0 dB. By comparison, the low-NA-POF NC 1000 is in the range of 10 dB, which is too much for deployment in practical applications. The ATM forum stipulates an admissible bending radius of 25 mm and at this radius the attenuation was already above the range of measurement. Meanwhile, DSIPOF offer significantly improved bending characteristics at comparable NA. Figures 2.147 and 2.148 demonstrate the losses over the inverse bending radius and the number of windings for a (genuine) low-NA-POF (NC 1000) and a standard NA-POF [Hen99].

146 12


loss [dB]

12 mm 10 turns

10

8 turns

8

15 mm 6 turns

6

4 turns 4

21 mm

2 39 mm 0

0.02

2 turns

26 mm 32 mm

0.03

0.04

0.05

0.06

0.07 0.08 0.09 inverse bend radius [mm-1]

Fig. 2.147: Bending loss of a PFU-CD-1000 ([Hen99])

Under UMD conditions, the bending losses should increase proportionally to the inverse bending radius. In practice, however, this only takes place below a bending radius of around 20 mm. It appears that the real equilibrium mode distribution reduces the losses above a certain radius. The reason for this is the smaller weighting of modes that have a large propagation angle, which, as already mentioned, are particularly sensitive to losses at bends. Basically the low-NA-POF in Fig. 2.148 shows the same behavior, albeit for significantly greater radii and due to the smaller NA. 16

loss [dB]

28 mm

10 turns

14 8 turns

12

32 mm

10

6 turns

8 4 turns

6 39 mm

4 2

2 turns

50 mm

0 0.020

0.022

0.024

0.026

0.028

Fig. 2.148: Bending loss of a NC-1000 ([Hen99])

0.03

0.032 0.034 0.036 inverse bend radius [mm-1]


147

2.6.2 Bending Losses in GI Fibers For graded index POF slightly different conditions apply for bending sensitivity compared with step index profile fibers. Here it is not the total reflection at the core-cladding interface but the continuous bending in the index profile that is responsible for light guiding. In addition, there is a fundamentally different distribution in the near and far field. Figure 2.149 shows a measurement for GI-POF according to [Ish95]. bend losses [dB] 20 10

GI-POF, doped by: MMA/DPS, AN = 0.29

5

MMA/BBP, AN = 0.21

2 1 0.5 0.2 0

10

15

20

25 30

bend radius [mm] 35 40 45 50 55

Fig. 2.149: Loss of two GI-POF ([Ish95]) for a 90° bend

Due to the different dopants used, the two samples with a core diameter of 0.5 mm each have a different NA, which has a very significant effect on the bending losses. Despite the smaller core diameter the losses for a 25 mm bend are still significantly higher than the values for a SI-POF or a DSI-POF. Here, too, a reduction in the core diameters leads to lower bending losses. [Aru05] describes how the bending losses in PMMA GI POF can be significantly reduced. In addition to an optimized index profile an additional PVDF layer (polyvinylidenfluoride) was applied to the core with parabolic profile resulting in a semi-GI-POF which combines high bandwidth with low bending losses. The losses of a 90° bend are shown later in Fig. 2.205 compared with a conventional PMMA GI-POF. (The sample length was 100 m.) Even with a bending radius of 5 mm there was no measurable increase in attenuation. The different methods for reducing bending losses in PMMA GI-POF and PF-GI-POF are described in greater detail in Section 2.8 on fiber production. Examples of measurements are also shown. 2.6.3 Change of Bandwidth by Bends However, bends do not only contribute to additional losses, but also have an effect on bandwidth because certain mode groups are selectively attenuated. This effect is exploited in mode filters and mixers.

148


Figure 2.150 (according to [Rit93]) shows what the effect of a 720º bend at the beginning of a 50 m long POF link has on the measured bandwidth. In this case the light is launched with AN = 0.10. bandwidth [MHz] 160 ØPOF: 140

250 μm 500 μm

120

750 μm 100

1000 μm

80 60 0.00

0.02

0.04

0.06

0.08

0.10

0.12

rel. inverse bend radius [ØPOF-1] Fig. 2.150: Change of bandwidth by bending the fiber according to [Rit93]

Due to the low launch NA, the bandwidth is relatively large (80 MHz · 100 m). In the case of tight bending radii at the beginning of the fiber there is mode mixing so that the bandwidth is significantly reduced sometimes. This effect is naturally more pronounced for smaller diameters. In the illustration selected here above the inverse relative bending radius, relative to the core diameter, the effect of the core diameter should disappear. It seams to be, that the effect described above of the larger bandwidth for thinner fibers is already dominant here due to the more mode dependent processes. Comprehensive investigations of the effect of bends on the bandwidth of POF links were presented in [Mar00]. The test fiber consisted of a 100 m long standard NA-POF; 360º bends were inserted at the beginning of the link, after 25 m, after 50 m, after 75 m or at the end of the link. The source consisted of a 655 nm laser diode, the NA of which could be adapted through different optics from 0.10 to 0.65. The bandwidth and the attenuation of the overall link were measured without bends and with bending radii of 6.4 mm, 11.1 mm and 13.8 mm. The results are shown in Fig. 2.151. When light is launched into the fiber using a large NA, the original bandwidth of approximately 33 MHz can be increased significantly. However, large improvements with small bending radii occur at the expense of large additional losses. The biggest gain in bandwidth is obtained with a bend in the middle because this means that many modes of the first 50 m are filtered out and EMD is not completely regained in the remaining 50 m. The changes in attenuation are largely independent of the length since the mode field is well filled out everywhere.

149




bandwidth over 100 m [MHz] 100 90 80 70 60 50 40 30 -1 -2 -3 -4 -5 loss [dB]

0

25

50

75

100

0

bend position [m] radius 6.4 mm

25 50 75 bend position [m]

radius 11.1 mm

100

radius 13.8 mm

bandwidth without bendings Fig. 2.151: Influence of a bend to bandwidth and attenuation ([Mar00])

When light is launched into the fiber using smaller NA, the relative gain in bandwidth compared to the original - approximately - 60 MHz is not as big. Therefore the optimum position for the bend is clearly nearer to the end since the mode field must first be filled. Again, tight radii have more effect. The additional attenuation increases significantly when the bends are moved to the end, since at the beginning of the fiber there are hardly any higher mode families in existence. These results also confirm clearly for the existing assumptions with respect to mode propagation in a coupling length of some 10 m. 2.6.4 Bends on PCS, Multicore Fibers and thin POF A very simple method to decrease bending radii is to reduce the core diameter while otherwise retaining identical parameters. If you wish to maintain the advantage of the simple handling of ready-made thick fibers, then there is the possibility of fiber bundles or multicore fibers respectively. Fig. 2.152 and 2.153 show the measured bending losses, each with a bend of 360° in the middle of the sample, with UMD launch and measured with an integrating sphere. A 10 m long fiber was used for the MC-GOF. The range of the bending radii lay between 2 mm and 100 mm. The bending attenuation measured lies below 0.1 dB.

150


0.06

bending loss [dB]

0.05 0.04 0.03 0.02 one bend by 360°

0.01 0.00

inverse bending radius [mm-1] 0.0

0.1

0.2

0.3

0.4

0.5

0.6

Fig. 2.152: Bending losses of MC-GOF, Schott

The bending losses of the MC-POF were measured on a 100 m long sample in order to guarantee as much mode equilibrium as possible. The bend (360°) was made in the middle of the fiber length. Due to the different relations between mode coupling and absorption the EMD conditions for 520 nm and 650 nm only differ slightly. That is the reason for the somewhat different bending losses. bending loss [dB] 1.00

100 m fiber length at 650 nm at 520 nm

0.10

bending radius [mm] 0.01 0

5

10

15

20

25

30

Fig. 2.153: Bending losses of MC-POF, 37 cores, 1 mm total diameter

In many areas the 200 μm PCS is used because it permits smaller bending radii. Fig. 2.154 illustrates quite graphically that the same physical characteristics are also valid for these fibers. Here the bending losses are given versus the relative bending radius in relation to the fiber diameter. The numbers in brackets indicate the bending radius in millimeters for the PCS. Both fibers thus have in relative terms an identical bending sensitivity.


151

bending loss [dB] 4.0 3.5

200 μm PCS

3.0

1 mm POF

2.5 2.0 1.5 1.0 0.5

bending radius [u Kern]

0.0 0

5

10

15

20

25

30

35

40

45

50

(0)

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

Fig. 2.154: Bending losses of PCS and POF in comparison

Thin POF could be used as an alternative to PCS in many areas when tight bending radii are indeed required, but the attenuation and the temperature range of the POF are satisfactory. A comparison between a 250 μm SI-POF and a 200 μm PCS, measured at 650 nm with full launch for 5 m long samples, is illustrated in Fig. 2.155. 10.0 bending loss [dB] 3.0 1.0

250 μm POF

0.3 0.1

200 μm PCS

0.03 bending radius [mm] 0.01 0

1

2

3

4

5

6

7

8

9

10

Fig. 2.155: Bending losses of small diameter POF and PCS in comparison

The somewhat thicker POF also has somewhat higher bending losses. A tenth of a dB is attained for the POF at a bending radius of 8 mm and 6 mm for PCS. The bending losses of three different SI-POFs with different NAs are compared in Fig. 2.156. The lowest losses are shown by the 300 μm thick POF with a high

152


NA. The 250 μm and 500 μm thick POFs have almost identical bending attenuations. It is thus indicative that the NA is by far the most important factor for the bending losses. Consequently, you should always choose fibers with the largest possible NA for particularly tight radii, unless you decide to go back to multicore fibers. In addition, the latter have the advantage of offering an even greater bandwidth. 10.00

bending losses [dB]

250 μm POF (AN = 0.63) 500 μm POF (AN = 0.50) 300 μm POF (AN = 0.63)

1.00

0.10

0.01 bending radius [mm]

0.001 0

1

2

3

4

5

6

7

8

9

10 11 12 13

Fig. 2.156: Comparison of bending losses of various SI-POF (different NA)

More recent measurements of bending losses of four different SI fibers, each with cladding and made available from Toray Germany, are shown in Fig. 2.157. Fibers with a large NA (0.63) were used for this measurement. They allow considerably smaller bending radii without remarkably decreasing the attenuation and bandwidth.

3.0

7.5 u r

D [dB]

1.0

7.5 u r 7.3 u r 8.0 u r

0.3 0.1 750 μm

250 μm

0.03

1000 μm

500 μm

0.01 0.3

r [mm] 1.0

3.0

10.0

Fig. 2.157: Comparison of bending losses of various SI-POF

30.0


153

Bending radii are drawn in the picture with which a bend (360°) results in exactly 1 dB additional attenuation. With the four fibers with their 250 μm to 1000 μm core diameters this is the case each with a seven-fold to eight-fold fiber radius, i.e. a bending radius between 0.9 mm and 4 mm. As a comparison, the bending losses of a 125 μm SI-POF ([Witt04]) are shown in Fig. 2.158. 2.0

bending losses [dB]

1.0 0.5 0.2 bending radius [mm] 0.1 0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Fig. 2.158: Bend losses of a 125 μm SI-POF ([Witt04])

Optimedia has made available samples of a thinner PMMA GI-POF. The bending losses of this fiber with overfilled launch (LED) and a launch with a laser (AN = 0.10) are shown in Fig. 2.159. Both measurements were carried out at 650 nm with a 5 m long fiber. bending losses [dB]

5.0

OM-Giga 500 μm/750 μm 360°-bends O = 650 nm 5 m fiber

2.0 overfilled (LED) 1.0 0.5 laser launch 0.2 0.1 0

20

40

60

80 100 bending radius [mm]

Fig. 2.159: Bending losses of a 500 μm PMMA GI-POF

A bending radius of 15 mm is still not a problem for collimated light whereas a high bending attenuation arises below a bending radius of 30 mm with an LED launch. You could argue, of course, that laser sources should always be used for GI fibers in order to utilize the high bandwidth. Nevertheless, it is imperative that manufacturers reduce the bending losses.

154


Finally, some results from a project work [Bau06] are shown. First, Fig. 2.160 compares the bending losses of three Toray fibers with different core diameters: 500 μm, 750 μm and 1,000 μm. The NA of the three fibers is the same. As expected, the bending radius for a given attenuation is reduced nearly proportional to the fiber diameter. Only with very thin fibers does the effect of stronger mode-dependent attenuation make itself noticeable. 10

bending losses [dB]

1

fiber type: Toray PFU AN = 0.47 measured with 650 nm LED 1 bend 360°, 10 m fiber

1,000 μm

0.1

750 μm 500 μm

0.01

bending radius [mm] 0.001 0

10

20

30

40

50

Fig. 2.160: Bending losses of various standard-NA-POF

The bending losses of 1 mm POF from three manufacturers are compared in Fig. 2.161. Since the NAs of the fibers are not exactly equal, the bending attenuations differ somewhat. In practice, however, these small deviations should hardly play a role.

10

bending losses [dB]

fiber type: St.-NA measured with 650 nm LED 1 bend 360°, 10 m fiber

1

0.1

0.01

0

10

20

30


Fig. 2.161: Bending losses of various standard-NA-POF

2.7 Materials for POF

155

2.7 Materials used for POF 2.7.1 PMMA The material most frequently used for polymer fibers is the thermoplastics PMMA (Polymethylmethacrylate), better known as Plexiglas. Figure 2.162 shows the structure of the monomer and its polymer chain. MMA

PMMA H

H C H H C C H C O H C O H H

CH3 C

CH3 CH2

C O

CH2

C C

OCH3 O

CH3

CH3 C

CH2

CH2

C

C OCH3 O

C

OCH3 O

OCH3

Fig. 2.162: Molecular structure of PMMA

PMMA is produced from ethylene, hydrocyanic acid and methyl alcohol. It is resistant to water, lyes, diluted acids, petrol, mineral oil and turpentine oil. PMMA is an organic compound forming long chains with typical molecule weights around 105. Essential from the point of view of optical transparency of the material is the amorphous structure of the polymerized material. The density of PMMA is 1.18 g/cm3. Its tensile strength is approximately 7-8 kN/cm2 ([SNS52]). The refractive index of PMMA is 1.492 and the glass transition temperature Tg lies between +95°C and +125°C. At room temperature and 50% relative humidity the material can absorb up to 1.5% water, which also affects the attenuation characteristics. Table 2.17 presents further properties of PMMA: Table 2.17: Properties of PMMA (typical values) Parameter refractive index glass transition temperature Tg density absorption of water up to saturation thermal conductivity: thermal heat expansion coefficient: Rockwell hardness (M), Shore hardness (D) tensile strength resistivity breakdown strength spontaneous combustion temperature

Unit °C g/cm³ % W/mK mm/mK N/mm² Ohmcm kV/mm °C

Value 1.492 115 1.18 0.5 0.17 0.07 95 70 76 1015 20 - 25 approx. 430

156


As can be seen in the illustration, each MMA monomer has a total of eight C-H bonds. The vibrations of this compound, or more precisely its harmonic waves are a main cause for the losses encountered in PMMA polymer fibers. The attenuation resulting from absorption at the respective wavelength is shown in [Mur96] and [Koi96c] (see Fig. 2.163 and table 2.18). In particular the harmonic waves at 627 nm (6th harmonic wave) and 736 nm (5th harmonic wave) essentially determine the level of attenuation within the application range of PMMA-POF because these are not narrow absorption lines but relatively wide bands. Further causes for attenuation will be discussed in the chapter titled Characteristics.

108 attenuation [dB/km] 106

molecule

104

C-H

102

C-D C-F

100

C - Cl

10-2 10-4 10-6 10-8 500

1000

1500

2000 wavelength [nm]

Fig. 2.163: Absorption lines of C-X-bounds according to [Gra99] and [Mur96]

Quite early in the history of this technology, the idea came up to reduce the absorption losses of polymer fibers by using different materials in which less or no C-H bonds were present. However, it is not easy to eliminate these; instead, the hydrogen atoms are replaced by other atoms of the 7th main group. A heavier core will result in a lower vibration frequency, thus moving the attenuation bands to a larger wavelength. The illustration shows the attenuation bands for deuterium (heavy hydrogen with the atomic weight 2), fluorine (atomic weight 19) and chlorine (atomic weight 35 or 37, see also [Bau94]). Generally, the materials for polymer fibers can be divided into three groups: ¾compounds containing hydrogen ¾compounds with partial substitution of hydrogen ¾compounds with complete substitution of hydrogen


157

Table 2.18: Absorption bands position of carbon bonds ([Gra99]) Oscillation

Q0 Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 Q9

C-H O [nm] 3,390 1,729 1,176 901 736 627 549

C-D O [nm] 4,484 2,276 1,541 1,174 954 808 704 626 566

C-F O [nm] 8,000 4,016 2,688 2,024 1,626 1,361 1,171 1,029 919 830

C-Cl O [nm] 12,987 6,533 4,318 3,306 2,661 2,231 1,924 1,694 1,515 1,372

C=0 O [nm] 5,417 2,727 1,830 1,382 1,113 934 806 710 635

O-H O [nm] 2,818 1,438 979 750 613 523

2.7.2 POF for Higher Temperatures Fibers with high resistance to heat are especially needed for use in certain areas of automotive engineering (engine compartment) and automation technology. In the passenger compartment of a vehicle a maximum of +85°C will arise. PMMA-POF can easily be used with such temperatures. In the area near the center console or under the roof temperatures can also go up to over +100°C and near the engine to +125°C. Summaries of the data published so far and of comprehensive investigations at the POF-AC Nürnberg can also be found in [Poi03a] and [Poi03b]. On the whole the following methods for increasing the resistance to heat of polymer fibers have been presented: ¾Cross-linking of PMMA: cross-linking between polymer chains can be generated by chemical effects or by UV irradiation which results in a rise of Tg. At the same time, however, the scattering and the mechanical characteristics become worse. ¾Polycarbonate: PC has a considerably greater Tg compared with PMMA and is likewise transparent. Fibers made of this material have been produced on a large scale. PC fibers, however, age relatively quickly in combination with humidity. ¾Elastomers: fibers made of this material could be used up to +170°C and show very low attenuation. So far, they have only been produced as laboratory samples. ¾Alternative polymers: a series of other polymers such as cyclical polyolefins have Tg up to +200°C. When determining the thermal stability, a maximum increase in the kilometric attenuation is established over a maximum period of aging. In case the aging procedures are thermally activated, then the permissible operating period decreases almost logarithmically to the temperature. An example of the behavior of a stan-

158


dard PMMA-POF can be seen in Fig. 2.164 (measurements were made at the POF-AC). The increase in losses is represented here vs. the temperature. With an approx. 10 K increase in temperature the speed of ageing increases about one order of magnitude. 1000

increase of attenuation coefficient dB/(km1000 h)

100

520 nm 590 nm 650 nm

10

temperature [°C] 1

70

75

80

85

90

95

Fig. 2.164: PMMA-POF ageing

2.7.2.1 Cross-Linked PMMA One of the most obvious methods for more heat-resistant POFs is the use of crosslinked PMMA, generally referred to as modified PMMA. The attenuation curves of such fibers are summarized in Fig. 2.165. The fibers of the PHK Series are sold by Toray ([Tor96a] and [LC00a]). Important parameters are: ¾Core/cladding: PMMA/fluoropolymer ¾Diameter. 0.5 mm, 0.75 mm. 1.0 mm and 1.5 mm ¾NA/aperture angle: 0.54/65° ¾Lowest attenuation at 650 nm (for 1 mm): 150°C Manufacturers of such polymers are among others Ticona and JSR. It is not foreseeable when test fibers made from this very promising material system will be produced again. 2.7.2.5 Comparison of High-Temperature POF So far the following temperature-resistant fibers have been summarily described: ¾Cross-linked PMMA (>130°C) ¾Polycarbonate (115°C) ¾Partially fluorinated polycarbonate (145°C) ¾Silicone elastomers (>150°C) ¾Thermoplastic resins (145°C) ¾ARTONTM (Fujitsu) (170°C) The data of these different fibers have been compiled in Tables 2.19 to 2.21. temporary data sheet, the fiber is not presently available **) modified PC - partially fluorinated according to the authors’ information ***) different data on materials, but with identical attenuation curves *)


165

Table 2.19: Polycarbonate-POF Mitsubishi Producer Furukawa Furukawa**) Laser FH 4001B*) [Irie94] [Hatt98] Comp. TM [Hatt98] [Nish98] core diameter 910 μm 500 μm 1 mm 910r50 μm 940r20 μm cladding thickness 40-50 μm 30 μm n. a. n. a. n. a. NA 0.75 0.54 n. a. 0.30 0.61 800@770 2000@633 400@660 460@650 800@770 x dB/km @ y nm 1500@780 700@760 300@780 bandwidth n. a. n. a. n. a. n. a. 200MHz100m max. temperature +125°C +125°C +125°C +145°C n. a. ***) D-POF, PC-AF core material n = 1.582 n = 1.586 PC(A) PC cladding material n = 1.392 n = 1.491 n. a. n. a. n. a.

Parameter

Table 2.20: Properties of modified PMMA-POF Parameter

principle core diameter cladding thickness NA x dB/km @ y nm bandwidth max. temperature core material cladding material jacket

Toray PHKSCD1001-22 mod. PMMA n. a. n. a. 0.54 300@650 n. a. +115°C PMMA n. a. PP

Hitachi H-POF cross linked PMMA 1 mm 250 μm 0.65 540@660 30 MHz100m +130°C PMMA P-FEP ETFE

Tver-POF (Sample 2002) Copolymer 1 mm 30 μm >0.50 800@660 n. a. +130°C PMMA n. a. n. a.

Toray [Tan94a] Copolymer 1 mm n. a. n. a. 250@650 n. a. Tg = 135°C Copolymer n. a. n. a.

Table 2.21: Properties of different high temperature-POF Parameter principle core diameter cladding thickness

NA x dB/km @ y nm bandwidth max. temp. core material cladding material jacket

Hitachi HPOF-S silicone 1.0 mm 0.25 mm 0.65 800@660 25 MHzkm n. a. n. a.

Hitachi [Sas88] resin 1 mm 0.5 mm 0.62 660@650 900@780 n. a. >150°C ester based thermosetting resin

Bridgestone [Ish92b] silicone n. a. n. a. 0.54 700@660 450@770 n. a. +150°C silicone

Zeidler Fujitsu [Zei03] [Suk94] elastomer elastomer 1 mm 1 mm n. a. n. a. 0.44/0.25 n. a. 800@770 800@680 n. a. +150°C elastomer

n. a. Tg = 171°C ARTON

P-FEP

ethylen tetrafluoride propylene hexafluoride copolymer

n. a.

elastomer fluorcopol.

n. a.

Tefzel (ETFE)

n. a.

n. a.

without

n. a.

166


Fig. 2.174 shows an aging experiment at +130°C with different fibers described above. The most suitable ones at these temperatures were evidently the EOF and the PC-POF. 10,000

attenuation [dB/km]

5,000 T = +130°C TVER 2002

2,000 1,000

FH 4001

500

PHKS HPOF-S

200

measuring time [hours] 100 0

100

200

300

400

500

600

700

Fig. 2.174: Ageing of various POF at high temperatures

The PC-POF from Mitsubishi (FH4001) only shows a moderate increase while the two POFs made of cross-linked PMMA aged more quickly. The EOF even gets better during the measurement period. Particularly noticeable is the clear drop in attenuation after 15 hours. This was the point at which the temperature was raised in the climate test chamber. It was noticeable that the bandwidth of the EOF had dramatically diminished after this treatment. The combination of both events provides the explanation that the adhesion of the cladding onto the core was clearly improved by the high temperature so that even higher modes can now be guided. 2.7.3 Polystyrene-Polymer Fibers Another candidate for the production of polymer optical fibers is polystyrene (PS), the molecular structure of which is shown in Fig. 2.175 ([Ram99]). H C C

H C H H C C H C H C H C H

H C H C

H C C C H

Fig. 2.175: Molecule structure of PS

H C H C H C H n


167

Theoretically, the attenuation of PS is partly below that of PMMA, as the following theoretical estimate of losses in [Kai89] shows - without taking into account propagation effects and the effects of claddings (see table 2.22). Table 2.22: Theoretical attenuation of different polymers according to [Kai89] Material Wavelength Rayleigh- UV-AbC-HSum Total Scattering sorption Absorption PMMA 520 nm 28 dB/km 0 dB/km 1 dB/km 29 dB/km 570 nm 20 dB/km 0 dB/km 7 dB/km 27 dB/km 650 nm 12 dB/km 0 dB/km 88 dB/km 100 dB/km PS 552 nm 95 dB/km 22 dB/km 0 dB/km 117 dB/km 580 nm 78 dB/km 11 dB/km 4 dB/km 93 dB/km 624 nm 58 dB/km 4 dB/km 22 dB/km 84 dB/km 672 nm 43 dB/km 2 dB/km 24 dB/km 69 dB/km PMMA680 nm 10 dB/km 0 dB/km 0 dB/km 10 dB/km d8 780 nm 6 dB/km 0 dB/km 9 dB/km 15 dB/km 850 nm 4 dB/km 0 dB/km 36 dB/km 40 dB/km

To date, PS-POF have been manufactured e.g. by Toray (first PS-POF 1972), NTT (1982) and CIS in Tver (1993). The initial fibers had an attenuation of over 1,000 dB/km; later on it was possible to reduce this to 114 dB/km at 670 nm ([Koi95]). The NA of these fibers which can be used at temperatures up to 70°C is 0.56, i.e. a little higher than that for the standard PMMA-POF. Figure 2.216 shows the attenuation behavior of a PS-POF ([Ram99], red curve and [Zub001b]). 1000

attenuation [dB/km]

800 600 400

200 [Zub01b] wavelength [nm] 100 500

550

600

650

700

750

800

Fig. 2.176: Attenuation spectrum of PS-POF acc. to [Ram99] and [Zub01b]

850

168


The refractive index of PS is n = 1.59 so that it is possible to use PMMA for the optical cladding (n = 1.49), as is possible for PC (n = 1.58). The glass transition temperature of PS is approx. 100°C and therefore approx. 5 K lower than that of PMMA. Hitherto there has been no reason to replace the PMMA-POF by PS so that this material is not of any practical significance. 2.7.4 Deuterated Polymers As has been illustrated in Fig. 2.163, a significant reduction in the absorption losses of polymers can only be achieved by substituting the hydrogen with heavy atoms. This would seem to be achieved most simply by replacing it with deuterium. This isotope has twice the atomic mass compared to hydrogen. In nature, approximately 0.0156% of all hydrogen atoms are deuterium (1 atom in every 6,400). Chemically, deuterium behaves the same way as hydrogen so that it simply makes sense to use so-called heavy water (D2O) as a base material for this synthesis. Table 2.23 shows data of different POF based on deuterated polymers. Table 2.23: Data of deuterated materials Ref.

Year Producer

[Koi95] [Koi96c] [Lev93]

1977 Du Pont 1982 NTT 1993 CIS

[Koi92] 1993 Keio Univ. [Khoe94] [Kon02] 2002 Keio Univ.

[Kon03] [Kon04]

2003 Keio Univ. 2004 Keio Univ.

Attenuation dB/km 180 20 120 180 56 94 58 109 127 58 80

at: nm 790 680 650 850 688 780 650 780 580 650 650

Remarks first deuterated SI-POF SI-POF core: 200-1000 μm, AN = 0.48, to 70°C core: 500 μm, MMA-BBP-d8, 2.000 MHz km g = 3.4; 511 MHz 300 m Tg = 105°C g = 2.0; 1020 MHz 250 m g = 2.3; 1200 MHz 300 m

According to [Koi95], the first deuterated SI-POF was produced by DuPont in 1977. In 1982, NTT ([Koi96c]) produced a SI-POF in deuterated material with a minimum attenuation of 20 dB/km at 680 nm. It was not until the year 2000 that this attenuation value was improved with the introduction of LucinaTM-POF. Figure 2.177 shows further attenuation curves for POF made with deuterated polymers; all examples are GI fibers. Using POF made with deuterated polymers would offer a number of advantages. Chemically these materials behave identically to the substances made from "normal" hydrogen. The attenuation is approximately one order of magnitude less than the values achieved for PMMA fibers. The behavior over temperature and the options for index profile design should be the same as those of PMMA-POF. However, the decisive disadvantage is that there is always water vapor present in the atmosphere which will be absorbed by the fibers. This will lead to a situation,


169

where which protons (normal hydrogen nuclei) slowly replace the deuterium so that the absorption losses will increase again. Although it is possible to solve the problem with a watertight coating of the fiber (including all connections), this would defeat the object of obtaining a particularly low priced cable system. 10,000

attenuation [dB/km]

5,000 2,000 1,000

[Koi95] [Ish92a] [Koi96b] [Koi96d] [Mur96]

500 200 100 50 20

wavelength [nm]

10 500

600

700

800

900 1000 1100 1200 1300 1400

Fig. 2.177: Loss spectra of GI-POF (deuterated, 1996)

In the past few years work has once again been conducted in Japan on the production of deuterated POF. GI fibers exclusively have been investigated - see [Kon02], [Kon03] and [Kon04]. The attenuation of these fibers from [Kon04] is compared in Fig. 2.178 with the values from 1995 and those of a PMMA-POF. 1500

attenuation [dB/km]

1000

PMMA

500

d8-PMMA 2002

0 450

550

650

Fig. 2.178: Loss spectra of GI-POF ([Kon02])

1995 750 850 wavelength [nm]

170


Different production versions are compared in [Kon02]. The effect of an additional PMMA cladding is investigated among other things. The best results compared with a pure PMMA-POF are shown in Fig. 2.179. With about 60 dB/km and 650 nm the attenuation ranges approximately between pure PMMA and PF-GI fibers. On the other hand, the attenuation of the PMMA POF at 520 nm also does not lie much higher. 10.000 5.000

attenuation [dB/km]

2.000

PMMA

PMMA

1.000 500 200 100

d8-POF

50 20 10 500

wavelength [nm] 550

600

650

700

750

800

Fig. 2.179: Attenuation of deuterated POF ([Kon04])

Since 2003, Fujifilm has been announcing the development of a new fiber “Lumistar” in the versions I, V and X. According to their own statements this is: “the first POF with a large diameter which is able to transmit over 1 Gbit/s”. This is somewhat exaggerated, of course, since PMMA GI-POF and MC-POF have been able to do this for many years. power [dB] 0.0 1.9 GHz100 m 3 dB-bandwidth

-1.0 -2.0 -3.0 -4.0 -5.0

frequency [GHz] -6.0 0.0

0.5

1.0

1.5

2.0

2.5

Fig. 2.180: Frequency response of the Lumistar GI-POF ([Nak05b])

3.0


171

Details of a fiber with a core diameter of 500 μm and a cladding diameter of 750 μm are described in [Nak05b]. The bandwidth of the fiber is 1.9 GHz over 100 m. Figure 2.180 shows the frequency response. Furthermore, the work shows that the index profile produced by gel polymerization technology also remains stable after 2000 hours of aging at +90°C (Fig. 2.181) which is very astonishing.

Fig. 2.181: Refractive index profile of the Lumistar GI-POF after ageing (+90°C)

Parameters for the Lumistar fibers are mentioned in different sources. According to this information a particularly low-attenuation polymer is used. Since the company works closely with Keio University, where until 2004 there were reports on the development of deuterated fibers with very similar parameters, we must assume that we are dealing here with d8 PMMA-POF. Table 2.24: Data of the d8-POF Lumistar

core material core diameter cladding diameter attenuation bandwidth

Lumistar-I

Lumistar-V

Lumistar-X

n. a. 500 μm 750 μm 160 dB/km (650 nm)

n. a. 300 μm 316 μm 180 dB/km (650 nm)

n. a. 120 μm 500 μm 95 % O2 >95 % O2

Thermal Thermal Linear Value Ho Conductivity ExpansionMJ·kg-1 W·K-1·m-1 Coefficient K-1 17 - 25 0.17 10 - 20·10-5 17 - 25 0.17 10 - 20·10-5 15 - 20 0.17 10 - 20·10-5 42 - 44 0.30 20 - 50·10-5 35 - 40 0.30 20 - 50·10-5 42 - 44 0.40 40 - 45·10-5 42 - 44 0.19 15·10-5 29 - 30 0.23 7 - 10·10-5 23 - 27 0.25 15 - 20·10-5 42 - 44 0.30 20 - 30·10-5 19 - 23 n. a. n. a. 5 0.26 8 - 11·10-5 5 0.26 6 - 15·10-5

*) E: extrusion, S: injection molding, V: vulcanization, W: wrap technology Table 2.28: Materials for POF jackets (physical/chemical properties) Low TempeShort Name Melting Temperature rature Limit

from 130°C PVC from 130°C PVC 90° PVC flame ret. from 130°C PE LD; MD 90-110°C

-10°C -10°C -10°C -50°C

Density g·m-3

Corrosive Harmfull Agents in the Flue Gas

J-Rays Resistance

1.20-1.50 1.20-1.50 1.30-1.60 0.87

yes yes yes no

d 10 Mrad d 10 Mrad d 10 Mrad d 100 Mrad


PE flame ret. PE HD PP PA-6 PUR VPE EVA FEP PTFE

from 110°C 125-135°C from 145°C from 175°C from 150°C 255-275°C 325-330°C

-50°C -50°C -20°C -50°C -50°C -50°C -50°C -65°C -65°C

0.98 0.95-0.98 0.91 1.10-1.15 1.15-1.20 0.92 1.30-1.50 2.00-2.30 2.00-2.30

yes no no ? no no no yes yes

d d d d d d d d d

179

50 Mrad 100 Mrad 10 Mrad 10 Mrad 500 Mrad 100 Mrad 100 Mrad 0.1 Mrad 0.1 Mrad

Table 2.29: Materials for POF jackets (physical/chemical properties) Short Name

Oil and fuel Weather Shore-Hardness resistance Resistance 1) = A; 2) = D

PVC middling PVC 90° middling PVC flame ret. middling PE LD; MD bad PE flame ret. bad PE HD middling PP middling PA-6 middling PUR good VPE middl. /good EVA bad FEP very good PTFE very good

good good good medium medium medium medium good excellent good good excellent excellent

70-951) 70-951) 80-901) 43-502) 502) 60-622) 40-602) 40-75 75-1001) 40-502) 70-901) 55-602) 55-652)

Tensile Strength

Extension Break

10-20 N·mm-2 150-350 % 10-20 N·mm-2 150-350 % 10-20 N·mm-2 150-250 % 15-20 N·mm-2 300 % 15-20 N·mm-2 300 % 15-25 N·mm-2 300 % 30-50 N·mm-2 300 % 70-120 N·mm-2 50-200 % 35-45 N·mm-2 300 % 12-20 N·mm-2 300 % 5-15 N·mm-2 300 % 15-25 N·mm-2 250 % 80 N·mm-2 50 %

Table 2.30: Materials for POF jackets (electrical properties) Short Name

PVC PVC 90° PVC flame ret. PE LD; MD PE flame ret. PE HD PP PA-6 PUR VPE EVA FEP PTFE

Loss Factor tanG at 20°C and 800 Hz 20 - 100·10-3 50 - 100·10-3 70 - 150·10-3 0.2; 0.4·10-3 1.1·10-3 0.3·10-3 0.5·10-3 30 - 50·10-3 30·10-3 0.5·10-3 20 - 30·10-3 0.0003·10-3 0.0003·10-3

Permittivity at 20°C and 800 Hz

Resistivity at 20°C

4-6 4-6 5-7 2.3 3 2.3 2.3 - 2.5 3-7 8 2.4 - 3.8 4-6 2.1 2.1

1013 :·cm 1013 :·cm 1013 :·cm 1016 :·cm 1016 :·cm 1016 :·cm 1016 :·cm 1014 :·cm 1012 :·cm 1016 :·cm 1012 :·cm 1016 :·cm 1017 :·cm

180

2.8 Fiber and Cable Production

2.8 Fiber and Cable Production The processes for producing POF have been continuously improved in the last few years. The fundamental methods have indeed always remained the same, but various details have been improved. A very comprehensive treatment of POF production and its history can be found in [Nal04]. Many fine points concerning the materials can also be found in [Har99]. As opposed to the production of glass fibers there is a number of unusual features with POF. First of all, the polymer chemistry involved, in part very complicated and with its occasional safety aspects, has to be mastered. On the other hand the process temperatures are very much lower - almost always below +200°C. The demands on POF production can be sub-divided into four areas: ¾The core material must be produced uniformly without any impurities, air bubbles, etc. and with a correct distribution of the molecular masses. ¾The fiber must be drawn or extruded exactly. ¾For SI fibers a suitable cladding material with low refractive index and an attenuation not too high must be found and applied. In doing so, one must guarantee that the interface is sufficiently smooth and that the cladding has a good wringing fit. ¾For graded index fibers a copolymer or a dopant must be found in order to be able to vary - usually increase - the refractive index. A suitable process is needed in order to distribute this material over the core cross-section so that you have a parabolic refractive index profile. There are other steps, of course, such as the application of additional protective layers, the production of duplex or ribbon cables and quality control. 2.8.1 Production Processes for POF Today glass fibers are produced in two different ways. The typically 125 μm thin fibers for telecommunication applications are produced - up to more than 1000 km - from a preform. Light guiding fibers are drawn directly from molten glass. Even with polymer fibers one differentiates between continuous methods, spinning or extruding, and the drawing out of the preform. In the preform method a cylinder is produced that already has the index profile of core and cladding while having a much larger diameter. During the drawing process, the diameter is reduced until the desired size has been reached (Fig. 2.191, see e.g. [Wei98]). Ideally, the index profile should be maintained during this process but at a proportionally reduced scale. The length of the fiber per preform is determined as follows: Length of fiber = preform length · (preform diameter/fiber diameter)2


181

This method is applied generally for glass fibers. Automated processes are then applied to make several 100 km of fiber out of each preform, as the following example shows: Length of glass fiber = 2 m preform (5 cm preform diameter/125μm)2 = 320 km

It is easy to see that the large core diameter of common POF is not favorable for this process since only a few km of fiber can be produced from each preform, for example: Length of POF = 1 m preform · (5 cm preform diameter/1 mm)2 = 2.5 km

Drawing speeds for glass fibers today can attain 10 m/s; with POF about 0.2 to 0.5 m/s. mounting with feed mechanism preform

oven take up drum diameter control unit

Fig. 2.191: Production of POF from a preform

In addition to being able to draw the complete fiber out of the preform there is also the possibility of producing the core as a polymer cylinder and then applying the cladding by extrusion or enameling. The advantage here is that the polymerization of the core material can proceed under very much better controlled conditions. This process is used with PCS. A silica glass core is drawn out to 200 μm - or to other thicknesses as well - and is then surrounded by a polymer cladding, typically 15 μm thick. Understandably, the glass and the polymer have to be processed using different procedures. Other versions are discontinuous production in which polymerization first takes place in the reactor and then the resulting block is extruded at low temperature, a so-called batch extrusion.

182


N2 vacuumpump reactor mixer

monomer

initiator polymerization controller

heater cooler

cladding polymer POF with cladding Fig. 2.192: Batch-extrusion according to [Hess04]

The monomer, the initior and the polymerization controller are first distilled by a vacuum pump. After the polymerization is finished, nitrogen pushes the polymers through the nozzle and the cladding is then immediately applied. In addition, Mitsubishi has developed a method with which the polymerization, described in [Nal04], can take place photochemically. Figure 2.193 from [Hess04] shows such a method. The core and cladding materials are pushed through a nozzle by a pump and a mixer. The cross-linking then takes place with a UV lamp. This process could prove to be quite suitable, especially for heat-resistant POF.

mixture

spinning cladding material

core material

nozzle UV-light for crosslinking

Fig. 2.193: Polymer crosslinking

take up drum


183

When extrusion techniques are applied, the POF is produced in a continuous process directly from monomers. For SI-POF this process is very simple. Figure 2.194 shows such an arrangement (e.g. [Ram99], [Wei98]). POLYMER filler

heated vessel core extruder

diameter control

fiber

conveyor pump

cladding extruder

Fig. 2.194: Production of SI-POF through extrusion

Such a system is also described in [Hac01]. According to the author the cladding materials used are Poly(3FMA) with n = 1.40 and PVF with n = 1.42. The polymerization takes place at about 150°C. With the drop in pressure when leaving the reactor the remaining monomer is vaporized and can be returned. The cladding is extruded at about +200°C. This temperature lies far above the glass transition temperature for PMMA. Thus is a critical step in the process in which the quick cooling of the fiber must be guaranteed. On the other hand, the cladding is only about 10 μm thick so that the thermal load is limited. monomer, initiator, polymerization controller

reactor cladding extruder heating

fiber pump extruder take up drum

Fig. 2.195: Extrusion of a POF according to [Hac01]

184


This process is also discussed in [Hess04]. The monomer is polymerized to about 80% in the reactor. The advantage of this standard process for SI-POF lies in the very slight contamination of the polymers caused by the process. A modification of the process is presented in [Poi06d]. The new components in the process are: ¾The core material is PMMA granulate which is crushed before extrusion and effectively cleaned. ¾The extrusion head is to be kept free of metal and any impurities whatsoever if possible. ¾The turbo pump used makes a particularly even transport possible. In addition, two further processes are mentioned in [Wei98]. In the thrust extrusion technique, polymerization is carried out in a closed heated container from which the fiber is subsequently expelled through a nozzle at high pressure. The cladding is applied directly within the nozzle. This is a non-continuous process just like the preform technique. In the spin-melt process, a volume of ready-to-use polymer pellets is melted and pressed through a spin head that incorporates many holes. The holes serve to form the core and apply the cladding. This process is very efficient but also very expensive. 2.8.2 Production of Graded Index Profiles In order to guarantee the optimal functioning of graded index and multi-step index fibers, the best index profile possible should be realized. The developmental goal of the past few years has been to attain as much as possible with minimum effort and to continuously produce GI fibers. A number of different processes for the manufacture of graded profiles are described in the technical literature: ¾Interfacial gel polymerization technique ¾Centrifuging ¾Photo-chemical reactions ¾Extrusion of many layers In most of these techniques the principle is to initially create a preform of up to 50 mm diameter and then to subsequently draw this preform down to the desired fiber size. Some of these methods are described below. 2.8.2.1 Interfacial Gel Polymerization Technique This method was developed by Prof. Koike of the Keio University (for an example see [Koi92]). In this process a tube is initially manufactured with PMMA. This tube is then filled with a mixture of two different monomers M1 (high refractive index and large molecules) and M2 (smaller refractive index and smaller molecules). Initially the inner wall of the PMMA tube is slightly liquefied in an oven


185

that has been typically heated to 80°C. This results in a layer of gel and accelerates polymerization. The smaller molecule M1 can more easily diffuse into this layer of gel so that the concentration of M2 increases more and more towards the middle. The index profile is thus formed in accordance with the resulting concentration gradient. For manufacturing a PMMA-GI-POF, [Koi92] proposes that MMA (M1) be supplemented with monomers VB, VPAc, BzA, PhMA and BzMA. The material that was finally used is BzA because its reactivity is comparable with that of MMA. The 15 mm - 22 mm thick preform is then drawn at temperatures between 190°C and 280°C to produce fibers ranging from of 0.2 mm - 1.5 mm in diameter. Figure 2.196 illustrates the principle (see also [Ish95]).

PMMA tube filled with a MMA/BzA mix

80°C melting of the PMMA tube formation of a gel layer

the gel layer moves to the center concentration of M2 increases from outer to the the center Fig. 2.196: GI profile formation by gel polymerization technique

[Koi95] describes this method in more detail. The PMMA tube is produced by rotating a glass reactor at 3,000 min-1 at 70°C that is partially filled with MMA. The polymerization process for the core takes place at a speed of 50 min-1 and a temperature of 95°C and requires approximately 24 hours to complete. [Ish95] describes the production of a PMMA GI-POF with DPS as dopants. For traditional materials such as BB or BBP, one obtains fibers with a NA of 0.17 - 0.21, whereas with DPS a NA of 0.29 is possible. The greater NA improves the bending characteristics and makes the launching of light easier.

186


2.8.2.2 Creating the Index Profiles by Centrifuging Several publications ([Dui96], [Dui98] and [Chen00]) propose utilizing the density difference of the different monomers to create the index profile through centrifugal force in a fast centrifugal process. [Chen00] compares the density and refractive index of different materials for this purpose (Table 2.31). Table 2.31: Refractive index and density of different polymers ([Chen00]) Molecule

Density

n

MMA

-3

0.936 g/cm

1.490

DOP

0.981 g/cm-3

1.486

BIE

-3

0.982 g/cm

1.564

Molecule

Density

n

BB

-3

1.120 g/cm

1.568

PMMA

1.190 g/cm-3

1.490

1.254 g/cm

-3

1.373

-3

1.422 1.538

TFPMA

BzMA

-3

1.040 g/cm

1.568

PTFPMA

1.496 g/cm

VB

1.070 g/cm-3

1.578

DBME

2.180 g/cm-3

The production of the preform is carried out in two steps. Once the monomer mixture has been filled into a tube, the GI profile is formed at room temperature. Then the temperature is increased so that polymerization takes place. Rotation continues during this process. Then the fiber is drawn from this preform. In this process the rotation speeds must be up to 50,000 min-1. Even for a preform with 10 mm diameter the centrifugal acceleration (a = M2r) already equals 14,000 times the acceleration due to gravity. At the University of Eindhoven an ultra centrifuge operating at 50,000 min-1 has been constructed for preforms up to 50 mm in diameter which produce a centrifugal acceleration of 70,000 g. In the first trials, GI cylinders were produced from PTFPMA and MMA. The process for forming the GI profile took 24 hours. This was followed by a period of 12 hours during which the polymerization process was carried out at 60°C to 80°C. The refractive index difference achieved was approximately 0.009. No research reports have as yet been published on the production of fibers from such preforms. 2.8.2.3 Combined Diffusion and Rotation The combination of diffusion and rotation for producing PMMA-GI preforms is described in [Park01]. The monomer is filled into a cylindrical glass reactor in the middle of which a rod made of a material with a high refractive index is located. This material diffuses slowly into the surrounding medium. Both parts can rotate at different speeds: the reactor at 500 to 1000 RPM and the rod at 6 to 60 RPM. The idea for different rotation speeds comes from determining the average of concentration fluctuations so that an ideal rotation-symmetrical profile comes about. After a few hours the preform is thermally polymerized. Figure 2.197 shows the principle and an index profile. A fiber with a 1 mm core diameter was produced through thermal drawing from the preform described above. The bandwidth-length product amounts to 1.2 GHz · 100 m, measured with a 650 nm InGaAsP laser on a 50 m long fiber.

2.8 Fiber and Cable Production solid copolymer with higher refractive index

1.0

phase 1 room temperature laminar mix of the phases rotating reactor with monomer, initiator and polymerization controller

n

phase 2 heated

n

concentration after 5 hours

0.8 0.6

polymerization copolymer is diffused into the monomer mix

187

0.4 final GI-preform

n

0.2 0.0 0.0

0.2

0.4

0.6 0.8 1.0 relative radius

Fig. 2.197: Fabrication of GI-POF-preforms according [Park01]

2.8.2.4 Photochemical Generation of the Index Profile According to [Nal04] the first GI-POFs were also produced by photo-copolymerization, introduced in 1981 by Koike. A thin glass tube is filled with a mixture of MMA, vinylbenzoate (VB as dopant) and benzoyl peroxide (as initiator). The glass tube rotates during the UV irradiation. Since the UV radiation is higher at the edge a gel phase forms here through faster polymerization. The VB concentration will be greater in the center since MMA has a faster reaction speed. The tube is irradiated from bottom to top and then polymerized out at high temperatures. This procedure did not result in any usable fibers. [Miy99] proposes a method for the production of index profiles by means of a photo-chemical reaction. In this process, PMMA is doped with DMAPN ((4-N,Ndimethylaminophenyl)-N’-phenylnitrone). During exposure to ultra violet radiation (380 nm) the refractive index is reduced by up to 0.028, sufficient for GI-POF. In the experiment, thin films of a few micrometer thickness were used. Fibers have not yet been produced. It is likely that a problem would be the depth of penetration of the radiation which is significantly less than the intended fiber radius. Nevertheless, this process is of great interest since it works fast and makes continuous fiber production possible. 2.8.2.5 Extrusion of Many Layers This multi-step index POF has hitherto been produced at two institutes (ResearchProduction Center, RPC Tver) and Mitsubishi Rayon. The process corresponds to the production of SI-POF or DSI-POF except that several extruders must be combined with one another. Figure 2.198 shows the index profile of an MSI-POF according to [Lev99]. The curve drawn corresponds to that of an ideal parabola. In the core area the deviations of the real structure are relatively small.

188


refractive index [a.u.] 60 50 40 30 20 10 0 0

100

200

300 400 500 distance to the fiber axis [μm]

Fig. 2.198: Index profile of a MSI-POF ([Lev99])

2.8.2.6 Production of Semi-GI-PCS The production of the preform for semi-GI-PCS does not in effect differ from the manufacturing methods for normal glass fibers. The usual process is MCVD (modified chemical vapor deposition). A mixture of SiCl4 and O2 are introduced into a heated quartz glass tube and SiO2 is formed by the chemical reaction. By adding chlorine, boron, germanium or phosphorus, you can continuously change the refractive index (Fig. 2.199). After cooling off, the tube with the inner layer will be collapsed, i.e. the hole disappears, and is drawn into a fiber. As opposed to classic glass fibers the PCS has an optical cladding made of polymers, not of glass, thus making a considerably greater refractive index jump possible.

O2

porous preform

burner SiCl4

gas mixing

rail

controller GeCl4 Fig. 2.199: Fabrication of glass fiber preforms (by OVD)

ceramic or graphite rod


189

2.8.2.7 Polymerization in a Centrifuge A new method for producing PMMA GI-POF ready for production has been developed over the past few years by the South Korean company Optimedia under the direction of Prof. C. W. Park. The production principle is based on copolymerization. As opposed to doping there is the advantage of the glass transition temperature not dropping as much. The polymer mixture is filled into a rotating tube and polymerized thermally or by UV irradiation. The polymer composition can be changed in steps or continuously. The rotation here does not serve the purpose of separating the materials, but only for achieving rotational symmetry. There are correspondingly fewer demands on the rotation speed. Figure 2.200 shows the set-up. A detailed description can be found in [Park06a].

Fig. 2.200: Rotating cylinder for GI-preform fabrication ([Park06a])

You can see quite well under a microscope that the fiber is built up of many layers. Nevertheless, the index profile is almost ideally parabolic and does not show any steps - see Fig. 2.201 acc. to [Park06a]. An attenuation spectrum of the OM-Giga, 1 mm GI-POF (data provided by the distributor Fiberfin) is shown in Fig. 2.202. At 650 nm the losses are below 200 dB/km. 1.525

refractive index

1.520 1.515 1.510 1.505 1.500 AN: 0.30

1.495

normalized radius [mm]

1.490 0.0

0.1

0.2

0.3

0.4

0.5

Fig. 2.201: Refractive index profile of a PMMA-GI-POF made by Optimedia ([Park06a])

190


5000 attenuation [dB/km] 2000 1000 500 200 100 400

wavelength [nm] 500

600

700

800

900

Fig. 2.202: Attenuation spectrum of a PMMA GI-POF made by Optimedia (Fiberfin)

2.8.2.8 Continuous Production at Chromis Fiberoptics While there are continuous production processes for SI-POF, PF-GI-POF could only be produced until just recently from preforms. Chromis Fiberoptics - previously Lucent, OFS - has developed a process for the continuous production of such fibers ([Rat03], [Whi03], [Whi04a], [Whi05], [Park05b] and [Pol06a]). First a SI fiber of CYTOP material with a doped core is produced in a double extruder. The fiber is wound around a heated cylinder. Here the dopant diffuses outwardly resulting in the GI profile. The 500 μm PMMA protective layer is then applied and the fiber can be wound up. The fibers almost attain the parameters of POF from Asahi Glass which has had about 10 years of experience in the field. cladding extruder (CYTOP)

protective layer extruder

core extruder (CYTOP + dopant)

coextrusion head heated tube

stepindex profile dopant diffusion GI-POF indexprofil index difference

coextrusion head diameter control

capstan -100

-50

to the take up drum Fig. 2.203: Continuous PF-GI-POF fabrication ([Pol06a])

0

50

100

radius (μm)


191

The insert shows the final index profile with an approximately parabolic curve. The manufacturer indicates the bandwidth-length product of the fiber as being 400 MHz · km. 2.8.2.9 GI-POF with Additional Cladding As already indicated above, a reduction in the bending losses plays a great role with polymer fibers. For SI fibers a considerable improvement could be achieved by means of a second cladding. Extremely small bending radii can be attained through fiber bundles or multi-core fibers respectively. For graded index fibers as well, an additional cladding layer with a smaller refractive index evidently offers clear advantages in regard to the bending behavior without dramatically reducing the bandwidth. A PF-GI-POF with an additional 6 μm thick cladding layer is introduced in [Oni04] and [Sato05]. Figure 2.204 shows the measured bending losses for three different fibers with different index jumps between the edge of the core and the additional cladding (around ¨n = 0.002, ¨n = 0.005 and ¨n = 0.014). Even with an index jump of 0.005 a bending radius of 10 mm with an attenuation below 0.1 dB can be attained. The bandwidth-length product of the fiber lies between 1,800 MHz · km and 2,700 MHz · km. The fiber attenuation amounts to 30 dB/km at 850 nm, measured with ODTR.

180°-bending loss [dB] 1.2

1.355 n

1.350

1.0

1.345

0.8

1.340 1.335

0.6

1.330

x [μm]

1.325

0.4

0 20 40 60 80 100 120 140 160 180 200

0.2 0.0

bending radius [mm] 0

10

20

30

40

50

60

Fig. 2.204: Reduction of the bending losses due to a Semi-GI profile ([Sato05])

This method can also be employed for PMMA-GI fibers. The results for a 1 mm thick fiber are presented in [Aru05]. The attainable bending radius drops to below 5 mm with an additional PVDF cladding (polyvinylidene fluoride, n = 1.42). The bandwidth-length product of the fiber is 1,500 MHz · 100 m and remains quite constant up to 10 mm. It only drops under full launch and with a 5 mm bending radius to 500 MHz · 100 m. The attenuation at a 90° bend is compared to a conventional PMMA GI-POF in Fig. 2.205.

192


2.0

bend loss [dB] PVDF clad GI-POF NA of the GI core region = 0.17

1.5

PMMA based GI-POF NA of the GI core = 0.21

1.0 0.5 0.0 bend radius [mm] -0.5 0

10

20

30

40

50

f

Fig. 2.205: Bend losses in Semi-GI-POF according to [Aru05]

In addition to the extra cladding layer a so-called W-profile for GI fibers has also been developed. Here the goal is to improve the attainable bandwidth. Measurements on PMMA GI-POF with this W-profile and different index exponents are presented in [Tak05b]. The W-profile is characterized by a very steep index drop directly at the core-cladding interface. Figure 2.206 shows the index curve.

Fig. 2.206: W-profile for PMMA-GI fibers ([Tak05b])

Furthermore, fibers with a NA of 0.20 and a ȡ-parameter (index exponent of the rise outside the core-cladding interface layer) have been produced with index exponents between 1.9 and 5.2. Figure 2.207 shows the theoretically calculated and measured bandwidths.


5.0

193

3 dB bandwidth [GHz100 m]

3.0

W-shaped POF

2.0 1.0 GI-POF

0.5

calculated for GI-POF

0.3 0.2 1.5

2.0

2.5

3.0

3.5

4.0 4.5 5.0 5.5 profile index exponent g

Fig. 2.207: Bandwidths of PMMA-GI-POF, improvement by W-profile [Tak05b]

PF-GI-POF with optimized index profiles are presented in [Ebi05]. Their bandwidth attain that of MM-GOF and in the short-wave range even surpasses it (Table 2.32). The high bandwidth is attained through the approximately ideal index coefficients of 2.05, i.e. in combination with the low chromatic dispersion of the material. Table 2.32: Bandwidths comparison of GI-GOF and POF according to [Ebi05] Bandwidth

wavelength

650 nm

780 nm

850 nm

PF GI-POF

8.39 GHz

8.50 GHz

9.54 GHz

SiO2-GI-GOF

5.27 GHz

7.34 GHz

9.31 GHz

Figure 2.208 shows the best attenuation values over time for some of the fibers listed above. PMMA fibers (SI and GI) reached their theoretically maximum possibilities in the mid-80s. Since then, other index profiles (MSI, MC, DSI) have also reached this order of magnitude (approx. 130 dB/km at 650 nm and 80 dB/km at 570 nm). Any differences in measured values and specifications are more likely to result from different measuring conditions than from differences in quality. The PF fibers have been continually improved, at least as far as the laboratory results are concerned. The best values were attained in 2003 with about 8 dB, almost one magnitude still above the theoretical limits. In the past three years no further progress has been made with the attenuation. On the other hand, there has been some success in attaining a high launch-independent bandwidth with optimized refractive index profiles and in reducing the bending sensitivity.

194


attenuation [dB/km] 1,000 500 GI-PMMA

200 100 50

20 10

SI-PMMA at 650 nm SI-PMMA at 570 nm SI-d8 at 680 nm PF-GI at 1.300 nm d8-GI at 688 nm

5 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 year Fig. 2.208: Development of POF attenuation until the year 2005

2.8.3 Cable Manufacturing This chapter discusses the structure and properties of various cable structures with POF wires. Different applications place different demands on the mechanical shielding of the polymer optical fiber. SI-POF (Step Index Polymer Optical Fiber) is a promising medium for relatively short transmission distances of 100 m. Polymer plastics such as polymethylmethacrylate (PMMA) or polycarbonate (PC) are used as the primary core material for manufacturing these fibers. Fluorinated polymers, silicone or fluorinated PMMA materials are used as cladding material with a reduced refractive index of ncladding ~ 1.42 as compared with the core material ncore > 1.48 (Fig. 2.209). Due to the large refractive index difference, numerical apertures of up to 0.50 are attained. Various manufacturer versions of optical fibers are shown in Fig. 2.210, in which glass or plastic are combined for the core and cladding material. The relatively thin glass fibers are mechanically fragile and must therefore be protected by a multilayer cable construction. The POF is so flexible that a simple jacketing of the optical cladding suffices as a cable construction.


d

0.98 mm

D

1.00 mm

ncore

1.492

ncladding

1.416

NA

0.47

r

195

ncladding ncore

D

n

d

core material: Polymethylmethacrylat (PMMA) cladding material: fluorinated PMMA Fig. 2.209: Typical SI-POF parameters

Glass fibers with polymer optical cladding represent an intermediate step. They also have a relatively simple construction (two-layer plastic coating around the optical cladding). The large core diameter allows only step-index profiles. singlemode glass fiber optical core optical cladding 10/ 125/ 250 μm

primary coating secondary coating

multimode glass fiber

strength member outer jacket glass fiber with polymer cladding50/ 125/ 250 μm

200/ 230 μm

polymer fiber

980/ 1000 μm

0 mm

0.5 mm

1.0 mm

Fig. 2.210: Comparison of different kinds of optical fibers

196


Until recently, step-index profile fibers were manufactured almost exclusively from polymer plastics having a typical outer diameter of 1 mm. These SI-POF exhibit significant transmission ranges with a minimum of attenuation for wavelengths between 400 nm and 900 nm (Fig. 2.211). The effective spectral loss windows are at 520 nm, 570 nm, 650 nm, and 760 nm. With improved purity, homogeneity and deuterated or fluorinated polymers, it is possible to reduce attenuation to 10 dB/km, as has already been described in Chapter 2.7.5. 10,000

attenuation [dB/km]

attenuation minimum

5,000 PC 2,000 1,000 500

PMMA

200 100 50 450

500

550

600

650

700

750

800

850

wavelength [nm] Fig. 2.211: Attenuation spectrum of different POF made from PMMA or PC

Polymer optical fibers that are flexible and break-resistant can be produced with a relatively large diameter (up to 1.5 mm or even more) and are thus easy to handle and to install. The large core diameters in combination with the numerical aperture make simple connection fittings and equipment possible with low demands on precision. 2.8.3.1 Cable Construction with SI-POF Elements SI-POF cables or lines must always be flexible when laid/installed at the end user place. SI-POF must also be flexible for mobile applications. The flexibility of a line or cable depends on the number and dimensions of the stranding units with the number of the layer changes of the individual stranding elements. The shorter the pitch length is and the larger the number of layer changes, the larger the flexibility of the stranding unit. The pitch length of the individual POF wires or the stranding elements with the proper diameter has a major influence on the flexibility of the stranding elements. The shorter the pitch length, the more flexible the stranding unit will be (Fig. 2.212).


197

flexibility

length of lay Fig. 2.212: Schematic diagram of the relationship between the pitch length and the flexibility of the stranding construction

2.8.3.2 Non-Stranded SI-POF Cables SI-POF Simplex Cable When processed into a cable with the respective strain relief, the SI-POF can be coated with a diffusion lock made from metal over the first cladding, if required. An absolute diffusion lock can be attained exclusively with a closed tube, for example with laser-welded metal tubing. The metal strip material for laser welding can be made of aluminum, copper or high-grade steel. The foil thickness is typically between 50 μm and 150 μm for welding. For overlapping with or without gluing, the metal foils have a sandwich layer construction, i.e., 9 μm / 20 μm / 9 μm = metal / plastic carrier strip/ metal. A jacked is extruded onto the traction elements in combination with the metal diffusion locks. This layer is practically always flexible and sturdy; polyurethane or polyethylene are the preferred materials. The next illustration (Fig. 2.213) shows two typical SI-POF simplex cable constructions.

outer sheath metal band inner coating cladding fiber core 2.2 mm Fig. 2.213: Structure of optical fibers with internal cladding

2.3 mm

198


SI-POF Duplex Cable The simplest form of a duplex cable is the combination of two parallel POF wires that are protected by shielding and equipped with traction elements. Various construction options for a duplex cable or duplex line are possible. Two very well known cable constructions are shown in Fig. 2.214.

foil tape lapping POF-element inner coating outer sheath

5 mm

strain relief element/ rip cord

2.5 mm 5 mm

Fig. 2.214: SI-POF duplex cable in a round cable and flat cable form

With these duplex cable constructions, care must be taken to ensure that the strain-relief elements in the plugs or on the connectors are included in processing. This is necessary because the temperature influence on the SI-POF wires is constructed in such a way that optimum temperature characteristics are ensured in the temperature range from -40°C through +80°C. SI-POF and GI-POF Ribbon Cable A ribbon cable with n SI-POF elements can be constructed as an extension to a duplex cable. The SI-POF elements are lined up in parallel as a comb and combined in either groups of 5 or 10 elements. A thin protective coating is extruded over this ribbon cable with the respective traction and support elements in one work cycle. Various SI-POF ribbon cable constructions with a modular design are illustrated in Fig. 2.215. twin group

2.3

2.3 5.2

5 cables group outer sheath strain relief element POF inner coating

13 mm

10 cables group

26 mm Fig. 2.215: SI-POF ribbon cable with traction and support elements


199

The cross-sections of two POF ribbon cables from [Boc04] are shown in Fig. 2.216. The individual fibers have each been extruded in a joint acrylic cladding.

Fig. 2.216: Ribbon with four 500 μm SI-POF (above) und eight 120 μm/500 μm GI-POF (below, [Boc04])

For the OVAL project (see Chap. 6) of the POF-AC Nürnberg Nexans had produced 8-strand ribbon cables made of SI- and GI-POF each with a 500 μm diameter. The cross-section of a prototype with PMMA-GI-POF (Optimedia) is shown in Fig. 2.217.

Fig. 2.217: POF-ribbon cable with eight 500 μm OM-Giga-fibers (dimensioning in μm)

The spacing between the individual fibers deviates only slightly from 500 μm. Only in a vertical position great deviations do arise which can easily be avoided by better guiding of the individual fibers in the extrusion tool. In order to investigate the influence of the ribbon cable production on the optical parameters, the spectral attenuation and the bandwidth were determined on the SI-POF ribbon cables. The results are shown in Fig. 2.218 and 2.219.

200


1000 attenuation [dB/km] 800 600 fiber 1 fiber 3 fiber 5 fiber 7

400 300

fiber 2 fiber 4 fiber 6 fiber 8

200 wavelength [nm] 100 450

500

550

600

650

700

750

800

Fig. 2.218: Single fiber attenuation in the ribbon cable

The attenuations of the 8 fibers agreed within the usual measurement error of ±0.5 dB. There were also no significant deviations in the frequency response in Fig. 2.219. +5

rel. level [dB]

0 -5 -10 -15 -20 -25 -30 -35

1

frequency [MHz] 3

10

30

100

300

1000

Fig. 2.219: Frequency response of the fibers in the ribbon cable

In one last experiment we investigated whether the ribbon cable production increased the mode mixture in the fibers. We determined the far field width of the individual fibers and ribbon cables with under filled launch for different lengths. The experiment on the ribbon cables was repeated after annealing (120 min. at +90°C) and aging (200 hours). As can be seen in Fig. 2.220, it took practically the same length of time in all four cases to achieve equilibrium mode distribution. In other words, the ribbon cables did not influence the mode mixing processes.


36

FWHMeff [°]

201

high NA-fiber NAlaunch = 0.10

32 28 24

fiber ribbon cable annealed aged

20 16

lPOF [m] 12 0.1

0.3

1.0

3.0

10.0

30.0

100.0

Fig. 2.220: Effect of mode mixing in fiber ribbons ([Har06])

SI-POF Hybrid Cable Hybrid cables are characterized by the fact that they are constructed from a combination of SI-POF elements with cooper-insulated wires that can be joined together individually or in pairs. Furthermore, there are hybrid cable combinations in a coaxial construction with a metallic tube, the so-called POF-CMT element (CMT = Corrugated Micro Tube). The illustrations shown in Fig. 2.221 point out possible combinations with SI-POF copper elements or SI-POF aluminum elements in a coaxial construction.

basic element POF

CMT

hybrid-cable single

duplex

triax

isolation 2 ... 3 mm

3 ... 4 mm

7 ... 9 mm

4 ... 5 mm

Fig. 2.221: New design for POF with CMT as electrical conductor

The advantage of such hybrid cable constructions is the possibility of supplying current directly to the transmitter and/or receiver of the individual SI-POF elements ([Ziem99a], [Ziem99b]). The connector combination for hybrid cable constructions are well known and are used in the automotive field. Apart from the coaxial hybrid solution, the layer-stranded hybrid solution is also well known (Fig. 2.222 and 2.223).

202

2.8 Fiber and Cable Production copper wire

POF 980/1000 μm

foil

support element

POF 980/ 1000 μm

strain element copper wire inner coating

inner coating

6.5 mm

outer sheath

outer sheath

7.5 mm Fig. 2.222: Layer-stranded POF-Cu cables (principle)

In these cases, insulated copper wires and POF wires are processed either into a group of four or as stranded layers with several stranding elements. The copper wires are used with diameters of 0.5 mm to 1.5 mm. Thicker copper wires are processed as braided wires, because the flexibility of the cable usually does not meet customers’ requirements.

Fig. 2.223: Hybrid POF-Copper cable

2.8.3.3 Stranded SI-POF Cables Introduction SI-POF cables or SI-POF lines are products that must bend easily when they are used and when they are processed. This requirement must be met for the manu-


203

facturing process or for transport purposes or for winding the cables or lines on production-machine reels or shipping reels or when sold in rings. The individual SI-POF elements are twisted in a screw-like fashion around an imaginary centerline. Twisting is necessary in order for the manufactured products to be flexible and portable. The advantage of twisting is that the stranding element is stretched and compressed alternatively on the inner and outer side of a curved section (Fig. 2.224). If the section in which a SI-POF stranding element is wrapped 360° around a twist axis that is considerably smaller than the curved section, the strain and pressure in a stranded construction are constant and it is possible to bend this SI-POF cable without deformation.

Fig. 2.224: Comparison of cable constructions with short or long lay lengths in terms of the bending characteristics

The flexibility of an SI-POF cable or SI-POF line is a function of the geometric dimension of the stranding elements and of the change in layers present in a cable construction. For example, a large number of layer changes results in a greater flexibility of the SI-POF cable construction. The SI-POF stranding elements are wrapped spirally around the twist axis in various machine configurations. The foundation for these various machine designs is always the result of a rotary motion with a linear motion. This can be seen schematically in Fig. 2.225.

204


1

2

5

3 4

s n1

n2

d

DA 1. 2. 3. 4. 5.

d: diameter of the rotor s: pitch length stranding unit stranding elements n1: rotational speed of the stranding basket n2: rotational direction stranding unit DA : diameter of the capstan gear and speed of the stranding basket stranding axis capstan gear

Fig. 2.225: Schematic diagram of the spiral-shaped strands

The option of being able to twist SI-POF elements together is determined by the following parameters. ¾Pitch length ¾Lay direction ¾Multiplication factor ¾Number of strands 2.8.3.4 Principles of Stranding Pitch Length The pitch length is the distance between two points on the twist axis. Within these two points, the SI-POF element has been rotated 360° around the twist axis. The lay length is calculated from the following variables: s

where

DA: n1: n2: vm:

D A S n2 n1

[mm]

s

v m 1000 n1

Diameter of the capstan gear Rotational speed of the stranded basket Rotational speed of the capstan gear The machine’s pull-off speed

During the manufacture of twisted SI-POF cables or SI-POF lines, the lay length s must be determined very exactly because of the precise geometry involved. This means that for stranding machines for SI-POF elements that are twisted via a capstan gear or caterpillar, the diameter of the stranding elements must be taken into account. In practice, a deviating diameter for the SI-POF stran-


205

ding construction is the result and increases the pitch length manufactured. The geometric assignment is easy to see in the enclosed illustration (Fig. 2.226); the manufacturing pitch length SH is calculated from it.

d 3 1

1: capstan gear 2: fiber loop 3: POF

2 DA Fig. 2.226: Diagram for explaining the concept of 'manufacturing pitch length'

The manufacturing pitch length is calculated from the following parameters: sH

sH: s: DA: d:

D d s A DA

Manufactured pitch length Pitch length in machines Diameter of the capstan gear Diameter of the stranded unit

Lay Direction The rotational direction of the stranding basket determines the lay direction. The following distinction is made depending on the sense of direction of the helix: ¾Z-lay means a right-handed thread ¾S-lay means a left-handed thread (Fig. 2.227)

Fig. 2.227: Schematic explanation of the lay direction

206


The following diagram (Fig. 2.228) illustrates how an SZ stranding is to be interpreted. It can be seen that, after a number of rotations, the lay direction is changed. In contrast to classic basket stranding, SZ stranding has the advantage of having a pull-off speed that is 5-20 times faster.

S

Z

S

Z

S

Fig. 2.228: Explanation of the lay direction schematically

Economic and engineering stranded cable products are manufactured exclusively using the SZ stranding method, i.e. also for POF applications. In classic production, SI-POF stranding elements constructed from several stranding layers are given alternatively a Z and an S direction. This cable construction element - the SZ-stranding method - for SI-POF results in a very compact geometric shape of the stranding construction, which allows it to cushion well both traverse and longitudinal forces. The stranding element is to ensure that the optical transmission values are retained during the manufacturing process of the cable product and to ensure that there are no changes after laying the cables and in subsequent operation. Multiplication Factor The helical SI-POF stranding element (Fig. 2.228) is longer in the stranded unit. The stranding method always leads to an increase in material consumption. The ratio of the laid length L of the SI-POF stranding element to the lay length of the stranded unit results in the well-known multiplication factor f = L/s. The multiplication factor f is determined from the pitch length and the average diameter Dm in the stranding layer. The multiplication factor can be easily derived from the triangle shown in Fig. 2.229. ( S Dm )2 s2 and f

L

with

L: f: Dm: s:

L s

( S Dm )2 s2 s

2

§ S Dm · ¨ ¸ 1 © s ¹

Laid length L = s/cos Z Multiplication factor Average diameter of the stranded layer Pitch length of each stranded layer

For relatively large pitch lengths (Dm « s), the calculation can be simplified as follows: f | 1 S Dm / s 2 /2


207

d L

SDm

s Z

Dm

s

D

Fig. 2.229: Graphical representation of the SI-POF stranding element

Number of Strands To characterize the bending properties of an SI-POF stranding element v, the number of strands is formed from the quotient of the pitch length and the average diameter Dm (v = s/Dm). s: Pitch length of each stranded layer Dm: average diameter of this stranded layer v: Number of strands Production developments in stranded cable constructions or SI-POF cable constructions have lead to the number of strands being v > 8. By using the number of strands v, the multiplication factor f can be easily calculated. 2

f

§ S· ¨ ¸ 1 ©v¹

S2 v 2 | 1 S²/2v² v

Layer Structure Standard SI-POF elements have a simple geometric shape but have an exact diameter. This makes it easy to calculate SI-POF cables or SI-POF lines. An SI-POF cable in its classic form, i.e. with a core element, has the same diameter as the SIPOF element; it can be constructed in a circular fashion with 6 SI-POF elements in the same layer. The cladding lines are in contact with each other. Two different core layers have been adopted schematically in Fig. 2.230. The other layers are calculated and shown. In Table 2.33 and Table 2.34, the number of elements and the diameters have been compiled for a general case and for the case with d = 2.3 mm respectively, whereby the variables have the following meaning: n: z: 6z: d: Dm: D:

Layer number Number of elements per position Total number of the elements to the layer n Diameter of the cable unit average diameter of the unit Diameter of the layer

208


d = 2.3mm

Dm1 Dm2

D2

Dm3

D1

Dm2

D2

Dm3

D3

D3

Fig. 2.230: SI-POF cable (layer structure) Table 2.33: Dimensions of layer-stranded POF cables in general

n 1. 2. 3. 4. 5. 6.

z 1 6 12 18 24 30

Dm 2·d 4·d 6·d 8·d 10 · d

D 1·d 3·d 5·d 7·d 9·d 11 · d

n 1. 2. 3. 4. 5. 6.

6z 1 7 19 37 61 91

z 2 8 14 20 26 32

Dm 1·d 3·d 5·d 7·d 9·d 11 · d

D 2·d 4·d 6·d 8·d 10 · d 12 · d

6z 2 10 24 44 70 102

Table 2.34: Dimensions of layer-stranded POF cables with d = 2.3 mm

n 1. 2. 3. 4. 5. 6.

z 1 6 12 18 24 30

Dm 4.6 mm 9.2 mm 13.8 mm 18.4 mm 23.0 mm

D 2.3 mm 6.9 mm 11.5 mm 16.1 mm 20.7 mm 25.3 mm

6z 1 7 19 37 61 91

n 1. 2. 3. 4. 5. 6.

z 2 8 14 20 26 32

Dm 2.3 mm 6.9 mm 11.5 mm 16.1 mm 20.7 mm 25.3 mm

D 4.6 mm 9.2 mm 13.8 mm 18.4 mm 23.0 mm 27.6 mm

6z 2 10 24 44 70 102

Cable Materials The specification profile for SI-POF cable or SI-POF lines in various fields of applications such as in industry, in office environments or in the automotive field place the highest demands on the material components. Thermoplastic materials (polymers) are preferred that have been mounted to the cable using an extrusion process. Excellent mechanical properties are needed so that the values listed below are ensured when SI-POF cable or SI-POF lines are installed.


209

¾Abrasion ¾Repeating bending characteristics ¾Torsion ¾Acceleration ¾Hammer blow ¾Small bending radii Especially in the automotive field, the material must be highly resistant to the following properties: ¾Resistance to oil ¾Cooling lubricant resistance ¾Steam ¾Hot gases The demand for materials that are temperature resistant comes from users. These customers are in the automotive field, in industry or in the cable-installation field for buildings. Special halogen-free material properties are desired in order to provide on-site safety to customers and consumers alike. Today’s selection of modern plastic insulation and cladding mixtures, which in part can be improved through various methods of crosslinking, should and must protect the SI-POF cables or SI-POF lines in all types of applications. In case of an accident, special plastic optical fiber cables are to have emergency running properties. SI-POF hybrid cable constructions ensure this reliability to a very high degree. The mechanical properties of thermoplastic materials such as ¾Hardness ¾Density ¾Tensile strength ¾Elongation at break ¾Tensile stress value ¾Compression strain ¾Impact resistance ¾Electrical properties can be found in the relevant data specifications of the standardized norms or the data specifications of the chemical industry. Preferred plastic materials are: ¾Polyethylene ¾Polypropylene ¾Polyurethane ¾Cross-linked thermoplastics The properties that have been improved by cross-linking are those of thermal resistance and higher mechanical strength. In addition, the resistance to solvents has also been increased, which can be seen by the fact that less swelling and cracking occur for polymers with residual tensile stress.

210


The essential physical properties of some of the important materials are listed in section 3.3.6. A very good alternative is a combination of plastic and metal, for example, with the corrugated micro tube. Metal in the most varied constructions, whether as a steel alloy, in aluminum or in copper keeps the SI-POF in an expanded temperature range protected against mechanical and thermal strain. 2.8.3.5 Corrugated Micro Tube Cables Corrugated micro tubes have been used to protect cables for quite some time. Nexans was the first company to encase polymer optical fiber wires for manufacturing resistant cables. Because of the small diameter of the POF, special corrugated micro tubes (CMT) were needed. More detailed descriptions of the mechanical and thermal properties are found in [Schei98], [Zam99], [Ziem99a], [Ziem99b] and [Zam00a]. Figure 2.231 illustrates a POF wire with aluminum corrugated tube.

Fig. 2.231: POF wires with corrugated micro tubes

Possible applications for CMT cables will be discussed later in Chapter 8.1.1.7. The manufacturing process for corrugated tubes is described below. Corrugated Tube Process The UNIWEMA (Universal Corrugated Tube Machine) has become a standard piece of equipment for cable plants worldwide. The origins of the corrugated tube process go back to the 1940’s. The corrugated tube process as practiced today is a butt-welding process for small dimensions (for example POF wires). A thin metal strip is formed around a cable core and formed into a small metal tube. The strip edges that form a butt joint are welded into a tube cladding by a laser beam under protective gas (argon and/or helium) and then corrugated in a spiral-shaped way or as rings (Fig. 2.232).


211

Fig. 2.232: Corrugated tube for POF

The UNIWEMA is used to weld copper, aluminum and steel strips or steel alloys or alternative materials. The machine creates smooth and corrugated metal tubing in an economical manner. The tube welding process is continuous and fast. All weldable metals such as copper, aluminum, steel and their alloys can be processed. The process can be used for manufacturing small metal tubes for core diameters ranging from 1 mm to 500 mm. Strip thickness’ of 0.05 mm to 4.0 mm are welded with a laser using the WIG process. Neither burrs nor bulges are produced at the welding seam (Fig. 2.233).

Fig. 2.233: Welding seams in laser welding

212


Due to the concentrated thermal effect of the welding source, the welding zone is limited on the metal edges. The heat is quickly dissipated over the tube. Since the welding zone is covered by a protective gas shield, the formation of an oxide layer is prevented. Corrugated Tube for POF Applications Metal tubes manufacture in compliance with the UNIWEMA procedure (Fig. 2.234) can be used for all POF cables. This applies to metal tubes of steel and welded special steel alloys as well as smooth or corrugated copper or aluminum tubes that have been welded lengthwise. Corrugated copper tubes are used wherever a particularly high conductivity or large dissipation of heat is required. Due to its comparably small weight when used with thin metal strips, corrugated cable tubes can be easily transported and installed. The corrugated tubing is easy to bend and particularly resistant to external deformation in the radial direction. It is absolutely hermetic. This makes it possible to operate corrugated cable tubes, and POF elements under pressure and in a vacuum.

Fig. 2.234: Laser welding device ([LZH01])

Laser Welding The laser beam is monochromatic and coherent and can be easily focused. As a result, a high power density can be achieved at the processing point - the V-seam between the strip edges (Fig. 2.235).


213

Fig. 2.235: Welding seam with laser beam ([LZH01])

By applying the auxiliary gases argon and/or helium in such a way that the beam power is absorbed in the capillaries, the coupling properties of the plasma can be controlled. The actual welding joint is produced by the melt converging behind the capillaries (Fig. 2.236). laser beam

laser beam

metal vapour laser induced plasma welding zone (fluid) welding zone (solid)

conduction limited welding

vapour (plasma) channel

direction of welding

Fig. 2.236: Principle of laser welding

welding zone (fluid) welding zone (solid)

keyhole welding

214


Keyhole welding causes the process heat to be uniformly distributed at minimum levels over the entire welding zone (Fig. 2.237). Typical welding joints are butt-welded or overlapping welding seams, weldable materials steel, special steel, brass, copper, aluminum and special metal alloys. Thin clad metal foils made of aluminum/plastic/aluminum can be used for laser welding. Fluted steel sheets can be welded overlapping or butt-jointed with a YAG laser or a diode laser.

ND:YAG-laser beam source

laser control device

controller

data acquisation

process computer laser fiber quotient pyrometer partially transparent optic O reflects Nd:YAG radiation O transmits heat radiation

laser optics beam-material interaction zone

detected heat radiation laser beam

modified track

workpiece feed direction

Fig. 2.237: Structure of a laser welding system

2.9 Microstructured Fibers

215

2.9 Microstructured Fibers In addition to classic optical fibers which consist of a core and cladding there are also microstructured fibers in which the wave guiding does not rely on a refractive index profile, but on holes along the entire length of the fiber. Normally, wave guiding in optical fibers is based on the effect of total reflection in the general sense of the term. The core consists of a material with a higher refractive index than the surrounding cladding material. In this fiber configuration special field distributions, so-called modes or eigenmodes, can be guided within the fiber. These modes experience an effective refractive index of the fiber, which lies between the maximum refractive index of the core and that of the cladding material. In 1996, J. Knight et. al. demonstrated a new kind of optical fiber, the wave guide characteristics of which were no longer based on a rotation-symmetrical refractive index. This created a variety of completely new possibilities and novel functions ([Kni96] and [Kni97]). These fibers now only consist of one material, usually silica glass, and have a structure of the cross-section with air holes. The holes in this structure are as a rule considerably smaller than the wavelength of light so that they do not act like objects on which light is reflected or scattered. Instead they change the refraction characteristics of the material. The material is changed in such a way that it acquires new kinds of characteristics. Relatively simple and specific characteristics can be created with these fibers, e.g. for dispersion, dispersion slope, modal field radius and others. For some years now microstructured fibers have also been made of polymer. These fibers with low temperature processes can be produced on the basis of the low melting point of polymers and other characteristics, thus resulting in possibilities for new kinds of fiber geometries and also potentially new applications. In the following section we would like to deal with the fundamental wave guiding mechanisms. The different types of fibers and their specific characteristics will be introduced and the methods for producing these different types of fibers will then be shown. We would particularly like to take a close look at the differences between microstructured fibers made of glass and polymers. Applications which are possible with these fibers and are presently the subject of research will then be introduced. Some of these applications can even be obtained commercially now. Finally, the present state of development will be discussed and we will venture a prognosis as to where the limits for such fibers may lie in the future. 2.9.1 Kinds of Wave Guiding Wave guiding in microstructured fibers is determined by the structure of the crosssection along the entire fiber. Holes which locally vary the refractive index very strongly are normally put into the fiber along its entire length. These areas with noticeably different refractive indices are very small in relation to the wavelength so that they cannot be resolved by light and only have an indirect influence on the propagation characteristics of the light.

216


There are two fundamental mechanisms which exercise this influence: holes either act as a kind of doping by changing the effective refractive index of the material in average ([Gho99]) or they are put into a regular, grid-shaped arrangement so that they act like a kind of meta-material ([Cre99]. Other materials which have a greatly differing refractive index from that of the core material can also be used). Such fibers can exhibit effects with a great degree of wavelength dependence since such arrangements have similar characteristics as e.g. Bragg gratings, in which the light at certain wavelengths can be constructive or destructive overlapped. The two-dimensional pendant to such a Bragg grating are the Bragg fibers in which concentric areas with greatly differing refractive indices alternate at regular intervals ([Yeh78]). Constructive overlapping waves can come about at certain wavelengths thus resulting in wave guiding. At other wavelengths light is not guided. One can therefore surmise that such fibers are capable of having strong wavelength-dependent characteristics. A new kind of wave guiding occurs in such fibers with regular structures. This wave guiding is possible in cores made of air as opposed to those fibers based on total internal reflection. For wave guiding with total internal reflection it is essential that the core material has an effective refractive index which is higher than that of the cladding. This is not necessary with fibers having a “photonic band gap”. Because of the regular structure within the fiber band structures are formed analogous to electrical semiconductors in which certain energy states of light waves are allowed and others are rejected resulting in light waves which can remain within the material and others which cannot. When there are light waves which have permissible energy states within the core area, but not in the cladding, then the light must stay in the core and is guided through this band gap since they cannot exit into the cladding. 2.9.1.1 Effective Refractive Index Fibers based on the effect of an effective refractive index can intuitively be understood most easily. They are doped with the material by introducing air or other materials. The holes made must be very small in relation to the wavelength and should be as randomly arranged as possible. The effective refractive index then results from the volume ratio of the two materials (e.g. one talks about air fraction). The greater the proportion of air, the smaller is the effective refractive index of the material. Fibers based on an effective refractive index should have holes relatively small in relation to the wavelength of the light so that the holes as such can no longer be resolved. Also, these holes should be introduced into the material in an irregular manner as possible so that the geometry and arrangement of the holes do not have any influence on the characteristics of the material (see Fig. 2.228). Such fibers are basically not different from traditional fibers in which the core has a higher refractive index than the cladding. Consequently, there is a form of total reflection. Since the effective refractive index can fundamentally only be reduced by doping with air, the cladding area in such fibers is normally structured with holes. The core is mostly undoped glass. Such fibers can be described as


217

being similar to normal step index glass fibers, whereby the fiber parameter depends on the wavelength of the light ([Mor03a ] and [Mor05b]). The reason for this is that the influence of the holes varies greatly depending on the wavelength of the light which also depends on the relationship between the hole diameter and the wavelength and whether light can resolve the holes.

Fig. 2.238: MPOF with effective refractive index according to [Lar02a]

2.9.1.2 Photonic Band Gaps In addition to the fibers whose refractive index profile arises from the effective refractive index resulting from the holes there is also wave guiding on the basis of a photonic band gap ([Cre99]). Fibers based on the principle of a photonic band gap behave fundamentally differently from the fibers with an effective refractive index just discussed. These fibers must have holes introduced in a specific periodic arrangement so that a kind of meta-crystal comes about. According to the Bloch theorem the neighboring holes act like elementary cells which are repeated regularly in several dimensions resulting in new kinds of characteristics for the meta-crystal. Just as with semiconductors, energy bands can be formed which originate from the periodic structure of the material. In semiconductors these are the periodically arranged atoms of the semiconductor material; in fibers with a photonic band gap it is the periodically arranged holes. In such fibers the light guiding comes about when the light of a certain wavelength, and thus photons with a specific energy, possess allowed energy states in the core area while the same energy states are not permitted in the cladding area. Thus, the photons in this energy state can only stay in the core area of the fiber (see Fig. 2.239).

218


effective index neff

nSiO2 1.4 1.3 1.2 1.1 1.0

n air PBGF-mode

0

1

2 3 4 5 6 7 normalized frequency /O

Fig. 2.239: Distribution of intensity in a large-mode-area-laser fiber according to [Lim03] (left). Effective refractive index of the radiation modes of the cladding (grey) with position of the bound defect mode in the band gap (center) and the magnetic field strength of the linear polarized fundamental mode for ȁ = 2.27 μm, d = 1.993 μm, D = 4.54 μm at Ȝ = 1,55 μm with neff = 0.977 (right).

The form of the energy bands, i.e. the energy areas, which correspond to the permissible energy states is greatly dependent on the arrangement of the individual holes. Even small deviations can lead to great changes in the energy bands so that with this kind of fiber only slight tolerances are allowed in the arrangement of the holes. Nevertheless, these fibers permit greater possibilities for structuring ([Arg06]). As a consequence, propagation characteristics such as dispersion, dispersion slope, effective area, etc. can have relatively large dimensions. Especially for very narrow-band applications, e.g. sharp-edged filters, fibers with photonic band gaps can be employed quite well. This is also true for high-performance applications in which the linear characteristics of the hole core are used ([Lim03], [Mat05b] and [Nie06]).

Fig. 2.240: Air-hole - MPOF with 220 μm outer diameter/5 μm hole distance, [Eij03a]


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2.9.1.3 Bragg Fibers Bragg fibers consist of concentric rings with different refractive indices. These rings act like a Bragg grating in radius direction so that they reflect certain wavelengths which are adapted to the spacing between the rings while letting other wavelengths through. This results in wave guiding in only those wavelengths which the Bragg rings reflect. With all other wavelengths no wave guiding takes place. The fibers thus act like a filter and only let light through with very specific wavelengths ([Yeh78]).

Fig. 2.241: Cross section of a Bragg-fiber according to [Arg06]

The rings can be produced in a variety of different ways. Refractive index profiles can be produced which have higher or lower refractive indices with specific radii. Microstructured fibers, however, are also possible where the rings with different refractive indices are realized by hole structures. In this case rings with holes are arranged at regular distances from the fiber axis which, because of the effective index of this layer, acts like a layer with reduced refractive index. Bragg fibers behave similarly to fibers with photonic band gap. They are also based on the exact arrangement of the holes or the layers with different refractive indices respectively. If the geometry is followed exactly very sharp-edged filters can be produced or fibers which are very selective in regard to the wavelength. 2.9.1.4 Hole-Assisted Fibers In addition to these new kinds of fibers whose waveguide characteristics are based solely on the structures introduced, hybrid fibers have also been introduced which represent a cross between conventional fibers with refractive index profiles and microstructured fibers ([Has01]). These fibers have the same wave guiding as with conventional fibers. However, the additional holes change the propagation characteristics so that you get other degrees of freedom in fiber design. In particular

220


ring-shaped hole structures are arranged around the core in order to reduce the bending sensitivity of the fibers ([Guan04] and [Nak03b]). The outer structure acts like an additional step in the refractive index profile which should hold part of the output emitted in the bend in the cladding area. This measure is supposed to increase the wave guidance without having to make compromises concerning the propagation characteristics of the fiber.

Fig. 2.242: Cross section of a hole-assisted fiber according to [Guan04]

2.9.2 Production Methods Microstructured fibers can be produced in very different ways. Various production methods are possible with glass and polymer fibers. 2.9.2.1 Microstructured Glass Fibers The first microstructured fibers were made of glass ([Kni96] and [Kni97]). Since glass has a very high melting point the production possibilities are limited. The fibers are mostly produced using the so-called stack-and-draw technique in which small glass tubes with different diameters - the number depends on the desired hole diameter - are put together in a bundle. Depending on the type of fiber, either a filled glass rod (effective index) or another small glass tube (photonic band gap) is used for the core. These small tubes combined then form the preform. They are melted and drawn into a fiber. A fiber cladding is generally drawn over the entire preform which then forms the outer area of the fiber. This only serves the purpose of stabilizing the fiber. The fact that the small round glass tubes are combined into a preform generally only allows a few arrangements: rectangular, hexagonal or so-called honey comb structures. Even if you decide on hexagonal structures when arranging the holes, the hole spacing and the hole sizes can be put into a rather large range which can lead to diverse design possibilities.


221

Fig. 2.243: Cross section of a microstructured glass fiber, fabricated by stack-and-drawtechnology ([Ort04])

Glass melting at low temperatures can also be extruded, whereby the glass is either melted or liquefied. The ensuing viscous fluid can then be pressed through specifically arranged nozzles which have the structure of the desired preform. This preform can then be used to immediately draw the fiber or to make a preform. This method of making preforms in effect allows the production of as many hole geometries as one likes. In principle, round holes and any kind of arrangement can be produced in this way. However, this production method is limited to glass with a low melting point. Consequently, silica glass, for example, cannot be processed. The production engineering of microstructured fibers has improved tremendously in the past few years. Whereas the first fibers still had attenuations of several 100 dB/km, today fibers based on an effective index with attenuations per unit length can be produced below 0.3 dB/km at a wavelength of 1.55 μm ([Taj03]). Photonic band gap fibers permit attenuations per unit length up to 13 dB/km ([Smi03]). 2.9.2.2 Microstructured Polymer Fibers (MPOF) Fibers made of plastics can be produced in a variety of different ways, especially since they can be processed at much lower temperatures. Whereas glass fibers can be drawn at temperatures around 2000°C, MPOF can already be drawn at 200°C ([Lyy04]). This not only allows simpler production techniques, but also permits the introduction of other materials into the fiber which would otherwise decompose, e.g. dyes ([Lar04]). However, there are also some disadvantages in regard to increased attenuation, lower operating temperatures, other operating wavelengths, etc. ([Lar06a]).

222


Microstructured polymer fibers can also be extruded and then drawn into fibers. The same limitations regarding geometry and production tolerances are valid for them as for glass fibers. Researchers at the University of Sydney ([Bar04c] and [Lar01b]) have developed a particular kind of preform production in which a massive cylinder made of polymer is structured using drills with different diameters. At present, preforms up 65 mm in length can be structured with this method, otherwise the drills would be too long. As many geometries as one may wish can be produced in which both the arrangement and the hole diameter can be freely chosen. Presentday production processes have hole diameters between 1 mm and 10 mm with minimum spacing in between of about 100 μm which then shrink to their original size through drawing. New kinds of process techniques can even produce elliptical holes which give the fiber an intrinsic double refraction. Preforms can either be poured into molds or around capillary tubes and then drawn into fibers ([Zha06]).

Fig. 2.244: Preforms of MPOF ([Lwin06], [Poi06e])

Other materials can be introduced into the fiber in addition to the holes. Fibers with metal wires for the poling of the material have been demonstrated as well as fibers with liquids in the capillary for controlling the propagation characteristics and doping materials for changing the optical and electrical characteristics ([Cox03b] and [Cox06]). After the first MPOF was introduced at the end of 2001 ([Lar01b]), the technology has continued to develop at an amazing pace. The fibers introduced back then still had an attenuation of 30,000 dB/km. In the course of time the individual process parameters have been continuously improved so that the attenuation could be steadily reduced. The process parameters optimized include conditions when drilling the preforms, rinsing and cleaning steps as well as drawing parameters. The best microstructured polymer fibers today have an attenuation of 200 dB/km and are thus not very far away from conventional polymer fibers which have an attenuation of about 120 dB/km at a wavelength of 650 nm.


100

223

achieved attenuation [dB/m] Sept. 2001

10

1 April 2005 0.1

0

5

10

15

20 25 30 35 40 45 months for the first publications

Fig. 2.245: Development of the attenuation of the MPOF 2001-2005 ([Lwin05])

2.9.2.3 End Surface Preparation Microtome cutting has proved to be a useful method for working on the end surfaces of conventional polymer fibers. This method of work only produces unsatisfactory results with microstructured fibers since the fine, step-like structure at the end of the fiber in the holes can lead to defects and irregularities (see Fig. 2.246). These structures can be seen at the ends of all such fibers and on conventional polymer fibers, too. Nevertheless, it can be seen that the mechanical characteristics in particular of the MPOF intensify the step-like effect. These filigree structures absorb the lateral forces and give in again after each thrust.

Fig. 2.246: Singlemode-MPOF cut by microtome, 1000-fold magnification

The direct cutting of the fiber with conventional cutting pliers can destroy the fine structures because of these lateral forces.

224


Fig. 2.247: Singlemode-MPOF cut by MOST®-tongs, 100-fold magnification

Other processing methods such as hot plate or subsequent polishing have also been investigated, but did not deliver any good results. The hot plate technique leads to inclusions at the end surfaces so that the original geometry can no longer be recognized. On the other hand, polishing leads to the deposition of rubbed off shavings and their removal into the holes. A reproducible coupling is therefore not possible since the influence of these inclusions or that of the deposited foreign matter in the structure’s holes is not controllable. Better processing characteristics are shown by those MPOF which are surrounded by another, so-called buffer layer made of hard polyester. This layer absorbs a large part of the mechanical forces when cutting and prevents the breaking of the fine webs within the structure. Since such fibers consist almost exclusively of polymer they can almost be worked on like polymer fibers. Figure 2.248 shows the end surface of such an embedded fiber with a buffer layer. You can see that the fiber is not embedded centrically which leads in practical use to a lateral misalignment of the plugs and thus to plug losses and power redistribution. In the future you can expect, however, that the dimensions of the fibers will become greater and that the fibers can be better centered with new drawing techniques.

Fig. 2.248: End face of an embedded fiber with buffer layer; 100-fold magnification


225

No practical solution exists yet which can provide for good reproducibility and a high degree of reliability. Processing methods still have to be found for both practical and laboratory use which can meet the necessary requirements. In the case of termination in the field the end faces must allow acceptable losses; in the laboratory, preparation with high reproducibility is necessary. Both kinds of preparation still have to be developed. 2.9.3 Applications for Microstructured Fibers Microstructured fibers allow a number of applications since their characteristics can be adapted to wide areas as desired because of the additional degree of freedom in design and production. For example, waveguide characteristics such as chromatic dispersion and its slope can be adjusted as well as the mode field diameter. Other materials or fluids can be introduced into the fiber through the holes running along the fiber. These materials can change the propagation characteristics through which tunable components or sensors are made possible. Some possible applications for microstructured fibers are subsequently described. This list does not make any claim to being complete, but is solely intended to present the best-known applications as well as the commercial applications available today. 2.9.3.1 Dispersion Compensation The first applications for microstructured glass fibers were the compensation for dispersion or its slope respectively. Because of the additional possibilities for fiber design, selection of the number of holes, their size and distance from one another, wavelength-dependent effects in particular such as chromatic dispersion can be adjusted very well. As already described above, the holes have weaker wave guiding at small wavelengths because the light can enter the bridges between the holes. This causes a different kind of wave guiding so that the fiber behaves as if it had another fiber parameter. By skillfully selecting the diameter of the holes and their spacing, the dispersion and higher orders can be adjusted very well. Dispersion-compensating microstructured glass fibers are commercially available today. 2.9.3.2 Endlessly Singlemode Microstructured fibers also allow applications which are not possible with conventional fibers. Such an application are the so-called endlessly singlemode fibers which have one mode in the entire wavelength spectrum and do not have a cut-off frequency. This characteristic can come about when the wave guiding changes with the wavelength. In step index fibers the existence of one mode is clearly determined by the fiber parameter V which is proportional to the core diameter, the numerical aperture and the reciprocal value of the wavelength used. Thick fibers with large numerical apertures are characterized by a large fiber parameter V. Fibers are only guide

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only one mode for V < 2.405, the first zero of the Bessel function of zeroth order. If the wavelength selected is large enough then V will become small enough at some point so that the fiber becomes singlemode. In microstructured fibers the fiber parameter is not simply anti-proportional to the wavelength since the holes in the cladding area act differently with large wavelengths than with small ones leading to a wavelength-dependent numerical aperture so that fibers can be produced which are singlemode for all wavelengths ([Bir97], [Mor03b] and [Zag04]). 2.9.3.3 Birefringence Since microstructured fibers are not rotation-symmetrical such as conventional fibers with a refractive index profile, for example, they tend to be birefringent. Typical hexagonal structures do not exhibit any birefringence. However, when this symmetry is disrupted, e.g. through production tolerances, then these fibers are birefringent. This effect is used positively in some fibers, whereby the high birefringence causes the fibers to retain their polarization ([Ort04]). In the case of very great differences between the propagation constants of both polarizations they can then only very weakly interact with each other and exchange power. When only one polarization is launched into the fiber, then the power in this polarization is retained and is propagated in this way to the end of the fiber.

Fig. 2.249: High birefringent MPOF by incorporated asymmetry ([Issa04b])

The effect of birefringence can be generated in microstructured fibers in two ways: either the holes are arranged asymmetrically so that a geometric birefrin-


227

gence occurs which can be created in a controlled and thermally stable manner, or the holes are elliptical and not round which contributes to the birefringence ([Issa04b]). It is more difficult to control this kind of birefringence, but it does allow complete freedom of fiber design because the arrangement of the holes and their size can be freely chosen. 2.9.3.4 Highly Nonlinear Fibers The nonlinear characteristics of fibers are influenced on the one hand by the nonlinearity of the material and on the other by the level of confinement which is described by the so-called effective mode area. With very strong wave guiding, light is guided into the center of the core and the optical power can propagate in the area of the core-cladding interface layer or even in the cladding. Here the light is strongly concentrated in a small area of the core which results in very high intensities with the same power which can lead to nonlinear behavior within the fiber. Such strong wave guiding can only be attained by means of big differences in the refractive index between the core and cladding. In conventional fibers the differences in refractive index are in the range of a few percentage points. Microstructured fibers on the other hand consist of areas of glass with a refractive index of about nglass | 1.5 and holes, which are generally of air (nair | 1). The very high confinement can be achieved by this very high contrast in refractive index. In fibers based on this effective refractive index, the cladding has to have a very high air fraction. The proportion of air in relation to the entire volume has to be so high that the effective refractive index lies near the value for air. Fibers with effective area up to Aeff | 2.85 μm2 have been realized using this process ([Lee02]). In addition, other materials such as Bi2O3 can be used which have a highly nonlinear susceptibility Ȥ3. With such materials nonlinear parameters of Ȗ = 1100 W-1km-1 can be produced ([Lee06c]). 2.9.3.5 Control of the Effective Area Fibers with a particularly high nonlinearity are needed for all optical signal processing. There are, however, a number of applications in which the nonlinear effects should be particularly weak so that the light propagation in such fibers is not disrupted. In such fibers the opposite path is taken as with highly nonlinear fibers: the material used should be as slightly nonlinear as possible and the effective area of the fiber should be as large as possible so that the intensity within the fiber remains low at the given luminous efficiency. Even if the difference in refractive index between the core material and the holes continues to remain large you can still see to it through skillful fiber design that the light is guided relatively weakly and the mode field takes up as large an area as possible. In general, these fibers have a very low air-fill factor so that the effective refractive index in the cladding area lies only slightly below that of the core. Fibers with effective areas of Aeff | 100 μm2 have been presented by [Kim06c] and [Sai06].

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This technique can also be used for controlling the form of the mode field in order to adapt it to other types of fibers and thus minimize coupling losses at the connector. For example, Furukawa introduced such fibers at the ECOC in 2004 ([Guan04]) the mode fields of which are adapted to standard singlemode fibers. 2.9.3.6 Filters Microstructured fibers can show very strong wavelength-dependent effects. As described above, the dispersion can be adapted to a wide area, but other wavelength-dependent characteristics can be specifically designed, e.g. group velocity or even the attenuation per unit length of the fiber. Fibers with an effective refractive index permit the relatively simple adaptation of the group velocity with which one can generate all-pass filters with specific phase responses. Fibers based on a photonic band gap can have very sharply delimited wavelength ranges with which light is guided. Thus, filters with specific amplitude response and sharp edges can be produced ([Vill03], [Kim05c], [Kim06d] and [Sai05]). 2.9.3.7 Sensor Technology, Tunable Elements The characteristics of microstructured fibers can be manipulated in many ways. In particular materials can be introduced into the holes along the fiber which can change the characteristics of the microstructured fiber through their different refractive indices. These materials can be gases or liquids which are guided through the fiber and can alter the characteristics when the composition is changed ([Car06b]). With such methods you can also analyze liquids such as blood in the human body. Polymer fibers are especially attractive for this kind of application because glass can split and would thus be considered too dangerous in the human body. You can also intentionally change the characteristics by means of the controlled introduction of liquids. Thus, sensors have been introduced which are based exactly on this phenomenon, e.g. a liquid is pushed into the capillaries in the cladding area when the temperature rises, thereby changing the propagation characteristics of the fiber ([Jen05]). Consequently, the dispersion ([Gun06]) or the band gap ([Sun06]) can be adjusted to a lesser or greater extent by introducing liquids. Pressure sensors represent another application. Since the geometry of the holes has a great influence on the fiber’s propagation characteristics, lateral pressures can have a very noticeable effect on its behavior ([Eij03b]). Especially fibers based on a photonic band gap react very sensitively to changes in the geometry. As a result, microstructured fibers can be produced which work like filters, the passband of which is changed when pressure is exerted.


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2.9.3.8 Double-Core and Multi-Core Fibers Most microstructured fibers consist of a cladding area in which the holes are arranged symmetrically or asymmetrically. In fibers based on an effective refractive index the core consists of an area in which a hole has been left out of the arrangement. The core is thus a kind of imperfection within the photonic crystal. In this way two or more cores can be produced by introducing two or more imperfections within the cladding area in which the light can propagate instead of having just one hole in the middle. Each individual location where a hole has been left out and the core material exists can be viewed as a separate fiber in which light can be propagated. If the individual cores are placed far enough apart, they either do not influence each other at all or only slightly. Such fibers with several cores can be used for parallel data transmission ([Eij06a]). The arrangement of the individual cores is retained and so these fibers can be used like a well-ordered fiber bundle. However, these fibers have a considerably smaller diameter and can be laid like individual fibers ([Eij03b] and [Pad04]).

Fig. 2.250: Double core-MPOF with 9.6 μm spacing between the cores ([Eij03b])

2.9.3.9 Imaging As we have seen above, microstructured fibers can be produced with more than one core for parallel data transmission. If you continue to increase the number of cores, you can use the same method to produce image guides in which every individual core transmits a part of the image (a pixel). As mentioned above, the arrangement of the holes stays the same and the cores along the fiber are retained. Each individual pixel reaches the end of the fiber in its definite position so that the image is retained ([Eij04c]).

230


Fig. 2.251: Image guide-MPOF ([Eij04c])

2.9.3.10 Multimode Graded Index Fibers The fibers introduced so far are relatively thin singlemode fibers. In addition to these fibers, graded index multimode fibers made of polymer have also been developed, so-called GI-MPOF ([Kle03b] and [Eij04d]), which have the large core diameter of a polymer fiber and the effective graded index profile of a multimode glass fiber (see Fig. 2.252).

Fig. 2.252: Schematic cross section of a GI-MPOF ([Kle04b] and [Lwin06])


231

Polymer fibers offer a number of advantages, especially with fibers having large core diameters compared to glass fibers (these advantages are also valid for other types of fibers). These considerably larger core diameters are possible without the fiber becoming inflexible. For this reason graded index polymer fibers have been produced for some years now which attain core diameters into the millimeter range. However, these fibers have a refractive index profile in the core which has been adjusted through doping and they are quite difficult to produce when the core diameters are very large. Another advantage of these microstructured fibers is the lack of doping material which results in these fibers having very good thermal and aging stability of the profile. Graded index profile polymer fibers which already exist are not particularly thermo-stable. With aging and especially in combination with increased temperatures they exhibit a flattening of the profile through diffusion of the doping material. This leads to an alignment of the concentrations of the doping materials resulting in a leveling out of the profile.

Fig. 2.253: Cross section of a graded index profile multimode polymer fiber (GI-MPOF) with 135 μm core- and 520 μm outer diameter ([Eij04d]) and of a MPOF according to [Lwin06]

Figure 2.253 shows a multimode fiber in which the effective refractive index continuously decreases with increasing distance to the fiber axis. If you take an average of the entire circumference of the refractive index, then you have a parabolic refractive index profile in the radius direction. Measurements have shown that these fibers have a similar propagation behavior as a conventional multimode fiber. However, the differences lie in the detail. If you stimulate the GI-MPOF with a small spot, for example, the fiber behaves differently, depending on whether or not the light hits a hole or the core material; something that cannot happen in conventional fibers. For this reason greater research and development in measurement techniques and characterization are necessary before the GI-MPOF is widely used in commercial applications.

3. Passive Components for Optical Fibers

3.1 Connection Technology for Optical Fibers The components required in a transmission system include plug-in connectors for coupling cables or fibers. One of the biggest advantages of polymer optical fibers in contrast to other cable types is the potential for very simple connector fittings. Copper cables for high data transfer rates mostly require the connection of several twisted pairs that must, in part, be individually shielded (Fig. 3.1). At frequencies of several 100 MHz, cutting open the shielding over a distance of one centimeter results in a noticeable drop in quality of the connection.

Fig. 3.1: Copper data cable with separately shielded twisted pairs

Glass fibers have a core diameter between 10 μm and 200 μm. This requires precise guides that are provided by metal, ceramics or high-grade plastics. Furthermore, glass fibers cannot simply be cut. The face must either be precisely broken by carving with a diamond blade or else polished after cutting. Further advantages for POF result from the material itself. The surface of plastics can be smoothed by both cutting and simple polishing. In addition, a thermal smoothing of the surface is also possible for PMMA.

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3.1 Connection Technology for Optical Fibers

3.1.1 Connectors for Polymer Optical Fibers In recent years, a wide range of connector types has been developed specifically for various applications. These fall into the following categories: ¾Special plug-in connections for POF, e.g. V-pin, DNP ¾Plug-in connectors developed for glass fibers have been adapted for POF, e.g. FSMA, ST ¾LAN plug-in connectors which have identical dimensions for both copper and optical fibers (SC-RJ, RCC45) ¾Connectors for special standards (D2B, F07) ¾Connector systems without plugs (optical clamps) ¾Hybrid connector, generally a combination of copper lines and POF (e.g. MOST) Several connectors are available in metal or plastic constructions depending on the requirements for mechanical stability. Most systems can still be obtained in simplex or duplex versions. At this point we would now like to define the term “connector attenuation”. Strictly speaking, a connector does not have any defined losses, only a fiber-tofiber connection has losses. The light losses are caused by: ¾inexact alignment of the fibers to each other, whereby the parameters of the fiber, the connector and the coupling could be responsible, ¾the fiber parameters not being adapted to each other, e.g. different numerical apertures and ¾direct losses at the fiber end face through reflection, scattering and absorption (see Fig. 3.2). You can thus see immediately the great dilemma of the connector manufacturers who are not at all responsible for an essential part of the losses. In most cases you cannot even be sure that the connectors and couplers all come from the same supplier. The greatest contradiction, however, lies in the specifications of the POF itself. As will be shown later on, the IEC has approved wide ranges for the fiber parameters, but still insists on including plug-in connections in the power rating calculation. That would be like approving on the one hand surge impedance of 100 ± 30 ȍ for a gigabit Ethernet on symmetric lines and demanding on the other a reflection attenuation of at least 20 dB and a connector attenuation of < 0.1 dB. For this reason attempts are being made to reduce the characterization of a connector to the effects at the connector end face. This involves precision in positioning the fiber in the connector or in the coupler and the quality of the surface treatment. Another thing we have to mention is that the connector attenuation with all multimode fibers depends on the mode distribution and thus on the launch conditions and the measuring fiber length. In this respect there are no specified directives.


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reasons for losses in the connector: 1. unequal fiber parameters (NA, diameter)

2. not presice position of the fiber inside the connector

3. mechanical tolerances of fiber and connector

4. not perfect guidance of the connector in the adapter

5. losses on the fiber end faces (reflection, scattering and absorption)

Fig. 3.2: Reasons for connector losses

3.1.2 Surface Preparation of POF Connectors The choice of the end face preparation is of great importance (see [Moll00]). The following procedures have proven themselves: ¾Cutting and polishing: The POF is roughly cut at the connector face and subsequently sanded down to the face with sand paper. Using fine-grained polishing paper, the surface quality can be further improved. For normal demands it usually suffices treating the surface with 3 μm polishing paper after the first cut. If the connector attenuation should be minimal and for measuring purposes we recommend polishing it with fiber grades of polishing paper, first 10 μm, then 3 μm and finally 0.3 μm. ¾Hot-plate: The POF is cut before the connector facet at a defined projection length. Afterwards, the connector is pressed within a guide against a hot mirror. The connector has a ring-shaped bulged opening on the face in which the projected material is pressed. After the mirror has cooled down, the connector is withdrawn.

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¾Cutting: Using a thin blade (usually a razor blade), the POF is cut in a guide at a perpendicular angle. This method is often used for connections without a special connector. The blades must not be used more than once at one point. ¾Laser cutting: The POF is cut vertically with a laser (for example, CO2 laser). This procedure is only feasible for ready-made cables. ¾Microtome cut: Extremely thin slices of a sample can be cut off with a microtome, for example, for microscopy. If you cut off several thin slices of a POF the remaining surface is then extremely smooth and even. This procedure is very expensive if you use a diamond blade and is consequently only employed as a reference method or for special measurements. ¾POF-Press-Cut (PPC): [Moll00] and [Fei00] contain descriptions of how cracking can be avoided in PMMAs when a suitable pressure is being applied to perform the cutting. Thus surfaces are possible whose losses are near the theoretical limit (Fresnel reflection), as shown in Fig. 3.3. The following diagram shows the losses of a plug-in connection which are counted back to one surface. Geometric influences have been eliminated through optimized coupling. loss per end face [dB] 0.70 0.68 0.60 0.47 0.50 0.35

0.40

0.30

0.24

0.30 0.20 0.10 0.00 conventional cot

Hot plate

POF press sanding/ microtom cut with cut polishing diamond blade finishing method

Fig. 3.3: Comparison of the connector losses for various procedures according to [Moll00]

The theoretical limit is 0.17 dB by Fresnel reflection. Simple cut POFs have a loss of almost seven tenths dB. By means of PPC and multistep polishing almost ideal end faces can be attained. The microtome cut here lies at only 0.07 dB above the Fresnel limit. However, it is the most costly procedure and used only for measurement purposes.


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3.1.2.1 POF Preparation by Cutting and Polishing The individual steps for assembling a POF connector by polishing are shown in Fig. 3.4. After the POF is cut to size a short piece of cladding is stripped off. The connector is pushed onto the fiber and fixed in position. With V-pin connectors this can be done by simply snapping them shut or by crimping a metal ring on them. With other types of connectors a kind of clip is pushed into the connector in order to fix the fiber in position. Another method, albeit too costly for POF, is to glue the fiber into the connector. After cutting off the excess fiber core, except for a little bit left over, cutting and polishing take place - in several steps if necessary. The connector is usually held in place by a special locator (crimping tool).

1. POF with coating

2. removed coating

3. connector mounted

4. POF cutted

5. grinding end face

6. polished end face

Fig. 3.4: Surface preparation by grinding/polishing

The advantages of this procedure lie in the very limited number of tools needed: ¾a scissors or pliers ¾a stripper for the fiber cladding ¾polishing fixture ¾regular abrasive paper ¾polishing paper, in several grades if necessary With a certain degree of practice you can make a usable plug in 30 seconds and a very good one in two minutes.

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3.1.2.2 Hot Plate Surface Preparation Another surface treatment method rests on the thermal smoothing of the fiber core. Except for a small slot on the end face the connector has the same mechanical assembly as the polishing connector. Figure 3.5 shows the sequence of steps.

1. POF with coating

2. removed coating

3. connector mounted

4. POF cutted with defined overlength

5. surface pressed to a hot plate

6. connector is ready after cooling

Fig. 3.5: Surface preparation by means of a hot-plate

3.1.2.3 POF Press-Cut Procedure A detailed investigation and description can be found in [Fei01a] as to what happens with the simple cut of a PMMA POF. In short, a break develops before the blade which can spread quickly through the material and can wander off to the side. The result is a surface which for the most part has been cut quite cleanly, but in the last third has an irregular break. Figure 3.6 shows such a surface and a detailed view of the edge of the break. Under a fluorescence microscope you can see the cracks particularly well. A rather surprising result of Feistner’s experiments was the fact that when treating the surface with hot plate the cracks do not disappear, but virtually melt in several micrometers deep. The cross-section of such an end face with a detailed view of the micro-cracks enclosed below the surface is illustrated in Fig. 3.7 from [Fei01a].


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Fig. 3.6: Irregularly broken POF due to the cutting process ([Fei01a])

Fig. 3.7: Cutted and hot plated POF with cracks in fluorescence light ([Fei01a])

Proceeding from these results, the POF press-cut procedure was developed at the POF-AC in which the fiber is pushed to the right and left of the blade when being cut. blade Fradial

Fradial compressed area Æ higher resistance uncompressed area Æ lower resistance

Fradial

Fradial

Fig. 3.8: Principle of the PPC method

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The crack is thus always forced to propagate evenly in front of the blade. The result is a POF which has a good uniform cut over its entire cross-section (principle is in Fig. 3.8). The cut with the PPC experimental set-up is compared with a conventional cut in Fig. 3.9. In the meantime two manufacturers have turned this procedure into commercial products. Figure 3.10 shows the cutting tool from the Rennsteig company based on this principle.

Fig. 3.9: POF-cut end by conventional blade and by PPC

Fig. 3.10: Cutting tongs for POF (Rennsteig)

3.1.2.4 POF Preparation by Milling A very quick and reliable method for preparing POF connectors is milling with a fast rotating blade or cutting tool. A steel cutting tool suffices for medium demands, for the highest demands in measurement techniques and component production, however, a diamond can also be used.


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The fiber itself as well as a fiber already secured in a connector can be worked on using this method. You can prevent the connector itself from being milled with guides and limit stops. The advantage of this procedure is the high reproducibility. An example of such a device is shown in Fig. 3.11.

Fig. 3.11: Milling machine for preparation of POF (DieMount)

3.1.3 Overview of Connector Systems In the first edition of this book a good half dozen families of connectors were described. In the meantime, additional versions have come along. It is not possible to review all products here. Many of the manufacturers have made data available in an overview of the industry under www.pofatlas.de. A number of connector systems for polymer optical fibers will be shown in the following pictures. Each system has been optimized for specific applications. There is no universal connector type as is the case with glass fibers or copper cables. 3.1.3.1 The V-Pin Connector System One of the connector systems specifically developed for POF and PCS was the V-pin system from Hewlett-Packard (for a while Agilent, now Avago). The V stands for versatile. In many experimental set-ups and in some applications with a low number of pieces the system has been eagerly used. The original version of the connector was intended to be crimped. There is a plastic ferrule (in different colors) and a soft-metal crimp ring which is pushed together with special pliers. An inline coupling is also available. Furthermore, there

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is a duplex connector version available. These plugs and the active components available for them are described in [HP06] and [HP03]. The emitter and receiver versions HFBR-0507 and HFBR-15X7/25X7 are designed for data rates up to 155 Mbit/s for all these connector systems. The simplex and duplex connectors, the inline coupling and an active component are illustrated in Figs. 3.12 - 3.14.

Fig. 3.12: V-pin-crimp connector system and coupling (above: crimped simplex connector with coupling, below left: duplex connector, below right: connector with latch)

Later on the system was expanded with a crimpless version which included a plastic part which locks in place when snap shut and fixes the fiber in position. If these two parts are connected, then you automatically get a duplex connector (Fig. 3.13). The fiber spacing naturally corresponds exactly to the spacing between the two connected HFBR components. Moreover, there is a connector with a latch so that the extraction force can be increased.

Fig. 3.13: V-Pin crimpless connector system (duplex in Fig. 3.42)


243

Fig. 3.14: V-Pin connector with latch, connector with an active component

The American company Fiberfin offers a compatible connector family with a metal ferrule (Fig. 3.15).

Fig. 3.15: Fiberfin- connector system (presented at the International POF Conference’2004)

The POF is cut off in front of the connector when being fixed in position and cut/polished with the polishing fixture (see next section). An identical connector can be used for 200-230 μm PCS which, however, has to be crimped twice. First the connector is attached to the 500 μm primary coating and then crimped onto the 2.2 mm outer sheath (Fig. 3.16).

Fig. 3.16: V-Pin connector for PCS

The coupling for PCS only differs from that of POF through the metal piece which reduces the tolerances. The active components can be used in part alternatively for POF or PCS.

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3.1.3.2 FSMA Connector One of the most widely used connectors for POF and PCS is also the SMA in its fiber version (FSMA). This connector is always a screw type. Consequently it is very reliable and provides reproducible transitions. As a metal connector it has low tolerances and the connector losses for POF are below 1 dB. It is too expensive, too big and too complicated for use in large-scale applications. It is standard, however, in measurement techniques and in many industrial applications. Different FSMA connectors for POF can be seen in the following photos. There are connectors for polishing and as hot plate version. The fiber can be fixed in position by adhesive, crimping or also a detachable clamping device. Different kinds of kink protection are available. Finally, the connector can be made of metal, plastic or a combination of both.

Fig. 3.17: FSMA-connector for crimping/gluing

Fig. 3.18: FSMA-connector for clamping (reusable)

Figure 3.19 shows a front-view of the FSMA connector versions for preparation by polishing and the hot plate procedure. In the hot plate version a small slot, which can also be slanted, is located around the fiber core hole. The core material sticking out is pushed into this open space when heated (see Fig. 3.5). As we shall see later, this causes a little additional attenuation.


245

Fig. 3.19: FSMA-connector for polishing (left) / hotplate (right), front view

Fig. 3.20: FSMA-connector in plastic variants and FSMA-connector with kink protection

Fig. 3.21: FSMA- inline-coupling and LED-receptacle

3.1.3.3 The DNP System In the middle of the 1990s AMP developed the so-called dry non-polish (DNP) system. This connector was always treated by hot plate. The system consisted of a standard size and a mini-connector, available in both simplex and duplex. There was also a coupling for both versions. A metal sleeve in the connector fixed the fibers in place. Small barbs held the fibers tight after they had been pushed into the connector using the corresponding pliers. The fiber-fiber coupling in which two smoothly cut POFs could be firmly connected without a connector was extremely practical. The system is no longer sold.

246


Fig. 3.22: DNP-connector (simplex, duplex, coupling)

Fig. 3.23: Mini-DNP-connector (above simplex-connector and coupling, duplex-connector, below with coupling)

Fig. 3.24: DNP fiber-fiber connection

3.1.3.4 F05 and F07 The best-known connector for 1 mm polymer fibers for the private user is the F05. It is used for example in the Toshiba’s Toslink system for connecting digital audio components (TOCP155, see Fig. 3.25). The connector face is essentially standardized with this type. There is a great variety of versions for the connector body. They differ in shape, size, color, kink protection and the design of the clamping mechanism.


247

Fig. 3.25: F05 connector and coupling

Fig. 3.26: F05 connector variants

The duplex version of this connector is the F07 which is also used as a standard connector in the ATM Forum (PN connector). There are different options with or without latch. With the F05 and F07 the end face is almost always treated with hot plate. Occasionally there are also versions for cutting and polishing.

Fig. 3.27: F07-connector and coupling

3.1.3.5 ST and SC Connectors Other connector versions such as ST and SC have been known for many years in the field of glass fibers and are also offered for POF.

248


Fig. 3.28: Different ST-connectors

Fig. 3.29: Coupling for ST-connector

Fig. 3.30: SC-connector for GI-POF (AGC)

Fig. 3.31: Coupling for SC-connectors


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3.1.3.6 Connectors for Future In-House Networks With household networking the demands on mechanical stability and temperature are not as tough as in automation or in vehicle networks. On the other hand the connectors have to be particularly simple and you have to be able to install them easily without automatic equipment. In a DKE workgroup (Deutsche Kommission für Elektrotechnik / German Committee for Electrical Engineering) about 20 companies in the POF field are currently discussing recommendations for future plugin connectors. The way things stand now the duplex connectors SMI, SC-RJ and EM-RJ will be recommended. All three versions are capable of small form factor (SFF) which means that they do not require more cross-section than an RJ45 connector. SMI Connector The SMI connector is a duplex connector for SI and GI-POF. It is already included in various committees and standards, not least in IEEE 1394. Hot plate as well as cutting/polishing can be used for the surface preparation. A number of manufacturers offer transceivers.

Fig. 3.32: Different SMI-connectors

SC-RJ The SC-RJ connector for glass fiber applications is well known and is especially popular in local networks. In any event it would not take much for many installers to get used to it. The SC connector is a duplex connector with an interlocking device. It is also available as a simplex version so that two simplex connectors can be plugged into a duplex coupling. A connector, a coupling and a transceiver from Reichle & De-Massari ([Chr05]) are illustrated in Fig. 3.33.

Fig. 3.33: SC-RJ-connector (RDM) with transceiver and coupling

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For glass fibers here are connector versions for singlemode fibers, green for angled polished, and blue as physical contact, and beige for multimode fibers. The connector is described in standard IEC 60873-14, parts 1 to 3. For POF the color combination suggested is black/white. A simplex connector and another transceiver prototype from [Dre05] are shown in Fig. 3.34.

Fig. 3.34: SC-Simplex-connector and transceiver ([Dre05])

EM-RJ The EM-RJ introduced in [Neh06a] is a duplex connector which can be cut and fitted on location with connectors and has housing dimensions of the RJ45 (acc. to EN 60603-7). It is available in the safety classes IP20 to IP67 for home and industrial applications. In addition to the connectors boxes, couplings and distribution panels are also available. The ferrules are made of metal, but in the future should also be available in plastic. It is also possible to insert 8 metal contacts into the connector so that it can be used as a hybrid Ethernet connector. The connector and a Fast Ethernet transceiver (Euromicron) are shown in Fig. 3.35.

Fig. 3.35: EM-RJ with Fast-Ethernet-transceiver

3.1.3.7 Connectors for Vehicle Networks At the end of the 1990s a number of different connectors were developed for use in vehicle networks. In addition to lower attenuation the goal was primarily to achieve a very moderately priced set-up as well as being able to assemble them on automatic equipment. Harting developed a metal connector with which the POF was directly cut off together with the tip of the ferrule in order to get a perfectly even end face. Figure 3.36 from [Bru00] shows the automatic machine with a finished cable.


251

Fig. 3.36: Harting connector and machine for assembling [Bru00]

The manufacturer FCI came up with another concept in which the connector consists of a single piece of plastic. Gear teeth which can be directly crimped onto the primary coating are located in the back. The end face is treated by hot plate. Figure 3.37 shows some of these connectors.

Fig. 3.37: FCI connector for direct crimping

A completely new connector system was developed by Tyco-AMP for the D2B bus. A kind of transparent plastic cup filled with an index matching gel is placed on the fiber. In this way surfaces which are not ideal can be compensated for. Further details concerning the system are not publicly available. The connector developed for MOST (see Chap. 8.1.1) has attained considerably greater importance. It consists of a metal or plastic ferrule which is crimped or spliced with a laser on the primary coating. The connector end face is milled with a fast rotating saw blade. Two versions of this ferrule and a cutaway view are shown in Fig. 3.38. The ferrules and electrical contacts in different combinations are built into hybrid plug-in connectors. There are a number of different versions for straight and angled connectors, in-line couplings and sockets on the control units (Fig. 3.39).

252

3.1 Connection Technology for Optical Fibers ferrule

fiber inner 1.00 mm coating 1.51 mm

outer sheath 2.30 mm

Fig. 3.38: MOST-ferrules left: made from plastic for laser welding according to [Eng00] center: made from metal for mechanical crimping according to [Eng00] right: cutaway view ([Sie00])

Fig. 3.39: MOST-connector system ([Sie00])

Fig. 3.40: MOST-hybrid connector

3.1.3.8 Other Connectors We can only provide a relatively incomplete overview of special connectors here such as how they are used in automation engineering. Hybrid connectors and versions with many fibers are used quite frequently. As a rule, high safety requirements necessitate stable housings and robust cables. Two examples of such connector systems can be seen in Fig. 3.41.


253

Fig. 3.41: Hybrid-POF-connector ([Kno03])

3.1.4 Processing Tools for POF Connectors A number of different tools are available for assembling the various connector types. Figure 3.42 shows the tool required for assembling a crimpless V-pin connector (see Fig. 3.12).

Fig. 3.42: Wire-end stripper and polishing fixture

A wire-end stripper is shown on the left side; it is used to remove the POF jacket. The polishing receptacle can be seen at the right; it is used to hold the connector for polishing the face. The pliers in Fig. 3.43 is additional required for the V-pin version with crimp ring. This is a combination tool for assembling connectors to POF and 200 μm PCS fibers.

Fig. 3.43: Crimping plears for V-pin connectors

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Complete installations sets are available for a number of POF connectors, some of which are illustrated here. Figure 3.44 shows the assembly and cutting locator for a Lucina duplex connector (Asahi Glass) and the assembly of a SC-RJ (Reichle & De-Massari).

Fig. 3.44: Assembling of POF-connectors (Lucina-fiber and duplex-POF in SC-RJ)

A tool as shown in Fig. 3.45 can be used to assemble connectors with the hotplate procedure. The power supply is (not visible) at the left. It periodically supplies power to the heated metal plate for a specified period of time. A red LED indicates the heating period. Afterwards, the plate is cooled by means of ventilation. The receptacle shown at the far left can be used to press several connectors perpendicular against the plate with high precision. This is done initially when the plate is cold. The connector must remain pressed against the mirror for the entire heating period. When the green LED lights up, the surface has sufficiently hardened so that the connector can be removed.

Fig. 3.45: Hot-Plate-tool (Siemens/FO-Systems/Leoni)


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The receptacles in Figs. 3.46 and 3.47 serve to polish various connectors. They guarantee that the face is always perpendicular to the polishing paper and the sand paper. This also prevents excessive grinding on the connector itself.

Fig. 3.46: Polishing fixtures for simplex and duplex connectors (AMP)

Fig. 3.47: Polishing fixtures for ST connectors and FSMA connectors (at the right)

Figures 3.48 and 3.49 show a tool for stripping the POF and a crimping tool for the TCP sleeve from FCI. The special feature of the stripper is that it can precisely guide the fibers in such a way that damage to the optical cladding can be prevented securely.

Fig. 3.48: Wire-end stripper tools for POF with wire guide and stop

256


Fig. 3.49: Crimping tool for TCP-ferrule (FCI)

Figure 3.50 shows three simple cutting tools to cut POF with or without jacket using a razor blade. The cut is relatively rough; the tool can be used once only for each hole. These are devices for home installations that do not involve any particularly high quality requirements.

Fig. 3.50: Simple hand-tools for cutting POF

Automatic machines occupy a particularly important place in the processing of POF. For vehicle networks such as MOST and Byteflight the individual cables are prefabricated before they are integrated into the cable harness. The automatic machines cut a cable to length, strip the jacket, attach metal ferrules, treat the end faces and the cable is finished in under 2 seconds.

Fig. 3.51: Machine for POF-cable fabrication, detail of the machine ([Mei02b])


257

The automatic machine, a detail of the machine, the principle of attaching the ferrules by crimping and the saw blade for treating the surface are illustrated in Figs. 3.51 and 3.52 from [Mei01a] and [Mei02b].

Fig. 3.52: Ferrule crimping, saw for end face processing ([Mei02b])

3.1.5 Connectors for Glass Fibers Today there are a number of connectors for glass fibers which for the most part have already been described in corresponding standards. As opposed to POF connectors, reflection loss plays a very important role in glass fibers. A low return loss can be attained through connectors with physical contact (PC, see Fig. 3.66) or through beveled end faces (APC: angled physical contact, see Fig. 3.53).

reflected rays will not be guided

contact areas with a 8° angle

Fig. 3.53: Principle of the APC-connector

The main difference however between the glass fiber connectors and the POF connectors is the much greater assembly time and effort caused by the lower tolerances of « 5 μm for MM-GOF and below 1 μm for SM-GOF. Figure 3.54 illustrates a typical assembly procedure for a duplex connector.

258


Fig. 3.54: Installation of a typical GOF connector ([Mye02])

Spring ferrules are almost always used with glass fibers in order to guarantee the physical contact with the end faces. Furthermore, glass fiber cables are usually equipped with strain relief, e.g. Kevlar strands, which have to be separately secured in order to protect the actual glass fibers from tensile load. On the other hand good glass fiber connections also attain losses in the range of 0.1 dB to 0.5 dB. Somewhere in the middle between POF and glass fibers lie PCS concerning the complexity of the plug-in connectors. An overview of the available plug-in connectors is provided by [Schö03]. The most important connectors with typical values are listed in Table 3.1. Table 3.1: Connectors for PCS according to [Schö03]

F05 crimping/cutting Dtyp = 1.5 dB FR = 8 lbs

F07 crimping/cutting Dtyp = 1.5 dB FR = 15 lbs

V-Pin crimping/cutting Dtyp = 2.0 dB FR = 10 lbs

SC/PC glueing/polishing Dtyp = 0.6 dB FR = 20 lbs

SMA ST FC/PC crimping/cutting glueing/polishing crimping/cutting Dtyp = 1.1 dB Dtyp = 0.6 dB Dtyp = 0.6 dB FR = 40 lbs FR = 40 lbs FR = 40 lbs Dtyp : typical connector loss, FR pull out force (1 lbs = 0.4536 kg)

3.2 Basis for Calculating Connector Losses

259

3.2 Basis for Calculating Connector Losses 3.2.1 Calculation of Connector Losses with Uniform Mode Distribution The following illustrations schematically show the different causes for connector attenuation. In all cases, Uniform Mode Distribution (UMD) is assumed. This means for a step-index profile fiber that both the near field as well as the far field are constant within the acceptance range as shown in Fig. 3.55. In a realistic POF, equilibrium mode distribution (EMD) is not identical with the UMD because mode dependent attenuation must also be considered. However, the calculation is initially made easier by assuming UMD. rel. level

rel. level

near field

-600 -400 -200

0

+200 +400 +600

far field

-30° -20° -10°

distance to the fiber axis [μm]

0

+10° +20° +30°

angle to the fiber axis [°]

Fig. 3.55: Near field and far field under UMD conditions

3.2.2 Differences in Core Diameter The first process under consideration concerns the difference between the core diameters of the POF deployed. In Fig. 3.56 below it is assumed that the light propagates from the left to the right. Apparently no loss arises where the output fiber (on the right, shown in blue in the front view) is larger than the input fiber (on the left, shown in yellow in the side view). However, if the output fiber is smaller, there is a loss of part of the light (for a more detailed description see [Schw98]).

Fig. 3.56: Connector loss through differences in core diameter

For UMD, it is easy to calculate the amount of attenuation. It corresponds exactly to the proportion of the excess circular area of the input fiber:

260


Area of the input fiber: Area of the output fiber: Attenuation:

A1 = d12S/4 A2 = d22S/4 D = 10 log (A1/A2) = 10 log (d12/d22)

F the values 931 μm to 1,029 μm, permitted in the ATM Forum this is a loss of 0.59 dB. The current standard EN 60793-2-40 allows a core diameter of 920 μm to 1,040 μm in class A4a, corresponding to permissible coupling losses of 1.06 dB. The user does not need to get nervous, however. The standard is limping far behind the technical development. Good manufacturers today attain tolerances considerably below 10 μm. 3.2.3 Differences in Numerical Aperture The difference in numerical aperture has a very similar effect. If the input fiber has a smaller aperture, the light is completely guided by the output fiber. However, if the NA of the output fiber is smaller, there will be some losses as shown in Fig. 3.57.

Tmax2

Tmax1

Fig. 3.57: Connector loss through differences in NA

When calculating the losses under UMD conditions, one looks at the solid angles of the far fields, which can be equated to the far field angle if the NA is not too large. Solid angle for input fiber: Solid angle for output fiber: Attenuation: Expressed by NA:

Tmax12S Tmax22S D = 10 log (:1/:2) = 10 log (Tmax12/Tmax22) D = 10 log (AN12/AN22)

For the worst case of the ATM Forum specification (AN = 0.35 or 0.30) this results in the loss of 1.34 dB, whereas the ATM Forum specification permits only 0.8 dB for the sum of losses from core diameter and NA differences. This difference can be explained by assuming EMD conditions where NA differences are far less critical. In EN 60793-2-40 the permissible values for the numerical apertures for the fiber classes A4a to A4c lie between 0.35 and 0.65, i.e. the actual NA has not yet been specified. For example, there is a theoretical loss of 5.38 dB with the coupling of a POF with an AN = 0.65 and full launch on a fiber with AN = 0.35. Evidently this relationship was unknown to the authors of this standard. The NAs of


261

the fibers of important manufacturers are steady between 0.47 and 0.51 which results in the possible coupling losses dropping to 0.71 dB. Under real conditions the mode distributions of the fibers are even closer together so that coupling losses are typically around some tenths of a dB. Figure 3.58 shows the theoretical losses in dependence of the relationship of the NA of the input to the output fiber. 6 5 4

coupling loss [dB]

3 2 1 0 0.8

NAin/NAout 1.0

1.2

1.4

1.6

1.8

2.0

Fig. 3.58: Coupling loss dependent on NA-ratio

3.2.4 Lateral Offset of the Fibers In the case of a lateral offset of fibers, the calculation of connector losses is just as simple (Fig. 3.59). Where distance x is not too great (compared with diameter d), the following applies: D = 10 log (A1/A2) = 10 log [(d²S/4)/(d²S/4-dx)] = 10 log [1/(1-4x/dS)]

Attenuation

x Fig. 3.59: Attenuation in the case of a lateral offset of fiber axes

The ATM Forum specification permits a lateral offset of x = 100 μm. At a core diameter of 931 μm this would result in approximately 0.64 dB (specified are 0.4 dB); for 980 μm it still is 0.6 dB. [FOP97] mentions the following precise formula, except that in the reference the last fraction has been printed in the reverse: D

ª S - 10 log « « 2 ¬ S - 2 arcsinx d - 2 x d 1 - x d

º » » ¼

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At a core diameter of 980 μm and an offset of x = 100 μm, one also obtains 0.60 dB which means that the approximation formula is completely adequate. Figure 3.60 shows a measurement example of the coupling attenuation with the lateral shifting of two fibers against each other. In this case a 1 mm standard POF and a 1 mm glass fiber bundle were used. Almost identical curves resulted for both fibers. In contrast to the theory a rounded curve can be seen around the zero point. The reason for this is that in the measurement the fibers did not touch each other exactly but were spaced about 100 μm apart. Consequently, the light spot is somewhat larger at the output fiber and very small shifts do not yet result in any losses. 5

loss [dB]

4

theory POF-POF GOF-GOF

3 2 1 0 -0.5

lateral misalignment [mm] -0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

Fig. 3.60: Excess loss due to lateral misalignment

3.2.5 Losses due to Rough Surfaces Another cause for attenuation at connectors is shown in Fig. 3.61. The surface of the POF can be rough due to processing, for example, grinding. This changes the light path and part of the power is lost through diffraction or scattering. The ATM Forum specifies 0.1 dB of losses since the POF surface should be of good quality due to hot plating.

Fig. 3.61: Connector attenuation due to roughness of the fiber's end faces

A specific problem of rough surfaces is that the light does not necessarily have to be lost immediately at the coupling spot. Rather, a part of the light can be converted into a large propagation angle. By means of mode-dependent attenuation it can then be lost little by little in the following meters.


263

3.2.6 Losses through Angles between the Fiber Axes Finally, Fig. 3.62 shows a deviation of the fiber axes as a possible cause for attenuation. Here, the ATM Forum allows a deviation of max. 1° corresponding to a loss of 0.1 dB.

H

Fig. 3.62: Connector attenuation caused by an angle between the fiber axes

This value is also relatively easy to calculate for small angles compared with the acceptance angle. The far field comprises an angle-range :, which approximately equals : = Tmax2S, provided that the NA is not too large. For an angle H (small against Tmax) between the axes, a solid angle area 2TmaxH is obscured. The following applies: D = 10 log [(Tmax2S)/(Tmax2S - 2TmaxH )] = 10 log [1/(1- 2H/TmaxS)]

Attenuation:

For an AN = 0.30 the acceptance angle of Tmax = 17° which results in an attenuation of 0.16 dB, again slightly above the specified value. However, under EMD conditions the additional loss to be expected is significantly smaller. A measurement example from [Schw98b] is shown in Fig. 3.63. Two standard POFs were tilted at angles up to 40º with the fiber spacing kept as low as possible. 10 coupling loss [dB] 9 8 7 6 5 4 3 2 1 0 -40 -20

St.-NA-POF theory 520 nm 570 nm 650 nm

0

20

40 angular misalignment [°]

Fig. 3.63: Coupling loss because of angular misalignment of the fibers [Schw98b]

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The POFs were about a meter long. The losses measured are clearly lower than the theoretical ones under UMD conditions. Under normal conditions there are hardly any modes at the higher angles. Small angle differences at the couplings do not immediately lead to initial losses. The measurement was conducted with three different wavelengths. The diverse results can be explained by the different far fields of the LEDs used. It also shows how important it can be to define the exact measurement conditions when stating connector losses. A detailed segment of the area tilted up to 10º is shown in Fig. 3.64. Here once again two POFs and two glass fiber bundles were used. You can easily see that the angle errors of 1º to 2º are perfectly tolerable for POF connectors. A diameter tolerance of e.g. 100 μm can be allowed with a 10 mm long connector holder. 1.0

loss [dB] theory POF-POF GOF-GOF

0.8 0.6 0.4 0.2 0.0 -10

-8

-6

-4

-2

0

2

4 6 8 10 angular misalignment [°]

Fig. 3.64: Excess loss caused by angular misalignment

3.2.7 Losses through Fresnel Reflection The value for the Fresnel losses results from the difference in refractive index for air and PMMA. One can always assume that between POF connectors there is an air gap between the cores which is substantially larger than the wavelength. Due to the large mode number and the usually incoherent sources used, interference effects can be neglected so that the reflection losses can be calculated in the same way as for independent PMMA/air transitions (see Fig. 3.65).

nPMMA = 1.49

first reflection Fig. 3.65: Loss by Fresnel reflection

second reflection

nair = 1.00


265

For a vertical incidence, the reflection coefficient compared to air is as follows: R

§ n -1· ¨ ¸ © n 1¹

2

0.04

T

1- R

The loss is therefore 0.35 dB for two interfaces (see also [Wei98]). In reality, the beams propagate not only vertically but also at different angles against the fiber axis so that the Fresnel losses are somewhat larger. The ATM Forum specifies a value of 0.3 dB. Normally, just as big a loss occurs with glass fibers. This is avoided through socalled physical contact (PC) in which the ferrules of both connectors are cut cambered so that the surfaces are round. Both fibers are pushed onto each other by springs in such a way that the gap in the core area completely disappears - or is at least small in regard to the light wavelength. Since glass is not very elastic, extremely high pressures are necessary. However, since the contact area has a diameter of only a few 10 μm, forces of a few Newtons are sufficient (principle in Fig. 3.66).

core cladding ferrule

F

F

Fig. 3.66: Principle of the PC connector

Still, a pressure of 70,000 bar is reached with a pressure force of 5 N and the contact surface of a 30 μm wide diameter. With polymer fibers the area is larger by a factor of 1000. Polymer is indeed somewhat softer, but much greater absolute force is necessary for POF. This is problematical because you cannot apply the forces over the soft cladding as you wish. You have to assume that there will always be an air gap over the greatest part of the cross-section in POF connectors. 3.2.8 Losses through Axial Distance of the Fibers Another loss mechanism described here has not been covered in the ATM Forum specification. In practice, the gap between the two fiber cores is often significant for fiber attenuation. Figure 3.67 shows the mechanism for a distances.

266


s Fig. 3.67: Connector attenuation caused by distance between fibers

According to [Wei98] the resulting loss is as follows: D

-10 log 1 - 2 s A N / 3 n d .

For d = 980 μm and s = 200 μm at AN = 0.30 the result would be 0.09 dB, as an example. This effect is of particular significance in the case of hot-plate connectors. With these connectors the excess material is pressed into grooves on the side. This destroys the core's light guiding capability for a short distance. As an approximation, this effect can be numerically regarded as a gap equivalent (Fig. 3.68).

s Fig. 3.68: Connector attenuation for connectors with a groove

Usually these grooves are approximately 0.2 mm deep, resulting in a distance of the intact fibers of 0.4 mm. A loss of 0.4 dB results for standard NA POF, whereas for DSI the value is 0.25 dB. An exact formula is given in [FOP97]: Į

2 ª§ s · º - 10 log «¨1 2 tan ș max ¸ » d «¬© ¹ »¼

For AN = 0.30 we obtain Tmax = 19.4°; since the groove is filled up, Tmax = 12.9° applies. At the stated values and a distance of 400 μm the calculated loss is 1.98 dB. The two values are significantly different. In [FOP97] the assumption was that the light will be evenly distributed on a circular disk expanding with the max. angle. Even for small distances this assumption is not adequate. We propose


267

here to use the following formula for small distances and UMD, assuming a linear decreasing light power for an over radiated ring and an under radiated ring (see Fig. 6.38). A1 = d2S/4 A2 = dSsAN/n (to be weighted with ¼) D = 10 log ((A1+A2)/A2) = 10 log (1 + (s AN)/(nd))

Area of the input fiber: Spilled over area: Loss:

For the stated values this would be 0.33 dB which is near the value given in [Wei98]. This approximation formula should be suitable for distances that are significantly less than 1 mm.

s intensity distribution

Fig. 3.69: Proposal for calculating the attenuation caused by fiber separation

The actual losses for standard and DSI-POF are shown in Figs. 3.70 and 3.71. They are clearly below the theoretical line for both fibers (with UMD assumption). The reason for the deviation is again the strong suppression of the higher order modes in the POF. 1.2

excess loss [dB]

St.-NA-POF

1.0 theory 520 nm 570 nm 650 nm

0.8 0.6 0.4 0.2 0.0 0.0

distance [mm] 0.2

0.4

0.6

0.8

1.0

Fig. 3.70: Connector losses caused by fiber separation for St.-POF according to [Schw98b]

268


1.2 excess loss [dB]

DSI-POF

1.0 theory 520 nm 570 nm 650 nm

0.8 0.6 0.4 0.2 0.0 0.0

distance [mm] 0.2

0.4

0.6

0.8

1.0

Fig. 3.71: Connector losses caused by fiber separation for DSI-POF according to [Schw98b]

3.2.9 Losses due to Different Causes It is necessary to point out, however, that it is not always automatically possible to add the individual contributions in linear fashion. For example, losses due to differences in core diameter and lateral offset may cancel each other out in part, as illustrated in Fig. 3.72. Here, the most unfavorable case is an output fiber with a smaller diameter (by 108 μm) and an offset of 100 μm (Table 3.1).

154 μm

46 μm

Fig. 3.72: Simultaneous effect of offset and difference in diameter

For the above values the losses are 0.59 dB for the difference in diameter and 0.29 dB for the offset, now only entered x = 46 μm. A similar ratio applies to the relationship between angle error and NA difference. Nevertheless, under UMD conditions the losses remain significantly larger than specified. We do not yet have a comprehensive model for calculating EMD in SI and DSI-POF or even for the connector attenuation. This is another open task for the standardizing bodies. However, when assuming EMD instead of UMD, 2.0 dB is a realistic value. With continuous improvement in POF technology, one could hope for an improved situation due to reduction in fiber tolerances for the diameter and also for the NA. It is not possible here to say anything about the likely specification of attenuation in GI fibers. The data to be expected from the fibers are largely not yet known. However, the diameter is likely to be between 100 μm and 150 μm with an NA of around 0.20 so that the requirements for connector precision will be more exacting by several orders of magnitude.

3.3 POF Couplers

269

3.3 POF Couplers 3.3.1 Construction of POF Couplers Among the passive components for POF transmission systems, couplers play a significant role that is borne out by the availability of a complete line of products. Extensive descriptions of how to construct couplers can, for example, be found in [FOP97] and [Wei98]. In recent years, coupler constructions have been introduced in various studies such as: [Kal92], [Rog93], [Yuu92], [Woe93], [Yuu94], [Li96], [Agu97], [Fau98], [Sug99], [Kob99], [Ern00], [Kaw00] and [Woe94]. The following diagrams show different principle possibilities for manufacturing 1 : X couplers (one input with several outputs). Figure 3.73 shows a face coupler in which both output fibers are coupled with a butt-joint and directly to the input fibers.

Fig. 3.73: Principle of the face coupler

The advantage of this arrangement is the very straightforward structure. Additionally, the fibers run in parallel so that there is no large mode dependency. The minimal loss of an Y-coupler is 3 dB (independent of the use as a splitter or coupler). Additional losses are caused here due to the surface of the coupled-in not being fully utilized. Mathematically, these are at least 1.08 dB, which is very acceptable. Added to that are losses brought on by less than perfect faces, which can be minimized by suitable index matching. Figure 3.74 shows a coupler with an optical waveguide element. It can be manufactured with injection molding equipment and subsequently cast with the fibers (for example, [Rog93]). The rectangular waveguide cross-section is easy to manufacture and does not create excessively high losses (0.48 dB).

Fig. 3.74: Principle of the Y-coupler with waveguide element

270

3.3 POF Couplers

With polished couplers (Fig. 3.75), matched polished fibers are glued in such a way that there are no protruding areas. Additional losses are created in particular by sudden changes in the guided angle range. These must be minimized by providing sufficient flat polished angles. At any rate, a certain dependency of the additional attenuation on the launch conditions remains.

Fig. 3.75: Principle of the polished coupler

If a coupler has several ports, polished couplers become too complex. Even planar mixing elements do not offer enough uniformity of attenuation to the outputs. Figure 3.76 illustrates a 1 : 7 coupler with cylindrical-mixing element. With 7, 19, 31 etc. output fibers, the excess loss is not that large.

Fig. 3.76: Principle of the coupler with mixing cylinder

To improve the uniformity of the attenuation, a curved element can be used instead of a straight mixing cylinder, which then functions as a mode mixer (Fig. 3.77 according to [Woe93a]).

Fig. 3.77: Principle of the coupler with curved mixing cylinder

3.3 POF Couplers

271

Good results can be attained at a bend of 180° with a radius of several 10 mm. The last example in Fig. 3.78 shows a coupler according to [Fau98] for which the mixing element is a hollow funnel with a wall thickness corresponding to the fiber thickness. Here too, the goal is - apart from the good uniformity - the reduction of mode dependency.

Fig. 3.78: Principle of the coupler with mixing element in a conical form

Further possibilities for the coupler design are polished couplers in which curved fibers are polished laterally or simply flat mixing elements. Melting couplers are based on welding fibers to couplers that are drawn in the coupling zone to adapt their diameters. Finally, couplers with reflecting elements are also conceivable. 3.3.2 Commercial Couplers The need for POF couplers is relatively small since there are not yet any largescale applications. Up to now couplers have been used in the field of measurement techniques and for special sensors. POF couplers for bidirectional transmission on one fiber could find wide-ranging applications in the future. Since POF couplers are relatively simple to produce, a number of manufacturers have attempted to enter the field over the years but none of them have been able to become well established. At least you can get these components today, but only if you order a certain minimum amount which is then made to order. Figure 3.79 shows two examples of a Y-coupler commercially available at the end of the 1990s. Another coupler in a 16 u 16 configuration is illustrated in Fig. 3.80 (such components were conceived of for passive star networks in mobile applications).

Fig. 3.79: POF-Y couplers (left Nichimen, right Microparts)

272

3.3 POF Couplers

Fig. 3.80: POF- 16 u 16 splitter (Nichimen, [Nich00])

Another coupler in 1 : 4 construction is shown in Fig. 3.81. This construction shape is manufactured with different fibers to specific customer’s wishes, including DSI-POF. They have been used for example in the POF multiplexers at BAM and at the POF-AC.

Fig. 3.81: 1 : 4-coupler by Nichimen

The Leonhardy Company has developed a coupler with a special waveguide (Fig. 3.82). This technology allows a reasonably priced series production and the first series have already been produced (see also [Fei01b]).

Fig. 3.82: Coupler structure from Leonhardy

3.3 POF Couplers

273

A very simple coupler for use in audio networks is sold by Hama in electronic specialist stores (Fig. 3.83). Four fibers with a diameter of 500 m μ are arranged in the component and distributed over the two outputs. The connections correspond to a F05 connector.

Fig. 3.83: POF-coupler by Hama

3.3.2.1 Polished Coupler from DieMount The principle of the polished coupler was described shortly above. Although it first appears to be relatively simple, the optimal conversion places great demands on the procedure. In addition to the optical quality of the boundary layers it is above all a question of a suitable shape of the piece of cut fiber. The DieMount company in Wernigerode has been selling POF couplers based on this principle for some years now [Kra04b] and [Kra05a]. The front part of a 1 mm POF cut to 50% is shown in Fig. 3.84.

Fig. 3.84: 50% polished POF (DieMount)

Fig. 3.85: POF polished coupler (DieMount)

274

3.3 POF Couplers

The typical additional losses of couplers manufactured in this way are below 2 dB including all interface losses. A frequency diagram of the measured additional losses is shown in Fig. 3.86. 24

no. of samples

20 16 12 8 4 0

0.9

1.1

1.3

1.5

1.7

1.9

2.1 2.3 2.5 excess loss [dB]

Fig. 3.86: Frequency distribution of the excess losses of POF-couplers

In addition to couplers for 1 mm SI-POF components for thinner fibers or GI fibers can be produced. Deviating splitting ratios, for example 80 : 20, can also easily be realized. 3.3.2.2 Moulded Couplers from IMM The Institute for Micro technology Mainz (IMM) introduced the production of passive components for POF and PCS on the basis of waveguides in [Klo03] and [Fre03]. The necessary moulds are produced by LIGA technology. The subsequent production of a coupler by filling the waveguide structure with a UV-hardening polymer is shown in Fig. 3.87.

Fig. 3.87: Fabrication of a waveguide coupler at IMM ([Klo03])

The finished component with a coupled 1 mm POF is shown in Fig. 3.88. In the experiment an average additional attenuation of 2.8 dB and a uniformity - in reference to the outputs - of better than 0.4 dB for the complete component was attained.

3.3 POF Couplers

275

Fig. 3.88: Finished waveguide coupler for 1 mm POF at IMM ([Klo03])

The disadvantage of this procedure is that it is only financially worth it when a large number of pieces is produced because of the relatively high tooling costs. Also, a certain additional loss cannot be avoided because of the crossover from round to rectangular waveguides and back. Nevertheless, the procedure promises reasonably priced production and good reproducibility when very large numbers of pieces are produced. 3.3.2.3 Waveguide Couplers from the University of Sendai A comparable attempt to produce POF couplers by means of polymer waveguides is described in [Miz06]. As opposed to the methods described above the authors use a simple photo-resist pattern (SU-8) for producing the waveguide structures. The losses in the waveguides produced lie at around 0.2 dB/cm at a wavelength of 650 nm. The optimal additional loss for the insertion of the waveguide in a 980 μ m POF route of 1.6 dB is the result with a waveguide cross-section of 900 u 900 μ m². A further additional loss of 1.0 dB results for the coupling structure.

Fig. 3.89: Detail of the coupler waveguide structure according to [Miz06]

276

3.4 Filters and Attenuators for POF

3.4 Filters and Attenuators for POF 3.4.1 Filters Optical filters have to fulfill numerous tasks in transmission systems and sensor applications. They can, for example, serve the purpose of suppressing interfering light or reducing near crosstalk in WDM systems. The different arrangements for multiplexers and filters are described in detail in Chapter 6. Fundamentally, all filters fall into two categories. With interference filters, which include optical grating, dielectric multi-layer structures and interferometers, a certain wavelength range can pass through while the rest is reflected. By means of correspondingly complex structures you can attain almost any desired spectral curve. Such components can also be used for multiplexers. Suitable additions to dye filters absorb undesired light and the spectral curves depend on the available dyes. These filters normally have worse parameters, but are many times over simpler and more reasonably priced. They are well suited for suppressing interfering light and have one great advantage in that they work independently of the angle of incidence. Table 3.2 summarizes some important characteristics. Table 3.2: Properties of optical filters Filter Type

Interference Filter

Dye Filter

principle

reflection transmission

absorption transmission

smallest spectral width

10 Gbit/s. These slip rings have to have a large, free inner diameter since they are primarily intended for use in medical applications (computer tomography). The POF-AC has been working together with the BLZ and the Schleifring and Spinner companies on this project. Fig. 3.117 shows one of the realized principles with which light is coupled into a halved optic fiber by means of a prism coupler. When in use, this prism head moves at several meters per second and glides at a distance of 0.1 μm over the optic fiber. The requirements here are comparable to those of DVD players of the next generation such as Blue-ray. The fiber used is a PF-GI-POF. On the one hand the high mode dispersion of the SI-POF can be eliminated and on the other long wave lasers from communication technology can be used.

3.6 Optical Slip Rings

291

Fig. 3.117: Prism head for light coupling

A demonstrator with a model about one-third the size was set up at the POF-AC which is supposed to prove the function. The central element next to the floating prism is the polymer fibers made of CYTOP® with graded index profile which are halved lengthwise. The principle and a photo of the model shown here with visible light can be seen in Fig. 3.118.

Fig. 3.118: Optical slip ring

292


3.6.5 The Mirror Groove Slip Ring The most recent development of the joint project between the POF-AC and the Schleifring und Instrument-Making Co. is a transmission system based on a mirror groove [Schl06] and [Schi07]. A laser beam is radiated at a very small angle into a circular slot. Figure 3.119 shows the principle of light guiding by repeated reflections and the rotating head for coupling in the light.

Fig. 3.119: Principle of the reflecting groove slip ring and the movable head for light coupling

Since the mirror groove has high reflectivity over a wide range, almost as many wave lengths as you wish can be used. Should you wish to use telecommunication components, i.e. lasers with 1.55 μm, then very high data rates can be achieved. At the same time the singlemode fiber connection of these components generates a very good collimated beam which hardly increases along the extent of the ring. The cross-section of the groove and the prototype of the entire system from [Schl06] are shown in Figs. 3.120 and 3.121.

Fig. 3.120: Cross section of the reflecting groove and the complete transformer


Fig. 3.121: Complete rotating slip ring (with electrical contacts in the foreground)

According to data from [Schl06] the following parameters can be realized: ¾Diameter: ¾data rate per channel: ¾data rate per channel: ¾higher data rates ¾max. rotation speed: ¾bit error probability:

0.6 m to 2 m 10 Gbit/s (currently) 40 Gbit/s (in development) through WDM and/or parallel channels 300 RPM < 10-12

293

4. Active Components for Optical Systems

In this chapter we will deal with active components, the most important components for transmission systems. Except for a few special applications semiconductor components are employed. First, we wish to talk about the theoretical basics, then summarize the most important types and structures of emitters and receiving diodes. Finally, we will describe the components available. Just as with fibers, technical development in the field of optoelectronic components is proceeding at such a rapid pace that we can only present a snapshot of developments here.

4.1 Emitters and Receivers An optical transmission system essentially consists of three components. The transmitter converts the electric sequence of signals into an optical one and inputs it into the optical transmission channel, in this case the polymer optical fibers. The transmission channel, which may contain further active or passive components in addition to the fibers, forwards the signal to the receiver. Here the signal is converted back into an electric signal that is then available for further processing. Usually, the goal is to make the electric signal received as similar to the starting signal as possible. Of course, the transmitter and the receiver play an important role in the process since they are primarily responsible for converting the signal (optical or electric voltage). At first, we will attempt to describe the possible transmitter elements. Today, semiconductor diodes are used nearly exclusively for optical communications. The reasons for this are: ¾Very small construction (considerably smaller than 1 mm³) ¾Very fast switching times (a few ns to less than 1/10 ns) ¾High efficiency (over 50% possible) ¾Virtually any wavelengths (from 200 nm to 10,000 nm) ¾Small spectral width of emission ¾Small emission angle ¾Small emitting surface (results in efficient coupling into the fibers) ¾Long service life and good reliability ¾Very large application temperature range ¾Economical to manufacture and process

296

4.1 Emitters and Receivers

Other laser sources are used only to a small degree in optical free-space connections or optical heterodyne systems. This is why the subject will not be further discussed here. Because of their low modulation speed and their size, none of the thermal light sources available today are suitable for optical communication. Organic light sources may become interesting alternatives particularly because of their potential for being manufactured inexpensively. 4.1.1 The Principle of Light Generation in Semiconductors Semiconductors differ from metal and insulators through their band structure. A valence band completely occupied with electrons in the ground state (0 K) is separated from an empty conduction band by an energetic gap (the band gap) having a width of WG. If an electron is lifted into the conduction band through thermal activation, a hole remains in the valence band (it is more efficient to fill the valence band with holes and the conduction band with electrons by using an external electrical source, i.e., through injection). Both particle types can recombine with each other, whereby a photon having a frequency of f = WG/h is emitted, and the electron falls back into the valence band. The band gap is dependent on the material and the state of motion (impulse) of both particles. The impulse p of a particle can be expressed as p = h/O = h k/2S. With direct semiconductors, the maximum of the valence band and the minimum of the conduction band (this is where the charge carriers tend to concentrate) are located directly on top of each other, i.e., at the same impulse value (Fig. 4.1, left illustration). Thus, recombination can occur very efficiently by emission of photons. With indirect semiconductors, the extreme values are found at different k-values (Fig. 4.1, on the right). During recombination, the electron must change its impulse, which is achieved through interaction with a phonon. Since three particles must interact, the radiating recombination is less probable and inefficient. Only a small number of the well-known semiconductors possess a direct band structure. With mixed semiconductors, i.e., a combination of more than two elements, there are often only certain areas in which the material is direct. Indirect semiconductors can also be used as light sources; nevertheless, they are inefficient and slow and will not be taken into consideration here. Some green LEDs, GaP for example, are produced from indirect semiconductors in which socalled deep defects are embedded through which the light is emitted. Recently, even silicon, the best-known indirect semiconductor was stimulated to emit light. In addition to their extremely low power efficiency these emitters are as a rule still too slow.


direct semiconductor

indirect semiconductor

W

---

297

-- - -

W - photon

conduction band

-

phonon

- - - photon

WG + + + ++ + + +

+ +++ ++ + +

k

k valence band

e.g. Si

e.g. GaAs Fig. 4.1: Direct and indirect semiconductors

To get an emission from a direct semiconductor, it generally suffices to provide current to it. In other words, to inject the one side with electrons and the other side with holes. However, this procedure is not very effective. The first light sources did not become possible until the p-n junction was developed. Semiconductors can be doped. Atoms of the material are replaced by other nuclei with excess or missing electrons. On the interface layer between a p-zone (with missing bonding electrons) und an n-zone (with excess binding electrons), the not binded electrons move into the empty holes thus creating a depletion zone (a few μm wide).

Eg [eV] 2.5 GaP

GaAs

InP

InAs GaSb

O [nm] 0,.5

AlAs

2.0 A lS b 1.5

GaAs

InP

1.0

1.0 GaSb 0.5

direct semicon. indirect semicon.

0.0 5.5

5.6

5.7

5.8

0.75

InAs lattice constant [Å] 5.9

6.0

6.1

Fig. 4.2: Lattice constants and band gaps of various semiconductors

1.5 2.0 3.0 5.0

298


When current is injected, it is exactly this zone that forms an area in which there is a large concentration of holes and electrons at the same time. Another effect is that, through doping, conductivity is increased by a high order of magnitude in contrast to a pure semiconductor. Figure 4.2 illustrates the range in which many optical semiconductors are direct. The colored strips mark the attenuation windows of the PMMA-POF. The material system (AlxGa1-x)yIn1-yP is of particular interest (band gap and lattice constant are shown in Fig. 4.3). With 50% part indium the semiconductor is lattice-matched to GaAs. Theoretically, the wavelength range of 525 nm to 656 nm can thus be covered, i.e. there where POF has its minima.

band gap [eV] 2.5

AlP

2.36 eV

indirekt

wavelength [nm] 500

AlxIn1-xP

2.3

GaP GaxIn1-xP

(AlxGa1-x)0,5In0,5P lattice matched to GaAs

2.1

600

direct

1.9

1.89 eV

550

AlxIn1-xP

650 700

1.7

750 800

1.5

GaxIn1-xP GaAs

1.3 5.4

5.5

AlP: Eg = 2.45 eV A0 = 5.4510 Å GaP: Eg = 2.26 eV A0 = 5.4512 Å

5.6

5.7

850 900 InP 950 5.8 5.9 lattice constant [Å]

Eg = 1.35 eV A0 = 5.8686 Å GaAs: Eg = 1.424 eV A0 = 5.6533 Å InP:

Fig. 4.3: Lattice constants and band gap energies in the AlP/GaAs material system ([Li05])

Figure 4.4 shows the corresponding lattice constants and band gaps for the GaN semiconductor system. In this case there is unfortunately no suitable, i.e. latticematched, substrate material. Usually, SiC as conduction material or sapphire as isolator is used. In both cases an interlayer has to be inserted in which the lattice constants have to be adapted to the LED material. This layer is full of defects and warping which are luckily electrically neutral unlike AIIIBv semiconductors. Only in the past few years has there been a theoretical explanation for this phenomenon.

299


The figure shows another necessary requirement for the construction of a luminous component. The various layers are applied to a substrate through different processes (epitaxy). The substrate is a carrier made of semiconductor material that was cut from a mono-crystal block (with a typical thickness of 100 μm to 300 μm). On the one hand, this carrier makes it possible to handle the otherwise extremely thin layer structure; on the other hand, it determines the crystal structure of the fully grown semiconductor layers.

6.5

wavelength [nm]

band gap [eV]

AlN: Eg = 6.20 eV A0 = 3.112 Å GaN: Eg = 5.35 eV A0 = 3.189 Å InN: Eg = 1.89 eV A0 = 3.544 Å

AlN

6.0 5.5 5.0 4.5

200

250

300

4.0

350

3.5 GaN

3.0

400

2.5

500

2.0 1.5 3.0

3.1

InN

ZnO

SiC

3.2

3.3

3.4

3.5

600 800 3.6

lattice constant [Å] Fig. 4.4: Parameters of the GaN material system

If the lattice constants of the substrate and the layers are perfectly identical then no forces arise in the crystal. Should the lattice constants differ to only a slight extent the layers applied are slightly deformed (strained). This lattice misfit can usually amount to a few parts per thousand. If it becomes larger, dislocations occur which can seriously influence the functioning of the component.

300


layers with matched lattice constant

layers with a small lattice mismatch stress is induced

layers with strong lattice mismatch dislocations occur

Fig. 4.5: Lattice (mis-)matching in semiconductors

The process of lattice matching is schematically illustrated in Fig. 4.5. A matched lattice element can be seen at the left. A strained layer is shown in the middle, and dislocation has occur on the right. In many cases, the band structure at such a dislocation is so strongly affected that charge carriers are lost without creating light. At first this reduces the efficiency and causes local heat development. The lattice defect can expand to such an extent that the component can be useless. Substrates cannot be easily manufactured from any semiconductor. Mixed semiconductors in particular are hardly ever used. Materials that are frequently used are for example: Si, GaAs, InP or Al2O3 (sapphire, for InGaN). The conductivity of the substrate material is also very important. If it is poor, both contacts must be created on top of the chip structure layers. 4.1.2 Structuring Semiconductor Components The properties of the semiconductor source are defined mainly by three essential design parameters. The choice of semiconductor material essentially determines the emission wavelength, as shown above. Furthermore, the choice between a direct semiconductor and indirect semiconductor is critical for efficiency and the modulating characteristics. Thirdly, there are various possibilities for spatially structuring the diode whereby the carrier density also can be influenced such as the light path within the component and the decoupling of the emission. A number of these structuring possibilities will be described below. Semiconductor diodes consist of a layer sequence of various materials in order to make light generation efficient. Thus, several requirements must be combined: The light emitting layer should have a band gap that corresponds to the desired wavelength. ¾ This material as well as the material of the other layers must be adapted to the substrate lattice. ¾ By doping, a p-n junction must be created near the light-emitting layer. All other layers must be good conductors. ¾It must be possible to manufacture all layers with an uniform process.

¾


301

Fortunately, many semiconductors can be mixed. For example, semiconductors can be manufactured from elements of the third main group (Ga, Al, In) and the fifth main group (As and P) by mixing each of their components in nearly any combination. These are known as AIII-BV-semiconductors (or in short as III-V semiconductors). Similar combinations can be found between elements of the second and sixth main group (II-VI semiconductors). One example of such a mixed semiconductor is the GaxAl(1-x)InyP(1-y). Both the lattice constant and band gap of this composition are dependent on the mixing parts x and y in a relatively complex way. By choosing a combination, the band gap can be selected in such a way that a lattice adaptation to a particular substrate such as InP is nevertheless ensured. This material is hampered by a number of physical and technological problems that limit the actual possibilities. Nevertheless, a whole line of successful material systems have been established that cover a wide spectral range from ultraviolet to the middle infrared wavelengths. Continued efforts to increase the carrier concentration in the active layer (the luminous layer) have led to a single and double hetero structure. With the latter, the active layer is enclosed by two cladding layers with an enlarged band gap (see Fig. 4.6). The charge carriers accumulate in the active layer and the efficiency is thus increased. energy W

Wg1 Wg2 Wg1

n-doped semiconductor band gap W g1

semiconductor with a smaller band gap W g2

p-doped semiconductor band gap W g1

Fig. 4.6: Double hetero structure principle

In laser diodes, it is not only the different band gap that is interesting, but also the different refractive indices. The intermediate layer forms an optical waveguide that also enhances efficiency. This effect does not play a role for surface emitters. Other possibilities for a vertical structuring are layers for a separate optical guide or the use of so-called quantum wells. Here, the layer thickness is reduced to the point that the charge carriers can only move in a single plane. This radically changes the band structure. Practice has shown SQW and MQW structures (single and multi quantum well) to have a high degree of efficiency and very stable wavelengths. However, they require very costly processes and cannot be implemented with just any material.

302


4.1.3 Structures of Semiconductor Transmitters 4.1.3.1 Luminescence Emitting Diode The oldest and simplest form of a light emitting semiconductor is the light emitting diode (LED). It essentially requires two layers of the same semiconductor, which form the p-n junction, as shown in Fig. 4.7. This is where the light is emitted. The internal efficiency can be as high as >50% ([Kra99] and [Hop00]). Since there is no guide whatsoever for the light, it is emitted in all directions. Due to the large refractive indices of common semiconductors (n = 3.5), only rays that strike the outer surface at a near vertical angle can leave the component. Taking into account the refraction that occurs at the surface, an LED has approximately the properties of a Lambert emitter [Ziem01].

upper contact cladding layers active layer substrate

bottom contact Fig. 4.7: Structure of a LED

If a conduction substrate is used, one contact can be attached to the bottom and a second one to the top of layer package. LED are often glued to the layer package face down on the metal carrier. Since the substrate is transparent for the light emitted, it does not effect the radiation. The heat generated due to it’s function can be dissipated better so that the potential optical power increases. The component is finally placed into a transparent package, so that the semiconductor is not subjected to humidity and is also protected against mechanical impact. By creating a lens, the emitting characteristics can be modified in a suitable manner. 4.1.3.2 Laser and Super Luminescence Diodes Laser diodes have practically the same layer structure as LED, i.e., a p-n junction usually combined with a double hetero structure, grown in planes on the substrate. Lasers, however, operate at considerably higher carrier concentrations. This makes it necessary to reduce the volume, where light is generated (for example through a small electric contact and lateral current barriers). Above a particular current den-


303

sity, the stimulated emission is so strong that losses in the component are exceeded. The last requirement is that a resonant cavity must be made. In the most simple of cases, it is formed by two parallel cleaved semiconductor edges (FabryPerot laser diode). Figure 4.8 illustrates the principle structure.

resonator mirrors upper contact active layer substrate bottom contact Fig. 4.8: Structure of a semiconductor laser

A laser has a series of advantages compared to a LED. Because of the stimulated emission involved here, the external efficiency is considerably higher. The high carrier density results in high modulation speeds. Light is emitted from a considerably smaller surface in a smaller angle range than with LED. The laser wavelength is not only determined by the semiconductor but also by the resonant cavity properties. Whereas an LED has a spectral width measured in some 10 nm, lasers have only a few nm or even less for singlemode lasers. An adverse effect is the horizontal light emission, the presence of a laser threshold and the high temperature dependency of some of the parameters. A super luminescence diode (SLED) has practically the same structure and is often referred to as an ELED (Edge Emitting LED). At least one side has an antireflection coating so that no resonant cavity is created. The component is still operated above the transparency concentration so that stimulated emission predominates. Since this is most efficient in the direction of the active layer, the SLED emits laterally, just like an LD. For a very readable introduction to the fundamentals of semiconductor physics, see [BS00]. anti reflection coated end faces upper contact active layer substrate bottom contact Fig. 4.9: Structure of a super luminescence diode

304


4.1.3.3 Surface Emitting Laser Vertical Cavity Surface Emitting Laser Diodes (VCSEL) are components with fascinating properties offering a multitude of design possibilities ([Wip98], [Wip99]). The principle structure is again identical to a normal double hetero structure. To put the component into operation, the resonant cavity is structured perpendicular to the active layer and not in the same direction. Since the active layer is only a few tenths of a micrometer thick, the lower reflection coefficients from the edge emitters are not large enough. Instead, the light must traverse the layer several times in order to attain a sufficient light amplification. Figure 4.10 demonstrates the structure of a VCSEL.

upper contact upper mirror active layer bottom mirror substrate bottom contact Fig. 4.10: Structure of a VCSEL

To limit the current, the area of the light emitting surface is practically always restricted by a limiting device acting on the conduction area under the upper contact (aperture diameter is typically some 10 μm). Thus, the active volume is much smaller than with conventional lasers. This results in threshold currents that are in the range of a few mA but can also amount to 100 μA. This also limits the output power to a few mW. Efficiency is also as high as with the best laser diodes. For data communication with POF, a laser power restriction for eye protection is in effect that makes typical VCSEL power completely adequate. Advantages of VSCEL technology are: ¾ ¾ ¾ ¾ ¾

The change in wavelength with temperature comes to approx. 1/3 of the value for LED. The laser emits light perpendicular to the surface. This makes it easier to couple the light to fibers and test the components on the wafer. The threshold current is very low which makes the power consumption of the transmitter very small. The VCSEL emits light at a small emission angle that is nearly circularly symmetrical and is thus ideal for coupling into the fibers. The spectrum of a VCSEL is very narrow compared to an LED.


305

The biggest problem in producing VCSEL is the mirrors. At times, they must reflect more than 99% of the light. To achieve this, various layers of semiconductor materials are applied alternatively. For both mirrors, this can mean more than 200 additional layers. Unfortunately, the selection of suitable semiconductors for short-wave ranges is very limited. As will be shown in Chapter 4.2.5, VCSEL may be the ideal source for POF systems. In the meantime very powerful VCSELs are available in the 780 nm to 850 nm range which also permit speeds up to 12 Gbit/s in addition to a temperature range of over +125°C and high efficiency. In the 650 nm range, however, things look a bit different. Because of the diminished thermal conductivity - caused by the Al share - the maximum application temperature drops. Commercial 650 nm VCSELs can only be used up to about +45°C. Recently at the University of Stuttgart a CW laser operation at 70°C was also achieved for red VCSELs. 4.1.3.4 Resonant Cavity LED In recent years, resonant cavity LED (RC-LED) were introduced, for example, by Mitel and Infineon ([Ste98], [Stre98a], [Stre98b], and [Schö99a]). The structure is similar to that of a VCSEL (see Fig. 4.11).

upper contact upper mirror active layer bottom mirror substrate bottom contact Fig. 4.11: Structure of a RC-LED

The component operates above the transparency concentration in the active layer. This means that stimulated emission predominates. The reflectivity of the mirror is so small that no laser operation occurs. An RC-LED operates without threshold current; it can be modulated very easy. The spectrum is wider than that of a VCSEL but just as small temperature-dependent. The efficiency of all RC-LED that have been manufactured to date just amounts to a few percent. They are suited for modulating up to several 100 Mbit/s. Red RC-LEDs are already being used in components for the automobile industry as well as in home networking. In the GaN material system, however, no efficient semiconductor mirrors have yet been produced. Green RC-LEDs with alternative mirror techniques have been realized by the Firecomms Company.

306


4.1.3.5 Non-Resonant Cavity LED Although related by name, non-resonant cavity LED (NRC-LED) operate on an entirely different principle. They were developed by IMEC of Belgium in cooperation with the University of Erlangen in Germany. upper contact

oxid apertur

polyimide

surface with etch pit p-AlGaAs

buttom contact GaAs n-AlGaAs polyimide/ gold mirror Si-substrate

Fig. 4.12: Structure of an NRC-LED

In contrast to the RC-LED, a resonant cavity is not formed. No noteworthy stimulated emissions occur. The principle is due to the fact that the efficiency of the light output is increased in contrast to conventional LED. Normally, only light rays with an angle of less than 17° to the normal can escape the semiconductor with n = 3.5. The remaining light is reflected. If the bottom of the chip is also mirrored, the radiation may travel back and forth between the carrier and the chip’s top side. With NRC-LED, the top surface is roughened by means of selective etching (see e.g. Fig. 4.13 from [Här03]). Light is reflected diffusively. Thus, light of any angle has a high chance of escaping the chip after a small number of reflections. External quantum efficiency of over 50% is possible. An additional lateral limitation of the emitting volume increases the efficiency and permits modulation data rates of up to 2 Gbit/s (see Chapter 4.2.6).

Fig. 4.13: Textured LED surface ([Här03])

4.2 Transmitting Diodes for Data Communication

307

Whereas the first NRC LEDs were realized in the wavelength range around 800 nm they can also be produced in the meantime in the visible range. Comparable decoupling power efficiencies of > 50% can also be attained with other methods. Slanted sides of the LED chip have proven to be very effective.

4.2 Transmitting Diodes for Data Communication The following sections describe examples of transmitting diodes which have been developed for POF systems or at least can be used for such. There is a considerable difference between semiconductors for glass fibers and those for POF, namely, the price. The great number of lasers available today for 850 nm to 1.55 ȝm have for the most part been specifically developed for data communication. All other applications are in comparison relatively few. The number of pieces was still relatively small at the beginning of development, but laser diodes sometimes cost several DM 10,000. Today the components are much cheaper, but the number of pieces is also considerably higher. The situation for POF components is entirely different. In the beginning POF systems had to be able to compete in price with other large-scale technologies. Therefore the price span for transmitting diodes often only amounted to a few 10 ct. The development costs for new components cannot be shifted to the first product generation. Most manufacturers only want to initiate the development of new components when production figures amounting to several million pieces annually are guaranteed. As long as there are no optimized POF transmitters available the expansion of the technology is being hampered. Fortunately, many POF applications can accept component parameters in many areas so that LEDs or lasers can be used from different large-scale applications. In the chapter on POF systems many experiments were presented in which LEDs for lighting purposes or lasers for laser pointers and barcode readers could be used. New transmitters were first developed for MOST networks since their use on a large-scale was very probable. The situation in Germany should change with the introductory use of POF in 2006. The increasing demand for POF systems will initiate developments among the different manufacturers which will lead to falling prices and quickly improved parameters and consequently inspire greater use of POF. 4.2.1 Red LEDs and SLEDs Since approximately the middle of the 1980s commercial POF components on the basis of red LEDs and SLEDs have been sold. In the first few years it was primarily GaAlAs LEDs whose emission wavelengths lay in the 660 nm to 670 nm range. Through the use of quaternary semiconductors the wavelength was gradually better adapted to the 650 nm minimum.

308


Ring-LED at 650 nm An LED especially adapted for POF is described in [Dut95] and [Yam95]. The emission wavelength of the double hetero structure LED is 655 nm and optimized for the attenuation minimum of the POF. Figure 4.14 shows the LED structure.

contact

metal

isolation

SiO2

contact layer p-GaAs buffer layer

p-AlGaAs 2.0 μm

cladding layer p-AlGaInP 1.0 μm active layer

AlGaInP

0.1 μm

cladding layer n-AlGaInP 1.0 μm buffer layer

n-GaAs

substrate

n-GaAs

Fig. 4.14: Red LED with ring contact according to [Dut95]

The special adaptation of the structure consists in the ring-like contacts (outer diameter 65 μm) instead of a contact attached to the center, as is common for LED. This adapts the near field better to the fiber coupling. When directly coupling with a 2 mm ball lens to a 1 mm POF, 35% of the output power can be launched into the POF. The coupling efficiency can be increased to 70% by attaching a specially mounted plastic lens. The power can thus reach 1.7 mW at 100 mA diode current. The spectral width is 25 nm; the modulation data rate up to 156 Mbit/s. SLED at 650 nm Super-luminescence diodes enable faster modulation and have a smaller emission angle than conventional LEDs. They were used in different POF transmitters at the end of the 1990s, for example, by Hewlett Packard and NEC (see also Fig. 4.71). MOST-LED A low temperature dependence of output power is especially decisive for use in mobile networks. A red LED with a particularly small change in the output power (only 2 dB) in the range of 20ºC to 125ºC was introduced in [Baur02] (see Fig. 4.15).


309

rel. opt. power [dB] 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 20

power in the POF: 150 μW at 10 mA (without lens) switching time: 12 ns 40

60

T [°C] 80

100

120

Fig. 4.15: Very temperature stable red LED for MOST

4.2.2 Red Laser Diodes Laser diodes in the range around 650 nm have been produced for a number of years in great quantities. The most important applications are in CD and DVD drives, laser pointers and scanners. Truly great modulation bandwidths are not required here. Since the lasers do not have to work continuously in most applications, the specified life spans are not always suitable for applications in data communication. The main disadvantage is usually the type of construction which has not been optimized for coupling with fibers. In general, these lasers work with AIIIBV semiconductors like GaAlAs or AlInGaP. Laser Diode, 650 nm In [Hon00], a 650 nm laser was introduced on the basis of AlInGaP. At room temperatures, the threshold current is only 9 mA. The laser can be used up to +90°C, which makes it promising for use in motor vehicles. The active layer has a MQW structure (multiple quantum well). To set the emission wavelength to 650 nm, the active layer is strain-compressed (SC-MQW). Figure 4.16 shows the temperaturedependent P-I characteristics according to [Hon00]. The maximum efficiency of the laser is 0.83 mW/mA (corresponds to an external quantum efficiency of 43.5%). A life time of more than 3,000 h was specified for an output power of 5 mW at 90°C. In [Hir97], an SQW laser (Single Quantum Well) is described with a wavelength of 638 nm at room temperature. The attenuation of the PMMA-POF is there approximately 209 dB/km (compared with 132 dB/km at 650 nm). As emission wavelength increases with temperature, the decreasing optical output power and sinking POF losses at increasing wavelength compensate each other. This laser would thus be very well suited for certain POF applications.

310


Popt. [mW] 9

90°C 80°C 100°C 110°C 120°C

8 70°C

7

50°C

6 5

25°C

4 3 2 1

laser current [mA]

0 0

10

20

30

40

50

60

70

80

90

Fig. 4.16: P-I-T-characteristics of a 650 nm laser

The active layer of the component described consists of an GaInP quantum well (tension-strained) in AlInGaP barrier layers (pressure-strained) encased with (Al0,7Ga0,3)0,5In0,5P cladding layers. For lasers with a 600 μm long resonant cavity, the maximum possible optical output power is 72 mW at a threshold current of 38 mA and a wall plug efficiency of 1 mW/mA. An optical power of 30 mW can be achieved at up to 75°C in CW operation. At an power of 30 mW and 50°C temperature, the service life is over 1,000 h. The possible performance of red laser diodes today is demonstrated in [Ohy99]. The laser described there is designed for DVD applications whereby high output power and a long service life are expected at a low price. The 655 nm laser is based on AlInGaP. The active layer is formed through a pressure-tensioned MQW on a disoriented GaAs substrate. The resonant cavity length is 500 μm. At 80°C and a constant optical output power of 5 mW, a service life of 92,000 h can be achieved. An power of 5 mW can be achieved up to +115°C. At room temperature, the threshold current is 36 mA. MQW-Laser at 650 nm In [Oka98] various MQW lasers are described that are based on tension-strained active layers made from GaInAsP/AlGaInP. Threshold currents between 4.5 mA and 23.4 mA are attained at wavelengths between 654 nm and 659 nm with different resonator lengths and mirror coatings. Power values of 30 mW for up to +90°C are possible. A laser that was adapted just for POF is introduced in [Mor95]. The LD possesses an active layer made of AlInGaP with MQW structure. The lateral current confinement is achieved by etched wells. At a threshold current of 24 mA (room temperature), the wavelength is 650 nm. The maximum operation temperature is +80°C. The maximum modulation bit rate is more than 4 Gbit/s.


311

In a series of studies, a 647 nm laser especially developed by NEC is used (for example [Yam94], [Ish95b] and [Koi96c]). If a GRIN lens is used, this laser makes it possible to launch +6.1 dBm into a 420 μm GI-POF. Several experiments were conducted with a modulation rate of 2.5 Gbit/s. Commercial Lasers at 650 nm Different manufacturers offer 650 nm lasers in different types of construction. The following data refer to types which have been tested at the POF-AC for use in POF systems. None of the manufacturers specifies a modulation bandwidth. All the lasers could be operated with at least 1,200 Mbit/s when driven with a 50 ȍ generator over a Bias-T. Important characteristics of the lasers tested are summarized in Table 4.1. Table 4.1: Data of typical laser diodes Laser Diode

Wavelength

Power

Ith

Max. Bit rate

650 nm 650 nm 650 nm 780 nm

7 mW 5 mW 10 mW 5 mW

50 mA 12 mA 30 mA 35 mA

1,300 Mbit/s 2,200 Mbit/s 1,600 Mbit/s 2,600 Mbit/s

SLD 1133VL (Sony) SLD-650-P5 (Union Optr.) L-4147-162 (Sanyo) RLD 78MA (Rohm)

For the Sanyo laser the curve for laser wavelength in dependence on the temperature is shown in Fig. 4.17 (the information has been taken from the data sheet). In addition, the attenuation of a PMMA POF at the corresponding wavelengths is entered in the diagram. 666

laser wavelength [nm]

664 662

DPOF = 234 dB/km DPOF = 217 dB/km

660

DPOF = 199 dB/km

P0 = 10 mW

658

DPOF = 181 dB/km

656

DPOF = 166 dB/km

654

DPOF = 150 dB/km

652

DPOF = 140 dB/km

temperature [°C]

650 0

10

20

30

40

50

60

70

Fig. 4.17: Change of the wavelength with the temperature (Sanyo LD)

At 70ºC the wavelength changes up to 664 nm. Here the POF-attenuation has already increased by 100 dB/km. The temperature coefficient of 0.18 nm/K is determined by the material of the red laser. You therefore have to take care that in

312


the middle of the intended operating temperature range the emission wavelength lies at exactly 650 nm, or as close as possible thereto. The temperature-dependent power-current characteristics of one of the lasers (from Union Optronics) is shown in Fig. 4.18. Between 10ºC and 70ºC the threshold current almost doubles and the differential power efficiency becomes less. 9

Popt [mW]

8

10°C 25°C 40°C 50°C 60°C

70°C

7 6 5 4 3 2 1 0

ILD [mA] 0

10

20

30

40

50

60

70

80

Fig. 4.18: Optical power-current-characteristics of a 650 nm laser (Union Optronics)

The spectral width of the laser normally lies in the range of about 2 nm. Chromatic dispersion therefore plays no role with PMMA fibers when laser diodes are used. Examples of the spectra are shown in Fig. 4.19. The lasers are spectrally multimodal. In the diagram the modes have not been resolved since they only lie a few tenths of a nanometer apart.

Fig. 4.19: Spectra of a 650 nm laser (Union Optronics)


313

Parameters such as the relative intensity noise or the polarization characteristics do not play any role in POF transmission systems. What is important are the radiation characteristics. Without image-forming optics all of these lasers show an elliptical far field with an expansion of about 6º u 30º. Almost the entire power can be coupled into a standard POF (see far field in Fig. 4.20). 1.0 0.9

Prel. 5 mW 25°C

0.8 0.7 0.6 0.5

vertical

0.4 0.3 0.2 horizontal

0.1 0.0 -40

T [°] -30

-20

-10

0

10

20

30

40

Fig. 4.20: Typical far field of a 650 nm laser (Sanyo)

Laser diodes are supplied in diverse types of housing. The set-up in a TO-18 housing is quite common. The laser chip is placed in a sealed housing. A window with a diameter of 1 mm is located at the top about 1 millimeter away from the emitting surface (Fig. 4.21). At a far field width of ±30º the light spot at the window is still only 1 mm large and can thus be readily coupled into a POF.

Fig. 4.21: TO-18 housing for laser set-up

The three pins are used for the power supply of the laser and for connecting a monitor photodiode. Common cathodes, anodes or also series connections are possible. From time to time a lens is used instead of a plane window which either focuses the light beam or generates parallel light (for laser pointers).

314


4.2.3 Blue and Green LEDs A particularly rapid development in the last few years has been that of LEDs on a GaN basis. The emission wavelength of pure GaN lies in the blue range. By admixing aluminum the emission wavelength can be enlarged to about 560 nm. As opposed to conventional AIIIBv semiconductors there is no lattice matched substrate material. Today GaN LEDs are produced on a sapphire or SiC substrate. By using suitable coating procedures you can prevent any dislocations from arising despite the large lattice mismatch in the active area. Generally, GaN LEDs can be modulated fast - up to some 100 Mbit/s -, have high power efficiency and good reliability. LEDs for Lighting Applications Since the middle of the 1990s with the availability of blue and later green LEDs on a GaN basis the possibility of using them in data transmission over POF opened up even if these diodes were developed exclusively for lighting and displays. Different systems will be described in detail later on in Chapter 6. The LEDs from the Nichia Company were chiefly used during this period. The temperature-dependent spectra of the type NSPG525 are shown in Fig. 4.22. 1.0

rel. power

-40°C

0.9 0.8 0.7 0.6

-20°C 0°C

NSPG525 Nichia 525 nm at 20 mA

20°C 40°C 60°C

0.5

80°C

0.4 0.3 0.2 0.1 0.0 460

480

500

520

540

560


Fig. 4.22: Spectra of the green LED NSPG525

As opposed to red LEDs there is almost no wavelength drift, e.g. only 2 nm over a range of 120 K. The relatively large spectral width of about 45 nm is disadvantageous since the chromatic dispersion of the PMMA POF with green is considerably greater than with red.


315

Another example is illustrated in Fig. 4.23 with NSPG510. The spectral width of 35 nm for this type is somewhat smaller and the temperature dependence is even lower. The advantages when using such an LED instead of red transmitters can easily be seen. First of all, the attenuation of the POF is lower by almost a factor of 2 and secondly the temperature coefficients for wavelength and output power play almost no role. 1.2

rel. opt. power

1.0

temperature -20°C to +70°C

0.8

LED current 20 mA 0.6

Nichia NSPG510

0.4 0.2 0.0

wavelength [nm] 450

500

550

600

650

Fig. 4.23: Spectra of the green LED NSPG510

The relative change in the output power in dependence of the temperature for the NSPG500 is shown in Fig. 4.24. The power only changes about 1.1 dB in the -20º C to +70º C range which is typical for home applications. 1.0 power [dB] NSPG 500S 0.5 0.0 -0.5 -1.0 temperature [°C] -1.5 -60

-40

-20

0

20

40

60

80

100

Fig. 4.24: Change of the optical power with the temperature for a GaN-LED

In the meantime, Nichia has also produced LED samples which have been optimized for POF transmission. Essentially, the chip surface has been reduced in size in order to maintain a lower diode capacitance.

316


The emission spectrum at room temperature and the transmission function, measured with a 50 ȍ source, are shown in Figs. 4.25 and 4.26. The modulation width of this LED amounts to about 150 MHz. Even higher values should be attainable with peaking and low-resistance driving. Consequently, GaN LEDS are clearly superior to their red “cousins”. 1.0 Prel. 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 400

Nichia LED sample T = 25°C ILED = 20 mA

O [nm] 450

500

550

600

650

Fig. 4.25: Emission spectrum of the fast green LED

0 frequency response [dB] -2 -4 -6 -8 T = 25°C -10 I LED = 20 mA -12 Imod < 5 mA -14 PPOF = 125 μW -16 RIN = 50 : -18 -20 10 1

f [MHz] 100

1000

Fig. 4.26: Modulation characteristic of the fast green LED

The highest wavelengths which have been attained with GaN LEDs so far amount to 562 nm, i.e. in the range of the absolute attenuation minimum for POF. The power efficiency is reduced with higher wavelengths, but the LEDs tested still emitted 1.9 mW at 50 mA. Figures 4.27 and 4.28 show the temperature-dependent spectra of the LED at 20 mA and the change in LED power with the temperature at 25ºC - measured directly and after 250 m of PMMA POF having taken the spectral filter effect into account.


1.8 rel. power 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 520 540

317

ILED = 20 mA temperature: -20°C to +70 °C

wavelength [nm] 560

580

600

620

Fig. 4.27: Spectra of 560 nm LED (sample from Nichia)

rel. change of attenuation [dB] 0.20 0.15 0.10 0.05 0.00 -0.05 -0.10 -0.15 -0.20 -0.25 -20 -10 0 10 20

LED power relative to 25°C LED-power relative to 25°C after 250 m POF

30

40

50 60 70 temperature [°C]

Fig. 4.28: Change of the optical power due to the temperature (560 nm LED)

Blue LEDs in the range of 430 nm to 470 nm have also already been used for POF systems. The attenuation of PMMA POF at 470 nm is indeed about 20 dB/km higher than at 520 nm, but the blue LEDs are considerably more efficient and as a rule can be modulated more quickly. One main reason may lie in their better conductivity. Furthermore, many blue LEDs show a clearly lower spectral width so that the chromatic dispersion is of no influence. The temperature-dependent spectra of a blue LED in the -20ºC to +70ºC range (type SHR470 from Sander Electronics) is shown in Fig. 4.29. The temperature coefficients for power and wavelength are even lower than with green LEDs, thus ideal for POF applications. In 2006, the POF-AC first succeeded in realizing an error-free transmission of data at over 1 Gbit/s with a blue LED (DieMount). Once the GaN LED manufacturers turn to optimizing POF transmitters, then further increases in performance are foreseeable.

318 1.1


rel. power SHR470, 20 mA T = -20°C .. +70°C

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 420

O [nm] 440

460

480

500

520

540

Fig. 4.29: Spectra of a blue LED (SHR470)

LEDs from the Agetha Project As part of the European Agetha Project (IST-1999-10292) yellow (570 nm) and green (510 nm) LEDs or VCSELs respectively were to be developed with particularly high temperature ranges up to +120ºC. A number of results were presented in [Lam01], [Lam02] and [Akh02]. The development of VCSELs was not successful, mainly because of the difficult production of the Bragg mirrors. The layers of a green RC-LED are shown in Fig. 4.30. n-contact upper mirror InGaN quantum wells p-contact AlGaN/GaN DBR GaN Sapphire AR-coating out coupled light Fig. 4.30: Set-up of a green RC-LED according to [Lam02]


319

Participants in the project were: ¾NMRC, Lee Maltings, Prospect Row, Cork, Ireland ¾CHREA, CNRS, Valbonne, France ¾Thales, Orsay, France ¾ETSI Telecomunicación, UPM, Madrid, Spain ¾Dept. of Physics, TCD, Dublin, Ireland ¾Dept. of Physics, Univ. of Surrey, United Kingdom ¾Infineon Technologies AG, Regensburg, Germany ¾Photonics Group, BAE Systems, Sowerby, Bristol, United Kingdom One of the results of the project was a 510 nm LED with 1.2 mW optical power at 20 mA and an power fluctuation of only 0.23 dB between 10ºC and 50ºC or 1.14 dB between -40ºC and +70ºC (Fig. 4.31). At the POF in 2002, the transmission of 200 Mbit/s over 100 m of PMMA POF was demonstrated. The project further showed that green LEDs can be used up to +200ºC. In addition to the setting up of semiconductor Bragg mirrors for RC-LEDs or VCSELs the use of dielectric mirrors (SiO2/TiO2) or metallic mirrors were investigated. norm. power 1.0 0.9 0.8 0.7 0.6 0.5

-40°C

0.4 +20°C 0.3 +70°C

0.2 0.1 0.0 470

480

490

500

510

520

530


Fig. 4.31: Spectra of a green LED according to [Lam02]

The current-light power characteristic curves of a green LED compared with a red RC LED are shown in Fig. 4.32 (from [Lam02]). The much more linear curve with the green LED is directly related to the lower temperature dependence.

320


1.0

10°C

PPOF [mW]

0.9

20°C

0.8 30°C

0.7 650 nm RC-LED

0.6

40°C

0.5

50°C 60°C

0.4 0.3 0.2 520 nm LED

0.1 0.0

I [mA] 0

10

20

30

40

50

60

Fig. 4.32: P-I-characteristics of a green LED compared with a RC-LED

4.2.4 Green Laser Diodes Lasers with a wavelength in the green attenuation minimum of the POF would of course be ideal transmitters. In the meantime, green laser pointers are part of the product range of any hobby shop. Hidden inside these laser pointers, however, is always an infrared laser source, the frequency of which has been doubled. “Real” green lasers have so far not been technically employed on a large scale. Lasers based on ZnSe have been successfully realized. The GaN material system has only permitted blue lasers so far. For POF systems efficient lasers at 520 nm and 560 nm would be suitable. Efficient lasers with ZnSe have already been developed whose service life, however, do not meet the practical requirements. Blue lasers on a GaN basis have been introduced into the market and are especially used for mass storage systems (DVD). Green lasers based on InGaN so far operate optically pumped. They definitely can be expected to be electrically powered in the near future. Green ZnSe Laser at 521 nm A 521 nm laser, constructed at the University of Würzburg in Germany and based on beryllium chalcogenide is described in [Leg98]. The active layer consists of 4 nm Zn0,65Cd0,35Se and is embedded in a ZnSe/Be0,06Zn0,94Se super lattice structure for optical guiding. Cladding layers of Be0,06Mg0,06Zn0,88Se are located on a GaAs substrate. At a ridge width of 1.5 μm (400 μm to 800 μm resonant cavity length), the threshold current is at 15 mA and the differential quantum efficiency at 21% with up to 10 mW optical output power (in pulsed mode with a pulse duty ratio of 1:20). In pulsed mode, a maximum operating temperature of +140°C was attained for a laser with a barrier width of 7 μm. At a barrier width of 1.5 μm, the temperature was still +100°C. The emission angles perpendicular and parallel to


321

the layer surface are ±10.5° and ±13° respectively. The service life is not specified. The optical parameters would be ideal for use in POF systems. Green ZnSe Laser at 528 nm [Stra00] describes the construction of a 528 nm laser. Figure 4.33 provides a schematic illustration of the laser. An improved contact structure results in a threshold current density of 42 A/cm² (compared with 235 A/cm² for a standard electrode). At a size of 20 × 1,000 μm² for the active zone, a threshold current of 8 mA is achieved. At room temperature and constant 1 mW output power, the service life came to 40 min in the experiment. This was an improvement of more than one order of magnitude compared with previous results. Pd/Au LiN

contact lateral isolation

ZnSe/ZnTe MQW contact layer ZnMgSSe cladding layer ZnSSe waveguide layer ZnCd0.25S0.07Se0.68 active layer ZnSSe waveguide layer ZnMgSSe cladding layer n-GaAs

substrate

Pd/Pt/Au

contact

Fig. 4.33: 528 nm ZnSe laser diode

4.2.5 Vertical Laser Diodes and RC-LED Extensive studies on the development of vertical laser diodes (VCSEL) in the red and near infrared spectral range were conducted at the Universities of Ulm and Stuttgart (for example [Ebe96], [Ebe98]). The best VCSEL are those currently available in the spectral range of 800 nm to 1.000 nm with AlInGaAs quantum wells. The most efficient components attain a 47% degree of conversion efficiency (optical power relative to the electrical dissipation power) and 50 mW of optical power or also threshold currents of 0.29 mA. In the short-wave range, 670 nm are achieved. Intensive studies are underway with red and green VCSEL. In the long wavelength range, 1,550 nm lasers are being developed, larger wavelengths (for example, 6 μm with IV-VI) semiconductors are also possible. 4.2.5.1 Red RC-LED In this section red RC-LEDs will first be described. As explained above, their setup corresponds to VCSELs in many details, but they do not work in laser operation. After several years of development they are today commercially available and surpass conventional LEDs in most output parameters.

322


RC-LED at 655 nm and 650 nm An RC-LED with an emission wavelength of 655 nm was introduced in [Saa01], i.e. adapted almost exactly to the attenuation minimum of POF. The resonator was detuned off-resonance at about 10 nm which meant that the band gap did not exactly agree with the resonance frequency of the Bragg mirrors. The lower mirror attained 99% reflection and the upper one only 60%. The active zone had a diameter of 84 μm, 15 μm and 300 μm. Thus, maximum power of 2.3 mW, 4.18 mW and 8.25 mW could be attained at 40 mA, 70 mA and 120 mA respecttively. The spectra of the RC-LED with a diameter of 84 μm of the emitting area are shown in Fig. 4.34.

9 8

Popt. [a.U.] 40 mA

7 6

30 mA

5 20 mA

4 3

10 mA

2 1

O [nm]

0 620

630

640

650

660

670

680

690

Fig. 4.34: Spectra of a 655 nm RC-LED

Further details on this diode are provided in [Gui00a] and [Dum01]. The sources developed at the Optoelectronics Research Center of Tampere University of Technology can be modulated to 1 Gbit/s (see also Chap. 6). 622 Mbit/s and 400 Mbit/s were transmitted over 1 m or 10 m respectively of POF. The relationship between the magnitude of the active zone, the optical power and the modulation bandwidth is shown in Table 4.2. Table 4.2: Parameters of 655 nm RC-LED according to [Dum01] RC-LED

ILED

Popt

BW3 dB

40 μm 84 μm 150 μm 150 μm

10 .. 15 mA 40 mA 70 mA 35 .. 45 mA

0.18 .. 0.20 mW 1.4 .. 1.5 mW 3.2 mW 2.5 mW

350 MHz 200 MHz 150 MHz 100 MHz


323

An RC-LED with a 650 nm wavelength is described in [Gui00b]. The external quantum efficiency is 3.25% (1.4 mW at 40 mA) with 200 MHz modulation bandwidth. Typical values are 30 MHz to 80 MHz for GaInAsP-LED. The active zone consists of a pressure-strained Ga0.45In0.55P quantum well. The lower and upper mirrors are formed from 32 and 6 to 12 layer pairs with 99% and >80% reflectivity respectively. A zone with an 84 μm diameter is produced for limiting the emission surface. RC-LEDs at 650 nm In [Gray00] a new 650 nm RC-LED is described for use in POF systems. The best RC-LED so far based on InGaP/AlGaInP reach an external quantum efficiency of 4.8%, and 0.5 mW of optical power at 5 mA diode current at an emission wavelength of 660 nm. The diode described has two mirrors with 32 periods (bottom) and 8 periods (top) made of Al0.5Ga0.5As/AlAs with a distance of one wavelength (and thus considerably fewer layer pairs than for the VCSEL described above). The active layer is formed by three In0.5Ga0.5P quantum wells, (Al0.5Ga0.5)0.51In0.49P barriers and (Al0.7Ga0.3)0.51In0.49P cladding layers (MQW structure). The shift to 650 nm is achieved by straining the active layer. A mesa 400 μm large was etched into the layer. The wavelength lies between 647 nm and 649 nm, depending on the angle of emission at a spectral width of 4 nm. At 1 mA current, the optical power is 0.1 mW, which corresponds to a quantum efficiency of 6%, with a diode voltage of 1.7 V. The principle of detuning of the mirror resonance against the emission wavelength of the active layer is described in [Gray01], whereby both Bragg mirrors have a somewhat too high wavelength at room temperature. When the temperature is increased, the emission wavelength of the active layer runs so-to-speak into the resonance. As a result the normal decrease in efficiency is compensated for by a wide temperature range. Another effect is that the emitted wavelength varies with the emission angle as shown in Fig. 4.35.

40

rel. power

25° 20°15° 30°

10°

30

5° 0°

35°

20

40° 45°

10

50° 55° 65°

0 620

625

630

60°

635

O [nm] 640

645

650

655

660

Fig. 4.35: Angle dependent spectra of a RC-LED with detuned mirrors ([Gray01])

324


Here the emitted spectra were measured angle dependent between 0º and 65º. At 0º the emission maximum lay at about 653 nm. On the other hand at 65º the maximum lay at 632 nm. You have to keep in mind that the differences in angle in the diode itself are smaller because of the light refraction when exiting the material. This effect can also be clearly seen in Fig. 4.36. Here the far field has been measured at different wavelengths. The higher wavelengths are transmitted from the center while the shorter wavelengths come out of the diode slanted. Popt [a.u.]

0°

4 -30°

30°

3

2

-60°

60° 652.5 nm 651.5 nm 650.5 nm 648.5 nm 646.5 nm 644.5 nm 642.5 nm

1

0 -90°

90°

Fig. 4.36: Wavelength dependent far field of the RC-LED with detuned mirrors

This diagram is particularly impressive because you can see that not only the power and spectrum of the detuned RC-LED change at different temperatures, but that the coupling power efficiency in a fiber can also change considerably and that these changes can proceed spectrally quite differently. This effect can be used to keep the power at the receiver largely temperature-independent, although it can only function well with one type of fiber within a specific length: for example, in vehicle networks in which the fibers are specified exactly and where there are no fibers longer than 10 m. RC-LEDs from Firecomms The Irish manufacturer Firecomms has been developing red RC-LEDs for many years which in the meantime are sold commercially. The temperature-dependent spectra of the type FC200R-010 (data sheet on the website, [Lam03d]) are shown in Fig. 4.37. This source is suitable for bit rates up to 250 Mbit/s. The spectral width of the RC-LED is 20 nm, which is relatively small. The wavelength shift is also smaller than for a normal LED (which is about 0.12 nm/K). The temperature-dependent P-I characteristic curves are shown in Fig. 4.38.


325

30 -10°C 0°C +10°C +20°C +30°C +40°C +50°C +60°C +70°C +80°C

Popt. [a.u.] 25 20

'O = 20 nm dO/dT = -0.082 nm/K

15 10 5

O [nm] 0

600

610

620

630

640

650

660

670

680

Fig. 4.37: Spectrum of a RC-LED

1.2

+10°C

PPOF [mW]

+20°C

1.0

+30°C +40°C

0.8

+50°C +60°C +70°C +80°C

0.6 0.4 0.2

I [mA] 0.0

0

10

20

30

40

50

Fig. 4.38: P-I-characteristics of the RC-LED

RC-LEDs for Mobile Networks RC-LEDs emitting at 650 nm for use in mobile networks have been presented in [Wir01a], [Wir01b], [Osr01] and [Baur02]. The goal is once again to keep the temperature coefficients as low as possible. The influence of the mirror detuning for these components on the far field is shown in Fig. 4.39 according to [Osr01]. In addition to the decrease in power with the temperature the coupling power efficiency into the POF also increases so that the changes in power at the receiver are kept to a minimum.

326 1.0 0.9


Prel

OHF00147

0.8 0.7

25°C

0.6

60°C

0.5

90°C

0.4 0.3 0.2 M [°]

0.1 0.0 -100 -80

-60

-40

-20

0

20

40

60

80

100

Fig. 4.39: Far field of a RC-LED at different temperaturs

The decrease in optical power with the temperature is shown in Fig. 4.40 according to [Baur02]. The drop in power up to 85ºC (working range for MOST networks) is less than 1 dB, up to 125ºC it is 3 dB (at present the power rating for MOST allows for up to 6 dB fluctuations in transmission power). 0.0

rel. opt. power [dB]

-0.5 -1.0 -1.5 -2.0 -2.5 -3.0 T [°C] -3.5

20

40

60

80

100

120

Fig. 4.40: Temperature dependence of an optimized RC-LED

As is shown in Fig. 4.41 from [Wir01b], the geometry of the electrical contacts also plays an important role. In order to maintain high optical power, large chip surfaces are used with large-area contacts which distribute the current (on the left) optimally. Small diodes which have less capacitance make sense for data transmission. Furthermore, the center of the emission area should not be covered by electrical contacts in order to enable an optimum coupling of the light into the fiber (on the right).


type A high power 300 u 300 μm²

327

type B data links : 80 μm²

Fig. 4.41: Various chip designs according to [Wir01b]

RC-LEDs with emission wavelengths of 605 nm and 632 nm are described in [Wir01b]. They attain an optimum power of 0.34 mW or 2.7 mW respectively, both with a current of 20 mA. RC-LEDs for High Data Rates Another red RC-LED (650 nm) which has been optimized for both high speed and for low temperature coefficients is described in [Chi05b]. The active surface of the LED has a diameter of 84 μm. Three versions with 1, 3 and 5 quantum wells have been produced, whereby the attainable power increases from 2 mW to 3mW or 3.2 mW respectively with the increasing number of layers. However, the modulation bandwidth drops from 235 MHz to 110 MHz or 60 MHz respectively. The fastest RC-LED can transmit 500 Mbit/s over 50 m of PMMA POF without any problems. At +85ºC the power coupled into the fiber drops to only 60% compared with room temperature. 4.2.5.2 Red VCSELs VCSELs at 650 nm would be the ideal transmission source for systems with PMMA POF. Encouraged by the rapid development of VCSELs in the near infrared range a number of institutes have been working on the problems since the mid1990s. As a result, a number of promotional projects have been financed, for example, HSPN and PAVNET in the USA (see Chap. 11). The first results lay in the 670 nm to 680 nm range. VCSEL at 690 nm [Saa00] describes a 690 nm VCSEL. Here 55½ and 38 layer pairs are used as a mirror. The mesas have a diameter of 34 μm to 50 μm at a current aperture diameter of 4 μm to 20 μm. 1.3 mA of threshold current and max. 0.56 mW of power at 5.6 mA (6.9% external efficiency) are attained. Laser operation is possible up to +45°C. The P-I characteristic curve of the VCSEL with a 10 μm aperture diameter is shown in Fig. 4.42. The greatest attainable power efficiency lays at 6.9% at 3.7 mA. This component is hardly suitable for POF since the attenuation amounts to over 300 dB/km.

328


1.0

opt. power [mW] 5°C

0.8

690 nm

15°C

0.6 25°C 0.4 35°C 0.2 45°C 0.0 0

1

2

3

4

5 6 current [mA]

Fig. 4.42: P-I-characteristics of a 690 nm VCSEL according to [Saa00]

VCSEL at 675 nm Figure 4.43 shows the characteristic curves of another 675 nm VCSEL [Lam00b]. The threshold current here is even lower. At 25ºC an optical power of over 1 mW is attained. However, laser operation can only be achieved up to around 50ºC. 1.8

opt. power [mW]

10°C

1.6 1.4

20°C

O = 675 nm

1.2 1.0

30°C

0.8 0.6

40°C

0.4 0.2

50°C

0.0 0

1

2

3

4

5

6

7

8 9 current [mA]

Fig. 4.43: P-I-(T) characteristics of a 675 nm VCSEL according to [Lam00b]

VCSEL at 674 nm Another VCSEL with an emission wavelength of 674 nm is described in [Tyn00b]. Here laser operation has even been attained up to 75ºC (Fig. 4.44). For a comparable type with an emission wavelength at 670 nm the maximum applica-


329

tion temperature still amounts to somewhat over 60ºC. The mirrors of the VCSEL consist of 35 pairs Al0.95Ga0.05As/Al0.5Ga0.5AS (top) and 54½ pairs (bottom). The active zone consists of four quantum wells. 1.0

opt. power [mW]

VCSEL 674 nm

0.8 10°C 20°C

0.6

30°C

0.4

40°C 50°C

0.2

60°C 75°C 70°C

0.0

0

1

2

3

4

5

6

7

8 9 10 current [mA]

Fig. 4.44: P-I(T)-characteristics of a 674 nm VCSEL according to [Tyn00]

VCSEL at 670 nm The temperature behavior of a 670 nm VCSEL (see [Tak99]) is shown in Fig. 4.45. The threshold current lies at only 4 mA and is clearly below the typical values of an edge emitter at 15 mA up to 60 mA. Laser operation is, however, only possible up to about 50ºC. If you take the typical temperature rise into consideration, for example, in a PC housing, such a component can hardly be used over 30ºC, in other words only in an air-conditioned environment. 1.0

opt. power [mW]

10°C

20°C 30°C

0°C

0.8 0.6

40°C

0.4 50°C

0.2 0.0

current [mA] 0

1

2

3

4

5

6

7

8

9

10

Fig. 4.45: P-I-(T) characteristics of a 670 nm VCSEL laser according to [Tak99b]

330


VCSEL at 665 nm [Lam00b] describes the structure of arrays from 665 nm VCSEL for parallel data communication with POF (see also [Lam00a]). The aim of the studies is the capability to manufacture VCSEL with >1 mW between 0°C and +50°C. In the component described, mirrors with 54 or 34 layer pairs are used. The active zone consists of 4 tension-strained GaInP quantum wells and AlInGaP barriers. The lateral current is confined with etched mesas (49 μm diameter) with an oxidized aperture (15 μm). The change in power of 2%/K is relatively high when heated. For a 10 μm aperture diameter, up to 2 mW is attained. Laser operation is possible up to +60°C. At 13 μm aperture, the threshold current is 1.9 mA. VCSELs at 650 nm - 670 nm A very comprehensive overview of the state of development of red VCSELs is given in [Schw03b]. The wavelength as well as the size of the emitting surface was varied for all the manufactured VCSELs by means of a corresponding current aperture. Figure 4.46 shows the electron microscope photograph of a VCSEL, in particular the upper mirror with aperture and contact.

10 μm Fig. 4.46: VCSEL detail photo (Univ. of Stuttgart)

The current-light intensity characteristic curve of such a VCSEL is shown in Fig. 4.47 i.e. for a laser with a 7 μm aperture and a 670 nm emission wavelength. The maximum output power lies at over 4 mW; laser operation (CW) is maintained up to 70ºC. Unfortunately, the attainable output power drops with decreasing wavelength. One main reason is the diminishing thermal conductivity of the material, particularly in the lower mirror. The output power attained at 20ºC in dependence on the emission wavelength is shown in Fig. 4.48.


331

4.5 4.0

Popt [mW]

3.5

0°C

cw operation O = 670 nm 7 μm VCSEL

3.0

10°C 20°C

2.5 30°C

2.0 40°C

1.5 1.0

50°C

60°C

0.5

I [mA]

70°C

0.0 0

2

4

6

8

10

12

Fig. 4.47: P-I-characteristics of a 670 nm VCSEL ([Schw03b])

12 11 10 9 8 7 6 5 4 3 2 1 0

Pmax (20°C) [mW]

wavelength [nm] 630

640

650

660

670

680

Fig. 4.48: Maximum CW output power for red VCSEL at 20°C

That the poor thermal conductivity is indeed responsible for the diminishing efficiency is proven by the measurement of the P-I characteristic curve with a small pulse duty factor. Laser activity of the 670 nm VCSEL is attained here up to a temperature of +150ºC in the active zone (Fig. 4.49). Usable 650 nm VCSELs would be the ideal source, especially for PMMA GI-POF. The results just shown reduce the problem “only” to efficient heat dissipation out of the active zone. A number of promising methods are currently under development.

332

0.30


Popt [mW] 23°C

pulse: 0.3 μs (200 Hz) 8.6 μm VCSEL, 670 nm

43°C

0.25 0.20 0.15

144°C 107°C

0.10 0.05

154°C

0.00 0

20

40

60

80

100

120

140

Fig. 4.49: P-I-characteristics for pulsed operation ([Schw03b])

The easiest method for reducing the temperature rise in the active zone during laser operation is to minimize the size. However, the attainable output power drops, too. The P-I characteristic curves of two 650 nm VCSELs with 20 μm and 7 μm large apertures are shown in Fig. 4.50. Whereas the output power drops to about 40%, the maximum temperature for laser operation rises to +65º C. 0.8 0.7

Popt [mW]

aperture: 20 μm 0.6 0

aperture: 7 μm 0°C .. +65°C

0.5 0.4 0.3 0.2 0.1 0.0 0

5

10

15

20 25 I [mA]

0

5

10 15 I [mA]

Fig. 4.50: Comparison between two 650 nm VCSEL with different apertures

The authors have come to the conclusion that possible parameters for red VCSELs are: ¾laser operation up to 110º C ¾up to 10 mW output power at room temperature ¾modulation bandwidths up to 10 GHz


333

Figure 4.51 summarizes the results achieved so far for maximum laser temperature and emission wavelength.

90

Tmax [°C] target

80 2000

70

2003

2006 2003

60

2000

50

1999

2000

2003

2000

40 center wavelength [nm] 30 645 650 655 660 665 670 675 680 685 690 Fig. 4.51: Overview of red VCSEL, presented up to now

4.2.5.3 VCSEL in the IR Region VCSEL at 970 nm [Ebe96] describes the use of 970 nm VCSEL for data transmission with glass fibers. At data rates of 10 Gbit/s, several kilometers were covered. The specific properties of VSCEL, in particular the radiation perpendicular to the layer surface at a small angle, make it possible to construct very inexpensive parallel optical connections. These components are particularly interesting for PF-GI-POF. VCSEL in the red PMMA attenuation window still have considerable reliability problems. Particularly the great temperature dependence that limits the range of applications to just under +50°C poses problems. VCSEL at 850 nm VCSELs with emission wavelengths of 850 nm can be used for relatively high ambient temperatures. A 850 nm VCSEL is presented for example in [Schn03] which shows a laser emission up to +145ºC, the P-I characteristic curve is in Fig. 4.52. In this wavelength range VCSELs can even be used without any power control over wide temperature ranges. The PMMA POF has losses of about 3 dB/m at 850 nm. The use of 850 nm sources nevertheless makes sense for example for connections between computer components or in mass storage. Even at 780 nm very good VCSELs are available. In this case PMMA fibers even permit lengths up to 30 m.

334


Fig. 4.52: P-I-characteristics of a 850 nm-VCSEL ([Schn03])

VCSEL at 782 nm In [Ueki99] a 782 nm VCSEL is introduced with a maximum output power of 3.4 mW at 10 mA. The threshold current is 0.61 mA (each at +20°C). At +60°C, 2 mW of optical output power is still attained. Longer wavelength VCSEL are thus also suited for considerably higher temperatures. The upper mirror of the VCSEL consists of 24 pairs, the lower of 40½ pairs Al0.3Ga0.7As/Al0.9Ga0.1As, with a distance of one wavelength. The active layer contains three quantum wells made of Al0.12Ga0.88As/Al0.3Ga0.7As. 4.2.6 Non Resonant Cavity LED NRC-LED at 850 nm To date (year 2000), non-resonant cavity LED have been described for wavelengths around 850 nm only. [Roo00] introduces an array of 850 nm NRC-LED with an active diameter of 30 μm and a distance of 100 μm. This enables the transmission of 1 Gbit/s across 10 cm image-guiding fibers (7 μm single-fiber diameter). A series of studies on NRC-LED were published by Windisch. An efficiency of up to 31% is cited in [Win99] for 870 nm NRC-LED. The roughened surface leads to a more diffuse scattering and improvement in efficiency. A mirror at the rear improves the efficiency. Further improvements of up to a 40% external quantum efficiency for an 870 nm NRC-LED are described in [Win00a], [Win00b] and [Win00c]. Various parameters were attained with various active layer thicknesses (10 nm, 20 nm and 30 nm) and various diameters of the etched mesa diodes (30 μm and 45 μm) as shown in Table 4.3.

335

4.2 Transmitting Diodes for Data Communication Table 4.3: Specifications for various NRC-LED Thickness Active Layer 30 nm 30 nm 20 nm 10 nm 10 nm

ØMesa

Max. Bit Rate

45 μm 45 μm 30 μm 30 μm 30 μm

580 Mbit/s 800 Mbit/s 1,100 Mbit/s 1,600 Mbit/s 2,000 Mbit/s

External Efficiency 36% 34% 31% 21% 2.5%

When using a NRC-LED with 20% efficiency, 1,200 Mbit/s can be transferred. A 100 μm glass ball lens can be used to improve the coupling of the 30 μm LED on a POF. Thus the launch efficiency in a POF with AN = 0.50 attains a value of 50%. Figure 4.53 shows the relationship between the bit rate attained and efficiency. A high efficiency is indeed achieved with larger active diameters, but the bit rate is somewhat smaller. Nevertheless, 1.25 Gbit/s can be attained with NRC LEDs with 20% external quantum efficiency.

40

external QE [%]

dAZ [nm]: 30 nm (1) 30 nm (2) 30 nm (3)

30

20 nm (2) 10 nm (2) 10 nm (3) 10 nm (4)

(1): Mesa = 45 μm (2-4): Mesa = 30 μm (1,2): QE opt. con. (3,4): low ser. R optim. (2): integr. micro lens

20

10

0 0.4

0.8

1.2

1.6

2.0

maximum bit rate [Gbit/s] Fig. 4.53: Bit rates and quantum efficiency of NRC LED

NRC-LED at 650 nm The first red NRC-LEDs were introduced in [Roo01]. The InGaP/AlInGaP LED at 650 nm attain an external quantum power efficiency of 31%, whereas the red LEDs up till then had only attained a maximum of 12%. For example, output power of 4 mA is attained with a current of 7 mA. The spectrum of NRC-LEDs is illustrated in Fig. 4.54.

336

1.0


rel. opt. power

0.8 0.6 0.4 0.2 0.0

wavelength [nm] 600

620

640

660

680

700

720

Fig. 4.54: Spectrum of a 650 nm NRC LED according to [Roo01]

NRC-LEDs at 623 nm and 610 nm Other NRC-LEDs in the visible range were introduced in [Lin01b]. An external power efficiency of about 30% is attained at 623 nm through a combination of surface structuring and optimized contact geometry. For a 610 nm diode an efficiency of 32 lm/W, i.e. a power efficiency of approximately 10%, is achieved. 4.2.7 Pyramid LEDs A simple method for increasing the coupling-out power efficiency of a LED was introduced in [Kra99], [Lew99] and [Här03]. Normally, a semiconductor has a refractive index of about 3.5. Total reflection already occurs above an angle of 17º. If the LED chip is as usual cubical, then the angle does not change even with reflection. Barely 3% of the emitted light can leave the LED through the upper boundary layer. By means of a transparent substrate with mirror coating and receptacle with a suitable funnel-shaped LED the light from all 6 boundary layers can be used which can increase the power efficiency to a good 15%. substrate n-contact

n-GaP

GaAlInP active zone p-GaP Fig. 4.55: Principle of the pyramid LED

4.3 Wavelengths for POF Sources

337

The pyramid LEDs make use of this effect just like the NRC-LEDs, namely changing the direction of light upon reflection. Intentionally slanted inner sides are produced instead of a roughened-up surface. The principle is demonstrated in Fig. 4.55 according to [Li05]. An external quantum power efficiency of 55% can be attained for red LEDs. For blue LEDs 37% can be attained. Some examples of such pyramid LEDs are shown in Fig. 4.56 from [Lew99] and [Här03].

Fig. 4.56: Examples for pyramid LED

4.3 Wavelengths for POF Sources To create efficient data transmission with a particular POF, sources will always first be sought whose emission wavelengths match the respective minimum attenuation values. For PMMA fibers, these are the ranges near 520 nm, 570 nm and 650 nm. In Fig. 4.57, the external efficiencies (data from the current datasheets of various manufacturers) are listed for available LED. The different materials are marked separately. 100%

external quantum efficiency

GaAlAs AlInGaP GaAsP GaN GaP GaAlP

GaAs InGaN SiC

10%

1.00%

0.10%

wavelength [nm] 0.01% 420 440 460 480 500 520 540 560 580 600 620 640 660 680 Fig. 4.57: Efficiency of various LED material systems (from datasheet information)

338

4.4 Receivers

Until the GaN/InGaN technology had been developed, the efficiency of LED in the direction of the short wavelength spectrum decreased markedly. Only SiCLED were available in the blue range; however, these were expensive and inefficient. In the mean time, very efficient LED have become available in the 370 nm through 540 nm range. For 560 nm too, samples have been produced. In the next few years, it is expected that the current gap will be closed into the red range so that sources with an efficiency >10% will be available for the entire visible spectral range. Apart from efficiency, it is obvious that the possible modulation speed also plays a decisive role. Figure 4.58 contains the quoted switching times for some LED from Fig. 4.57. 3,000 switching time [ns] 1,000

SiC GaN GaP

AlInGaP AlGaAs GaAsP

300 100 30 10 wavelength [nm] 3 460 480 500 520 540 560 580 600 620 640 660 Fig. 4.58: Switching times of various LED material systems

Diodes that have a lower efficiency usually exhibit very low switching speeds. This is particularly true of green LEDs in conventional technology. Diodes based on GaN are very well suited for data transmission due to their high switching speeds. The various diode constructions such as lasers, VCSEL, RC-LED, or NRCLED have until now only been realized in the red spectral range. Only LED are available in the yellow and green POF attenuation window. This situation will probably change in the next few years.

4.4 Receivers In addition to the transmission diodes the photodiodes are, of course, extremely important components for optical transmission systems. Besides the sensitivity, the speed of these components is important for the transmission performance.

4.4 Receivers

339

As opposed to transmission diodes only a material system actually can be used. Light quanta are absorbed in photodiodes and converted into electron-hole pairs. In principle, all photons whose energy lies above the band gap are captured. Another characteristic of photodiodes is that semiconductors with an indirect band structure can be used. The electron has to change its impulse only after it is generated in order to enter the minimum level of the conduction band. It can take as much time as it wants and also does not exert any influence on the photo current. That is why silicon presents itself for all wavelengths below approximately 1.1 μm. Practically all commercial POF receivers are indeed based on this most reasonably priced of all semiconductor materials. 4.4.1 Efficiency and Sensitivity Unlike solar cells, photodiodes in telecommunications engineering are always operated with a bias voltage. The photo current generated by the incident photons then represent the measured signal. The efficiency of a photodiode is described by the external quantum power efficiency Șext. It is common knowledge that the energy of a photon amounts to W = h · f. At a wavelength of 1.24 μm the photons have just 1 eV energy. Consequently, a light power of 1 W generates a photo current of just 1 A under ideal conditions. The following is true for all other wavelengths: IPh >A @ Popt >W @ Kext O 1.24 Pm

The smaller the light wavelength, the less the photo current per watt of optical power emerges just because each photon possesses more energy. This may first seem to be a paradox but you must keep in mind that a short-wavelength LED with the same power efficiency emits more optical power than long-wavelength diode. Consequently, a wavelength-independent relationship arises between the driving current of the LED and the photo current of the photodiode. 0.8

responsivity [A/W]

Kext. = 1.00

0.7 0.6 850 nm: = 0.63 A/W Kext. = 0.92

0.5 0.4

650 nm: = 0.47 A/W Kext. = 0.89

0.3 0.2

500 nm: = 0.32 A/W Kext. = 0.79

0.1

wavelength [nm]

0.0 400

500

600

700

800

900

1000

Fig. 4.59: Responsivity of a Si-pin photodiode (Hamamatsu S6801)

1100

340

4.4 Receivers

In the data sheets for photodiodes there is information about the responsivity , but seldom data about the quantum efficiency. This responsivity parameter describes the emerging photo current per light power in A/W or also in mA/mW. An example of the length-dependent responsivity is shown in Fig. 4.59 (from Hamamatsu data sheet). At about 950 nm this diode has the highest responsivity which is typical for silicon photodiodes, but even at 650 nm the quantum efficiency is close to 90%. Above 1,000 nm the efficiency quickly falls back to zero since the energy of the photons is too small. In addition to the actual efficiency there is another important parameter, namely the absorption length (depth of light penetration), which becomes larger and larger at longer wavelengths. If it exceeds the thickness of the absorbing layer, then the power efficiency drops because the light passes right through the photodiode. The dependence of the absorption length on the wavelength for different semiconductor materials is shown in Fig. 4.60. 10

3

absorption length D-1 [μm]

102 101

Si GaAs

100 10-1

Ge

In0.7Ga0.3 In0.53Ga0.47As As0.64P0.36

10-2 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 O [μm] Fig. 4.60: Absorption length for different semiconductor materials

Assuming you have a maximum thickness of the absorbing layer of 10 μm, then you can well use silicon in the range between 400 nm and 1000 nm. For glass fiber systems at 1.3 μm and 1.55 μm diodes on an InGaAs basis are generally used. 4.4.2 Photodiode Structures Different photodiode structures are employed in this technology. The three most important versions are:

4.4 Receivers

341

¾The pin-photodiode consists of an intrinsically doped interface layer between the p and n zones. The absorption primarily takes place in this area. ¾The avalanche photodiode (APD) has a highly doped layer in which the electrons produced are multiplied and accelerated by a strong local electric field. ¾With the metal-semiconductor-metal photodiode (MSM) there is no p-n junction. Finger-like electrodes are applied to an absorbing semiconductor surface. The bias voltage applied pulls off the ensuing charge carriers. The typical set-up of a pin-PD and an APD is illustrated in Figs. 4.61 and 4.62. Both type of construction can be realized with silicon as well as with other semiconductor materials. The internal gain of an APD can amount to 400. Since the multiplication factor is not the same for every generated electron, the APD produces additional noise. In real amplifiers, however, the noise of the following amplifier generally exceeds the amounts of noise in the photodiode by far. Since the APD already generates high amplification before the first stage, the electronic noise thus plays a much smaller role. On average, APD receivers are about 10 dB more sensitive than pinPD receivers. Si-nitrid metal contact passivation p-InP i-InGaAs absorbing layer n-InP

metal contact Fig. 4.61: Typical structure of a pin-PD

The advantage of a pin-diode lies in its easy use. It only requires a bias voltage, typically 5 V to 15 V. For an APD, the bias voltage has to be adjusted in such a way that the optimum gain factor is attained. The necessary bias voltages can reach some 100 V and are dependent on the temperature and the power (controls are necessary). Furthermore, APDs are considerably more expensive. All commercial POF systems work with pin-photodiodes. The contact structure of an MSM photodiode is shown in Fig. 4.63. The metal surfaces cause partial shading so that the efficiency is diminished. However, since the capacity of the diode can be much smaller than with a PIN diode of the same size, a greater transimpedance can then be used which improves the sensitivity.

342

4.4 Receivers metal ring

p+-InP p-InP Si-nitrid n-InP

p-n-juction

E-field

n-InGaAsP n--InGaAs absorbing layer n-InP n+-InP metal multiplication layer

Fig. 4.62: Typical structure of an APD

For very fast MSM PDs, i.e. up to 30 GHz, finger spacing in the area of 1 μm is necessary, whereby polarization dependencies do arise. MSM-PDs have not yet been employed commercially for POF systems, but should be in use in a few years.

Fig. 4.63: MSM photodiode (diode made by Astri HongKong)

A qualitative comparison of the most important characteristics of these three types of photodiodes can be found in Table 4.4. The pin-photodiode represents a good compromise in all parameters and is furthermore moderately priced. Table 4.4: Comparison of photodiode properties capacity SNR reverse voltage responsivity price

MSM +++ ++ -

PIN ++ ++ ++ ++ +++

APD + +++ --+++ +

4.4 Receivers

343

4.4.3 Junction Capacity and Bandwidth A pin-photodiode, just like any other semiconductor diode, has a junction capacity which depends on the reverse voltage applied. A typical example is shown in Fig. 4.64. This capacity is much larger than with diodes for glass fiber systems - at least above 3 pF with diodes up to a diameter of 800 μm - because of the large photodiode surfaces required. 7

capacity [pF]

6 5 4 reverse voltage [V] 3 0.1

0.3

1.0

3.0

10

30

100

Fig. 4.64: Junction capacity of a pin-diode (Hamamatsu S6801)

Together with the input resistance of the following stage this diode capacity forms a low-pass which as a rule limits the entire bandwidth of the receiver. In order to get a good signal-to-noise ratio, the impedance of the receiver should be as high as possible. A low diode capacity is therefore directly responsible for good receiver sensitivity. Consequently, for a long time it was supposed that the possible bit rate for 1 mm fibers was limited to a maximum of 150 Mbit/s. In the meantime, however, different laboratories have attained up to 2,500 Mbit/s with pin-photodiodes ranging in diameter from 600 μm to 800 μm (see Chap. 6). Even considerably higher bit rates should also be possible with multi-level transmission, adaptive equalization and multi-carrier transmission. Optimization of the fiber coupling to the photodiode promises further improvements. 4.4.4 Overview of Receivers There is as a rule very little mention of the construction of receivers in the various publications on POF systems. Many institutes use commercial receivers or at least amplifiers. Low-noise transimpedance amplifiers are available for almost every bit rate range. The greatest disadvantage is that these commercial components have been designed for a capacity of only a few tenths pF. If you couple photodiodes with considerably greater capacity, the bandwidth and sensitivity drop dramatically. Details on receiver circuits, to the extent that there is information on them, are listed in Chapter 6 on System Overview.

344

4.4 Receivers

An overview of the sensitivities attained so far with different bit rates as well as for different fiber diameters and various wavelengths is illustrated in Fig. 4.65. -8 -10 sensitivity [dBm] -12 -14 -16 -18 -20 -22 -24 -26 -28 -30 -32 -34 100 200 500

POF: 1.0 mm 0.5 mm 0.2 mm wavelength 470 nm 650 nm 850 nm 1300 nm bit rate [Mbit/s] 1,000

2,000

5,000

10,000

Fig. 4.65: Parameters of POF receivers up to now

The relationship between bit rate and sensitivity attained can clearly be seen. A ten-fold increase in the bit rate costs about 15 dB sensitivity because of the greater noise bandwidth as well as by the necessary reduction in the receiver resistance. Data rates above 3 Gbit/s have only been attained so far with relatively thin fibers. As is the case with transmitters, there are numerous technical starting points for greatly improving the parameters of receivers which should be implemented when large-scale use begins. 4.4.5 Commercial Products Putting together a nearly complete list of available types of diodes and receivers would go far beyond the scope of this book which is why we have not included such a compilation. This is the reason why we wish to present only two exemplary photodiodes which have been used for POF systems for years by various institutes.

Fig. 4.66: pin-diode SFH 250 by Infineon for 1.0 mm/2.2 mm POF (right with a housing for clamping the POF with a screw)

4.4 Receivers

345

As early as 10 years ago Siemens - now Infineon - developed the type SFH 250 in Fig. 4.66. This Si-pin-PD has about a 1 mm diameter. The plastic housing has a 2.2 mm bore so that a sheathed 1 mm POF can be directly centered. The diode is then ideally suited for systems without plug connectors. The important parameters for this diode are summarized in Table 4.5 (data from [Inf03]). Data rates up to 250 Mbit/s have been realized with different set-ups. This diode has also been used successfully for analog signals. Table 4.5: Parameters of the SFH 250 Parameter operation temperature max. reverse voltage wavelength of the best sensitivity sensitive range (S t 0.1 Smax) dark current (UR = 20 V) junction capacity (f = 1 MHz, UR = 0 V) junction capacity (f = 1 MHz, UR = 20 V) rise and fall time (10% - 90%, RL = 50 :, UR = 30 V, O = 880 nm) photo current (10 mW in the POF, UR = 5 V) for 660 nm and 950 nm

Symbol Top UR OS max O IR C0 C20 tr, tf

Unit °C V nm nm nA pF pF ns

Value -40 .. +85 30 850 400 .. 1100 1 (d 10) 11 2.3 10

IP

μA

3 (660 nm) 4 (950 nm)

Another readily used diode is the S5052 from Hamamatsu. The 800 μm large chip is encapsulated in a plastic housing with a 3 mm large lens which guarantees an optimum coupling to 1 mm thick fibers. The diode was operated at data rates up to 2,500 Mbit/s at the POF-AC. The important parameters are listed in Table 4.4 [Ham01]. Table 4.6: Parameters of the S5052 Parameter operation temperature max. reverse voltage wavelength for the best sensitivity sensitive range (S t 0.1 Smax) dark current (UR = 20 V) junction capacity (f = 1 MHz, UR = 5 V) bandwidth on 50 : (UR = 20 V) responsivity at 660 nm responsivity at 780 nm responsivity at 830 nm

Symbol Top UR OS max O IR C0 fc

Unit °C V nm nm nA pF MHz mA/mW mA/mW mA/mW

Value -25 .. +85 20 800 320 .. 1000 0.02 (d 0.3) 4 500 0.40 0.45 0.45

Receivers for polymer fibers are hardly obtainable as a single unit; they are mostly sold as a system together with the corresponding transmitters. Data on the components available have been summarized in the chapter after next.

346

4.4 Receivers

4.4.6 Improvement in Sensitivity As already mentioned above, many receivers use a lens to improve the sensitivity. If the photodiode is designed smaller than the fiber, you can have a higher transimpedance, but you lose more sensitivity because of the lost light. With the use of an appropriate lens a large part of the light guided in the fiber can be concentrated onto a smaller photodiode. Because of the POF’s large NA this works up to a reduction factor of 2 since anything above that would mean that the angle which appears would be too large for the lens and for the photodiode. In Fig. 4.67 from [Har01] the possible coupling power efficiency form a 1 mm standard POF (UMD launch) is shown in different-sized photodiodes with spherical lenses.

70 coupling efficiency [%] 60 50

PD [μm]

40

600 500 400 300 200

30 20 10 0

0

200

400

600

800 1000 1200 distance POF lens [μm]

Fig. 4.67: Coupling efficiency POF-PD with spherical lenses according to [Har01]

Using a 500 μm large photodiode, 50% of the light could be coupled in a photodiode with a correctly positioned spherical lens. Even if you could now select a four-fold greater load resistance at the receiver - because of the smaller capacity there would still not be any gain in the SNR: ¾loss through coupling: ¾gain through greater transimpedance: ¾loss through greater noise:

3 dB 6 dB 3 dB

On the other hand, higher data rates can generally be attained with the smaller photodiode so that it is worth using. Optical concentrators as non-imaging elements permit a more efficient coupling from the fiber into the photodiode as described in [Poi04a]. There are versions with mirrored surfaces as well as with wave guiding through total reflection. One version from [Ueh02b], [Ueh03] and [Mat02b] is shown in Fig. 4.68.

4.5 Transceivers

347

Fig. 4.68: Optical concentrators

Table 4.7 from [Poi4a] shows the theoretical power efficiency of different versions for the coupling of a 1 mm POF (UMD launch) to photodiodes of different sizes as compared with direct butt joints. Table 4.7: Options for photodiode coupling Reflective Dielectric Parabolic Coated Taper Taper Mirror 31.6 % 21.2 % 50.5 % 34.0 %

PD = 300 μm

Butt Coupling 8.3 %

PD = 400 μm

16.4 %

46.6 %

36.9 %

75.3 %

39.7 %

PD = 500 μm

23.4 %

60.6 %

54.4 %

83.2 %

41.4 %

Concept

Lens

The dielectric taper (reflection against air) attains the best power efficiency since total reflection is more efficient than metallic reflection. Theoretically, 75% of the light can be coupled (with a loss of 1.25 dB) in using a photodiode which is only 400 μm large (only 16% of the capacity). Without question future POF systems will increasingly use these methods since the concentrators can be mass produced very easily.

4.5 Transceivers 4.5.1 Components before 2000 In the past five years there have been diverse developments particularly in the field of POF transceivers. We were hardly able to present complete systems in the first edition of this book. The components then were sold only by small producers and for niche applications. Wiesemann & Theis have developed converters for RS232 or 10BaseT to duplex-POF. Figure 4.69 shows both components. 594 nm (T2P£) or 650 nm LEDs are used as the sources (see also [Leh00]).

348

4.5 Transceivers

Fig. 4.69: RS232 (left) and 10 Mbit/s Ethernet transceiver (right) by W&T

It is easy to couple the duplex-POF to both components by plugging in the cutoff cable. A simple release mechanism makes it easy to detach the components later. A version for 100 Mbit/s was announced. Figure 4.70 shows 3 transceivers for 125 Mbit/s and 155 Mbit/s made by Hewlett Packard, NEC and the University of Ulm (substituted to 520 nm LED). The HP transceiver is equipped with V-pin receptacles. It is also available for F07 connectors with the component HFBR-5527, just like for NEC transceivers.

Fig. 4.70: ATMF-compatible transceivers for 100BaseT and 155 Mbit/s ATM (from left to right: Hewlett Packard, NEC, Univ. Ulm)

All three components are equipped with a 1 u 9-pin contact and can be plugged into conventional PC cards or LAN components. Test networks were created for this purpose at the University of Ulm ([Som98a]) and at Deutsche Telekom in Berlin ([Lei98]). Table 4.8 summarizes the most important properties (according to [HP01], [HP02], [NEC1] and the results of a research project with the University of Ulm).

4.5 Transceivers

349

Table 4.8: 125/155 Mbit/s transceiver for POF (typical values) Parameters

HFBR 5527

manufacturer OSource application temp. max. data rate LED-NA spectral width transmitting power max. receiver power min. sensitivity tr/tf transmitter

NL2100

R-2526/NSPG500

Hewlett Packard NEC 650 ± 10 nm 650 ± 10 nm 0..70°C 0..70°C 125 Mbit/s 155 Mbit/s 0.30 0.21 21 nm 33 nm -4.3..-10.4 dBm -4.2..-5.7 dBm -7.5 dBm -1.0 dBm -27.5 dBm -25 dBm 2.1/2.8 ns 4.5/4.5 ns

tr/tf receiver minimum range

6.3/6.3 ns 25 m

Univ. of Ulm 520 ± 10 nm n. a. 155 Mbit/s 0.50 40 nm approx. -8 dBm -7.5 dBm -23 dBm n. a.

n. a. 50 m

n. a. 100 m (laboratory)

All three transceivers are based on the ATMF specification. Figure 4.71 shows the temperature-dependent spectrum of the NL-2100 transceiver. It has virtually all the properties of the HP transceiver. Neither HP nor NEC provides current information on possible new product developments. This also applies to the transceiver provided by NEC as a prototype for data rates of up to 250 Mbit/s.

1.0

Popt rel.

24°C 30°C 40°C 50°C 60°C 70°C

0.8 0.6 0.4 0.2 0.0 590

600

610

620

630

640

650

660

670

680

690

wavelength [nm] Fig. 4.71: Spectra of the transmitter LED in the transceiver NL2100

A series of different POF transceivers made by Hewlett Packard (later Agilent, now Avago) are now available for a wide range of data rates. These are listed in table 4.9.

350

4.5 Transceivers

Table 4.9: POF transceiver by HP (according to [Leh00]) Data Rate

Reach Transmitter (HFBR-) +25°C | 0-70°C

Receiver (HFBR-)

Connector

Osource

DC-40 kbit/s 120 m

110 m

1523/1533

2523/2533

v-pin

650 nm

DC-1 Mbit/s

55 m

45 m

1522/1532

2522/2532

v-pin

650 nm

DC-1 Mbit/s

75 m

70 m

1528/1538

2522/2532

v-pin

650 nm

DC-2 Mbit/s

45 m

42 m

1604/1614

2602/2612

SMA/ST

650 nm

DC-4 Mbit/s

50 m

40 m

1505A/1505B 2505A/2505B SMA/ST

650 nm

DC-5 Mbit/s

30 m

20 m

1521/1531

2521/2531

v-pin

650 nm

DC-10 Mbit/s 60 m

55 m

2528

2528

v-pin

650 nm

DC-32 Mbit/s

n. a.

75 m

1527/1537

2526/2536

v-pin

650 nm

DC-55 Mbit/s

n. a.

60 m

1527/1537

2526/2536

v-pin

650 nm

125 Mbit/s

n. a.

50 m

1527/1537

2526/2536

v-pin

650 nm

155 Mbit/s

n. a.

50 m

1527/1537

2526/2536

v-pin

650 nm

4.5.2 Fast Ethernet Transceiver In the last few years Fast Ethernet (125 Mbit/s physical data rate) has increasingly been used in automation instead of the conventional field bus systems. A number of components have been developed for this field. In addition, there are products which have been specifically developed for the home. In the first half of 2006 alone there were half a dozen new manufacturers. That is why the following summary can only be a short-lived snapshot and not a detailed listing of parameters and company addresses. However, this information can be found in the POF-Atlas (www.pofatlas.de). 4.5.2.1 POF Solutions from DieMount in Wernigerode A number of the most innovative developments for transceivers have been introduced in the last few years by the DieMount Company in Wernigerode (SaxonyAnhalt). The components are notable for three particular features. ¾The POF is fastened to the transceiver with a simple screw mechanism without connectors which simplifies installation. ¾The LEDs are coupled into the POF in a particularly efficient manner by using a micro-mirror (see Chap. 6). ¾Both transmission directions can run on one fiber (simplex systems) by using their own patented coupler with low losses and high return loss. A duplex and a simplex version of the POF mounts on the transceivers are shown in Fig. 4.72.

4.5 Transceivers

351

Fig. 4.72: Fiber fixations by DieMount

DieMount is the only manufacturer which sells transceivers with blue and red LEDs. A WDM version has also been developed for bi-directional transmission. The simplex components achieve ranges of 70 m or 30 m at wavelengths of 470 nm or 645 nm respectively. As duplex versions each is capable of a transmission length of over 100 m. Different versions of the Fast Ethernet transceivers from DieMount are illustrated in Fig. 4.73. In the meantime these products are also being sold under other brand names by large electronic companies. In addition to the external media converters presented here PC plug-in boards and complete switches are also sold.

Fig. 4.73: Fast-Ethernet converters by DieMount

352

4.5 Transceivers

4.5.2.2 Optical Clamps from Ratioplast The manufacturer Ratioplast has been supplying solutions for automation applications on a POF basis for a long time. One of their own developments is a socalled optical clamp which is also a solution for connector-free POF installation (Fig. 4.74). The cut-off POF is held in place by a small clamp on the side.

Fig. 4.74: Optical clamp for POF from Ratioplast

Versions for Fast Ethernet with a red LED and a range of 70 m and for 10 Mbit/s Ethernet with 520 nm LED and up to 200 m transmission length are available as media converters. The components can be seen in Fig. 4.75 ([Thi04]).

Fig. 4.75: Media converters for 10 und 100 Mbit/s from Ratioplast

4.5.2.3 Transceiver Family from Avago We discussed the V-pin product family from Avago (Hewlett Packard) earlier in this book. Practically all versions are available as individual transmitters and receivers as well as transceivers in standard construction sizes (1 u 9 pin transceiver). Figure 4.76 shows a test PC-board with a transmitter-receiver pair and a transceiver.

Fig. 4.76: POF transceiver from Avago

4.5 Transceivers

353

4.5.2.4 Home Installation by RDM The Swiss manufacturer Reichle &De-Massari has developed two different concepts for inhouses cabling with POF. First there are connectors, tools and a transceiver for the SC-RJ connector [Rich05b] and secondly a combination of data cable and POF is offered with the RCC-45 system ([Rich04], see Fig. 4.77).

Fig. 4.77: SC-RJ connector and Fast-Ethernet transceiver (left) and RCC-45 hybrid connector with data cable and duplex POF from Reichle & DeMassari

4.5.2.5 POF Transceivers from Infineon/Siemens At the 21st meeting of the ITG Sub Committee “Polymer Optical Fibers” Infineon Technologies presented its new POF transceiver for the first time (Fig. 4.78, [Lück06]). The clamp principle developed by Ratioplast is used here.

Fig. 4.78: Optical clamp and transceiver (prototype) from Infineon

Infineon expects strong growth in the POF market in the next few years for which the introduction of VDSL and the increase in IPTV products are responsible. (The estimated growth rate for IPTV-capable connections will be 92% annually.) Infineon gives a price of $12 USD for the complete electronic components (POF transceiver and ADM6992SX Chip when mass produced (Fig. 4.79 from [Inf06]).

Fig. 4.79: POF transceiver - printed circuit board design

354

4.5 Transceivers

Since October 2006 the system, now under the Siemens label, can be ordered from the website of the Deutsche Telekom. Included in the set (Fig. 4.80) are copper cables, power supplies, a cutting tool and 30 m of duplex POF (with 1 mm core diameter and 1.5 mm jacket) in addition to the two media converters (Fig. 4.81).

Fig. 4.80: Siemens Gigaset Optical LAN-Adapter, offered by the Deutsche Telekom (right: media converter for Fast-Ethernet)

4.5.3 Other Systems POF transceivers are also sold for other different interfaces in addition to the Fast Ethernet components. Particularly in the Asian countries house networking is not carried out in pure IP networks, but in accordance with the IEEE 1394 standard. This standard permits real-time transmission with guaranteed data rates and maximum delay times in contrast to IP and has been designed for operation without a main server. The components are still somewhat more expensive today because of low-volume production. Whether Ethernet or IEEE 1394 is used plays no role whatsoever for POF and the transceivers. Both systems can work up to 100 Mbit/s and use 4B5B coding. 4.5.3.1 Comoss The Taiwanese manufacturer Comoss sells a complete assortment of POF components for IEEE 1394 systems on the basis of the SMI connector (Fig. 4.81). Media converters as well as transceivers with red LEDs with a range up to 50 m and data rates of S100 and S200 are available.

4.5 Transceivers

355

Fig. 4.81: IEEE 1394 components with SMI connector

4.5.3.2 IEEE 1394, MOST and Fast Ethernet from Firecomms The information on the Irish manufacturer Firecomms is described in detail in Chapter 6 on “Systems”. The characteristics of VCSEL, RC-LEDs and green LEDs were discussed in the section on active components. An entire series of products with outstanding parameters, especially on the basis of red RC-LEDs, is available. The Fast Ethernet transceivers reach a transmission length of 100 m [OTS06c]. Even the S200 version equipped with SMI connectors for IEEE 1394 is available for a range of 50 m and meets the corresponding specifications (Fig. 4.82, [OTS06b]). The Netopia Company has announced it will use Firecomms transceivers for its VDSL systems in home networking (Fig. 4.88).

Fig. 4.82: IEEE 1394 S200 transceiver with SMI connector from Firecomms

The transceivers designed for use with MOST have been certified in the meantime and exceed, for example, the required power budget by 8 dB. The switching times of 4.2 ns make it possible to use them up to 250 Mbit/s. The temperature range can also be enlarged. A POF version without connectors is also available from Firecomms (Fig. 4.83). Table 4.10 summarizes some parameters of the Firecomms transceivers.

Fig. 4.83: IEEE 1394 transceiver OptoLock from Firecomms without connector

356

4.5 Transceivers

Table 4.10: Parameters of Firecomms POF products (typical values from data sheets)

max. data rate wavelength max. POF length operating temp. optical power sensitivity tr and tf

Mbit/s nm m °C dBm dBm ns

FOT-Pair

OptoLock

MOST

1394, S200

125 650 100 -20 .. +70 -8.5 .. -2 -24 .. -28 2.0 / 2.0

250 660 n. a. -40 .. +70 -7.0 .. -2.0 -24 1.5 / 2.0

50 650 n. a. -40 .. +95 -7.0 .. -2.0 -28 3.0 / 2.0

250 660 n. a. -40 .. +70 -7.0 .. -2.0 -27 1.5 / 2.0

4.5.3.3 Japanese Manufacturers Numerous Japanese manufacturers sell POF transceivers for Fast Ethernet, especially for IEEE 1394 in the speed ranges S100 to S400. Among these firms are Hitachi, Mitsubishi (Fig. 4.84), Sharp, Hamamatsu, Toshiba and Sony.

Fig. 4.84: POF transceiver Hitachi DC9500 from 1999 (left), Fast Ethernet POF media converter Mitsubishi OMCP-ETH100SA for S200 (middle and right)

Unfortunately, it is hardly possible to obtain any information about new developments. As a rule there are few European distributors and orders are only accepted with advance payment. Nevertheless, we must assume that very intensive work is presently being carried out in Japan on the development of new POF transceivers. 4.5.3.4 Fast Ethernet, Ethernet and Video from Luceat The Italian manufacturer Luceat sells a wide assortment of POF components. In addition to transceivers for 10 Mbit Ethernet with a 510 nm LED and a range of 200 m [Luc04a] and 100 Mbit/s Ethernet with a 650 nm LED and a range of 100 m [Luc04b] converters for RS232 interfaces with a range of up to 400 m are sold ([Luc04d], see Fig. 4.85).

4.5 Transceivers

357

Fig. 4.85: Media converters for 10/100 Mbit/s Ethernet and RS232 from Luceat

In addition, Luceat sells systems for analog video transmission in different versions with transmission lengths up to 250 m on the basis of green LEDs (Fig. 4.87, [Luc04c]).

Fig. 4.87: Analog video transmission components from Luceat

4.5.3.5 DSL Modem with POF A big problem in present-day POF applications is the lack of devices with their own POF interfaces, thus requiring the use of external media converters and additional power supplies (also see Chap. 8). Netopia is the first manufacturer to sell a DSL modem with built-in POF interface (Fig. 4.88, transceiver from Firecomms, [OTS06c]).

Fig. 4.88: Netopia DSL modem with POF interface

5. Planar Waveguides

Optical waveguides can be described as transparent structures which are more or less put onto solid carriers. In principle, they function just like fibers and are also described by the same parameters. However, there are also some fundamental differences: ¾Waveguides are not produced ready-made by the meter which are then cut to the necessary length, but as optical lines with exact predetermined course and length. ¾Typical lengths range between several meters and under a millimeter which is why attenuation is not normally the most important parameter. ¾Because of their simple method of production square as well as trapezoidal waveguides are produced instead of the round cross sections. Waveguides can be produced from quite diverse materials, e.g. from silica, from LiNbO3, from GaAlAs or also from other optical semiconductors. Since there are various publications on the diverse shapes and applications, we only wish to deal in this chapter with waveguides made of polymers. Polymer waveguides cannot only be in the shape of fibers as described in the preceding chapters, but are also produced simply und reasonably priced as planar (flat) structures. Various features of polymers are very interesting: ¾Great changes in the refractive index with the temperature, thus making thermo-optical switches possible. ¾Large non-linear coefficients enable extremely fast optical switches and multiplexers. ¾Optical waveguides can be produced simply by casting. ¾Diverse materials enable very great differences in refractive index which make, for example, very small bending radii possible. The following paragraphs show exemplarily some possible applications of singlemode and multimode polymer waveguides. In fact, these are not actual polymer fibers, but are technically related to them. Furthermore, interesting options result for combining planar waveguides and polymer fibers.

360

5.1 Materials for Waveguide Structures

5.1 Materials for Waveguide Structures A comprehensive overview of polymer materials for the manufacture of optical waveguides is presented in the dissertation ([Gra99], Table 5.1). In addition to the polymers discussed here we will also investigate waveguides made of inorganic glass. Table 5.1: Polymers for optical waveguides ([Gra99]) Attenuation at Attenuation at 670/850 nm 1300 nm

Attenuation at 1500 nm

EGDMA / TFPMA / PMMA

0.3 dB/cm

> 1.0 dB/cm

PFPMA / TeCEA

0.2 dB/cm

0.7 dB/cm

Material PMMA / BDK

0,9 dB/cm

partially fluorinated acrylate

0.06 dB/cm

polyisocyanourate polyimide

0.8 dB/cm 0.3 - 0.5 dB/cm 0.3 - 0.5 dB/cm

polysterene polycarbonate BCB polycylobutene

1.5 dB/cm

photo bleachable polymer (Akzo) composite (Ormocer)

< 0.1 dB/cm < 0.4 dB/cm

< 0.1 dB/cm < 0.3 dB/cm

As you can see with the losses stated in dB/cm, the transparency of these structures is not comparable to that of optical fibers. The main reasons for the very much greater optical losses lie above all in the non-cylindrical waveguide geometry and in the poorer surface quality of the core-cladding-interface layer. If you take into consideration, however, that only a few centimeters in length are needed for typical applications, then the losses in the magnitude shown are justifiable. PMMA at 1.3 μm and 1.55 μm is completely useless for optical fibers since the attenuation is too high. However, it can be used as waveguide material since the demands on the material here are not too great. Nevertheless, materials with lower attenuation, especially at 1.55 μm, are being searched for. As with fibers, an effective variation is the use of partially or completely fluorinated polymers. CYTOP® material also can be employed here. The positions of vibration bands for different carbon compounds are shown in Fig. 5.1 (detailed description in [Gra99]). The fluorinated compounds have the lowest vibration frequency - in the mid-infrared range. In addition to the relatively high attenuation, another inhibiting factor is the lack of thermal endurance of the PMMA. For many polymer materials with high Tg, the use as fiber materials fails because of the losses. However, it can make sense to use them in waveguide structures.

=CH2 -CH3

C-Br

C-Cl

361

C-C C-N C-O

-CH2-

2

C-F

C-S

-C N -C CC=O C-C C=N -CH2-CH3

-SH

-O H -NH2 CH -CN

5.2 Production of Polymer Waveguides

3

4

5

6

7

8 9 10

20 wavelength [μm]

Fig. 5.1: Position of vibration bands in polymers according to [Gra99]

Other materials for optical waveguides from developments of the Fraunhofer Institute HHI Berlin are described in [Keil05]: ¾Triazine-acrylate MA2, 0.45 dB/cm losses at 1.55 μm ¾Triazine-acrylate MA3, 0.28 dB/cm losses at 1.55 μm ¾ZPU12 (Korea), 0.50 dB/cm losses at 1.55 μm ¾CYTOP® (AGC Japan), 0.12 dB/cm losses at 1.55 μm

5.2 Production of Polymer Waveguides In [Gra99] different production methods for polymer waveguides are described in detail and compared - work at the University of Saarland. ¾ion etching of polyimides ¾ion irradiation ¾photo-structuring (mask aligner, laser writing) ¾induced diffusion of doped polymers ¾injection-molding ¾molecular orientation of the doping ¾photo bleaching ¾electron beam structuring (photoresist) ¾sol-gel technology ¾casting technology Some advantages in the production of polymer waveguides, compared to other materials, are the low process temperatures and the possibility of casting which particularly makes sense in mass production. A basic comparison between organic and inorganic material systems is shown in Table 5.2 ([Gra99]), whereby the author concerns himself solely with single-mode waveguides which are intended for use as components in glass fiber communications systems.

362


Table 5.2: Comparison of waveguide material systems [Gra99] Material system

Parameters

therm. expansion thermo optical coefficient critical energy of phonons intrinsic absorption @ 1.5 μm technological method

temperature stability

inorganic

organic

10-6/K (low) 10-6/K (low) < 1,300 cm-1 (small) 0.01 dB/cm (low) FHD, RIE hot compression molding several 100°C

10-4/K (high) 10-4/K (high) < 2,500 cm-1 (low) 0.50 dB/cm (high) molding method photolithographiy about 100°C

This work describes, for example, the production of waveguides from aqueous synthesized composite materials - combination of organic and inorganic components (Fig. 5.2). The stamp used can be made of silicon or silica - produced by anisotropic etching - or from nickel (formed galvanically). molding

filling

solvent-free sol molded composite layer

composite layer substrate pressure stamp

substrate pressure

cover glass

removing of the cover glass deforming

depositing of the covering layer waveguide

Fig. 5.2: Schematic sequence of waveguide production from aqueous synthesized composite ([Gra99])

The material systems described include the combination MPTS/MAS/Zr(PrO)4 and the anhydrous synthesized composite M115. The following Tables 5.3 and 5.4 show the optical losses of the first system and compare the characteristics of both variants. Table 5.3: Attenuation of the MPTS/MAS/Zr(PrO)4 material system at different wavelengths ([Gra99]). Wavelength losses

632.8 nm 0.35 dB/cm

780 nm 0.17 dB/cm

1320 nm 0.55 dB/cm

1550 nm 2.41 dB/cm


363

Table 5.4: Overview of the optical characteristics and production technology of passive optical light waveguides ([Gra99]) Material System

Optical Properties

Manufacturing Process

MPTS/ MAS/ Zr(PrO)4

n | 1.5; 'n | 0.005 attenuation (waveguide layer) D (633 nm) | 0.35 dB/cm D (780 nm) | 0.17 dB/cm D (1320 nm) | 0.55 dB/cm D (1550 nm) | 2.41 dB/cm

M115 derivate

n | 1.4; 'n | 0.005 attenuation (bulk) D (1320 nm) | 0.10 dB/cm D (1550 nm) | 0.40 dB/cm attenuation (stripe waveguide) D (1550 nm) | 1.00 dB/cm

process technology: planar optical waveguide 3-layer waveguide end face polishing stripe waveguide (critical processing material modification necessary) process technology: planar optical waveguide microstructure molding stripe waveguide

Another overview of the production methods for polymer optical waveguides was compiled in the dissertation [Hen04] at the University of Karlsruhe. Possible methods mentioned are: ¾photo-chemical structuring: photo locking, photo-polymerization, photolysis ¾ablative / etching methods: laser ablation, reactive ion etching (RIE) ¾replication methods: injection molding, hot embossing, injection embossing The photo locking method is shown as an example in Fig. 5.3.

UV-light

mask photoinitiator, monomer cladding substrate

annealing

photoinitiator, monomer

cladding waveguide cladding

Fig. 5.3: Schematic depiction of the photo locking method ([Hen04])

364

5.3 Singlemode Waveguides

With this method a substrate is coated with a mixture of a monomer and a photosensitive material. By means of a mask a selective change in the refractive index of the waveguide layer has brought about by inserting the initiators in the polymer matrix. After exposure the remaining volatile molecules are removed from the unexposed areas through annealing. After another cladding layer is put on the waveguide is formed. This method is used commercially for the production of multimode waveguides under the brand name Polyguide from DuPont. PMMA can serve as a substrate. The photoinitiators are, e.g. ketones and benzoins. The possible refractive index changes lie at 0.001 to 0.010. According to [Keil96b] this method is used for thermo-optical switches, whereby the waveguides show losses of 0.3 dB/cm (at 1.3 μm) and 0.8 dB/cm (at 1.55 μm). Another method for producing waveguides mentioned in [Hen04] is selective polymerization. In this case the waveguide is formed through photochemically initiated polymerization. However, the unexposed parts are subsequently removed. With ormocers, an inorganic-organic hybrid polymer of the Fraunhofer Gesellschaft, losses of 0.32 dB/cm (at 1.32 μm) and 0.66 dB/cm (at 1.55 μm) were achieved. On the other hand with photo bleaching the refractive index outside the later waveguides is lowered through a photochemical reaction. The non-linear characteristics of different dyes are used here. Waveguide attenuations of 0.8 dB/cm at 1.31 μm were reached. A method for the production of waveguides in a kind of printing process is described in [Kal03b], whereby the polymer is melted and then applied directly onto the carrier by means of an approximately 10 μm thick micropipette. Afterwards the hardening is effected by means of UV light. A second polymer serves as the upper cladding material. Single-mode waveguides can successfully be produced with a cross-section of about 16 u 0.8 μm. The production of waveguides by UV-induced refractive index changes is described in [Bru06]. Through radiation with light in wavelengths of 200 nm 260 nm 7.5 μm wide waveguides are written into PMMA. At a wavelength of 1.55 μm the waveguides achieve losses below 1 dB/cm. Different kinds of couplers were also realized.

5.3 Singlemode Waveguides The exceptional quality of singlemode waveguides is their small cross-section. Just as with fibers the condition for singlemode capability is a V-parameter below 2.405. In order to be able to realize relatively small bending radii in the range of a few millimeters, the refractive index difference between core and cladding should be as great as possible - several percentage points. The result is a typical crosssection of 5 u 5 μm which poses in effect no technical problems in production. Practically all waveguide components are, however, intended for use in singlemode fiber systems and thus must be able to be coupled to them. In order to


365

reduce coupling losses, the cross-section of the waveguides must either be fit to or so-called tapers for transforming the mode field have to be used. Not all production techniques are equally suitable for singlemode and multimode waveguides. The casting methods in particular are more advantageous for thicker structures. The production of waveguides in a sensor application (interferometry) is described in [Kor04], whereby a grooved structure is written into the glass through laser ablatation (UV-excimer laser). The groove is subsequently filled with a highly viscous polymer as the waveguide core. A second technique is based on the UV-polymerization of thin layers, whereby the ridge waveguides produced are then surrounded by a second material with a lower refractive index. The third method investigated produced linear waveguides by modifying the refractive index by means of a fs-pulse laser. Details of the waveguide structures are shown in Fig. 5.4.

Fig. 5.4: Details of the waveguides from [Kor04]

The task of particularly temperature-stable optical waveguides (> 300ºC) is covered in [Xu00]. Two recently developed deuterated silicone polymers (DSBP1 and DSBP2) serve as the basis. They show a very good transparency in the near infrared range (0.2 dB/cm at a wavelength of 1.55 μm). The refractive index can be changed by mixing the polymers so that index differences of ¨n = 0.32% to 1.2% (at 1.55 μm) result. A ridge waveguide structure was produced on a Si substrate. The work describes two variations: ¾core: n = 1.520, w = 6 μm, h = 4 μm, d = 2 μm, 5 μm cladding layer n = 1.507 ¾core: n = 1.520, w = 6 μm, h = 3 μm, d = 3 μm, 5 μm cladding layer n = 1.507 The setup of the waveguides is shown in Fig. 5.5. The structuring results from photolithography; the waveguide is generated through reactive ion beam etching (RIE). Thereafter the waveguide is filled with the polymer and covered.

366


covering layer core d

h SiO2-substrate w

Fig. 5.5: Waveguide structure according to [Xu00]

The losses of both waveguides lie at 0.42 and 0.46 dB/cm respectively (at a wavelength of 1.55 μm). Particularly temperature-stable polymer waveguides are also described in [Kang02]. Fluorinated poly(arylene ether sulfide) (FPAESI) and fluorinated poly (arylene ether sulfide fluorene) (FPAESF) serve as materials. For singlemode waveguides with a cross-section of 6 u 7 μm2 losses of 0.4 dB/cm at 1.55 μm were reached. Ageing over 1,000 h at +100ºC did not result in any increase in the attenuation. Another method for the production of polymer waveguides is described in [Sum04]. Again a sheet of glass is used as a substrate. The core is made of PMMA or SU-8 while NOA-88 is used for the cladding. The waveguide structures are written with a proton beam (2 MeV, 2 pA proton current 1.875 · 1013 protons/cm2). The production process is illustrated in Fig. 5.6 (waveguide cross-section is 5 u 5 μm). irradiation of SU-8 by 2 MeV-protons chemical developing and removing of the non irradiated areas SU-8 layer glass substrate depositing of the NOA 88layer as optical cladding

glass covering NOA 88 glass substrate

Fig. 5.6: Production of waveguide structures ([Sum04])

Microscope photos of the waveguides produced are shown in Fig. 5.7. At 633 nm the attenuation amounts to 0.19 ± 0.03 dB/cm. The refractive indices of the glass substrate, of the SU-8 core and the NOA-88 cladding amount to 1.514; 1.595 and 1.555 (at 633 nm) which corresponds to a NA of about 0.35.


367

Fig. 5.7: Proton-written polymer waveguide ([Sum04])

In addition to linear waveguides Y-splitters are also realized. The setup of such a splitter is shown in Fig. 5.8. The spectral transmission for a 2 mm long waveguide is indicated in Fig. 5.9. 75 μm

1400 μm

350 μm

175 μm

linear taper

linear region

190 μm cosinus S-bend

linear region

Fig. 5.8: Structure of a Y-splitter ([Sum04])

According to the information for the particle stream and the radiation performance used (12 mill. particles/second) the writing of a coupler may have taken several minutes. The power output of both arms lay in a ratio of 46 : 54. 100

transmission [%]

80 60 40 20 0

wavelength [nm] 400

600

800

1200

1400

1600

Fig. 5.9: Spectral attenuation of the cross-linked SU-8 (2 mm, [Sum04])

368

5.4 Multimode Waveguides

5.4 Multimode Waveguides Multi-mode polymer waveguides, just like multimode fibers, have the disadvantage of a limited bandwidth due to mode dispersion. The decisive factor is once again the numerical aperture. However, since optic waveguides are typically used in the length range below one meter, bit rates of many Gbit/s can be transmitted without any problem. Measurements of the bandwidth are also hardly mentioned in the technical literature since normally the values lie far outside the measurement possibilities. In demonstrators bit rates up to about 10 Gbit/s have so far been realized. The main reason for the use of thick waveguides lies in the tolerances (see also Section 5.6). When they are integrated into electrical printed circuit boards, conventional automatic placement machines should be used for passive placement on the optical components, whereby waveguide cross-sections of at least 50 u 50 μm² are necessary. Concepts for planar multimode waveguides, for example, were introduced in [Schm00]. In this case TOPAS® 6017, APEC®HAT 9371 and PMMI®8817 were used as materials. The waveguide cross-section is about 100 μm u 250 μm. Nothing has yet been said about the attenuation. While singlemode waveguides are primarily used as functional components in classic glass fiber systems, among other things as couplers, filters, switches or also as possible amplifiers, multimode waveguides serve above all as high-bit rate data channels. Production techniques are primarily casting or the photochemical structuring of thin layers. Fig. 5.10 from [Schr02] shows the most important production methods for multimode polymer waveguides. hot embossing

photolithography

embossing of the channels

layer structure

n1

emossing tool substrate

deforming

n2 n1

structure filling n1

layer structure core cladding substrate

exposure

n1

n2

n2 n1

direct laser writing micro casting

core material

n2 n1

direct laser writing photomask

n1

waveguide casting core n2 material casting form application of the substrate

core cladding substrate

light source

n1 n2

development

development

deforming

n2 n1

n2 n1

substrate material

n2 n1

n1 n2 n1

caping

caping

overcladding cladding material

n1 n2 n1

cladding material

n1 n2 n1

cladding material

caping n1 n2 n1

cladding material

Fig. 5.10: Overview of the production methods for polymer waveguides ([Schr02], [Sche05])


369

This work [Schr02] also describes the setup of a complete demonstrator as part of the BMBF project EOCB (Electro-Optical Circuit Board). Up to 1.25 Gbit/s were transmitted over each of 4 parallel channels with a spacing of 250 μm between each (50 μm cross-section). The cross-sections of polymer multimode waveguides are mostly square or trapezoidal. In calculations made in the C-Lab Paderborn additional losses of this geometry are calculated ([Bie02]). The advantage of trapezoidal-shaped structures lies in the easier ability to cast and the resulting smaller roughness. For a waveguide of 30 cm in length additional losses of 0.38 dB/cm and 0.66 dB/cm at edge angles of 5º and 10º respectively result from simulations. This can normally be ignored in regard to losses because of the surface roughness. The use of silicones for multimode waveguides is described in [Ney05]. The high temperature stability of this group of materials simplifies above all the laminating into printed circuit boards. At 850 nm losses are only 0.03 dB/cm - comparable to the attenuation of a PMMA-POF of about 3000 dB/km at 850 nm. Table 5.5 shows the parameters of the polymers for waveguides described thus far. Table 5.5: Overview of polymers for waveguides in printed circuit boards ([Ney05]) Company

Material

Luvantix

Epoxy

Thermal Stability [°C] > 250

Optical Losses @ 850 nm [dB/cm] 0.04

KIST

Epoxy

220

0.36

NTT

Epoxy

> 200

0.10

Zen Photonics

Acrylate

> 250

0.05

IBM

Acrylate

> 250

0.04

DaimlerChrysler

unknown

> 250

0.04

RPO

Siloxane

> 250

0.10

Dow Corning

Siloxane

> 200

0.06

Shipley

Siloxane

> 250

< 0.10

The silicone waveguides are produced by means of a combined casting-doctor blade process (Fig. 5.12). First, a mold is produced. The photo lack (SU8) is not applied as usual by spin coating, but by means of a doctor blade technique. Exposure results through a mask. After development a master form is completed. A more stable copy is produced galvanically for mass production. The core polymer (n = 1.43) is then filled in the channels of the preform. A substrate (n = 1.41) is applied to the cured cores. A conventional printed circuit board can be used as carrier on which the copper has been removed in the area of the waveguides, this resulting in a thickness of the optical substrate of 35 μm. After renewed curing the component is removed from the form and in another step in the process the superstrate is applied as optical cladding, if necessary again using printed circuit board carriers. An alternative method involves the embedding of the waveguides

370


between polyimide foils. The waveguides thus created have a cross-section of 70 u 70 μm² with a NA of 0.26. One advantage of this process is that the 45º mirrors required for coupling can already be created in the preform. 10 Gbit/s (BER > 10-12) can be transmitted over a 12 cm long waveguide. 0.10

before 2 h/180°C

attenuation [dB/cm]

0.08

5 min/220°C 5 min/260°C

0.06 0.04 0.02 0.00

1

2

3

4

5

6

7

8 9 10 waveguide number

Fig. 5.11: Temperature behavior of silicone waveguides (polyimide embedding)

depositing of the core polymere

preform

squeegee curing preform

WG-cores

substrate carrier Cu preform

substrate grouting, curing

substrate deforming

superstrate-carrier

superstrate grouting, curing

superstrate polymer Fig. 5.12: Production of waveguides according to [Ney05]

5.5 Functional Components as Waveguides

371

5.5 Functional Components as Waveguides 5.5.1 Thermo-Optical Switches A number of studies have been carried out for constructing components for systems with singlemode glass fibers. A few examples we wish to mention are the works of the Heinrich Hertz Institute (HHI) in Berlin ([Keil96], [Keil97], [Keil99], [Keil05]). Singlemode waveguides on a polymer basis have a crosssection of about 10 u 10 μm². One of their advantages is the fact that they can easily be produced through casting. The great dependence of the refractive index on temperature can be bothersome, but also advantageous for the production of switches and the fact that tunable filters can be fabricated. Figure 5.13 shows the schematic setup of a thermo-optical switch with polymer waveguides. Heating electrodes are located over the core areas. The refractive index can be changed in such a way through heating that the power only leaves from one output because of the altered wave propagation. heating electrodes teflon layer PMMA waveguide cladding material SiOx Si carrier

heating electrodes

PMMA waveguide

Fig. 5.13: Thermo-optical switches with polymer waveguides ([Keil96], [Keil05])

In the HHI different switches have been developed and presented in the configurations 1 u 2, 2 u 2, 1 u 4 and 4 u 4 with crosstalk lying below -30 dB. The attenuation of the waveguides is about 0.7 dB/cm so that the insertion losses of the complete switches are very small. One advantage of this technology is the very small necessary switching power of a few 10 mW. The switching speed lies in the range of a few milliseconds. The dependence of transmission of such a switch on the applied heat power is shown in Fig. 5.14 from [Keil05].

372

5.5 Functional Components as Waveguides transmission [dB] 0

-10 -20 -30 -40 direction

-50 -60

P1 S1 P1 S2 P2 S1

-70

P2 S2 electr. power [mW]

-80 0

20

40

60

80

100

120

Fig. 5.14: Transmission of a thermo-optical switch

A normal 3 dB splitter is available without heating. With increasing heat power the component switches more and more efficiently, whereby above 60 mW the separation amounts to approximately 50 dB. The insertion loss of this component in a singlemode glass fiber lay at only 1.1 dB. Similar parameters have also been attained with a four-fold array of 2 u 2 switches (Fig. 5.15).

Fig. 5.15: Four-fold thermo-optical switches made of polymer waveguides (HHI 2005)


373

A multitude of other waveguide components on a polymer basis has in the meantime been developed at the Fraunhofer Institute (acc. to [Keil05]), including configurable add-drop multiplexers. Another work which has dealt with thermo-optical switches on the basis of polymer waveguides is [Yang02]. In a no driven state the x-configuration acts as a crossing with >29 dB crosstalk attenuation. With an electrical heat power of 132 mW the element switches over and attains a crosstalk attenuation of over 28 dB between the two channels. The waveguides have a 7 u 7 μm2 cross-section and are made of the polymers Ultradel 9120 and 9020 (n = 1.535 and 1.527). 5.5.2 Modulators The setup of an electro-optical modulator on the basis of polymer waveguides is described in [Len05]. The great refractive index difference of the waveguide materials (1.55 to 1.48) permits particularly thin layers (cross-section of 0.3 u 5 μm), thus reducing the entire modulator thickness between the electrodes to 8 μm (Fig. 5.16). A phase change by 180º is already reached with an applied voltage of 0.8 V - 5 V are typical. A bandwidth of 150 GHz to 205 GHz can be attained with this component. In comparison, a LiNb03 modulator has a bandwidth of 70 GHz to 105 GHz.

waveguides of the interferometer

HV

Fig. 5.16: Setup of a polymer waveguide modulator

5.5.3 Coupling Components The setup of passive couplers made of polymer waveguides is described in [Mule03]. For production, a 6 to 7 μm thick silica layer is first grown on a Si wafer. After passivation, the material Unity 200P is applied as a 10 to 14 μm thick layer by spin coating and cured at 110ºC. The waveguides are written by UV light at a wavelength of 365 nm. The cladding material is Avatrel 2090P. One exceptional feature of this procedure is the formation of air-cladding regions around the waveguides (1 μm thick spacing) which improves the waveguiding and thus allows tighter bends. Examples of 1 u 2 splitter structures are shown in Fig. 5.17.

374


Fig. 5.17: Coupling structures as planar waveguides from [Mule03]

The waveguides in the picture are each 2 μm wide and 0.9 μm thick with spacing of 4 to 6 μm. The losses of the waveguides lie at 0.43 - 1.22 dB/cm and the uniformity of the splitter outputs at 0.23 - 1.30 dB. The work previously cited ([Hen04]) also describes the production of couplers. Fig. 5.18 shows such a splitter component. A best value of 0.9 dB/cm (at 1.55μm) was achieved as attenuation for the waveguides. The insertion loss of the y-coupler amounts to about 6.5 dB which means an additional attenuation of about 1 dB compared to the waveguide losses.

Fig. 5.18: Waveguide coupler ([Hen04])

5.5.4 Waveguide Gratings The production of waveguide gratings (AWG: arrayed waveguide gratings) as multiplexers and demultiplexers in WDM systems is described in [Dre06]. One great problem with conventional AWGs, those made of glass as well as of polymers, is the temperature dependence of the refractive index, from which a wavelength shift of the transmission channels results. Temperature-independent transmission characteristics can be achieved by means of a suitable compensation of thermal expansion coefficients and refractive index dependencies. Examples of conventional and compensated AWGs are shown in Fig. 5.19.

5.6 Waveguides as Interconnection Solutions

375

Fig. 5.19: Temperature-independent AWG (IZM and HHI, [Dre06])

5.6 Waveguides as Interconnection Solutions In this section examples of a still relatively seldom used application of optical polymer waveguides is presented which could, however, develop an enormous potential in the near future. The goal of these developments is often characterized as an “optical printed circuit board”. The idea is to integrate optical waveguides onto a conventional printed circuit board. Whereas singlemode polymer waveguides compete with other technologies, e.g. glass, InP, Si/SiO2, etc., in their applications, multimode waveguides from polymer materials open up completely new fields of application which are very useful because almost any large cross-sections can be produced simply and inexpensively. We can only present a few examples in this book for reasons of space. However, the essential parameters and problems are quite similar. 5.6.1 Optical Backplane Systems from DaimlerChrysler For some years now work has been carried on in DaimlerChrysler’s research center on the development of computer backplanes in combination with electrical and optical lines (see e.g. [Gut99], [Moi00a], [Moi00b], [Moi00c], [Rode97], [Mon00], and [Kru00]. This work has been motivated by the fact that an ever greater number of data connections in PCs can be found on wider and wider busses. Optical solutions can prevent the problems of crosstalk considerably. Figure 5.20 shows the principal setup of a PC with an electrical - optical backplane. A detailed illustration in the following Fig. 5.20 shows how the plug-in boards can be coupled into the backplane without contact. Using lenses, a collimated beam is generated which is then again focused onto the waveguide, whereby the distance between the backplane and the board is mostly uncritical.

376


backplane with combined terminals

optical lines electrical lines plug-in cards Fig. 5.20: Plug-in cards and backplane with polymer waveguides

receiver module

transmitter module laserdiode

lenses

plug-in card

photodiode

backplane interface

micro mirror mounting board

low loss multimode polymer waveguide

waveguide cladding layers

Fig. 5.21: Principle of the coupling of an optical backplane using lenses

In one of the first experiments the waveguide was formed by means of a 1 mm St.-NA POF. Ball lenses with a diameter of 5 mm were used for the imaging. The tolerance for the lateral shifting of the boards - with less than 1 dB loss - was 500 μm; the permissible angle error was 1.5º. 780 nm laser diodes with an optical output power of 1 mW were used as transmitters. The sensitivity of the Si-PD was -14 dBm at a data rate of 1 Gbit/s. Since the loss of the entire optical path was only 3 dB, the power margin amounted to 11 dB.


377

In later works waveguides with a 200 u 200 μm² cross-section have been presented. Wavelengths between 650 and 850 nm had losses lower than 3 dB/m. The additional attenuation is about 1 dB/cm for a bending radius of 15 mm. The numerical aperture of the waveguide is 0.35. The usable temperature range lies between -40ºC and +85ºC. For waveguides with a length of 55 cm the complete attenuation only amounted to 2.5 dB. In addition to straight and bent waveguides, crossings and couplers can also be realized so that complete optical networks can be set up. By using VCSEL and MSM photodiodes (diameter of 300 μm) the sensitivity can be improved to -20 dBm at 2.5 Gbit/s. Figure 5.22 shows the design for a 56 u 1 Gbit/s connection [Moi00c].

Fig. 5.22: Example for the application of an optical backplane with 56 channels

The following Figures 5.23 to 5.25 demonstrate further details for realizing optical backplanes. First of all Fig. 5.23 shows a carrier with different waveguides (straight and as a 4 u 4 network).

Fig. 5.23: Parallel optical waveguides (partially with couplers)

Figure 5.24 shows the setup of a single transceiver with LD, PD and the lenses for the coupling (on the left). On the right you can see the photos for combining several transceivers as well as the individual lenses. Fig. 5.25 depicts a bent polymer waveguide with a rectangular cross-section (the laser cannot be seen). Such components are above all useful for the routing of light.

378


Fig. 5.24: Transceiver design for optical backplanes

Fig. 5.25: Bent planar waveguide

The license to produce optical backplanes was granted to ERNI and Varioprint as cooperation partners from 2002 on ([Ern02]). 5.6.2 Systems from the University of Ulm The results of waveguide production at the University of Ulm are described in [Med00]. On the basis of 855 nm VCSEL a complete transmission system was set up. The polymer waveguides have a cross-section of 120 u 130 μm2 at lengths of 43 mm and 46 mm respectively. The minimal attenuation was 0.5 dB/cm. With the aid of a 50 μm large germanium detector it was possible to transfer data at 3 Gbit/s error free. The optical guides remain stable at +160ºC and can thus be laminated into printed circuit boards. Two examples of waveguide structures are shown in Fig. 5.26.


379

Fig. 5.26: Polymer waveguides ([Med00])

5.6.3 Electro-optical PCB from the University of Siegen The concept of a printed circuit board with integrated multimode waveguides on a polymer basis was presented in [Gri06]. The advantage of the multimode technique was elucidated in the lecture. Despite the relatively large tolerance of conventional placement machines (approx. 50 μm) a passive adjustment of the active elements is possible to relatively thick waveguides. Hot embossing is mentioned as the first production technology. Attainable losses are indicated at 0.2 dB/cm. Fig. 5.27 shows the cross-section of several waveguides.

Fig. 5.27: Hot embossed polymer waveguides ([Gri06])

Compared to this method, the laser direct-writing of waveguides permits losses of 0.03 dB/cm and thus connection lengths in the meter range (Fig. 5.28). This latter process does permit any number of structures, but is slower and necessitates insufficient temperature characteristics.

Fig. 5.28: Optical polymer waveguides written by laser and produced photolithographically ([Gri06])

380


Other methods include pressing the waveguides into copper (0.1 dB/cm) and the production method mentioned above by means of photolithographic processes (0.1 dB/cm). In addition to the waveguides themselves the coupling of active components also plays an important role. A concept is presented in [Gri06] in which light is coupled from the waveguides to the active components by a 45º mirror. The connection can be made by using plugs. Reasonable demands on stability and tolerances can only be attained by using multimode waveguides. 5.6.4 IBM Research Center Zurich /ETH Zurich Interconnection solutions with polymer waveguides including practical demonstrations have been investigated in [Schm05] (IBM Research Institute Zurich). The deliberations here proceed from the point of view that the data rate that can be transmitted drops in electrical connections with the square of the length and that the cross-section required increases linear with the data rate. Especially with very high data rates in the Gbit/s range, optical solutions promise advantages for both space requirements and power consumption. The recommended solutions work at a wavelength of 850 nm for which polymer waveguides are particularly well suited. With the use of new optimized CMOS drivers low power consumption is possible (only 100 mW for 4 channels; transmitter and receiver at 2.5 mW/Gbit/s.) The transmission of 10 Gbit/s using multimode glass fibers is shown in Fig. 5.29.

Fig. 5.29: Eye diagram at 10 Gbit/s over 5 m of MM GOF at 850 nm

A layer up to 30 u 40 cm² is placed on a substrate in order to produce the waveguides. The following lithography is carried out by means of UV light. The typical cross-section of the waveguides is 50 μm. However, up to 16 channels per mm can also be produced by reducing the spacing and the cross-section to 35 μm. Furthermore, two-dimensional arrays are also possible.


381

The characteristics attained with the waveguides are: ¾straights: attenuations of 0.028 to 0.040 dB/cm for all core sizes ¾curves: 0.1 dB per 180º bending with a 20 mm radius ¾crossings: 0.02 dB/crossing ¾splitters: 0.1 dB per 50:50 splitter ¾dispersion: open eyes at 12.5 Gbit/s with a 1 m long spiral (50 μm core) ¾potential up to 40 Gbit/s (measured with ultra short pulses) System demonstrators were presented in 2003 and 2004. The waveguides were integrated into a FR4 printed circuit board. In the first version the opto-electrical module was coupled using lenses (90º deflection). The transmitters tested allowed data rates up to 12.5 Gbit/s; the receivers up to 10 Gbit/s. 5 Gbit/s were attained for card-to-card connections. In the second version the optical deflection was not used and the transmitter/receiver was connected more through a direct butt coupling to the waveguides. A concept for the passive positioning of the active components was presented.

Fig. 5.30: Eye diagram of the transmitter (12.5 Gbit/s), polymer waveguides ([Schm05])

Fig. 5.31: Polymer waveguides with different spacing ([Schm05])

Details of these investigations were also published in [Lenz 05] by the ETH Zurich. Waveguides with a cross-section of up to 100 μm are described. It was pointed out in this work that one of the limiting factors for the integrated optics was the small packing fraction which resulted from the minimal bending radii. One solution could be photonic crystal structures which permit approximately right-angled bends.

382


Fig. 5.32: Y-coupler on the basis of a photonic crystal (ETHZ, [Lenz05])

5.6.5 Results of the NeGIT Project A similar concept for optical PCBs is described in [Bau05] by the Fraunhofer Institute for reliability and micro-integration (IZM Berlin). The aim of the NeGIT project (New Generation Interconnection Technology) supported by the BMBF is primarily to develop plug connectors in order to be able to run complete optical lines over the backplanes of computer components (Fig. 5.33). connecting system for opt. coupling of the backplane optical backplane

plugable optoelectric module

circuit board with integrated optical waveguides

Abb 5.33: Principle of an optical backplane connection according to [Bau05]

This proposal, similar to other systems, also works at a wavelength of 850 nm with available VCSEL transmitters. The material used is photo patternable epoxy resins which can also withstand high temperatures during printed circuit board


383

production. A typical waveguide cross-section with a width or height respectively of 50 μm is shown in Fig. 5.34.

Fig. 5.34: Index profile of an optical waveguide and cross-section ([Bau05])

The transmission of such a waveguide is shown in Fig. 5.35 (50 u 50 μm² waveguide with NA = 0.18). The spin coating process used permits layers with a thickness of 20 μm to 120 μm.

transmission [%] 90 80 70 60 50 40 30 20 10 0 0.2

0.4

0.6

0.8

1.0

1.2

1.4 1.6 1.8 wavelength [μm]

Fig. 5.35: Transmission spectrum of a UV-cured epoxy resin ([Bau05])

The waveguides produced also do not reveal any clear change in transmission after the printed circuit boards have been laminated and the soldering tests as well as the temperature cycle tests (-40ºC/+125ºC, > 200 cycles) have been carried out. On the contrary, there was even a decrease, Fig. 5.36.

384


laminate solder

attenuation [dB/cm] 0.25

100 cycles 204 cycles

0.20 0.15 0.10 0.05 0.00 1

2

3

4

5

6

7

8

9 10 11 12 waveguide number

Fig. 5.36: Waveguide attenuation after lamination/soldering and temperature load ([Bau05])

Other up-to-date details have been published, for example in [Schr05b], [Schr06a], [Schr06b], and [Micr06]. The goals for developing optical waveguides are as follows: ¾reduction of the attenuation < 0.1 dB/cm at 850 nm in laminated state ¾increase in reproducibility ¾new methods, e.g. UV direct writing for long waveguides ¾reduced waveguide cross-sections for new applications ¾economical production ¾splitters and crossings ¾45º mirror surfaces for coupling The Microresist company involved in the NeGIT Project uses the combination of EpoCore and EpoClad. The attainable data are indicated in [Micr06] (examples of waveguide structures generated are shown in Fig. 5.37): ¾polymer ¾waveguide ¾glass temperature ¾substrate ¾lamination ¾standard tests ¾optical attenuation

epoxy resin refr. index: EpoCore 1.58, EpoClad 1.57 @ 830 nm > 180º standard FR4 (10 u 10 cm, 8 inch) standard temperature > 185º, pressure 23 kp/cm² reflow: 3 u 15 s at T = 230ºC; TCT 240 u -40ºC / +120ºC |0.2 dB/cm at 850 nm


385

Fig. 5.37: Waveguides, 50 μm lines, 200 μm spacing; wafers with waveguide structure; waveguides with smooth surface and vertical edges (from [Micr06])

A module for coupling the active modules to the optical printed circuit board is shown in Fig. 5.38. Holes are drilled into the printed circuit board into which the module is projected. The light is deflected by means of 45º mirrors.

Fig. 5.38: Coupling of an optical PCB (IZM, [Schr06a])

6. System Design

6.1 Link Power Budgets of Optical Transmission Systems An optical transmission system can be basically divided into three sections: ¾The transmitter, usually a semiconductor source. ¾The transmission link, i.e. an optical fiber with connectors, couplers, elements for launching and coupling out. ¾ The optical receiver, again a semiconductor component with amplifier. 6.1.1 Changes of the Transmitted Power In order to calculate the link power budget, one needs to know the range of fluctuation of the source output power. The attenuation of the link must then be calculated. The two taken together indicate the range of the optical power arriving at the receiver. If this exceeds the dynamic range of the receiver, the system will not operate reliably. The optical power of a semiconductor source is not constant. It is particularly dependent on the temperature and it generally loses power over time. If one intends to deploy a system within a certain temperature range, for example between -20°C and +70°C, the output power fluctuations of the source must be considered in this range. Figure 6.1 shows typical power current characteristics of LD and LED, the most frequent sources used in POF systems, in relation to the temperature (schematic illustration). -20°C 0°C +25°C +50°C +70°C

Popt [mW] 2.0 1.5 1.0

-20°C 0°C

Popt [mW] 5

+25°C

4

+50°C

3

+70°C

2 0.5 current [mA]

0.0 0

10

20

30

40

50

1

current [mA]

0 0

10

20

30

Fig. 6.1: Typical P-I(T) characteristics of a LED (left) and a LD (right)

40

50

388

6.1 Link Power Budget of Optical Transmission Systems

Essentially, the LED shows a flattening of the characteristics curve, i.e. a reduction in efficiency. In the case of the LD, there is also the fact that the threshold current Ith, the start of the laser operation, shifts to higher currents. Later on we will demonstrate that this phenomenon requires very special precautions to facilitate using LD as transmitters. The next step is to establish how the source is to be operated. In the case of the LED, we assume that it is operated with a constant current. That means that we can determine the change of optical power from the P-I(T) curve, see Fig. 6.2. -20°C 0°C +25°C +2 +50°C +1 +70°C

Popt [mW] 2.0 1.5

Popt [dB]

0 1.0

'Popt -1

0.5 0.0

-2 0

10

20

30

40 50 current [mA]

-3 -40

-20

0

20

40

60 T [°C]

Fig. 6.2: Typical P-I(T) characteristics curves for LED and determining the change in Popt

The figure is based on a reference power at 25°C (0 dB). The vertical doublepointed arrow represents the total possible power change within the permissible temperature range. Typical values for the LED power variation between -20°C and +70°C range from 1 dB up to 5 dB. In addition, there are up to 3 dB for changes in output power due to the increasing aging of the source (see also [Schö00]). 6.1.2 Sensitivity of the Receiver Likewise, the sensitivity of receivers is not constant but depends on various factors. For example, one of these is temperature which particularly has an effect on the noise of the amplifier as well as the wavelength of the light received, which in turn is affected by the temperature of the source. The proposed use of the receiver must be clear in order to evaluate it. If it is used for the analogue transmission of a signal, a certain signal-to-noise ratio (SNR) must be complied with as well as a certain degree of linearity which is indicated by the distortions power. A general requirement of digital systems is that a certain probability of bit errors (bit error ratio: BER) is not exceeded. Figure 6.3 shows the typical BER of a system in relation to the power at the receiver. For example, in the case of data transmissions a BER = 10-9 is accepted. That means that the sensitivity of the receiver shown is -32 dBm (0.63 μW). However,


389

the figure also shows a second characteristic of the receiver, the dynamic range. When the power received is too large, it is possible that the amplifier may be overloaded. This will also lead to a deterioration of the signal, even exceeding the permitted BER. In the example shown, this occurs at -12 dBm (63 μW). This means that the dynamic range of the receiver is 20 dB. It follows that it is important for the system design to ensure that the optical power at the receiver always corresponds at least to the sensitivity and that it also does not exceed the top end of the dynamic range. Since transmission systems often have to work with very different cable lengths, the latter requirement is not always easy to meet. BER 10 -5 10 -6 10 -7 10 -8 10 -9 10 -10

limited by receiver overload

limited by noise sensitivity -32 dBm

dynamic range 20 dB

10 -11 10 -12 -36 -34 -32 -30 -28 -26 -24 -22 -20 -18 -16 -14 -12 -10 -8 Preceiver [dBm] Fig. 6.3: Sensitivity and dynamic range of a receiver

Of course, the simplest way for establishing the sensitivity of the receiver is through measurement. The set-up for this test consists of a transmitter (of the best possible quality), the transmission link, the receiver to be tested and a variable optical attenuator (VOA). This attenuation element should allow the tunable setting of a selected attenuation; otherwise it will be necessary to take a parallel measurement of the received power via a coupler, as schematically shown in Fig. 6.4. source

receiver under test variable attenuator

source

receiver under test variable attenuator

splitter

measurement of optical power

Fig. 6.4: Measuring receiver sensitivity with/without reference receiver

390


The second option for determining the receiver's sensitivity, which is particularly important during the design phase, is to perform a theoretical calculation. Assuming that there is sufficient bandwidth and linearity, the noise of the first amplifier stage is usually the limiting factor. As a simple method, one can use the current noise density at the amplifier input which is expressed as pA/Hz. The calculation example below (Table 6.1) shows the necessary steps: Table 6.1: Calculating sensitivity from the current noise density Parameter current noise density at the receiver input bit rate in the system electrical filter bandwidth (t ½ bit rate) current noise at the receiver input required electrical SNR minimum required photo current responsivity of the photo diode at Osource minimum required optical power sensitivity of the receiver

Formula/ Calculation BR 'f IRMS = 'f SNR Iph = IRMS 10(SNR/20) Popt = Iph /

Value 10 pA/Hz 155 Mbit/s 100 MHz 100 nA 12 dB 400 nA 0.4 mA/mW 1 μW -30 dBm

Up to this point we have established the necessary basic data for the transmitter and the receiver for calculating the complete link power budget, as illustrated schematically once again in Fig. 6.5. If the transmitter output power is within the dynamic range (case a) in every case, the sum of losses on the fiber link may be between 0 dB and the maximum value shown. If the maximum possible transmitted output power is over the limit of the dynamic range (case b), a minimum link attenuation, if necessary by providing an additional attenuator, has to be guaranteed. opt. source power

a) specified range of link attenuation

opt. source power

b) specified range of link attenuation

minimum link attenuation

min. receiver sensitivity

-32

-30

-28

max. received power

-26

-24

-22

-20

-18

Fig. 6.5: Determining the permissible link attenuation

-16

-14

-12 -10 -8 Preceiver [dBm]


391

6.1.3 Attenuation of the Fiber Link When calculating the link power budget, the greatest attenuation must be placed on the fiber link. Normally, the system provider has the least influence on this part. In addition, this is the part where the environmental effects are greatest. In this book we cover passive POF transmission systems. Although it is possible to conceive of POF amplifiers, these will not be of any practical significance in the foreseeable future. This means that there will only be loss-making elements between the transmitter and the receiver. Figure 6.6 shows an illustration of all important elements.

3. fiber line 7. receiver

4. connectors

1. source 2. source-POFcoupling

5. passive elements as couplers, filters

6. POF-receiver coupling

Fig. 6.6: Lossy elements within a POF link

6.1.3.1 Coupling Losses from the Transmitter into the POF The first loss we encounter is located at the coupling interface between the transmitter and the fiber. First of all, the transmitter has a certain emitting area and divergence (angle of emission). Since it is not usual to install the fiber directly onto the transmitter but to leave a certain protective distance, it is not possible for all of the light to fall onto the front face of the fiber. Secondly, the fiber is limited in its acceptance angle. Any light falling onto the front face of the fiber at a greater angle will be not guided and radiated. Furthermore, anything less than an ideal surface as well as the refractive index difference between air and PMMA will cause the light to be partially reflected so that it too is lost - see Fig. 6.7.

2. 1. POF

3. source

4.

Fig. 6.7: Causes for losses when coupling to the POF

1. correct coupled light 2. light, reflected at the front surface 3. light, not falling on the front surface 4. light outside the acceptance range of the fiber

392


The most critical of all source parameters is the angle of emission or, more precisely, the far field, i.e. the emitted power in relation to the angle with the optical axis. A standard NA POF has an acceptance angle of approximately ±28°. For a DSI-POF this value is reduced to ±17°. For DSI-MC-POF or GI-POF it is only ±11°. However, the LED used for cost reasons in POF systems emit at a much wider angle. To a certain degree it is possible to reduce the emission angle of a LED by means of lenses. There are a number of LED available which have different emission angles. These are achieved through different designs of the LED housing, which also has the function of acting as the lens. According to the laws of optics, the product of image size and numerical aperture cannot be reduced. This means that a reduction in the angle will result in an increase in the image of the LED chip on the front face of the fiber. Typical LED chips are 200 μm to 300 μm in size. Thanks to a POF diameter of 1 mm there is some room here for maneuvering, as is schematically illustrated in Figs. 6.8 and 6.9.

f

f LED

POF

lens Fig. 6.8: Imaging of the LED chip on to the POF with a 1:1 magnification

f

f LED lens

POF

f

f LED

POF

lens

Fig. 6.9: Imaging of the LED chip on to the POF with a reduction (left) and enlargement (right) of the chip image on the POF

One can see that in the case of the reduction of the LED image, the angle range of the rays increases. Conversely, in the case of a magnification, the angle range becomes smaller. The illustration also shows a second effect too. Typical LED emit at such a large angle that they can hardly be captured by normal lenses. That means that the aperture of the lens determines how much light can be launched. For example, the authors used plano-convex lenses for this purpose with a focal length of 13 mm at 21.4 mm effective diameter. By arranging two lenses behind each other it was possible to place the LED approximately at the focal point so


393

that the useable lens NA was approximately 0.8. However, a much more efficient coupling of the LED can be achieved if the chip is equipped with an appropriate micro-lens fitted by the manufacturer, as shown in Fig. 6.10.

micro lens

LED

POF

Fig. 6.10: Projection of the LED chip on to the POF via a micro-lens

With direct butt coupling of a LED to the POF, the typical losses are in the range of 10 dB to 12 dB. When the imaging is optimized, it is possible to reduce these losses to within a range of 4 dB to 5 dB. Any improved values can only be achieved where specially optimized components are used. Such components are, for example, VCSEL or special LED, as will be described later on. 6.1.3.2 Losses in the Fiber Link In a homogenous fiber in which the light propagates in equilibrium mode distribution, the same proportion of light will be lost in each unit length. Over the whole length there is therefore an exponential decrease which appears linear in the logarithmic expression. The slope of this straight line is the attenuation coefficient in dB/km. For the POF the main causes of attenuation are: ¾Rayleigh scattering ¾Absorption, primarily at the C-H bonds ¾Losses through geometric imperfections at the core/cladding interface ¾Losses through the attenuation in the optical cladding The first two processes are volume related, i.e. their effect is more or less the same for all modes. The latter two processes, however, are largely dependent on the angle at which the light propagates within the POF. For example, [Paar92] establishes the amount of attenuation through the optical cladding by assuming a cladding material attenuation of 50,000 dB/km (see Fig. 6.11). In contrast to glass fibers, the attenuation of different modes varies greatly in the POF. This means that over the length of the fiber, the effective attenuation, i.e. the average attenuation through all modes, varies significantly. This process is further changed through mode conversion at the fiber bends and mode mixing where links are not homogenous. Another factor is that so-called leaky modes which can

394


have propagation angles of up to 90° relative to the fiber axis contribute significantly to light propagation in POF, as was shown in [Bun99a] and [Bun99b]. 90 80

excess loss by cladding attenuation [dB/km]

70 60 50

POF with AN = 0.48 core diameter: 980 μm acceptance angle: ± 28.6°

40 30 20 10 0

4

8

12

16

20

24

28

launch angle [°] Fig. 6.11: Additional attenuation through cladding losses according to [Paar92]

However, it is generally true that when light is being launched with a small NA, a significantly lower level of attenuation can be measured over the first few 10 m, whereas when light is launched with a large NA, the level of attenuation is significantly higher. Numerically this effect is expressed by an attenuation coefficient for equilibrium mode distribution and by an additional value for the overall attenuation deviation resulting from light launching using a different NA. However, the exact determination of this value is still open and current standards only describe it in basic terms. Attenuation data for POF quoted in data sheets have mostly been established with collimated light and are therefore only of limited use in practical application. 6.1.3.3 Connector Losses It is very rare that a long fiber link can be installed as one complete cable. For this reason, it is necessary to connect individual lengths of fiber with plug connectors or by splicing them. In singlemode glass fiber technology, splicing technology is well developed, allowing a connection of fibers with dB losses of only a few percentile fractions. Plug connectors also have a low attenuation amounting to just a few tenths of dB because the fibers are polished very precisely and pressed together at the end faces without any air gap. A major advantage of POF lies in the economic aspect of connector technology. However, the core diameter, a large NA, high permissible tolerances and simple connectors also result in relatively high losses for plug connections (see section on


395

passive components). In the following section, the possible causes for attenuation are described in detail. No practical splicing or bonding technology has as yet been developed for PMMA-POF. 6.1.3.4 Passive Component Losses Apart from connectors, the most frequently used passive components for POF systems are couplers and filters. One can distinguish between symmetric couplers, in which all inputs and outputs have equal status, and asymmetric couplers, in which the splitting rations are different. For symmetric couplers with N arms the minimum losses are: D = 10 log N [dB] It does not matter whether we are talking here about couplers in the sense of elements being joined together (combiners) or about splitters, as follows from the reversibility of the light path. For a clearer understanding, one should imagine a 2-to-1 coupler as a 2-to-2 coupler with unused output. Where a system is intended to connect several transmitters with several receivers, there are basically two feasible solutions, i.e. spatially separated couplers and dividers or one central coupler, as shown in Fig. 6.12.

distributed coupler

central coupler Fig. 6.12: Options for MP-MP structures (MP: Multi Point)

In the top arrangement, the loss through the couplers is at least 12 dB, i.e. 6 dB through the coupling of the four arms and a further 6 dB through their subsequent division. In the bottom arrangement, it is possible to reduce the loss to a total of 6 dB as shown in Fig. 6.13 with the detailed illustration of the coupler. While there is in fact only one single fiber between coupling and division point in the top arrangement, there actually are four fibers or a correspondingly wider wave guide provided in the bottom arrangement.

396 1

6.1 Link Power Budget of Optical Transmission Systems 1

1/4

1/2

1/4

1/2

1/4

1/4

1/4

1/4

1/4

1/4

Fig. 6.13: Central 4 u 4 couplers

The version on the left shows a 4 u 4 coupler that is composed of x-couplers. The central connection is actually not a coupler, but a crossing of two wave guides. The example of one path shows that the power is evenly distributed over all paths. A 4 u 4 coupler featuring an appropriately wide wave guide as a central element is shown on the right. This has to have the correct dimension as it is instrumental in ensuring that all inputs are evenly distributed to all outputs. By comparison, Fig. 6.14 shows an 8 u 8 coupler, illustrating the increasing complexity with the rising number of ports.

Fig. 6.14: Central 8 u 8 coupler made up of x-couplers

This short overview of coupler technology shows that one should not use general formulas unless the topology applied is completely clear. Filters are always useful in POF systems when wavelength multiplex is used. A number of different solutions have already been presented in Chapter 3.4. The large diameter of the POF makes it possible to fit filters directly between the end faces of two connectors, as shown in Fig. 6.15. Here, the losses occur in the area of the connector attenuation. The second option is the widening of the ray with the help of a lens or mirror and placing the filter element into the parallel ray (Fig. 6.16). Due to the large NA of POF, lenses are often not in a position to efficiently capture the light so that losses of 5 dB are typical.


coupling POF with adapter connector

colored foil

397

finished filter

POF with connector

Fig. 6.15: Filters in a POF connector

filter

POF

POF lens Fig. 6.16: Filter with beam expansion

6.1.3.5 Coupling Losses between POF and Receiver Coupling a photodiode to a POF appears relatively simple at first. In contrast to LED, the far field of a POF is a relatively well known factor, being, for example, within the range of ±30° for a standard NA POF. This means that one simply needs to place a sufficiently large LED relatively close to the end of the POF in order to capture practically all of the light that is coupled out. One main problem, however, is the fact that in the case of photodiodes the area directly determines the diode capacity CPD. It is important here that this capacity, together with the input resistance R of the amplifier, forms a low-pass filter, the critical frequency of which is formed by the product CPDR. R determines what voltage can be generated by a given photo current Iph a Popt. The electrical power is proportional to U² which also means that it is proportional to R². However, since the noise power is only proportional to R, the signal-to-noise ratio increases in direct proportion to R. This means that a larger diode area will limit either the bandwidth or the sensitivity, depending on the choice of R. In practice, it is the diode's capacity that primarily presents the limiting parameter, at least for 1 mm POF systems. One will therefore endeavor to use as small a photodiode as possible to which the POF is coupled via a micro-lens. Commonly available receivers operate with diodes of approx. 0.7 mm diameter. For the reduced diode capacity, losses in the range of 2 dB are accepted for coupling via a lens.

398


6.1.4 The Link Power Budget of the ATM Forum Specification Many of the processes described above in general terms have been studied in various standards using exact quantities. Between 1996 and 1999, the ATM Forum has established a specification for the transmission of 155 Mbit/s over 50 m of PMMA-POF ([ATM96a], [ATM96b] and [ATM99]). In these documents the different contributions to the link power budget are described in great detail so that they can be used here as a very informative example. 6.1.4.1 Loss Analysis by the ATM Forum We will use the pie chart below to represent the different proportional shares of the link power budget. The complete circumference corresponds to the range between maximum transmission power and minimum sensitivity. The respective segments represent the proportional shares of the link power budget. Figure 6.17 shows the link power budget according to [ATM96b] for a 155 Mbit/s connection over max. 50 m. 22

2

20 4 18 6 16 8 14 12

10

LED power variations mode dependent loss loss of POF at 650 nm loss at connectors source spectral width and drift of center wavelength influence of temperature and humidity fiber bends

cumulated loss [dB]

Fig. 6.17: Link power budget according to the ATM Forum specification

We will investigate the individual contributions in more details below. The total link power budget available is 23 dB. The maximum permissible optical power of the transmitter at 650 nm is -2 dBm which is determined by eye safety and the LED current, e.g. 30 mA for HP components. The guaranteed sensitivity of the receiver is -25 dBm (3 μW). The maximum permissible receiver input is identical to the maximum transmitter power at -2 dBm which means that the receiver cannot be overloaded. 6.1.4.2 Changes in the Transmission Power The specification allows a maximum of 6.0 dB of possible changes in the LED power due to temperature, aging of the source and manufacturing tolerances. This means that the guaranteed minimum output power is -8 dBm (158 μW) for a max. value of -2 dBm.


399

The main part of the permissible changes is determined by the fluctuation of the LED power caused by temperature. Figure 6.18 shows the spectrum of a 650 nm LED, as used in components by Hewlett Packard (Agilent, Avago), in relation to the temperature (according to [HP04]). The figure clearly shows the reduction in emitted power at higher temperatures. Another effect is the shifting of the emission wavelength towards greater values. As we will see later on, this process also results in additional losses. 1.4

-40°C

opt. power [a.U.]

1.2

0°C

1.0

+25°C

0.8

+70°C

0.6

+85°C 0.4 0.2 0.0 610

620

630

640

650

660


Fig. 6.18: Changes in the LED spectrum under the influence of temperature ([HP04])

The power level's high degree of sensitivity to temperature is primarily determined by the selection of the semiconductor material. [Nak97] quotes a difference of approximately 4.5 dB for the change in optical power of a GaAlAs LED between -20°C and +70°C, as shown in Fig. 6.19. This value is a little lower for quaternary materials (AlInGaP). rel. opt. power [a.u.] 1.5 approx 4.5 dB 1.0 GaAlAs LED

0.5 temperature [°C] -20

0

20

40

60

80

Fig. 6.19: Change in output power of a GaAlAs LED under the influence of temperature changes according to [Nak97]

400


[Schö99a] presents new LED for use in POF systems in automotive applications where temperature resistance is of particular importance. The LED shown here (DH-MQW: Double Heterostructure Multi Quantum Well) feature less than 1 dB of output power change between -20°C and +70°C. The amount of power reduction due to aging of the LED is determined primarily by the operating temperature and the operating current. Halving the current or reducing the operating temperature by 10 K can increase the service life by approximately one order of magnitude. The degree of production tolerance is proportional to the care and effort afforded in manufacture. It may be necessary to select the appropriate components. 6.1.4.3 Attenuation of the Polymer Optical Fiber Link The ATM Forum specification allows for a link attenuation of 13 dB for a link of 50 m length. This is sub-divided as follows: ¾7.8 dB attenuation of the POF at room temperature (156 dB/km) for 650 nm ¾0.5 dB additional loss through launching from a divergent source with max. AN = 0.30 ¾3.4 dB additional attenuation due to the spectral characteristics of the source (max. 40 nm width and ±10 nm deviation from the center wavelength) ¾1.3 dB additional attenuation through external factors Whereas the first value is easy to establish and, above all, can also be measured relatively easily, it is necessary to investigate in greater detail the additional loss that results from the characteristics of the spectral source. Fiber Attenuation at the Source Wavelength Figure 6.20 compares the attenuation spectra of different 1 mm SI-POF made from PMMA. 10,000

attenuation [dB/km]

5,000 2,000 1,000 500

Toray St.-NA 1995 PCU-CD1002 Mitsubishi St.-NA 1995 Eska Extra Asahi Low-NA 1996 NC-1000 Toray DSI 1997 PMU-CD1002 Mitsubishi DSI 1997 Eska Mega Asahi St.-NA 1996 TC-1000

200 100 50 350

wavelength [nm] 400

450

500

550

600

650

700

Fig. 6.20: Attenuation spectra of different 1 mm PMMA-POF

750

800

850

900


401

At a wavelength of 650 nm, all fibers meet the ATM Forum requirements with an attenuation of less than 156 dB/km, as is shown in greater detail in the illustration of different fibers in Fig. 6.21. Below 550 nm, clear differences can be recognized between the different types of fiber. There are hardly any measurements of fiber attenuation under conditions of equilibrium mode distribution. Manufacturers usually specify fibers with collimated light or excitation with a small NA. Detailed studies on the attenuation in polymer optical fibers, particularly in regard to the launching conditions, are described in [Kell98], [Pfl99], [Hen99], [Pei00a] and [Pei00b]. 400

POF attenuation [dB/km]

350 GH 4001 SH 4001

300

MH 4001 250

MH 4002

ATM-Forum specification

PMU-1001

200

TC-1000 GH 4001

150 100 630

635

640

645

650

655

660

665

670

675

680

wavelength [nm] Fig. 6.21: Attenuation of different POF around 650 nm

Spectral Filter Effect for Broad Sources In practice, POF systems are not designed with ideal sources having constant wavelengths of 650 nm. Instead they have LED with a certain spectral width (Full Width at Half Maximum: FWHM) and permissible deviations from the specified wavelength. The effective attenuation must be calculated for a given spectrum. This is carried out as described in table 6.2. Since the attenuation of the POF increases rapidly on both sides of the 650 nm minimum, the effective attenuation for a wide spectrum source will always be larger. In this context the so-called filter effect occurs. This means that the LED spectrum will be clearly changed in shape when passing through a long fiber link. Light at 650 nm undergoes relatively little attenuation while the spectral parts neighboring this value suffer from higher losses. This makes the LED spectrum narrower. If the center wavelength of the LED is not exactly at 650 nm, the spectral maximum is shifted towards the POF attenuation minimum.

402


Table 6.2: Steps for the calculation of the effective POF attenuation with LED Parameter normalized LED spectrum

Formula / Calculation PLED(O)

LED power

Unit 1/nm 1

f

P0 =

³ PLED (O)dO = 1

O 0

spectral POF attenuation

dB/km

D(O) l P´LED (O)

link length LED spectrum after the POF LED power after the POF

km 1/nm 1

f

Pc(l) =

³ PLED (O) 10

( D( O ) / 10l)

dO

O 0

effective attenuation

Deff = 10 log(P´/P0)/l

dB/km

effective excess loss

Dexcess = Deff - D(O)

dB/km

Figure 6.22 shows the change of a Gaussian spectrum with 40 nm spectral width and 660 nm center wavelength in linear scale for a transmission over 50 m POF. The shift towards 650 nm can be clearly seen. 500 POF-loss [dB/km] 450

1.0 0.9

rel. power [a.u.]

0m

400

0.8 0.7

350

10 m

300

0.6 20 m

0.5 0.4

250 200

30 m

150

0.3 40 m

0.2

100 50

0.1 0.0 620

50 m 630

640

650

660

670


Fig. 6.22: Deformation of the spectrum of a 660 nm LED through POF attenuation

Figure 6.23 shows the filter effect for lengths up to 200 m of POF in logarithmic scaling. For this purpose a measured LED spectrum as well as the measured spectral POF attenuation were used to mathematically determine the spectra after passing through different fiber lengths. Here too, one can clearly recognize the shift towards 650 nm as well as the narrowing of the spectrum.


0

403

rel. power [dB]

-10

-30

0 m POF 10 m POF 20 m POF 50 m POF

-40

75 m POF

-50

100 m POF

-20

-60

150 m POF

wavelength [nm]

-70 620

200 m POF

630

640

650

660

670

680

690

700

Fig. 6.23: Change of the spectrum of a LED (FH511) through POF attenuation

Due to the filter effect, the effective attenuation of the fiber is no longer proportional to its length. A relatively wide LED spectrum at the beginning of a POF experiences a relatively high averaged attenuation since a great degree of power is placed in spectral areas with high POF attenuation. After some distance the spectrum has become narrower and its center of gravity is close to the attenuation minimum so that the effective attenuation decreases. This indicates the difficulties experienced when measuring POF attenuation with LED sources as well as with correct specification of the effective attenuation. The ATM Forum specification allows sources with a spectral width of max. 40 nm and a center wavelength of 650 r 10 nm. No statements are made about the shape of the LED spectrum. However, most LED can be approximated quite well to Gaussian shaped spectra. In order to calculate the effective additional loss, we will use the Gaussian LED spectra and the POF attenuation curve according to [Wei98] (losses at 650 nm: 132 dB/km). Figure 6.24 shows the results of the calculation for a spectral width of 40 nm and deviations from the center wavelengths 650 nm up to 20 nm, i.e. for center wavelengths of 630 nm, 640 nm, 650 nm, 660 nm and 670 nm (calculations as explained in Table 6.2). 8.0

eff. excess loss [dB] 630 nm 640 nm 650 nm 660 nm 670 nm

7.0 6.0 5.0 4.0 3.0

ATM-Forum specification

2.0 1.0 0.0

POF length [m] 0

10

20

30

40

50

60

70

80

Fig. 6.24: Additional effective losses of a 40 nm wide LED source

90

100

404


The limit value of the ATM Forum for a maximum of 50 m has been entered. The specification allows 3.4 dB of additional loss. The diagram shows 3.61 dB as the max. value for the curves with 640 nm to 660 nm center wavelength, whereby, however, a value of 132 dB/km is assumed for 650 nm, which means only 6.6 dB for 50 m instead of the 7.8 dB stated in the specification. One can see very clearly the flattening of the curve due to the filter effect, i.e. the convergence of the effective attenuation to the value at the attenuation minimum. Figure 6.25 shows comparable results for a source which again is of Gaussian shape, this time with only 20 nm full width at half maximum and center wavelengths between 630 nm and 670 nm. 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0

eff. excess loss [dB] 630 nm 640 nm 650 nm 660 nm 670 nm

POF length [m] 0

10

20

30

40

50

60

70

80

90

100

Fig. 6.25: Additional effective losses of a 20 nm wide LED source

In this case, the value stays significantly below the limit set by the ATM Forum. For sources between 640 nm and 660 nm center wavelength, the narrower spectrum is associated with smaller additional losses. However, for sources with 630 nm and 670 nm wavelength the additional attenuation is significantly larger compared with those sources having a width of 40 nm. Lastly, Fig. 6.26 shows the additional losses for monochromatic sources, for example, singlemode lasers. 24.0 eff. excess loss [dB] 22.0 630 nm 20.0 18.0 640 nm 16.0 650 nm 14.0 660 nm 12.0 670 nm 10.0 8.0 6.0 4.0 2.0 0.0 0 10 20 30

POF length [m] 40

50

60

70

Fig. 6.26: Additional losses from a monochromatic source

80

90

100


405

As will be readily appreciated, there is no filter effect in this case, since only the kilometric attenuation of the source wavelength is of importance. For 660 nm the additional loss is 3.35 dB again, i.e. near the limit of the specification. For greater deviations from the wavelength of the attenuation minimum, the additional losses rise rapidly in accordance with the attenuation characteristics of the POF. This leads us to the three basic statements below: ¾Wide sources create additional attenuation since a large part of the spectrum is located in spectral areas with high POF attenuation. ¾Deviations of the source center wavelength from the attenuation minimum lead to additional losses due to increasing POF losses. ¾If a source is significantly outside the attenuation minimum, a wide spectrum limits the additional losses up to a certain degree, since parts of the spectrum lie in low-loss areas. In the case of very long links, the additional losses increase less than proportionally due to the filter effect. The last point also provides an explanation of why the effects of center wavelength and spectral width cannot be established separately but must always be considered in their (non-linear) relation to each other. Mode Filter Effect Apart from the spectral filter effect, i.e. varying attenuation for the different wavelengths, we must also consider the mode filter effect. As already described above, different modes have different attenuations. For example, Fig. 6.11 from [Paar92] illustrated the relation to the propagation angle relative to the fiber axis for meridional rays. It makes sense to state the attenuation of a POF for conditions of equilibrium mode distribution. If the source has a different light launching characteristic, the actual attenuation may be larger - particularly for wide light launching angles - or smaller. The latter applies in particular for laser light with low divergence which is launched parallel to the fiber axis. The ATM Forum specifies a maximum additional attenuation of 0.5 dB for the launching with sources with a maximum NA of 0.30. After some 10 m of fiber link distance, the condition automatically balances out due to mode conversion and mode coupling in the POF. This means that the mode filter effect is limited to the beginning of the fiber link. It is therefore customary not to measure this in dB/km, but to describe it as a cumulative additional attenuation. In [Kle00] the effect of the propagation angle on the attenuation, relative to the fiber axis, has been investigated experimentally. For this purpose, a thin laser beam (594 nm wavelength) is launched centrally into the POF front face under different angles. The output power at the fiber end is determined with the help of an integration sphere in order to capture all modes created. Figure 6.27 shows the results for a standard NA POF. Rays launched at angles between approximately -10° and +10° relative to the fiber axis show no discernible change in attenuation, but at angles of ±20° the additional effective attenuation is already some 10 dB/km, i.e. one third of the specified value. These rays are still clearly within the accepted range of the POF which means that the additional attenuation cannot be explained on the basic of

406


ray optics but must be due to losses at the core/cladding interface. Up to the limit of the acceptance range, the additional losses increase to some 100 dB/km, i.e. a multiple of the basic attenuation. The measurement at 50 m length reveals significantly lower attenuations. This can be explained by the fact that part of the input power launched at the wide angle is shifted along the fiber path to smaller propagation angles due to mode conversion. Therefore, the values no longer reflect the conditions of exactly one angle of propagation but instead an average of the modes generated along the path.

excess loss [dB/km] 500 10 m 400

O= 594 nm

300 50 m

200 100 0 -30

-20

-10

0

10 20 30 propagation angle [°]

Fig. 6.27: Angle-related additional attenuation of standard NA POF according to [Kle00]

Figure 6.28 shows the comparable results for the measurement with a doublestep index profile POF which basically shows the same behavior. In contrast to the standard POF, however, high additional attenuations in the range between ±20° and ±30° can be seen. These angle ranges carry light which is no longer guided by the interface between core and inside cladding but is still completely reflected at the interface between the inner and outer cladding. The significantly higher attenuation of the inner cladding compared to that of the core causes the high additional loss. It is precisely this effect that ensures that after some 10 m there is only light present in the acceptance range of the inner cladding. In this case too, mode conversion leads to a reduction in the additional losses after some length of the POF. The measurement of angle-related additional attenuation according to the method developed by Klein facilitates very swift and explanatory insight into the function of mode-related attenuation and mode conversion which are jointly responsible for the mode filter effect. One can only hope that this method will be adopted into the specifications for measuring POF.


600

excess loss [dB/km]

407

10 m

500 O= 594 nm

400 300

50 m 200 100 0 -30

-20

-10

0

10 20 30 propagation angle [°]

Fig. 6.28: Angle-related additional attenuation for a DSI-POF according to [Kle00]

6.1.4.4 Connector Losses As already described in the overview of the loss mechanisms, it is often necessary to insert connectors for the installation of cables. The ATM Forum allows a maximum of two connectors with a maximum of 2.0 dB insertion loss permitted at each connection. Determining the connector loss is not easy, both from the aspect of measuring technology as well as from the aspect of theory. Again, the deviation from the correct mode distribution is one of the reasons for recurring problems. The main cause for additional attenuation in a connector is represented by the mechanical properties of the plug/coupler combination as well as the tolerances of the fibers used. Table 6.3 below shows the losses established for an SI-POF with AN = 0.30 in accordance with the ATM Forum specification. Table 6.3: Calculation of maximum connector loss

Maximum Connector Loss (acc. to ATM Forum specification, SWG Phy. Layer , RBB, Doc. 95-1469) cause parameter attenuation attenuation of connector lateral offset 0.1 mm max. 0.4 dB roughness of fiber end face 5 μm 0.1 dB angle between fiber axes 1° 0.1 dB Fresnel losses n = 1.49 0.3 dB external effects1) 0.3 dB losses due to fiber characteristics core diameters and differences in NA 0.8 dB total 2.0 dB 1)

20 times plug/unplug, vibration at 10-2000 Hz, 15g, +70°C temp. for 96 h; -25°C temp. for 96 h, -25°C/+70°C, 10 cycles, +25°C/+65°C, 90-96% RH, 10 cycles 2) core diameter 931 μm to 1029 μm, NA from 0.30 to 0.35

408


As was shown in Chapter 3, the protruding material, which arises with the hot plate process, acts as an axial distance of some tenths of a millimeter which results in additional attenuation of typically 0.4 dB. It is not clear why the ATM Forum specification does not take this value into account despite the recommendation for the hot plate process. Table 6.4 summarizes once again the losses considered here. Table 6.4: Overview over the connector losses under UMD conditions Loss Mechanism

ATMForum

Assumptions

Calculation with UMD (AN = 0.30)

differences in the core diameter

0.80 dB

dmin = 931 μm dmax = 1,039 μm

0.59 dB

differences in the numerical aperture

-

AN min = 0.30 AN max = 0.35

1.34 dB

lateral displacement

0.40 dB

x = 100 μm

0.64 dB

angle displacement

0.10 dB

max. 1°

0.16 dB

Fresnel reflection

0.30 dB

n = 1.492

0.35 dB

axial displacement

n. a.

s = 400 μm

0.33 dB

external influences

0.30 dB

the same

0.30 dB

end face roughness

0.10 dB

the same

0.10 dB

sum

2.00 dB

3.81 dB

If the losses due to roughness and external mechanical losses are assessed right away, then the overall result is almost a doubling of the attenuations with the UMD conditions accepted here compared with the ATM Forum specification. 6.1.4.5 Additional Losses through External Influences Fiber Bending along the Path No fiber can be installed exactly straight so that bending must be taken into account in all cases. Each bend leads to additional losses in the case of multi-mode fibers. It is easy to imagine that a light beam within the acceptance range of the fiber can exceed the angle of the total reflection at a bend and is therefore attenuated. The ATM Forum allows an additional attenuation of 0.5 dB for bends. The number of bends permitted is 10 bends at 90° each and a minimum bending radius of 25.4 mm. Figure 6.29 shows bending losses for a standard POF (Toray) according to data sheet [Tor96b]. The measurement of the attenuation of 15 bends of 90° each follows the first draft of the ATM Forum specification. The fiber presented met the limit requirement as indicated by the yellow arrow. In addition, the attenuation of a bend of 360°, i.e. a full circle, is drawn in. This cannot simply be determined by multiplying the attenuation of a 90° bend by 4.


409

0 bending loss [dB] -1 -2

PFU-UD1001 St.-NA one 360° bend

-3

PFU-UD1001 St.-NA 15 bends by 90°

-4 -5

bending radius [mm] -6 0

10

20

30

40

Fig. 6.29: Bending losses for a standard NA-POF according to [Tor96b]

If the bends are distributed over the length of the fiber, it is possible that mode conversion occurs between them. The first bend leads to the extraction of certain modes which exceed the acceptance range. If another bend follows immediately, the modal field has already adapted, thus usually lowering the attenuation. If the bend occurs at the same distance, the mode distribution has already changed towards equilibrium mode distribution. In addition, a subsequent bend usually bends in another direction. Effect of Temperature and Relative Humidity The ATM Forum specification allows an additional attenuation of a maximum of 0.8 dB for the effect of the climate on the POF link. That corresponds to a kilometric attenuation of 16 dB/km. The maximum values permitted are 95% relative humidity (RH) and +70°C temperature. The effects of relative humidity and temperature on the attenuation of POF are relatively complex so that a separate section is devoted to these aspects (see Chapter 9.6.1). In this section we only wish to mention the essential processes: ¾During increases in temperature up to +70°C in a dry environment, the attenuation of the POF fiber hardly changes. For bent fibers there is often even a reduction in the losses because the fiber tension is reduced. ¾With an increase in the relative humidity, in particular during higher temperatures, the core absorbs water which can lead to an increase in the attenuation of typically 10 dB/km. This effect is reversible which means that the fiber releases the water again in a dry environment. ¾During long term storage at high temperatures (above +70°C) the capability of absorbing water increases and thereby the additional attenuation in a humid environment. This process is also reversible.

410


¾Long storage at high temperature and relative humidity leads to a steady and irreversible aging of the fiber leading to a typical additional attenuation of 60 dB/km for each 1,000 h of aging (for +85°C/95% RH, see also [Ziem00b]). Above a certain threshold, increased aging sets in which will destroy the internal structure of the fiber. For +70°C this time span is over 20 years. Overall the aging effects for polymer optical fibers have not been systematically investigated to any great extent. It is quite probable that the current specifications will have to be adjusted since, under certain conditions, the additional attenuations calculated are exceeded under the specified conditions. On the other hand, it does not make much sense to stipulate a relative humidity of 95% at +75°C for the total maximum link length in buildings since this type of climate would make a normal building at least uninhabitable. Any short term exposures or the effect on short sections can easily be managed by current POF. We have now covered all aspects of the link power budget of the ATM Forum specification. It shows that many of the individually discussed mechanisms require relatively complex calculations. Overall one cannot completely follow budget considerations. In particular, several questions remain open in regard to the definition of additional loss due to spectral source characteristics, connectors and climatic effects. In the next chapter, we wish to show what improvements can be achieved in the link power budget through different components. The biggest potential is represented by the selection of source wavelength and the type of transmitter. 6.1.5 Choice of Wavelength for POF Systems Initially, it would appear that the selection of source wavelengths for POF systems is relatively easy, being determined by the minimum attenuation. However, closer study reveals that a multitude of criteria must be considered, the most important of which are listed below: ¾low POF attenuation for the LED wavelength ¾low effective additional loss, taking into account the spectrum ¾low drift of the wavelength with temperature ¾low spectral width ¾high optical power ¾small emitting surface area ¾small angle of emission ¾long service life ¾good efficiency ¾large modulation bandwidth ¾low price ¾good availability, preferably from several manufacturers ¾large operation temperature range ¾low temperature coefficients for wavelength and power

411


Further points can be added to this list. Many of the parameters lead to completely different solutions so that the final selection always represents a compromise. The best spectral characteristics, the largest modulation bandwidth and the highest power are offered by laser diodes. However, these are not always cheap and often have large temperature coefficients. Red LED are cheap and widely available but have unfavorable spectral characteristics. Green LED or red VCSEL have better parameters but are not yet widely available. In the following sections, we will be looking initially at LED as possible transmitters for POF systems. Subsequently, we will carry out a theoretical comparison of different semiconductor structures. 6.1.5.1 LED as Transmitters for POF Systems Different LED sources have been investigated, e.g. in [Schö00] and [Arn00]. We will show some results here in detail. Initially, we will only investigate the question of spectral additional attenuation. For 15 different LED delivered from Nichia, R&S Components, Farnel and Conrad, the spectra were measured at ambient temperatures of between -20°C and +70°C. For calculating the spectral excess loss, the typical attenuation curve was applied as shown in [Wei98] and presented here again in Fig. 6.30 for a clearer understanding of the results. The effective attenuation of the LED for a transmission over 50 m of POF was determined by calculation each time. The calculation follows the approach shown in Table 6.2 above, however, instead of a Gaussian curve, the exact LED spectrum was entered into the calculation. Any effects caused by different launching conditions were disregarded. Table 6.5 shows all the different types of LED that were considered.

5,000

POF attenuation [dB/km]

2,000 1,000 500 200 100 wavelength [nm] 50 400

450

500

550

600

650

700

750

800

Fig. 6.30: POF attenuation spectrum according to [Wei98] for calculating the effective attenuation (range 700 nm to 800 nm added according to [LC95])

412


Although these LED have a relatively high power for their wavelength, they do not in every case represent the maximum achievable values. One essential cause for this is the fact that most manufacturers of visible LED do not state the optical output power. A typical specification covers the illumination and the angle of emission. If the far field and spectrum are not known, these data only allow a limited conclusion regarding the optical power. Often, these data are also inaccurate and have been determined using different measuring conditions. Another factor is that many LED vary a great deal in their intensity from unit to unit so that only typical values are stated. Table 6.5: Different LED in the visible range

Type/ Supplier Sander Sander Sander RS Sander Conrad Nichia Nichia RS RS RS Farnell Conrad Farnell Conrad RS RS RS

Wavelength

Power at 20°C and 20 mA (total)

Spectral Width (20°C)

430 nm 450 nm 470 nm 470 nm 500 nm 520 nm 525 nm 560 nm 563 nm 583 nm 594 nm 609 nm 615 nm 621 nm 626 nm 650 nm 660 nm 700 nm

0.6 mW 2.2 mW 4.5 mW 0.05 mW 2.6 mW 2.0 mW 3.9 mW 1.9 mW 0.011 mW 0.12 mW 0.7 mW 2.4 mW 2.3 mW 2.9 mW 4.0 mW 0.45 mW 4.7 mW 0.50 mW

62 nm 32 nm 24 nm 68 nm 32 nm 38 nm 38 nm 42 nm 28 nm 32 nm 16 nm 17 nm 18 nm 18 nm 18 nm 42 nm 21 nm 66 nm

Material InGaN/GaN InGaN/GaN InGaN/GaN SiC InGaN/GaN InGaN/GaN InGaN/GaN InGaN/GaN no info. GaAsP AlInGaP AlInGaP AlInGaP AlInGaP AlInGaP no info. AlGaAs GaP

When these units were measured, differences up to a factor of 5 were discovered between the absolute measured power and the respective values calculated from the data given on the data sheets. It follows that the optimum selection of a suitable source must rely on practical trials. For the mass production of POF components specially optimized parts are used rather than LED “off the shelf”. All the elements listed here are standard LED in a 5 mm housing. The absolute power is only representative within limits while the spectral characteristics and temperature response are largely typical for the material and therefore allow a good comparison between the different LED groups.

413


As a first step, Fig. 6.31 shows the relation of optical power to the ambient temperature, integrated across the whole spectrum. The change between -20°C and +70°C is represented as an average value in dB/K. All LED show a decreasing output power for an increase in temperature. However, the size of the coefficient 'Popt/'T varies a great deal. For the LED investigated, the values range from -0.002 dB/K to -0.052 dB/K. GaN LED ranging from 470 nm through 560 nm show the least change in optical power in response to temperature. The 593 nm LED shows the strongest reduction in power. Sander 430 nm Sander 450 nm RS 470 nm Sander 470 nm Sander 500 nm Conrad 525 nm Nichia 525 nm Nichia 560 nm RS 563 nm RS 583 nm RS 593 nm Farnell 609 nm Farnell 621 nm Conrad 625 nm Conrad 640 nm RS 650 nm RS 660 nm RS 700 nm 0.00

'Popt/'T [dB/K] -0.01

-0.02

-0.03

-0.04

-0.05

-0.06

Fig. 6.31: Temperature coefficient of the optical power of different LED

In a further step, the effect of temperature dependency of spectra on the effective attenuation was investigated for a POF of 50 m in length. In order to perform the calculation, all spectra were first normalized to the same overall power so that only the width and spectral position are entered. The numerically determined values are shown in Table 6.6. In a number of cases, the change in the effective attenuation is negative which only means that the effective attenuation of the LED is smaller than the POF attenuation at the LED center wavelength. The loss of optical power increases with positive values which is primarily caused by the width of the spectrum. Figure 6.42 shows the ranges of additional loss for the LED in the wavelengths ranges between 430 nm and 593 nm. The POF attenuation spectrum is here overall relatively low and above all flat.

414


Table 6.6: Effect of the LED spectrum on effective POF attenuation

Type

Eff. POF Attenuation POF Effective Excess Loss Attenuation of the POF (over 50 m POF) min. max. max. min. at OLED

Sander 430 nm Sander 450 nm RS 470 nm Sander 470 nm Sander 500 nm Conrad 525 nm Nichia 525 nm Nichia 560 nm RS 563 nm RS 583 nm RS 593 nm Farnell 609 nm Farnell 621 nm Conrad 625 nm Conrad 640 nm RS 650 nm RS 660 nm RS 700 nm

5.50 dB 4.54 dB 4.72 dB 4.51 dB 4.04 dB 4.16 dB 4.24 dB 4.58 dB 4.33 dB 5.67 dB 6.17 dB 11.11 dB 12.83 dB 13.11 dB 10.93 dB 10.58 dB 10.27 dB 15.03 dB

5.45 dB 4.48 dB 4.62 dB 4.50 dB 4.04 dB 4.12 dB 4.21 dB 4.55 dB 4.25 dB 5.00 dB 4.88 dB 8.88 dB 12.22 dB 12.00 dB 9.19 dB 10.02 dB 8.59 dB 14.96 dB

Sander 430 nm

119.0 dB/km 101.0 dB/km 88.0 dB/km 88.0 dB/km 76.0 dB/km 76.5 dB/km 76.5 dB/km 73.0 dB/km 70.5 dB/km 80.0 dB/km 98.5 dB/km 249.0 dB/km 443.5 dB/km 433.0 dB/km 184.0 dB/km 132.0 dB/km 199.0 dB/km 498.0 dB/km

-0.45 dB -0.51 dB 0.32 dB 0.11 dB 0.24 dB 0.33 dB 0.41 dB 0.93 dB 0.80 dB 1.67 dB 1.24 dB -1.34 dB -9.34 dB -8.54 dB 1.73 dB 3.98 dB 0.32 dB -9.87 dB

-0.50 dB -0.57 dB 0.22 dB 0.10 dB 0.24 dB 0.29 dB 0.38 dB 0.90 dB 0.72 dB 1.00 dB -0.05 dB -3.57 dB -9.95 dB -9.65 dB -0.01 dB 3.42 dB -1.36 dB -9.94 dB

temperature range: -20°C to +75°C

Sander 450 nm RS 470 nm Sander 470 nm Sander 500 nm Conrad 525 nm Nichia 525 nm Nichia 560 nm RS 563 nm RS 583 nm RS 593 nm -1.0

-0.5

0.0

0.5 1.0 1.5 2.0 change of the effective loss [dB]

Fig. 6.32: Effect of the spectrum on the additional attenuation of a 50 m POF


415

All sources with values near the absolute minimum of 520 nm and 570 nm create additional losses. The sources with short wavelengths at 430 nm and 450 nm are on the downward slope of the first minimum and have a somewhat smaller excess loss due to the filter effect. The relatively large increase in the effective attenuation of the LED with 583 nm and 593 nm occurs because of the shift in the direction of significantly higher attenuations at the 620 nm maximum. Figure 6.33 shows the results for LED above 600 nm.

Farnell 609 nm Farnell 621 nm Conrad 625 nm Conrad 640 nm RS 650 nm RS 660 nm RS 700 nm -10

temperature range: -20°C to +75°C

-8

-6

-4

-2

0

2

4

6

change of the effective loss [dB] Fig. 6.33: Effect of the spectrum on the additional attenuation for a POF of 50 m length

In this range of wavelengths, the narrow minimum around 650 nm dominates the POF attenuation spectrum as well as the high attenuation peak at 620 nm and the steep increase beyond 650 nm. This is clearly proved by the great differences in Fig. 6.33. The LED at 621 nm and 625 nm achieve approximately 10 dB improvement due to the filter effect, albeit against a background of a very high value for attenuation, while the effective additional loss of the red LED with 650 nm is up to 4 dB. The varying fluctuations are due in particular to different temperature coefficients of the center wavelengths. In the last step we consider the effects of the change of spectral characteristics and decrease of the optical output power together. Figures 6.34 and 6.35 show the result for the wavelength ranges of 430 nm to 560 nm and 606 nm to 660 nm. As already described, the effective POF attenuation was here calculated from the integrated spectra of the LED before and after the POF link. When considering the permissible attenuation of the ATM Forum specification of 11.2 dB (7.8 dB basic attenuation plus 3.4 dB additional losses), the POF meets these requirements for all the LED, in particular since the change of the LED output power through temperature variation has already been taken into account (contained in the 6.0 dB of permitted fluctuation). In the range of 450 nm to 560 nm, the effective attenuations are between approximately 4 dB and 5 dB. For the LED at the blue and yellow edge, the effective attenuation increases to approximately 6.5 dB with a significant increase in sensitivity to temperature changes. The LED in the green window in particular promise significantly greater transmission lengths. Figure 6.35 shows the results of LED above 600 nm wavelength.

416

-3.8

6.1 Link Power Budget of Optical Transmission Systems effective POF loss [dB/50 m] Sander 500 nm Conrad 525 nm Nichia 525 nm Sander 470 nm Nichia 560 nm RS 563 nm Sander 450 nm RS 470 nm

-4.2 -4.6 -5.0 -5.4 -5.8 -6.2 -6.6

Sander 430 nm -20 -10

0

10

20

30

40

50

60

RS 583 nm 70

temperature [°C] Fig. 6.34: Effective attenuation of 50 m POF with different LED

The classic 660 nm LED still meet the requirements of the ATM Forum, albeit with a high sensitivity to changes in temperature, which is caused in particular by the LED wavelength running out of the attenuation minimum when heating up. The LED with 650 nm wavelength from RS shows even worse results due to a relatively wide spectrum and a big reduction in power with increasing temperature. The 640 nm LED by Conrad benefits from the tendency of POF attenuation to counteract a shift in the wavelength apart from the fact that the power is less sensitive to temperature. When heating up, the spectrum runs into the attenuation minimum which means that the effective attenuation over the whole range is significantly lower. -8.0

effective POF loss [dB/50 m]

-8.5 -9.0 -9.5

Conrad 640 nm

-10.0 -10.5 -11.0

RS 660 nm RS 650 nm Farnell 609 nm

-11.5 -12.0 -12.5 -13.0 -20 -10

Conrad 625 nm Farnell 621 nm 0

10

20

30

40

50

60 70 temperature [°C]

Fig. 6.35: Effective attenuation of 50 m POF with different LED


417

The maximum values of the LED at 621 nm and 625 nm lie practically exactly on the attenuation peak. Nevertheless, their effective attenuation is only a little bit higher than that of the 650 nm LED. The spectra are so wide that they reach into the areas on both sides of the peak with lower attenuation. The shift towards the red window is here compensated for by the reduction of power due to heating up in an almost ideal way. For links with fixed installation it would be possible to deploy a receiver with very low dynamic. However, the 609 nm LED shifts towards the attenuation maximum during heating up which is indicated by the big reduction of power. Some LED in the green and yellow window are compared again in Fig. 6.36. The 593 nm LED by RS has the lowest effective attenuation of all LED at low temperatures, however, during heating up the peak wavelength clearly moves towards an increase in attenuation up to 620 nm. In addition, there is a high temperature coefficient of the output power. -2.5 effective POF loss [dB/50 m] -3.0 -3.5 -4.0 -4.5 -5.0 -5.5 -6.0 -6.5 -7.0 -7.5 -8.0 -8.5 -9.0 -20 -10 0 10 20 30

Nichia 525 nm Nichia 560 nm RS 563 nm

RS 583 nm

40

50

RS 593 nm 60 70 temperature [°C]

Fig. 6.36: Effective attenuation of 50 m POF with different LED

We can conclude that selecting an LED only according to the parameters center wavelength and spectral width does not always lead to optimum results. Instead, one needs to determine the effective attenuation together with the attenuation curve of the POF deployed. GaN LED in the green and yellow range generally provide an advantage here, in the case they offer an adequate absolute power. The numerical values established here depend on the actual LED type and the POF type. Different types can lead to significantly different results especially in the area of steep edges within the attenuation spectrum. However, the basic tendency should remain the same. Figure 6.37 shows that for all ranges of visible light there are capable LED available (spectra have been normalized to reflect an equal maximum power).

418


Prel

400 420 440 460 480 500 520 540 560 580 600 620 640 660 680 700 720

wavelength [nm] Fig. 6.37: Overview of the spectra of different LED

6.1.5.2 Selection of the Type of Source In this section we will now discuss the optimum selection of the type of source. The following semiconductor structures are of interest for deployment in POF: ¾LED: ¾SLED: ¾LD: ¾VCSEL: ¾RC LED: ¾NRC LED: ¾P-LED:

Light emitting diodes Super light emitting diodes Laser diodes Vertical cavity surface emitting laser diodes Resonant cavity light emitting diodes Non resonant cavity light emitting diodes Polymer light emitting diodes

The detailed characteristics are described in the chapter on components. We wish to discuss here the effect on the calculation of the link power budget. So far only LED, LD and RC LED have been deployed in practice. Table 6.7 lists the characteristics of different semiconductor sources (typical values). Table 6.7: Comparison of parameters of different types of sources (various authors)

Property available at 650 nm available at 520 nm available at 570 nm threshold current Ith opt. power modulation up to [Gbit/s] spectral width [nm] 'O/'T [nm/K] 'Popt/'T [dB/K] emission angle [°] h/v emitting area [μm²]

LED

SLED

LD

VCSEL RC-LED NRC-LED

yes yes yes n. a. 2 mW 0.25 30 0.12

yes no no n. a. 3 mW 0.25 20 0.12

yes no no 40 mA 7 mW 4.0 2 0.18

yes*) no no 8 mA 1 mW 5.0 3 0.06

yes no no n. a. 2 mW 0.6 4 n. a.

no no no n. a. 2 mW 1.2 30 n. a.

-0.02

-0.03

-0.02

-0.08

-0.03

n. a.

50

50/10

60/8

10

8

50

200 u 200 10 u 0.3 3 u 0.3 10 u 10 30 u 30

*) only usable for up to +50°C

15 u 15


419

Practically all commercially available POF transmitters today use LED or SLED. System experiments with high data rates from about 1 Gbit/s are exclusively carried out with laser diodes because commercially available LED are not fast enough. VCSEL as well as RC-LED and NRC-LED show excellent characteristics in the laboratory which suggests future deployment in connection with POF. For the time being these technologies have been restricted to the red spectrum range. However, work is already proceeding on the development of green and yellow VCSEL/RC LED. 6.1.5.3 Typical Losses for LED Sources Figure 6.38 shows an idealized spectrum of a red LED. The assumed parameters of the source are: ¾Center wavelength at 25°C: ¾Temperature coefficient of wavelength: ¾Temperature coefficient of output power: ¾Shape of the optical spectrum: ¾Spectral width (FWHM):

665 nm 0.12 nm/K -0.03 dB/K Gaussian 40 nm

The picture shows the spectra of the LED for five different temperatures between -20°C and +70°C. The spectra of the LED are calculated and illustrated after a length of 50 m POF using the attenuation curve shown in Fig. 6.30. Prior to passing through the POF one can see the shift of the center wavelength because of the change in temperature. The maximum positions are shifted towards 650 nm due to the spectral filter effect of the POF. LED spectra rel. power 1.6 1.4

after 50 m POF rel. power 0.30 0.25

1.2 1.0

0.20

0.8

0.15

0.6

0.10

-20°C 0°C +25°C +50°C +70°C

0.4 0.2 0.0

0.05 0.00 600 620 640 660 680 700 720 600 620 640 660 680 700 720 wavelength [nm] wavelength [nm]

Fig. 6.38: Spectra of a LED before and after 50 m of PMMA-POF

420


The effective attenuation of the 50 m POF link is between 10.20 dB (-20°C) and 11.54 dB (+70°C), of which the basic attenuation at 665 nm results in 12.1 dB which means that a wide spectrum benefits the effective attenuation. In addition, there is a value of 2.70 dB for the change in LED output power. In total, the following link power budget results: A) B) C) D) E) F)

Basic attenuation at 665 nm for 50 m: Change of LED output power (rel. to 25°C): Effect of wavelength drift: Effect of spectral width: Therefore effective attenuation for 50 m: Therefore change in received power (rel. to 25°C):

12.10 dB -1.35 dB to 1.35 dB -2.15 dB to 1.50 dB 0.25 dB to -2.05 dB 10.20 dB to 11.55 dB 8.85 dB to 12.90 dB

Next we will consider three further LED on a theoretical basis. Red LED that are specially adapted for POF are as accurate as possible at 650 nm and have very small temperature coefficients. Yellow LED on AlInGaP basis are at the absolute attenuation minimum of the PMMA-POF but feature large temperature coefficients. Green LED on GaN basis are typically not very sensitive to temperature changes with respect to wavelength and power output. Table 6.8 lists the parameters and the link power budget contributions. Figure 6.39 shows the respective spectra for assumed Gaussian characteristics (max. power at 25°C standardized to 1 in each case). Again all values have been determined numerically with typical parameters for LED. The effects of aging and variation between specimens have been disregarded. The exact calculation is described in Table 6.2. This provides the basis for determining real components or other POF lengths. Table 6.8: Parameters for calculating the link power budget

Parameter

Red LED

Yellow LED

Green LED

Ocenter

650 nm

590 nm

520 nm

'O

30 nm

25 nm

40 nm

dO/dT dPopt/dT POF basic attenuation change of Popt effect of wavelength drift effect of the spectral width effective loss for 50 m POF change in received power

0.12 nm/K

0.12 nm/K

0.04 nm/K

-0.01 dB/K

-0.05 dB/K

-0.01 dB/K

6.60 dB ±0.45 dB 0.74 .. 1.69 dB 1.99 .. 0.98 dB 9.07 .. 9.33 dB 8.88 .. 9.72 dB

4.40 dB ±2.25 dB -0.30 .. 1.15 dB 0.29 .. 0.29 dB 4.39 .. 5.83 dB 2.14 .. 8.08 dB

3.65 dB ±0.45 dB 0.00 dB 0.37 .. 0.45 dB 4.03 .. 4.10 dB 3.58 .. 4.55 dB

With the optimized red LED a gain of more than 2 dB is obtained in the link power budget in addition to the amount for the decrease in fluctuation of the output power. Green LED have a low attenuation as well as the advantage of a very flat attenuation curve so that the spectral parameters only play a minor role.


421

Yellow LED at 580 nm to 590 nm are always characterized by being susceptible to temperature-related parameters. Ideal would be LED in the range around 560 nm which are now available as samples on GaN basis. 1.8

rel. power

-20°C 0°C +25°C +50°C +70°C

1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

460 480 500 520 540 560 580 600 620 640 660 680 700 wavelength [nm]

Fig. 6.39: Temperature dependent spectra of different LED

Figure 6.40 summarizes the link power budget for the effective POF attenuation of the four LED under consideration. 520 nm-LED

POF attenuation

590 nm-LED

spectral parameters

650 nm-LED

variations of the LED power

665 nm-LED 0

2

4

6

8

10 12 14 16 contribution to the power budget [dB]

Fig. 6.40: Comparison of different LED for a POF link of 50 m length

6.1.5.4 Lasers for POF Systems When using laser diodes, calculating the link power budget of POF systems becomes significantly easier. Due to the threshold current's high sensitivity to temperature, the output power of normal edge light-emitters must always be controlled. Many laser diodes feature a monitor photodiode at the rear chip edge, with which the emitted power can be directly monitored. The laser power is stabilized via a simple control loop.

422


In addition, the spectral width of laser diodes is much smaller than that of LED. The typical width of multi-mode laser diodes is just a few nanometers and therefore hardly of any significance for the effective attenuation. DFB laser diodes (with distributed feedback in the active zone) emit light in single modes with a width that can be completely neglected in POF systems. Somewhat more significant is the temperature-related drift of the wavelength. As an example, we wish to take a look here at an AlInGaP based laser from Toshiba [Tos98]. Between -10°C and +60°C the wavelength increases in a practically linear fashion from 664.5 nm to 677.0 nm which corresponds to a coefficient of 0.18 nm/K as is typical for Fabry-Perot laser diodes. For a POF length of 100 m this corresponds to an increase in attenuation of 5 dB (from 238 dB/km to 288 dB/km). When assuming this coefficient for the overall POF wavelength range for the lasers that are of interest here, the emission wavelength drifts by r8 nm at 25°C within the range of -20°C to +70°C. Figure 6.41 shows the changes in the effective attenuation in a 100 m long POF link for an ideal laser for a value of 25°C. For a laser at 650 nm the change is no less than 3.4 dB and 4.9 dB for -20°C or +70°C respectively. Components of the wavelength range around 520 nm or 560 nm would be very well suited again. DFB lasers are much less susceptible to the effect of temperature; for POF connections, however, they are too expensive by several orders of magnitude. 10 8 loss change relative to 25°C [dB/100 m] 6 4 2 0 -2 -4 change for -25°C -6 change for +70°C -8 -10 450 500 550

600 650 700 center wavelength at 25°C [nm]

Fig. 6.41: Change in POF attenuation due to a drift in the laser wavelength

6.1.5.5 VCSEL and RC-LED for POF Systems Vertical laser diodes and resonant cavity LED have their place between LED and lasers with respect to the effect of spectral parameters on the link power budget. Firstly, the wavelength of VCSEL is determined by the resonator, as is the case for DFB laser diodes. The same applies to RC LED more or less.


423

In addition, it is possible to partially compensate for the temperature dependency of the output power which is relatively high to start with. In order to achieve this, the resonator is designed in such a way that the resonance wavelength at low temperatures is slightly above the emission wavelength of the active layer. During laser operation, the wavelength is increased by the resonator but the efficiency is reduced. With increased temperatures the efficiency of the emission decreases but the adaptation of the resonator is improved so that the launched power remains nearly constant (for example see [Ebe98]). Another advantage of the VCSEL compared to edge emitting lasers is the very low threshold current, sometimes less than 1 mA which means that during operation the current is many times higher than the threshold current. Even if the threshold current changes markedly during changes in temperature, the VCSEL can still be operated at constant current as shown schematically in Fig. 6.42. Publications on POF systems with VCSEL or RC-LED have so far not contained complete data for the output power and spectrum under the influence of temperature so that it is not possible to carry out a representative calculation for the link power budget. However, it is possible to say that RC-LED have excellent characteristics with respect to changes in the output power (see [Schö99a]), whereas up to now it has not been possible to use red VCSEL up to 70°C.

1.8

Popt [mW]

VCSEL

Popt [mW]

-20°C 0°C 25°C 50°C 70°C

1.6 1.4 1.2 1.0

Laser

-20°C 0°C 25°C 50°C 70°C

0.8 0.6 0.4 0.2 current [mA]

0.0 0

1 2 3 4 5 6 constant current operation

7 30

current [mA] 35 40 45 50 55 60 constant power operation

65

Fig. 6.42: Operating modes for VCSEL and laser diodes

6.1.6 Definition of new LED Parameters Most conventional LEDs with a double-hetero structure have more or less Gaussian-shaped emission spectra which is why the values computed so far for the additional spectral attenuation were calculated with this distribution.

424


In the meantime there are new different LED versions which have strong asymmetrical spectra. This definitely has significant consequences for the effective attenuation. Tests were conducted in a joint project with Agilent to determine what the actual losses on a 10 m long fiber link could be in dependence of different spectral parameters. Peak wavelengths between 630 nm and 685 nm are permitted in the MOST specification whereby the maximum spectral width may only be 30 nm. The attenuation of 10 m of POF for LEDs with peak wavelengths (OPeak) and spectral widths ('O) in the permitted range are shown in Fig. 6.43. In each case a Gaussian-shaped spectrum was used. eff. loss [dB/10 m] 3.8 MOST limit: 3.3 dB 3.3 2.8 FWHM: 30 nm

2.3

Gaussian shaped spectrum

FWHM: 20 nm

1.8

FWHM: 12 nm FWHM: 4 nm

1.3 630

640

650

660

670

680 690 peak wavelength [nm]

Fig. 6.43: Effective POF attenuation for Gaussian like spectra

Exactly 3.3 dB were indeed attained with (OPeak) = 685 nm und 'O = 30 nm. At 630 nm the loss is still clearly smaller. The authors of this standard obviously assumed that the greatest attenuations will always arise at maximum spectral width. This is true for the long-wave side of the spectrum, but not for the shortwavelength. An LED with OPeak = 630 nm and 'O = 4 nm does indeed meet the MOST specification, but leads to 0.3 dB too much attenuation. This very theoretical case should not come about, however, since LEDs always have wider spectra. Furthermore, the short wavelengths only come about at low temperatures with which the efficiency of the LED increases. A much more realistic situation is when the LED spectrum is clearly asymmetrical. This could be the case with an S-LED. A spectrum was tried in Fig. 6.44 which consists of two differently steep Gaussian edges with a width ratio of 1:7. In the worst case an attenuation of 5 dB can arise (1.7 dB over the assumed maximum attenuation), although the LED formally corresponds to the specifications. The reason is that the greatest part of the light energy lies above the peak wavelength where the POF attenuation is the greatest.

6.1 Link Power Budget of Optical Transmission Systems 4.8

425

eff. loss [dB/10 m]

4.3 3.8 MOST limit: 3.3 dB 3.3 2.8

asymmetrical spectrum (1:7)

FWHM: 20 nm FWHM: 30 nm

2.3 FWHM: 12 nm

1.8

FWHM: 4 nm

1.3 630

640

650

660

peak wavelength [nm] 670

680

690

Fig. 6.44: Effective POF attenuation for asymmetrical spectra

The effective attenuation was calculated overall with 7 different spectra. The parts of the parameter field from OPeak and 'O in which the attenuation for at least one spectrum lies above 3.3 dB are marked in Fig. 6.45. A significant part of the range permitted by MOST is thereby covered. 60

FWHM [nm]

loss >3.3 dB for 10 m POF

50 40 30 specified area for MOST®-LED

20 10 0 600

620

640

660

Opeak [nm] 680

700

720

Fig. 6.45: Calculated areas for LED parameters giving >3.3 dB loss

If you wish to avoid either having to significantly limit the permissible parameter range or to allow more attenuation, then you have to describe the LEDs by means of parameters in which possible asymmetries in the spectrum have less consequences. The solution recommended, and in the meantime accepted by the MOST consortium, is to describe in the future LEDs through their spectral center of gravity wavelength and the effective width (50% width of the equivalent Gaussian distribution): O central

¦ Pi Oi ¦ Pi

and 'O eff

2.355 V

2.355

¦ Pi (Oi - Ocentral )2 ¦ Pi

426


Two examples of real LED spectra are shown in Fig. 6.46. In addition, the Gaussian curves which result from OCentral and 'Oeff are included. Determining this data is simply more practicable for LED manufacturers. 1.0

Popt (norm.)

Popt (norm.) RC-LED IMEC = 654.0nm OPeak FWHM = 9.5nm OCentral = 649.1nm FWHMeff = 17.8nm

NEC-NL2100 = 654.0nm OPeak FWHM = 19.1nm OCentral = 648.4nm FWHMeff = 28.6nm

0.8 0.6 0.4 0.2 0.0

580 600 620 640 660 680 700 620 630 640 650 660 670 680 O [nm] O [nm] Fig. 6.46: Comparison of the old and new parameters for 2 LED

If you take these two parameters as a basis then almost the entire range specified by MOST remains with losses below 3.3 dB. There could even be a much larger range permitted (Fig. 6.47). This change is practically irrelevant for conventional LEDs with symmetrical spectra. The calculability of POF losses could be considerably improved especially for RC LEDs with unusual spectra. 60

'Oeff [nm] (FWHMeff) possible area for the new LED parameters

50 40

loss >3.3 dB for 10 m POF

30 20 specified area for MOST®-LED

10 0

600

620

640

660

680

700

720 Ocentral [nm]

Fig. 6.47: Calculated areas for the new LED parameters giving >3.3 dB loss

The effects become even more dramatic for greater lengths. In the future, other standards will also have to make use of the new parameters introduced by MOST.

6.2 Examples of Link Power Budgets

427

6.2 Examples of Link Power Budgets 6.2.1 ATM Forum Specification The following is a list of some practical examples for calculating link power budgets. In Fig. 6.48 we show again the calculation for the link power budget in the ATM Forum specification for 155 Mbit/s over 50 m at 650 nm wavelength. 22

2

20 4 18 6 16 8 14

LED power variation mode dependent loss POF loss at 650 nm connector loss source spectral width and drift of the center wavelength influence of temperature and rel. humidity fiber bends

10

12

Fig. 6.48: ATM Forum link power budget for 155 Mbit/s

Apart from fluctuations in the LED power, connector losses, and attenuation of the POF with respect to spectral source characteristics, the modedependen loss and the effect of climate and bends have been taken into account. 22

LED power variation

2

20

mode dependent loss POF loss at 650 nm

4

18

connector loss

6

16

source spectral width and drift of the center wavelength influence of temperature and rel. humidity

14

8 12

10

fiber bends system margin

Fig. 6.49: Link power budget of a 100 m POF link with 520 nm LED

[Ziem98a] and [Ziem98b] propose using the first optical POF attenuation window for transmitting 155 Mbit/s. Due to the reduced attenuation at 520 nm and

428


especially because of the flatter minimum, the effective attenuation, as shown, is significantly lower compared to the red minimum and LED sources. In the author's estimation, it should be possible to achieve 100 m length without any problem, whereby an additional margin of 3.0 dB would remain in the system. Figure 6.49 shows the link power budget compared to the specification by the ATM Forum for 50 m. The maximum power was assumed to be identical with -2 dBm while the lower sensitivity of the receiver was entered with 1 dB reduction in the link power budget (assuming optimized PD). 6.2.2 IEEE 1394b A link power budget is also calculated for using POF in the standard IEEE 1394b (Fire-Wire or i.link, Fig. 6.50). Here, the maximum used data rate is 200 Mbit/s. Due to the 8B10B coding (NRZ), the physical data rate on the POF is 250 Mbit/s and correspondingly higher for the further steps in the hierarchy. The sensitivity of the broadband receiver is correspondingly worse so that the overall link power budget is only 19 dB compared to 23 dB of the ATM Forum. No fixed length is specified in IEEE 1394. Instead, the standard allows a maximum link attenuation of 9.1 dB corresponding to the values of the ATMF specification without connector but taking into account climatic factors and mode dependent loss. If connectors are used, again taking a maximum loss of 2.0 dB into consideration, the permissible length of the link is reduced correspondingly. A maximum of three connectors is possible which reduces the maximum length to 27 m (42 m for one connector and 34 m for two connectors). There is no margin provided in the system.

0

18

LED power variations

2 16 4

POF loss at 650 nm with mode dependent loss, influence of temperature and humidity and connector loss (optional up to three) source spectral width and drift of the center wavelength

14

fiber bends

6 12 10

8

Fig. 6.50: Specification IEEE 1394b for 250 Mbit/s at 650 nm


429

The effect of the spectral source parameter for the 50 m fiber length is entered at 3.4 dB and the values for the other lengths are reduced correspondingly, for 42 m: 2.9 dB, for 34 m: 2.3 dB and for 27 m: 1.6 dB. The calculation is based on 0.182 dB/m and 2.0 dB loss per connector and 12.5 dB overall loss. Currently, there are receivers on the market which are significantly better, also at a data rate of 250 Mbit/s. Therefore it would appear to make sense to revise the specification, at the latest when short wavelength POF windows can be considered or when deploying new source types (RC-LED, VCSEL). 6.2.3 D2B and MOST Further specifications of the link power budget exist for the bus systems used in the automotive field, i.e. D2B (Domestic Digital Bus) and MOST (Media Oriented System Transport). Figure 6.51 shows the link power budget for the D2B specification according to [Pet98]. The guaranteed LED power is -15 dBm. A maximum LED power has not been stated in the literature. When taking the typical temperature susceptibility and a specified temperature range of -40°C to +85°C into consideration, the value would probably be around -6 dBm.

11

POF loss at 650 nm (max. 8 m, 0.4 dB/m) LED-POF coupling

1

10

2

9

3

8 7

POF-PD coupling connector loss (optional one) system margin

4 6

5

Fig. 6.51: Link power budget of the D2B specification

The data rate for D2B is 5.65 Mbit/s, of which 4.2 Mbit/s are the payload. Due to bi-phase coding, the corresponding physical data rate on the POF is 11.3 Mbit/s. The architecture of D2B forms an active ring. This means that each component is equipped with one receiver and one transmitter. The components are switched in a ring so that each element has to pass on the signals for all following equipment. The specification does not take into account mode dependent loss, climatic effects or the effect of the spectral source parameters. On the other hand, the value

430


for the POF attenuation is a conservative one at 400 dB/km so that even considerable aging or a drift in the source wavelength to 670 nm can be tolerated. For a maximum link length of 8 m, these losses are not really critical. Their specification provides a large margin of 5.0 dB in the system. This probably reflects the intention to make appropriate allowance for the newness of the technology for the automotive field. Another factor is that the conditions for the cable installation during vehicle manufacture are much more difficult compared to the installation of cables in a building network, for example. Subsequent replacement of a defective cable is also expensive. The sensitivity of the receiver is specified as -26 dBm. Considering the low data rate (11.3 Mbit/s), this value is also rather conservative. Overall, the link power budget shows that a large part of risk of the cable installation and the design of the electrical interfaces has been moved to the POF link. However, this can be well accommodated by the link. It is likely that the existing reserves in the next generation will probably be used to increase the bit rates as is already heralded by the introduction of the MOST Standard. At approximately 21.2 Mbit/s of use data rate, the physical bit rate is approximately 50 Mbit/s due to the RZ coding. Figure 6.52 shows the link power budget for MOST (Media Oriented System Transport, for example, see [Tei00], [Pan99] and [Pan00]). The budget comprises approximately 23 dB. For the attenuation of the POF link a value of 16.5 dB is taken into consideration which includes losses at additional connections, in case prefabricated pigtails are used in installation. Nevertheless, a large value has been assumed considering the short lengths in a vehicle, for example compared to the value of 13 dB of link attenuation assumed in the ATM Forum specification for a link of 50 m length. The high value for attenuation takes into account the rough conditions for POF during installation and practical use in the vehicle as well as the necessary margin for long service life and the reliability requirements of the systems.

22 2 20 4 18

6

16

8 14

10

LED power variations influence of temperature and current supply to the electronics LED aging LED pigtail coupling POF loss at 650 nm (includes 4 dB for POF coupling to transceivers) pigtail to PD coupling

12 Fig. 6.52: Link power budget of the MOST specification ([Pan00])


431

6.2.4 ISDN over POF Finally, Fig. 6.53 shows an additional link power budget based on the author's own proposal for transmitting ISDN signals via POF ([Ziem00c], [Ziem00d] and [Ziem00e]). 42

44 0

2

40

LED power variations

4

mode dependent loss

6

38

POF loss at 560 nm (source with 560 ± 10 nm, FWHM max. 40 nm)

8

36

10

34

12

32

14

30 16

28 26

24

18 22

20

influence of aging and temperature water absorption connector loss (maximum three) fiber bends and system margin

Fig. 6.53: Link power budget for 250 m POF-ISDN with LED of 560 nm

The sources used here are 560 nm GaN-LED which are available as samples from Nichia. The maximum power launched into the POF is assumed to be -3 dBm. For a guaranteed receiver sensitivity of -48 dBm, the link power budget amounts to 45 dB. Due to the excellent temperature stability of GaN LED, only 3 dB have to be considered to for the fluctuations in power. 1.0 dB has been taken into account for the mode dependent loss. For a wavelength drift of 10 nm (GaN LED feature a much lower drift) and a maximum of 40 nm spectral width, a fiber attenuation of 80 dB/km is obtained corresponding to 20 dB for the maximum transmission length of 250 m assumed here. Following our own tests for climate exposure, 20 dB/km were entered for the effects of aging due to temperature and 20 dB/km for the effect of water absorption (5 dB for 250 m, see also [Ziem00b]). Allowing for 3 permissible connectors there remains a margin of 3.0 dB in the system which would cover, for example, bends in the fiber link. This proposal is new in the field of POF applications since it is the intention here to cover a large distance with a relatively small data rate (192 kbit/s). This makes the use of the first two POF windows mandatory. 6.2.5 Link Power Budget for Bi-Directional Transmission The T-Nova GmbH and Alcatel Autoelectric companies cooperated in investigating options for a high bit rate data transfer over short distances. In Chapter 5 a 520 nm/650 nm WDM system which has been realized in practice is described in

432


detail. Below we will demonstrate link power budgets for possible systems with asymmetric bit rates. If in the coming years the very high transmission rates under consideration here are achieved in vehicle networks, a highly asymmetrical requirement must be assumed. Conversely, this means that only small data rates will be achieved in backward direction. The transmission of different data streams via just one fiber as opposed to a solution with duplex fiber can be solved particularly effectively with WDM (see also [Ziem97b]). Two different concepts can be considered here: 6.2.5.1 Asymmetrical Couplers By using asymmetrical couplers, the channel with the lower rate experiences higher attenuation. Due to the greater sensitivity of the receiver (less noise bandwidth) it is, however, possible to ensure the same quality of transmission. The likely solution would be a 520 nm/650 nm multiplex as schematically shown in Fig. 6.54. 1 Gbit/s, 0 dBm 770 nm LD

100 Mbit/s, - 4 dBm 520 nm LED Y-splitter

Y-splitter 2 dB 5 dB

5 dB 1 mm SI-POF

WDM-filter

2 dB WDM-filter

Si-PD 100 Mbit/s, -26 dBm

Si-PD 1 Gbit/s, -16 dBm

Fig. 6.54: Concept for WDM with asymmetrical couplers

6.2.5.2 Symmetrical Couplers When using symmetrical couplers, the wavelength with the least fiber attenuation for the higher rated channel is used. For the lower rated channel one can accept the higher attenuation for a different wavelength due to the higher sensitivity, as demonstrated in Fig. 6.55. 650 nm LD 1 Gbit/s 0 dBm

Y-splitter 4 dB 4 dB

Si-PD 100 Mbit/s -28 dBm

Y-splitter 4 dB 1 mm SI-POF

WDM-filter

Fig. 6.55: Concept for WDM with symmetrical couplers

770 nm LED 100 Mbit/s -4 dBm

4 dB WDM-filter

Si-PD 1 Gbit/s -16 dBm


433

Figure 6.56 shows a diagram for the complete link power budget calculations of both systems. To date this set-up has not been implemented in practice. At the time of this concepts, there was still a problem of obtaining sufficiently fast receivers for POF of 1 mm. These systems should be considered as a theoretical concept design.

power budget calculation: LED power variation

loss 1. coupler

loss 2 connectors

POF link loss (10 m)

loss 2. coupler

margin

asymmetrical coupler: source powers

receiver sensitivities

770 nm LD 520 nm LED 0

-2

-4

-6

-8 -10 -12 -14 -16 -18 -20 -22 -24 -26 -28 optical power [dBm]

symmetrical coupler: source powers

receiver sensitivities

770 nm LED 650 nm LD 0

-2

-4

-6

-8 -10 -12 -14 -16 -18 -20 -22 -24 -26 -28 optical power [dBm]

Fig. 6.56: Link power budget for asymmetrical data rate WDM connections

434

6.3 Overview of POF Systems

6.3 Overview of POF Systems A detailed description of the transmission experiments with polymer fibers published up till then was presented in the first edition of this book. In the past few years the number of publications has, of course, increased considerably. A not insignificant share was the work of the POF Application Center Nürnberg (POF-AC), for example, with the first transmission of 2.5 Gbit/s over a 1 mm thick fiber. In order to maintain the character of an overview and reference work, we have attempted to retain the overview of systems to the extent possible. We can thereby naturally only take into consideration those experiments which have been published at important POF conferences, in the most important IEEE journals or on the Internet. Some experiments appear today to be less remarkable because all parameters have long since been surpassed, but we also wanted to show the development over the course of the past 15 years. The following sections contain summaries of transmission experiments in different groups. Within these sections there are again summaries of works of the same research group or thematically very similar publications. Otherwise, the chronological sequence formed the basis. The following are discussed in particular: ¾Systems with PMMA SI-POF at wavelengths around 650 nm ¾Systems with PMMA SI-POF with data rates of 500 Mbit/s and more ¾Systems with PMMA SI-POF at wavelengths below 600 nm ¾Systems with PMMA SI-POF at wavelengths in the near infrared range ¾Systems with PMMA GI-POF, MSI-POF and MC-POF ¾Systems with fluorinated POF ¾Wavelength multiplex systems with PMMA POF ¾Wavelength multiplex systems with PF GI-POF ¾Bidirectional systems with POF ¾Special systems, e.g. with analog signals At the end of each section the latest results of the POF-AC Nürnberg are presented and an overview in tables is shown. The most important parameters of each system appear in lists in order to simplify the overview. The glass fibers included for the first time in this book and a few other special fibers will then be treated in Section 6.4. The parameters for bit rate and the fiber length are shown in many diagrams. These two parameters alone naturally do not describe a system. Many of the experiments described took place in laboratories under ideal conditions. For commercial use margins for changes caused by temperature, aging and less than ideal fiber installation have to be taken into consideration. Under some circumstances limits also have to be taken into consideration for eye safety. Many systems would also be much too expensive and lavish for practical use, e.g. if they use APD with high bias voltages. Consequently, a simple comparison between the different results is not always given. The reader will thus find information in the text or in the references listed.


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6.3.1 Step Index Profile POF Systems at 650 nm Low-attenuation PMMA SI-POF have been commercially available since the middle of the seventies (see, for example [Sai92]). First a number of older experiments will be illustrated - without claiming to be comprehensive - that verify that the POF has been examined for short-link transmission for a long time. 6.3.1.1 The first SI-POF Systems One of the first descriptions of a POF transmission system available to the authors is illustrated in [Scho88].With this system 20 Mbit/s can be transmitted over 80 m POF (step index, PMMA). Fiber type: Length: Bit rate: Transmitter: Reference:

SI-POF 80 m 20 Mbit/s 650 nm [Scho88]

The international POF conference has been held annually since 1992. Since then, almost all important new developments in the field of POF are presented at this conference. An overview of the development so far in POF data transmission can be found, for example, in [Kuch94]. After that, Kaiser demonstrated the transmission of 140 Mbit/s over 110 m standard NA POF as early as 1990. Fiber type: Length: Bit rate: Transmitter: Reference: Company:

St.-SI-POF 110 m 140 Mbit/s 650 nm [Kuch94] in Kaiser 90

One of the first commercial POF transceivers was introduced by Hewlett Packard in 1992 [HP05] with 50 Mbit/s at a range of 15 m. Later on, a complete family of transmitters, receivers and transceivers for POF and PCS with its own plug system (described in the chapter on Components) came about. The field of POF belonged for a while to the Agilent Company and can in the meantime be found at Avago. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

St.-NA-POF 15 m 50 Mbit/s 660 nm Si-pin-PD [HP05] Hewlett Packard 1992

In 1992, Price described [Pri92] the transmission of a signal of 125 Mbit/s over 1 m, 25 m, 50 m, and 90 m SI-PMMA POF (Mitsubishi EH 4001). The fiber

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medium is rated by the manufacturer at 300 dB/km. The source was a laser diode NDL 3200 with a max. of 3 mW at 670 nm wavelength that was originally developed for barcode lasers. When modulated and launched with a collimated beam, the maximum power in the POF was still 0 dBm. A pin FET transistor arrangement was used as a receiver. At a diode capacity of 3 pF, the bandwidth of the receiver was 75 MHz (0.6 times the bit rate). The calculated sensitivity was -31.4 dBm at BER = 10-9. A value of -28.5 dBm was measured (with a 80 MHz preamplifier). This resulted in a coupling loss of 3.7 dB at the receiver. With measured losses of 276 dB/km, a maximum transmission link length of 90 m could be achieved with the power budget of 28.5 dBm. The system is shown schematically in Fig. 6.57. Fiber: Length: Bit rate: Transmitter: Receiver: Reference: Company:

St.-NA-POF EH 4001, 300 dB/km (Mitsubishi) 1 m/ 25 m/50 m/90 m 125 Mbit/s LD 670 nm; NDL 3200, 3 mW pin-FET-transistor-combination [Pri92] Kennedy & Donkin Systems Control Ltd.

670 nm LD NDL 3200 3 mW 125 Mbit/s

1 m, 25 m, 50 m, 90 m 980/1000 μm SI-POF ESKA EXTRA EH 4001 276 dB/km | 670 nm

Si-PD pin-FET 75 MHz

Fig. 6.57: Transmission system according to [Pri92]

In [Kit92] the Mitsubishi POF ESKA Premier was used which was newly developed at the time. At a NA of 0.51 and at losses of 135 dB/km at a wavelength of 650 nm, this fiber material can be used at temperatures up to +85°C. With collimated light, the attenuation of 65 dB/km at 570 nm and 124 dB/km at 650 nm was measured. Using the TOLD 9410 Toshiba laser diode and a receiver of its own, it was also possible to transmit 125 Mbit/s over 100 m, since the launched power was correspondingly higher (Fig. 6.58). Moreover, a system with a yellow LED was realized (see below). Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

SI-POF, Mitsubishi ESKA Premier 100 m 125 Mbit/s 650 nm LD, Toshiba TOLD 9410 own construction [Kit92] Mitsubishi


650 nm LD 125 Mbit/s

100 m SI-POF Mitsubishi, ESKA Premier

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own setup for 125 Mbit/s

Fig. 6.58: Transmission system according to [Kit92]

In [Fuk93] the use of various wavelengths is also described for POF transmission systems. At first an InGaAsP LED at 670 nm was used with which an average power of -12 dBm could be coupled into the fiber. 100 Mbit/s were transmitted over 30 m SI-POF. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

SI-POF 30 m 100 Mbit/s InGaAsP-LED, 670 nm, -12 dBm commercial Toshiba receiver, -22 dBm sensitivity [Fuk93] Toshiba

670 nm LED InGaAsP, 63 μW 30 m SI-POF

TORX 196

Fig. 6.59: Transmission system according to [Fuk93]

In 1994, Keio University ([Koi94]) presented a system with a data rate of 250 Mbit/s at a transmission length of 100 m over SI-POF. The author cites mode dispersion as the bandwidth limiting factor. A NEC laser diode with a maximum output power of +6.1 dBm (4 mW) was used as the source. Fiber type: Length: Bit rate: Transmitter: Reference: Company:

SI-POF 100 m 250 Mbit/s NEC, laser diode; +6.1 dBm [Koi94] Keio University

In [Tan94b] the transmission of a 400 Mbit/s signals over 50 m of SI-POF is described by Fujitsu. The PMMA-POF (ESKA EXTRA MH 4001 from Mitsubishi) used had an attenuation of 250 dB/km at a wavelength of 650 nm with NA = 0.50. A 0.5 mm APD served as the receiver. Fiber type: Length: Bit rate: Receiver: Reference: Company:

SI-POF, Eska Extra MH4001 50 m 400 Mbit/s 500 μm APD [Tan94b] Fujitsu

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NEC started using the newly developed low NA-POF for transmissions of 156 Mbit/s over 100 m using 650 nm LED (for example, [Koi97a]) as early as 1995. In [Kob97] a new type of LED is used for such systems that makes a coupling efficiency of 70% possible with simple plastics lenses through improved emission characteristics. Later a transceiver in a 1 u 9 pin type of construction was offered as a commercial product [NL2100]. It was followed by the NL2110 of the S200 type with a data rate of 250 Mbit/s over a maximum of 70 m. Today there is no longer any known work being done by NEC in the POF field. Fiber type: Length: Bit rate: Transmitter: References: Company:

Low-NA-POF 100 m 156 Mbit/s 650 nm LED, improved coupling efficiency by an optimized emission characteristic [Koi97a], [Kob97] NEC

Sony introduced a transceiver for IEEE 1394 in 1997 ([Sak97]). The SI-POF used had an attenuation of 160 dB/km and 130 MHz 100 m bandwidth at 650 nm. The silicon-pin diode was attached to a transimpedance amplifier, making it possible to achieve a sensitivity of -25 dBm at a BER = 10-10 (125 Mbit/s). At 200 Mbit/s, a transmission length of 70 m was achieved. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

DSI-POF, 160 dB/km, 130 MHz 100 m 70 m 200 Mbit/s 650 nm LD Si-pin-PD, -25 dBm at 125 Mbit/s, transimpedance amplifier [Sak97] Sony

A test network was set up at the University of Ulm for office cabling with SI-POF and later with DSI-POF. The first results were presented in 1998 ([Som98a]). A total of 33 POF connections were installed with lengths ranging between 5 m and 63 m (total installed cable length was 1,400 m). The bit rate was 125 Mbit/s. A 100BaseFX(POF) switch served as the central node. The switch and the PC cards were re-equipped with various commercially available POF transceivers in 1 u 9-pin form. Hewlett Packard HFBR 5527, NEC NL 2100 and self equipped receiver were used. For the fiber, Asahi AC-1000W, Mitsubishi MH 4002F and Toray PMU-CD 1002-22E according to the ATM forum specification were used. The measured effective attenuation with the transceivers used was from 196 dB/km to 205 dB/km. Fiber type: Length:

SI-POF, DSI-POF, Asahi AC-1000W, Mitsubishi MH 4002F, Toray PMU CD 1002-22E 5 m to 63 m


Bit rate: Transceiver:

Reference: Company:

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125 Mbit/s HFBR 5527, NL 2100 by NEC and transceivers with components of the HFBR 0507 series, built up at the University of Ulm [Som98a] University of Ulm

Another commercial transceiver was developed by Hamamatsu and presented in [Mai00]. After the enterprise initially produced components for its network of vehicles, there are now also diverse components available for Ethernet and IEEE 1394 applications. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

SI-POF 50 m 4 to 156 Mbit/s 650 nm LED, -2 dBm in the POF -22 dBm [Mai00] Hamamatsu

The area of vehicle networks is especially dealt with by [Num01a]. The RC-LED used has been optimized to a wide temperature range. All coupling losses have been taken into consideration in the computation of power budget. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

PMMA SI-POF, AN = 0.50 20 m 50 Mbit/s 650 nm RC-LED, 'O < 20 nm, 'O/'T < 0.1 nm/K, -6.8 dBm; temperature range -40°C to +85°C with 4.1 dB change 800 μm Si-PD, sensitivity: -29.1 dBm [Num01] Matsushita

For some years now the work of all important institutes have concentrated on higher transmission speeds or the latest fiber types respectively (see next section). In the area of SI-POF work has shifted to the development of better and more reasonably priced products, some examples of which can be found in the chapter on Components. In conclusion, a recent work should be mentioned which describes a new POF transceiver for Fast Ethernet. The SC plug which can be fit easily is new (Fig. 6.60). Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

PMMA SI-POF 70 m 125 Mbit/s 655 nm LED Si-pin-PD [Neh06a] Euromicron

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Fig. 6.60: EM-RJ connector and Fast-Ethernet transceiver

6.3.1.2 SI-POF Systems with over 500 Mbit/s Data transmission rates of up to 500 Mbit/s are of interest for a number of applications. Many of today’s devices in entertainment electronics have IEEE 1394S400 interfaces (data transmission rates of 500 Mbit/s) as standard components. Such high values are also expected in the automobile industry when uncompressed moving pictures are to be transmitted. This is necessary, for example, with driver assistance systems when the time delay is too great because of compression algorithms. Even today you can read in many publications that 1 mm of SI-POF is not suitable for such high speeds since the bandwidth is first of all too low and secondly because even the large photodiodes are not fast enough. The following examples from the past ten years show, however, that this opinion on the potential of POF has long since been revised. High data rate transmission experiments were introduced in a series of publications written by Bates, Yaseen, and Walker from the University of Essex ([Kuch94], [Wal93], [Yas93], [Bat92a], [Bat96b]) spanning the years 1992 to 1994. With data rates of 265 Mbit/s and 531 Mbit/s (1994), 100 m POF was covered. Figure 6.61 illustrates the principle test set-up. high pass filter

bit generator 100 m 980/1000 μm SI-POF ESKA EXTRA EH 4001 139 dB/km | 652 nm

652 nm LD

Si-PD

Fig. 6.61: High bit rate data transmission over SI-POF

The Mitsubishi ESKA EXTRA EH4001 was used as the fiber medium. It has 139 dB/km of attenuation at 652 nm. A Philips laser diode CQL82 with a wave-


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length of 652 nm served as the light source. The laser was operated at 290 K (17°C) with 36 mA of bias current. To increase the bit rate, a first order high-pass filter was pre-connected as the peaking filter. With the help of input optics, 2.7 mWp-p of power was achieved at launch of NA = 0.11. During modulation, the average power was -1.7 dBm (0.68 mW); with the peaking filter, the average power fell to -6.7 dBm (0.21 mW). An AEG-Telefunken BPW89 photodiode with 4.9 pF capacity at 20 V of reverse voltage was used as a receiver. The responsivity is 0.4 A/W at 650 nm (76% external efficiency). The coupling to the POF is done with a ball lens. A second high-pass filter was connected behind the receiver as a compensation filter for the mode dispersion. The receiver achieved -22.1 dBm sensitivity at BER = 10-9. As a result, a data rate of 265 Mbit/s was achieved. With an improved Hamamatsu S4752 photodiode with 1.6 pF capacity at 10 V reverse voltage and a diameter of 600 Pm of the active surface, the same sensitivity was achieved without lens and compensator. As is already known, the theoretical bandwidth of the standard NA-POF is approximately 40 MHz 100 m. Consequently, the high data rates of the experiments in question were not possible. Nevertheless, this value only applies to light propagation in the equilibrium mode distribution. Figure 6.62 shows which methods can be used to increase the bandwidth of the transmission system. launch with small NA high pass peaking filter

detection with low NA POF without connectors and bends

input

high pass for dispersion compensation

output

Fig. 6.62: Methods for bandwidth enhancement on SI-POF

Besides the compensation of the band limitations through transmitter, fiber medium and receiver with suitable high-pass filters, it is particularly helpful to reduce the number of the modes involved in data transmission and thereby the pulse broadening. The following methods can be used individually or in combination (values according to Bates): Launch with small NA = 0.11. Only a few modes with small mode delay will be excited. ¾Predistortion of the LD drive signal (peaking) with high-pass filter (33 pF ««51 :). ¾Output with small NA (modes with large mode delay are hidden). ¾Dispersion compensation behind the receiver with high-pass filter (8 pF ««20 :).

¾

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To the author's knowledge, the transmission of 531 Mbit/s over 100 m of SIPOF is the fastest system in terms of the bit rate length product so far. In [Bat96a] the theoretical limit was even estimated to be 1 Gbit/s over 100 m (which is clearly proven in the meantime). The practical application of such systems is, however, beset with difficulties since the filters must be dimensioned very precisely and the specific parameters must be adapted to each individual link. Through bendings and plug-in connections, the mode distribution is changed for POF in such a way that the gain in bandwidth through selective launching and detection lost at least in part. Fiber type: Length: Bit rate: Transmitter: Receiver:

References: Company:

SI-POF, Eska Extra EH4001, 139 dB/km @ 650 nm 100 m 265 Mbit/s, 531 Mbit/s Philips LD CQL82, 652 nm. -1.7 dBm AEG-Telefunken, BPW89, 0.4 A/W Hamamatsu S7452, : 600 μm (comparable with the present product: S7482) [Bat92], [Wal93], [Yas93], [Kuch94], [Bat96b] University of Essex

In cooperation with IBM and Keio University ([Kuch94]) a system was developed in 1994 in which a surface emitting laser with a wavelength of 670 nm was used as a transmitter. At a launch NA of 0.11, the averaged power in the fiber was -10 dBm. A 400 μm diameter Si-pin PD Hamamatsu S4753 served as the receiver that was coupled to a GRIN lens. The sensitivity was -23.3 dBm at 1 Gbit/s (BER = 1.5 · 10-9). A data rate of 531 Mbit/s was transmitted over 30 m of PMMA SI-POF. The used POF was an INFOLITE F120 (Hoechst Celanese) with a core diameter of 500 μm. The attenuation of this fiber is 130 dB/km at 650 nm or 300 dB/km at 670 nm. For 100 m of POF, the limit for the data rate was 300 Mbit/s for the selected test setup. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

500 μm SI-POF, Infolite F120 (Hoechst) 130 dB/km (650 nm); 300 dB/km (670 nm) 30 m 531 Mbit/s 300 Mbit/s over 100 m 670 nm VCSEL; -10 dBm 400 μm PD, Hamamatsu S4753, -23.3 dBm at 1 Gbit/s [Kuch94] IBM, Keio University

The Swedish company Mitel has been working on the development of a 650 nm VCSEL for use in POF systems for some time now. The resonant cavity LED (RC-LED) can be regarded as the preliminary stage of the VCSEL and already has a number of advantages such as the small radiation angle and the small wavelength drift during changes in temperature.


443

The use of 650 nm RC-LED for the transmission of 250 Mbit/s over 30 m SI-POF was demonstrated in [Stre98] and [Stre98b]. A BER of 3 10-10 was achieved in the test. The RC-LED had a DBR mirror made of AlGaAs and an active zone with 4 quantum wells of GaInP and barriers of AlGaInP. The cladding layers consist of AlGaInP. The emitting area has a diameter of 84 μm. At a center wavelength of 660 nm, the RC-LED has a spectral width of 3 nm and emits 3 mW optical power at 50 mA. The maximum power is 4.2 mW at 120 mA. The maximum external quantum efficiency (QEext) is about 3.2 %. The differential resistance is specified by the authors to be 3 :. A SI-POF with 980 μm core diameters and AN = 0.48 was used. The effective attenuation at 650 nm is 180 dB/km. The source was coupled to the POF directly without any optics. At a DC current of 60 mA, the launched power was -2.2 dBm (0.6 mW). A Si-pin photodiode (Tek P6701A) served as the receiver. The probability of errors was estimated from the measuring the Q-factor (6.2). The data transmission rate is limited by mode dispersion in the POF (fiber rise time = 2.85 ns, corresponding to 44 MHz 100 m). It was possible to transmit 512 Mbit/s over a 1 m POF link. Fiber type: Length: Bit rate: Transmitter: Receiver: References: Company:

SI-POF; AN = 0.48; 44 MHz 100 m 30 m; 1 m 250 Mbit/s; 512 Mbit/s 660 nm RC-LED 'O = 3 nm, 4.2 mW at 120 mA Si-pin photodiode (Tek P6701A) [Stre98a], [Stre98b] Mitel

In one recent study [Schu01a] commercially available RC-LEDs for use with lengths of 50 m to 100 m were investigated. Except for the use of a DSI-POF no other measures which increased the bandwidth were employed. With 50 m of DSI-POF the SNR loss (penalty) through mode dispersion at 500 Mbit/s still amounts to 7 dB. This system permits the error-free transmission of 250 Mbit/s and 125 Mbit/s with a standard POF over 50 m or 100 m respectively. Fiber type: Length: Bit rate:

Transmitter: Receiver:

Reference: Company:

PMMA DSI-POF, AN = 0.30 50 m 500 Mbit/s 125 Mbit/s over 100 m and 250 Mbit/s over 50 m St.-SI-POF 650 nm RC-LED Si-PD (tr, tf < 1 ns) pre amplifier Infineon FOA 1061 sensitivity: -11.25 dBm (7 dB penalty caused by the mode dispersion) [Schu01a] Infineon Technologies

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650 nm RC-LED

Mitsubishi Eska Mega 50 m DSI-POF, AN = 0.30

Si-PD

Fig. 6.63: Transmission of 500 Mbit/s over DSI-POF

The last system presented in this section was set up at the DaimlerChrysler Research Center in Ulm [Scha01]. It permitted the transmission of 500 Mbit/s over 30 m of standard POF with commercial components in the receiver. Fiber type: Length: Bit rate: Transmitter: Receiver:

Reference: Company:


PMMA SI-POF 30 m 500 Mbit/s 650 nm LD pin-H125G-010 by OSI Fibercomm (400 μm), MAX3761 pre amplifier -11.4 dBm sensitivity (butt coupling) [Scha01] Daimler Chrysler research center Ulm

30 m PMMA-POF AN = 0.50

Si-PD 400 μm

Fig. 6.64: Transmission of 500 Mbit/s over SI-POF with laser diode

The reaching of ever greater bit rates, on SI-POF, too, had a formative influence in the following years. The basis for this was less the development of better components than the increased interest in fast systems over short distances. 6.3.1.3 SI-POF Systems with more than 500 Mbit/s In order to achieve even higher data rates above 500 Mbit/s, the active components must above all be optimized. At present, such high data rates cannot at all be achieved with LEDs and only to a limited extent with RC-LEDs. Edge-emitting laser diodes with wavelengths of 650 nm are fast enough and offer high emission power, but are very complicated in practical use. Vertical laser diodes would be ideal POF transmitters, but suffer at present from too low operating temperatures (as described in the chapter on Components).


445

It is particularly important on the receiving side to use photodiodes with a lower capacity. Diodes with a diameter of 600 μm to 800 μm are generally used for 1 mm fibers and are coupled with suitable lenses. Diodes from Hamamatsu are used most frequently. In recent works by DieMount and Infineon similarly good results have also been achieved with other types. In [Gui00a] a 650 nm RC-LED with 622 Mbit/s was modulated. This data rate was transmitted in an experiment over 1 m of SI-POF with AN = 0.48. At 30 mA of bias current, the optical power of the RC-LED was 1.4 mW. When coupled directly, 30% of the power could launched into the POF. Fiber type: Length: Bit rate: Transmitter: Reference: Company:

SI-POF 1m 622 Mbit/s 650 nm RC-LED; 1.4 mW [Gui00a] University of Tampere

Different kinds of RC-LEDs with varying current apertures have been investigated in further experiments. Small active volumes result in greater bandwidths, simultaneously reducing, however, the output power. With optimized biasing current up to 1,000 Mbit/s can be transmitted. To the best knowledge of the authors this is the highest data rate ever achieved with RC LEDs. For a 10 m-long fiber (SI-POF) the transmitted bit rate was still 400 Mbit/s; when using a DSI-POF, 622 Mbit/s were possible. Fiber type: Length: Bit rate:

Transmitter:


SI-POF, AN = 0.48, DSI-POF 1 m, 10 m 622 Mbit/s over 1 m POF (BER < 1 · 10-11), 84 μm-LED, 30 mA Bias, 1 V 1,000 Mbit/s with higher BIAS 400 Mbit/s over 10 m (POF-NA: 0.48) 622 Mbit/s over 10 m DSI-POF RC-LED with 84 μm aperture, reaches 200 MHz bandwidth at 40 mA with 1.4 .. 1.5 mW 40 μm-RC-LED reaches 350 MHz at 0.18 .. 0.20 mW with 10 .. 15 mA current [Dum01], [Gui00a], [Gui00b] University of Tampere

655 nm RC-LED 622/1,000 Mbit/s 400 Mbit/s

SI-POF, AN = 0.48 1m 10 m

Fig. 6.65: Data transmission with a 655 nm RC-LED

Si-PD

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Fig. 6.66: Eye diagram at 622 Mbit/s over 10 m DSI-POF

The transmission of larger data rates for use in the automotive field is examined in [Scha00] (Fig. 6.67). Various PMMA SI-POF from Höchst, Toray and Siemens (with Mitsubishi core, developed especially for MOST) were tested. With a 670 nm VCSEL and a fiber-coupled power of 0.32 mW a data rate of 500 Mbit/s was transmitted over 10 m. In further tests a 650 nm laser with 50 μm glass fiber output (AN = 0.20) was used. A Tektronix converter P6701A with 850 MHz bandwidth served as the receiver. 400 Mbit/s were attained over 10 m and 20 m Höchst fiber medium (300 dB/km at 650 nm) and also 600 Mbit/s at 30 m. In all cases, the eye diagram was measured and not the BER (Fig. 6.68).

50 μm GOF

650 nm LD 400 Mbit/s 600 Mbit/s

670 nm VCSEL 500 Mbit/s 0.32 mW

receiver P6701A 850 MHz PMMA-SI-POF Höchst (300 dB/km), 10 m, 20 m Siemens, Toray (150 dB/km), 20 m, 30 m

Fig. 6.67: Transmission experiments with different POF according to [Scha00]

Fiber type: Length: Bit rate: Transmitter: Receiver: References: Company:

PMMA SI-POF from Höchst, Toray and Siemens 10 m, 20 m, 30 m up to 600 Mbit/s 670 nm VCSEL, 650 nm LD Tektronix P6701A, no BER measurement [Scha00], [Scha01] DaimlerChrysler, Research Center Ulm


447

Fig. 6.68: Bit sequence at 500 Mbit/s, PRBS [Scha01]

At the beginning of 2000 different transmission experiments with a 650 nm laser were conducted at the T-Nova GmbH. These experiments took place in cooperation with Nexans Autoelectric GmbH. The edge-emitting laser was coupled directly onto the POF. In order to attain high data rates despite the large surface area Si-pin photodiode, a low-ohmic broadband receiver (10 :) was set up. To compensate for the receiver bandwidth, a predistortion filter was implemented on the laser. The parameters of the laser used are: ¾Sony SLD 1133VL (for DVD, laser pointer and barcode reader), index guided ¾Material system: AlGaInP, SQW structure ¾Longitudinally single mode ¾O = 657.5 nm (at 20°C) ¾Max. 7 mW output power ¾Emission angle (FWHM): TA = 30°,TŒ = 8° ¾Bit rate with predistortion filter > 1,200 Mbit/s ¾Ith = 50 mA (at 20°C)

This makes it possible to conduct the following experiments: ¾1.200 Mbit/s over 10 m SI-POF, AN = 0.48, BER < 10-13 ¾ 800 Mbit/s over 20 m SI-POF, AN = 0.48, BER < 10-12 ¾ 800 Mbit/s over 50 m MC-DSI-POF, AN = 0.19, BER < 10-12

The objective of the experiments was to prove the transmission of high data rates over the length area up to 10 m typical for the automotive industry, as described in [Ziem00a] and [Ziem00f]. Even in these experiments, too, the system capacity for the SI-POF was more than the theoretical limits. The main reason was the laser's relatively small emission angle. However, since the setting of the equilibrium mode distribution required at least several 10 m, a practical application should be possible for the short distances.

448


Fiber type: Length: Bit rate: Transmitter:

Receiver: References: Company:

SI-POF, AN = 0.48 10 m, 20 m 1,200 Mbit/s, 800 Mbit/s Sony LD SLD 1133VL, 657 nm, 7 mW AlGaInP, SQW structure; emission angle (FWHM): TA = 30°,TŒ = 8°; bit rate with peaking filter > 1,200 Mbit/s: Ith = 50 mA (at 20°C) Hamamatsu S5052, low impedance receiver [Ziem00a], [Ziem00f], [Stei00b] Deutsche Telekom

The system set-up is shown in Fig. 6.69 and the eye diagram for the 1,200 Mbit/s signal after being transmitted 10 m in the next picture. The eye is still open relatively wide and it was possible to transmit error-free for several days. The fiber was cut to length and terminated with a v-pin connector to the laser side and with a FSMA connector to the receiver side. No coupling lenses or active adjusting units were used. At the time this result represented the highest published data rate over 1 mm fibers. 10 m 980/1000 μm SI-POF 160 dB/km at 657 nm

BIAS Peaking 657 nm LD 2 stages

F-SMA

V-pin

PD broadband S5052 receiver

Fig. 6.69: Transmission experiment at T-Nova Berlin

voltage [a.u.]

BERT: in 88 h: 0 errors

28 24 20 16 12 8 t [ns]

¤ Giehmann

4 0.00

0.42

0.83

Fig. 6.70: Eye diagram at 1,200 Mbit/s

1.25

1.67


449

Different experiments with Gbit/s transmission using POF were also conducted at the Fraunhofer Institute for Integrated Circuits (IIS) in Nuremberg. A commercial laser diode for DVD applications was used as the transmitter. A relatively small photodiode (330 μm active diameter) was used, resulting in a coupling loss of about 10 dB. A passive filter for dispersion compensation and a limiter amplifier were employed for improving the signal. Different varieties of fibers with lengths of up to 50 m were used. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

SI-POF, DSI-POF, MC-POF 15 m to 50 m 500 Mbit/s, 800 Mbit/s, 1,000 Mbit/s 650 nm DVD-LD 330 μm Si-pin-PD [Jun04d] Fraunhofer IIS

The different tests result in the transmission of: ¾1,000 Mbit/s over 15 m SI-POF direct ¾1,000 Mbit/s over 20 m SI-POF with equalizing filter ¾ 500 Mbit/s over 50 m SI-POF ¾ 500 Mbit/s over 50 m DSI-POF ¾1,000 Mbit/s over 50 m MC-POF (estimated theoretically, not realized due to the limited received power)

In Figures 6.71 and 6.72 the measuring setup and an eye diagram for a S800 signal (effectively 1,000 Mbit/s) are illustrated. As can be seen, noise represents the dominating system limitation. A definite improvement in the system could be achieved with an optimized photodiode coupling, e.g. with a CPC.

Bit Error Detector (BERT)

TP1 Laser Photodiode Driver & & TIA Laser 20 m POF

TP2 Dispersion Filter

Fig. 6.71: Measurement setup of the IIS for IEEE 1394-S800 transmission

450


Fig. 6.72: Eye at 1,000 Mbit/s over 20 m SI-POF with equalizing

The next two illustrations show an example of the S800 bit sequence after the limiter and for the media converter already set up.

Fig. 6.73: Bit sequence after equalizer and limiter

Fig. 6.74: Media converter plug-in card for POF


451

6.3.1.4 SI-POF Systems at the POF-AC Nürnberg

Since 2002, a testing system for various thick fibers has been set up at the Polymer Optical Fiber Application Center of the Nuremberg University of Applied Sciences. The fundamental principle is a simple laser driver based on a broadband MMIC amplifier and a broadband receiver using the Hamamatsu photodiode S5052 (active diameter of 800 μm). In one variant the photodiode worked directly on a 50 resistor. With a bandwidth of about 900 MHz the sensitivity of this variant (LIA) amounted to about -16 dBm (at 1 Gbit/s and a wavelength of 780 nm). Improved receiver versions with bipolar and FET transistors were set up in the first stage and later documented in a Master’s thesis. Thanks to increased transimpedance - between 500 and 1,000 - the sensitivity was improved to -22 dBm at 1 Gbit/s. However, the bandwidth of this receiver (TIA) with about 500 MHz was somewhat lower. In combination with equalizing filters better results could for the most part be achieved with the TIA variant (details on the receiver in [Sap04], [Vin04b] and [Vin05b]). Today a total of six different laser diodes are available for measurements: ¾657 nm laser diode, Sony SLD1133VL, max. 1.3 Gbit/s, +8.4 dBm ¾652 nm laser diode, Sanyo L-4147-162, max. 1.6 Gbit/s, +7.0 dBm ¾654 nm laser diode, Union Optronics SLD-650-P5, max. 2.7 Gbit/s, +10 dBm ¾665 nm VCSEL, Firecomms, max. 2.7 Gbit/s, +0 dBm ¾780 nm laser diode Rohm RLD 78MA, max. 2.7 Gbit/s, +7.0 dBm ¾850 nm VCSEL, max. 2.5 Gbit/s, -3.0 dBm

The bit error rate tester of the POF-AC permits data rates up to a maximum of 2,700 Mbit/s. In Figs. 6.75 and 6.76 the transmitter and receiver and a circuit diagram of the LIA receiver are shown.

Fig. 6.75: Transmitter and receiver for POF experiments

In two different series of measurements the capacity of DSI-POF was investigated. With both the LIA receiver and the Sony laser diode 550 Mbit/s were able to be transmitted ([Ziem03h]). Later, an improved power budget, i.e. more powerful laser diode and better sensitivity of the TIA, allowed the transmission of even 820 Mbit/s. In Figure 6.77 the system setup is shown (eye diagram in Fig. 2.142 after 100 m).

452


+Ucc

+Ucc

PD S5052

high pass filter

MMICamplifier GALI-51

+Ucc

+Ucc

+Ucc

high pass filter




SMAplug

Fig. 6.76: General layout of the receiver (first version, LIA)


BIAS LD 650 nm SLD 1133VL

DSI-POF, Asahi Chemical, AC-1000-I, AN = 0.25 100 m 550 Mbit/s, 820 Mbit/s 657 nm LD, Sony SLD 1133VL, +3.2 dBm in the POF 800 μm Si-pin-PD Hamamatsu S5052 received power after 100 m POF: -11.5 dBm [Ziem03h], [Vin05b] POF-AC 2003 and 2004

DSI-PMMA-POF Asahi AC 1000 core = 980 μm, AN = 0.25

Si pin PD S 5052

Fig. 6.77: Transmission system with DSI-POF

Above all the capacitance over short distances was investigated on standard SI-POF. The fiber used was Toray PFU-CD-1001. Over 10 m and 20 m first 1,220 Mbit/s and then 820 Mbit/s at 650 nm was transmitted ([Ziem03g], [Vin04b], [Vin05c], [Ziem05j]). For lengths over 25 m it is surely necessary to fall back on other index profiles for high bit rate applications. The limits of SI-POF with the components described above on the POF-AC will also be investigated in an internship experiment, whereby fiber pieces each 5 m in length are connected to FSMA plug connectors and the maximum bit rate is measured with each length. Measurements up to 85 m are possible - then with 15 plug connectors. Gigabit transmission is reached up to about 30 m to 35 m. The results of two internship work groups are shown in Figure 6.78. Fiber type: Length: Bit rate:

Toray PFU-CD1001, 980 μm PMMA SI-POF 15 m to 85 m (one connector after every 5 m) up to 1,500 Mbit/s (20 m)


Transmitter: Receiver: References: Company: 3000

650 nm LD (Sony, Union Optronics) 800 μm Si-pin-PD [Obe06], [Gott06] POF-AC Nürnberg

bit rate [Mbit/s]

max. values on fiber without connectors

1000

300

453

Siemens POF-AC

PMMA-SI-POF AN = 0.50 Toray 650 nm LD

fiber length [m] 100 10

20

50

100

Fig. 6.78: Data transmission on SI-POF (intership experiment)

Even better values can be attained with the same set-up when fewer plug-in connectors are used. A maximum bit rate of 760 Mbit/s over 100 m of standard POF could be attained with a two-stage passive equalizer (eye diagram in Fig. 6.79). This surpasses both the results with optical mode filtering (533 Mbit/s, [Bat96]) and the first calculations for multicarrier transmission up to 540 Mbit/s ([Ran06a]).

Fig. 6.79: Error free transmission of 760 Mbit/s over 100 m St.-POF

In the spring of 2007, systematic tests of the maximum capacities of different fibers were carried out with an improved receiver circuit ([Was07].) A maximum data rate of 910 Mbit/s at BER < 10-9 could be transmitted with a 650 nm laser

454


over 100 m of standard POF (one plug-in connection). If an error correction had been used, even about 1 Gbit/s could have been attained. The eye diagram of the measurement is shown in Fig. 6.80 (latest POF-AC results of summer 2007, over 1 Gbit/s, see next table).

Fig. 6.80: Error free transmission of 910 Mbit/s over 100 m St.-POF ([Was07])

The multicarrier system has been improved to such an extent in [Ran06b], [Ran06c] and [Ran07a] that in the meantime about 1 Gbit/s over 100 m of standard SI-POF can be transmitted. The idea behind this system is the combination of different procedures which are also well-known in radio communication. The range of frequencies up to 200 MHz is subdivided into different carriers. More power in the higher carrier frequencies partially compensates for the diminishing transmission function of the POF link (Fig. 6.81). 20 P [dBm]

transmitted spectrum

10 0 -10 -20 -30 -40 0

50

100

150

f [MHz] 200

Fig. 6.81: Equalizing of the frequency response in a multi carrier system


455

In order to attain greater spectral efficiency, each carrier is modulated with a higher-order QAM signal. In the example described 2 groups each having 40 carriers were used. The carrier distance was 2 MHz with a symbol rate of 1.8 Mbaud per carrier. In order to achieve as high a degree of modulation as possible, the crest factor was optimized by adapting the carrier phases. QAM 256 (8 bits/symbol) was used in the lower group and QAM 64 (6 bits/symbol) in the upper group which results in a total bit rate of: BR brutto

40 1.8 Mbaud 8

bit bit 40 1.8 Mbaud 6 symbol symbol

1,008 Mbit / s

The measurement technique at hand did not yet permit any real time demodulation. This is why only data packets were transmitted, recorded with a fast oscilloscope and demodulated in a PC. The constellation diagrams for 2 typical channels are shown in Fig. 6.82.

Fig. 6.82: Constellation diagrams for two carriers

This procedure achieves an overall spectral efficiency of 6.3 bit/s/Hz. The 100 m of SI-POF used has a loss of 14 dB with the 650 nm laser. A commercial TIA with a 1 mm large photodiode serves as the receiver. A bit error probability of below 10-3 could be calculated for all carriers from the error vector. By using an RS code (511,479) the bit error probability can be reduced to below 10-9. Then the net bit rate would be around 945 Mbit/s. Work on this system is being continued as part of the POF-ALL project (www.ist-pofall.org) in cooperation with the FhG IIS and the POF-AC Nürnberg. The system described with the use of forward error correction (FEC) is not exactly comparable to the experiments previously mentioned which were able to be significantly improved even more with FEC. The systems presented so far have been compiled once again in Table 6.9 and Fig. 6.83.

456


3,000

data rate [Mbit/s]

1,000

300

100

30 POF length [m]

10 1

2

5

10

20

50

100

200

Fig. 6.83: Overview of the SI/DSI-POF systems at 650 nm

It is quite obvious that the PMMA SI and DSI-POF cover an essentially greater area of use than most users perceive. Many of the experiments described above have indeed been conducted under ideal laboratory conditions, but on the other hand further improvements in the active components are possible and foreseeable. Table 6.9: POF transmission systems (SI-POF/DSI-POF) Ref.

Institute

[Scho88] [Kuch94] [HP05] [Pri92] [Kit92]

HP Mitsubishi

[Kuch94] [Fuk93] [Koi94] [Tan94b] [Kob97] [Yos96] [NL2110]

Kaiser 1990 Toshiba Keio Univ. Fujitsu NEC Asahi Glass NEC

Kennedy&Donkin

Fiber SI-POF SI-POF SI-POF SI-POF SI-POF SI-POF SI-POF SI-POF SI-POF Low-NA DSI-POF DSI-POF

Length Bit rate Capacity Transmitter [m] Mbit/s Mbit/skm 80 10 0.8 650 nm LED 100 140 14.0 650 nm LD 15 50 0.75 650 nm LED 90 125 11.3 670 nm LD 100 10 1.0 596 nm LED 125 12.5 650 nm LD 110 140 15.4 n. a. 30 100 3.0 670 nm LED 100 250 25.0 653 nm LD 50 400 20.0 650 nm LD 100 156 15.6 650 nm LED 50 155 7.8 650 nm SLED 70 250 17.5 650 nm SLED


457

Table 6.9: POF transmission systems (SI-POF/DSI-POF), cont. Ref.

Institute

[Sak97] [Som98a] [Mai00] [Num01a] [Neh06a] [Bat92a] [Yas93] [Kuch94]

Sony Univ. Ulm Hamamatsu Matsushita Euromikron Univ. Essex Univ. Essex IBM

[Stre98b]

Mitel

[Schu01b] Infineon

Fiber SI-POF DSI-POF SI-POF SI-POF SI-POF SI-POF SI-POF 500 μmSI SI-POF DSI-POF

[Scha01] [Gui00a] [Gui00b]

DaimlerChrysler Univ. Tampere Univ. Tampere

[Gui00b] [Scha00] [Scha00]

Univ. Tampere DSI-POF DaimlerChrysler SI-POF DaimlerChrysler SI-POF

SI-POF SI-POF SI-POF

[Ziem00a] Telekom

SI-POF

[Jun04d]

SI-POF

FhG IIS

[Ziem03h] POF-AC [Gott06]

DSI-POF

POF-AC

SI-POF

POF-AC/ TUSUR [Ran06a] Siemens [Ran06c] Siemens unpublished POF-AC

SI-POF

[Was07]

SI-POF SI-POF SI-POF

Length Bit rate Capacity [m] Mbit/s Mbit/skm 70 200 14.0 63 125 7.9 50 156 7.8 20 50 1.0 70 125 8.8 100 265 26.5 100 531 53.1 30 531 15.0 100 300 30.0 1 512 0.5 30 250 7.5 50 500 25.0 100 250 25.0 30 500 15.0 1 622 0.6 1 1,000 1.0 10 400 4.0 10 622 6.2 10 500 5.0 10 400 4.0 20 400 8.0 30 600 18.0 10 1,200 12.0 20 800 16.0 15 1,000 15.0 20 1,000 20.0 50 500 25.0 100 550 55.0 100 820 82.0 20 1,470 29.4 30 1,200 36.0 50 650 32.5 85 100 8.5 50 1,660 83.0 100 910 91.0 100 540 54.0 100 945 94.5 100 1,390 139.0

Transmitter 650 nm LD 650 nm SLED 650 nm LED 650 nm RC-LED 655 nm LED 652 nm LD 652 nm LD 670 nm VCSEL 660 nm RC-LED 650 nm RC-LED 650 nm LD 650 nm RC-LED 650 nm RC-LED 650 nm RC-LED 670 nm VCSEL 650 nm LD

650 nm LD 650 nm DVD-LD

657 nm LD 650 nm LD

650 nm LD 650 nm LD, FEC 650 nm LD, FEC 650 nm LD, NRZ*

* measured after the German edition deadline (special designed passive equalizer, BER = 10-9, no error correction, will be presented on the OFC’2008)

458


6.3.2 Systems with PMMA SI-POF at Wavelengths below 600 nm

The PMMA fiber has the lowest attenuation at wavelengths around 520 nm and 560 nm. Nevertheless most transmission systems and experiments were first carried out in the third attenuation window around 650 nm. The reason for this was the lack of suitable LEDs at the time of development of GaN technology in the second half of the 1990’s. The different stages of transmission systems with blue, green and yellow LEDs will be presented in the following sections. 6.3.2.1 Systems with AIII BV Semiconductor LEDs

Before the introduction of GaN LEDs green LEDs could only be produced on the basis of indirect semiconductors (GaP) which are not suitable for data transmission because of their poor efficiency and the slow switching times. However, wavelengths in the range down to 570 nm can be manufactured from InGaAlP or GaAsP respectively. These were already being used at the beginning of the 1990’s for POF. In a transmission experiment, 10 Mbit/s were realized over 100 m. In the test, a yellow LED HLMA-DL00 (596 nm) with -12.4 dBm power in the POF was used. The TORX 194 with -29.55 dBm sensitivity at a BER = 10-9 was used as a receiver. Since then, a 10Base-POF transceiver from W&T with comparable parameters has become commercially available (to the author's knowledge, the only product not offered at 650 nm at this time). Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company: 596 nm LED 10 Mbit/s

SI-POF, Mitsubishi ESKA Premier 100 m 10 Mbit/s LED HLMA-DL00 (596 nm); -12.4 dBm TORX-194, -29.55 dBm [Kit92] Mitsubishi

100 m SI-POF Mitsubishi, ESKA Premier

TORX 194 for 10 Mbit/s

Fig. 6.84: Transmission system according to [Kit92]

In a second attempt, a 573 nm InGaAlP LED with 0.7 % external efficiency, 12.5 nm spectral width and -20.5 dBm average power was used in the POF. A TORX 196 with -32.5 dBm sensitivity served as the receiver. With greater LED power, a range of 200 m was estimated, although at the time, no such sources were available. The advantages of newer GaN-LED will be described in the later chapters.


Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

459

SI-POF 100 m 10 Mbit/s InGaAlP-LED, 573 nm, 'O = 12.5 nm -17.5 dBm in the fiber at 100 mA TORX-196, -32.5 dBm sensitivity [Fuk93] Toshiba

573 nm LED InGaAlP, 9 μW

100 m SI-POF

TORX 196

Fig. 6.85: Transmission system according to [Fuk93]

6.3.2.2 Systems with GaN LEDs

In the period between 1995 and 1998 the Technology Center of the Deutsche Telekom in Berlin carried out extensive preliminary tests on the use of green LEDs in POF systems. Measurements with different optical parameters were conducted on the then new LED from Nichia. Among other things tested were the temperature independence of the optical performance and the center wavelengths. These values are much better for GaN LEDs than for conventional red LEDs. The results have been presented, for example, in [Ziem07a], [Ziem98a], [Ziem98b], [Ziem98d] and [Ziem98e]. An example is represented in Figure 6.86. The bandwidth of a red, green and yellow LED with identical drive circuits (20 mA biasing current and optimized pre equalizing filter) is shown. Because of its design (5 mm type) the green LED could be coupled into the POF with less power, but the bandwidth was almost twice as high as the two other types. 6 power [dBelectr.] 3 0 -3 -6 -9 -12 -15 -18 -21 -24 -27 0.2 0.5 1

HFBR-1527

NSPG 500 HLMA-DL00

2

5

10

Fig. 6.86: Bandwidth of different LED ([Ziem98d])

frequency [MHz] 20 50 100 200

460


During this period the setup of the transmission system was not part of the Berlin research group’s work. In a test made in 1999 in cooperation with the University of Ulm first 125 Mbit/s over 50 m and later 155 Mbit/s over 100 m of DSI-POF could be transmitted ([Daum01a]). In [Ino99a] the use of blue LED for a 125 Mbit/s - 100 m of POF-transmission was under study. The LED was available as a chip and emitted a maximum output power of 0.92 dBm (1.24 mW). Without peaking, -5.28 dBm (0.3 mW) was launched into the POF. With optimized lenses and peaking, -3.62 dBm (0.43 mW) were launched. The SI-POF used has an attenuation of 168 dB/km for the wavelength used. Thus -22.1 and -20.5 dBm are available after 100 m POF at the fiber output (without peaking/with peaking and lens optimization). The receiver has -21.1 or else -22.1 dBm sensitivity (without/with peaking) at BER = 10-12. The maximum modulating frequency of the blue LED is 120 MHz or 200 MHz respectively without/with peaking. The transmission can only be achieved with optimized launch. Although the effective attenuation of the fiber medium is lower than at 650 nm, the parameters are not yet sufficient for a practical application covering a range of 100 m. It should be observed that the limits for eye safety for blue light are more restrictive than for red light. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

Peaking

SI-POF, 168 dB/km at blue light 100 m 125 Mbit/s +0.92 dBm, 120 MHz bandwidth (200 MHz with peaking) -22.1 dBm sensitivity [Ino99] Optowave Inc.

blue GaN-LED 125 Mbit/s

100 m 980/1000 μm SI-POF 168 dB/km at blue light

receiver

Fig. 6.87: POF system with a blue GaN-LED according to [Ino99a]

The use of green and blue LED for POF transmission was also described in [Yago99]. The authors used commercial LED at 475 nm (as a chip) and 520 nm (in a housing). The available power after 50 m POF (Eska Mega from Mitsubishi) came to -14.6 dBm (blue) and -17.9 dBm (green). An input power of -5.1 dBm was specified for the blue LED. The bandwidths of various LED ranged between 70 MHz and 120 MHz so that in each case 125 Mbit/s could be transmitted. No special peaking was used; the NEC NL2100 (155 Mbit/s transceiver) was used as the receiver. The following transmission experiments were conducted:


461

¾475 nm, 50 m POF, BER = 10-12, received power: -14.6 dBm ¾475 nm, 100 m POF, BER = 10-12, received power: -22.1 dBm ¾520 nm, 50 m POF, BER = 10-12, received power: -16.3 dBm


DSI-POF Eska-Mega 50 m, 100 m 155 Mbit/s 475 nm LED; 520 nm LED NL2100, NEC [Yago99] Optowave Laboratory

In [Mat00b] the use of a green LED (520 nm) for transmitting 30 Mbit/s over 100 m of SI-POF (110 dB/km at 520 nm with NA = 0.51) is described. At approximately -1 dBm launched input power, the sensitivity is -20.8 dBm for BER = 10-9. The authors call their system „First Plastic Optical Fiber Transmission Experiment using 520 nm LEDs“ whereby they were a few years too late. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

SI-POF; 110 dB/km at 520 nm with AN = 0.51 100 m 30 Mbit/s 520 nm LED; -1 dBm -20.8 dBm sensitivity [Mat00] NTT Basic Research Labs

Within the scope of the European IST Agetha Project green and yellow LEDs and RC-LEDs were to be specially developed for use in vehicle networks. In addition to high data rates temperature characteristics in particular were to be optimized. Even if not all project goals could be attained, nevertheless, results for POF systems with short wavelengths, unattained up till then, were achieved. In the first stage 100 Mbit/s over 100 m of SI-POF were reached. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

PMMA SI-POF, 126 dB/km at 495 nm 100 m 100 Mbit/s 495 nm LED, 0.8 mW at 20 mA (1.6% quant. efficiency) TK = -0.4%/K, up to 200°C Si-pin-PD [Lam01] Firecomms

In the following year a transmission of 200 Mbit/s over a 100 m fiber was demonstrated. The power efficiency of the diode and the modulation speed were improved. Fiber type: Length: Bit rate:

PMMA SI-POF 100 m 200 Mbit/s

462


Transmitter: Receiver: References: Company:

510 nm LED, 1.2 mW at 20 mA, 5 QW, on sapphire substrate 200 Mbit/s at 50 mA, no peaking Si-pin-PD [Lam02], [Akh02] Firecomms

GaN-LED 495 nm, 100 Mbit/s 510 nm, 200 Mbit/s

100 m POF 126 dB/km at 495 nm

Si-PD

Fig. 6.88: Data transmission using green LED at Firecomms

Fig. 6.89: Eye diagram with green LED, 200 Mbit/s over 100 m

Up to 310 Mbit/s could be transmitted over short distances (10 m) with single LEDs (Agetha final report). In the meantime, Firecomms has been producing green transceivers as commercial products, for the time being only up to a data rate of 50 Mbit/s. The guaranteed data are mentioned below. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

DSI-POF Asahi AC-1000 100 m 60 Mbit/s 520 nm HSG-LED, -9.7 dBm in the fiber Si-pin-PD [Lam03a] Firecomms

Another company which has been investigating the transmission of data using green LEDs is Toyota. Using their own LED, they transmitted 125 Mbit/s over 60 m of DSI-POF. This LED is also characterized by very small temperature coefficients.



Receiver: Reference: Company:

463

DSI-POF Eska-Mega 60 m 125 Mbit/s 520 nm LED; E1L53-3G, +3.9 dBm, 'P/'T = -0.22 %/K, 'O/'T = 0.033 nm/K, 'O = 35 nm -4.5 dBm in the POF (20 mA), chip area 300 u 300 μm² Scientek APD-250 [Kat02] Toyota

Further experiments with data rates up to 250 Mbit/s followed over the next two years. Experiments involving different LEDs with small chip surfaces were carried out. Fiber type: Length: Bit rate: Transmitter:


DSI-POF Eska-Mega 20 m (estimated 80 m at 100 dB/km) 250 Mbit/s 515 nm LED; +1.1 dBm 'P/'T = -0.72 %/K, 'O/'T = 0.025 nm/K 38.2 nm spectral width, -5.6 dBm in the POF chip area 200 u 260 μm² TODX-2402, -18.6 dBm (BER = 10-12) [Kat04], [Kat05] Toyota also 250 Mbit/s over 20 m with 490 nm LED

Fig. 6.90: Green LED for data transmission

Fig. 6.91: Eye diagram for 250 Mbit/s transmission over 20 m POF

464


For some years now there has been new work going on in Italy (Luceat, ISMB: Instituto Mario Boella and Politechnico di Turino) with the goal of increasing the range of PMMA POF systems. In addition to using green LEDs optimized modulation procedures, error correction codes and multi level coding have been employed. One example is a four level transmission, whereby an additional 5S/6S code (out of 64 possible codes 54 are selected) in order to keep signals as free of direct current as possible. For 100 Mbit/s the result is a symbol rate of 60 Mbaud/s and 100 m of SI-POF can be bridged. In Figs. 6.92 and 6.93 the effect of the improvement in signal quality by means of the 5S6S code can be seen after 50 m and 200 m of POF respectively.

Fig. 6.92: Eyes: uncoded and coded for 50 Mbit/s over 100 m ([Gau05a])

Fig. 6.93: Eyes: 50 Mbit/s over 100 m (5S/6S) and 50 Mbit/s over 200 m [Gau05b]


Luceat SI-POF, AN = 0.50, 105 dB/km (green) attenuation: 17 dB (150 m); 21 dB (200 m) 100 m, 150 m, 200 m 50 Mbit/s (4- level code, 5S/6S coded) 4 PAM modulation green DieMount LED; +3 dBm (pigtail), 22 MHz; with compensation of the non linearity



465

Hamamatsu-PD + TIA S6468-02 [Gau04a], [Gau05a], [Gau05b] Politechnico di Turino, ISMB Turin

With a four-level code a maximum of 100 Mbit/s over 100 m has been able to be transmitted so far. Up to 150 Mbit/s can be attained using an eight-level code (the eye diagram after 50 m in the picture).

Fig. 6.94: Eye for 150 Mbit/s over 50 m and 100 Mbit/s over 100 m, [Gau04a]

At the 2006 POF Conference in Seoul a data transmission of 100 Mbit/s over 200 m of SI-POF was presented as the latest result ([Nes06a] and [Nes06b]). A green LED was used as a transmitter (DieMount, +3 dBm fiber-coupled power) as described in the previous systems. An adaptive equalizer was now used to compensate for the transmission behavior of the POF. Using 8-level coding the symbol rate was 33 Mbaud/s. The receiving signal was sampled at 66 MSample/s and further processed. A noise gap of 19 dB is needed with FEC. It was possible to attain 26 dB (margin of 7 dB) in experiments. The next step is to integrate the system into a FPGA. Fiber type: Length: Bit rate: Transmitter: Receiver: References: Company:

Luceat SI-POF, AN = 0.50, 105 dB/km (green) 200 m 100 Mbit/s (8 level coded, 33 MBaud/s) green DieMount LED; +3 dBm (pigtail) Hamamatsu-PD + TIA S6468-02 adaptive equalizer [Nes06a], [Nes06b] Politechnico di Turino

In another experiment a date rate of 10 Mbit/s was transmitted. One particularly powerful LED (DieMount), a large-surface Si pin-PD with a low-noise transimpedance receiver and a special code, allowed a range of 350 m. At the 2006 POF conference it was even possible to demonstrate a transmission over 425 m (with 8B10B coding, Reed-Solomon code for FEC, [Car06a]).

466


Fiber Type: Length: Bit rate: Transmitter: Receiver: References: Company:

Luceat SI-POF, AN = 0.50, 105 dB/km (green) 350 m, 425 m 10 Mbit/s green DieMount LED; +3 dBm (pigtail) Hamamatsu-PD + TIA S6468-02 (-37 dBm sensitivity) [Gau04a], [Car06a] Politechnico di Turino

An alternative to transmitting with multi level codes is the use of multicarrier procedures as employed for example with DSL. Teleconnect Dresden realized the transmission of Fast Ethernet over 200 m [Blu07] as part of the POF-ALL project. Here, too, a green LD was used in order to be able to utilize the low POF attenuation. A detailed description of this system can be found in Section 6.3.7.2. 6.3.2.3 Commercial Developments

Since about 2003 different commercial transceivers with GaN LEDs have been in development or are already available. Almost all of these products are intended for Ethernet and Fast Ethernet applications and are employed in the fields of automation and home networks. For example, Ratioplast offers 10 Mbit/s transceivers with ranges up to 200 m (Fig. 6.95). Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

PMMA SI-POF 200 m 10 Mbit/s 520 nm LED Si-pin-PD [Thi04] Ratioplast

Fig. 6.95: Transceiver with green LED


467

Luceat (Italy) also offers a comparable transceiver with green LED as a commercial product. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

PMMA SI-POF 25 m to 200 m 10 Mbit/s 520 nm LED Si-pin-PD [Luc04a] Luceat

Fig. 6.96: Transceiver with green LED by Luceat (Italy)

Astri Technology Centre in HongKong has developed various products for POF and above all for PCS. Presently in development are components for SI-POF on the basis of green LEDs. At the 2005 POF Conference a complete module was presented. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

PMMA SI-POF, 70 dB/km 40 m 20 Mbit/s 520 nm LED, transmitter module: 9.7 u 6.2 u 3.6 mm³ Si-pin-PD [Wip05] Astri Hong Kong

Some time ago Infineon Technologies also presented a development close to production for data transmission with green LEDs. The system is supposed to work at 125 Mbit/s over 100 m of DSI-POF and is characterized by low temperature coefficients. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

DSI-POF, Eska Mega 100 m 125 Mbit/s 510 nm LED, 200 μW, 0.23 %/K temperature dependency Si-pin-PD, -23 dBm sensitivity [Witt03] Infineon Technologies

468


DieMount has presented different transceiver developments. At present they have demonstrated a transmission of 125 Mbit/s over 100 m of DSI-POF. 50 Mbit/s over 200 m of POF is also possible with a more powerful (+1 dBm in the fiber), but somewhat slower LED. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

DSI-POF Asahi AC-1000 100 m 125 Mbit/s 520 nm LED, -3.5 dBm Si-pin-PD [Kra03] DieMount

Higher speeds are possible with a blue LED. In addition, these LEDs are generally more efficient, which possibly compensates for the somewhat higher attenuation and the poorer PD sensitivity. In one test a transmission of 125 Mbit/s over 150 m was achieved. The receiver contained a high pass for compensating the mode dispersion. The system is commercially available as a duplex version (with 2 fibers) and a range of 80 m is guaranteed. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

PMMA SI-POF 150 m (one connection) 125 Mbit/s, bi-directional over duplex fiber 470 nm LED Si-pin-PD [Kra04a] DieMount

Fig. 6.97: Eye diagram, 125 Mbit/s over 150 m SI-POF, one connection


469

6.3.2.4 POF-AC Systems

The POF-AC has carried out various experiments on the modulation limits of green and blue LEDs. With Nichia’s green LED a maximum data rate of 380 Mbit/s was able to be attained (modulation over bias-T, driven with 50 generator). In Fig. 6.98 a bit sequence with 250 Mbit/s after 10 m of SI-POF is shown. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

1 mm SI-POF 10 m 250 Mbit/s 525 nm LED (Nichia), 8 mW at 20 mA 130 MHz modulation bandwidth at 50 : Si-pin-PD [Ziem03e] POF-AC also 380 Mbit/s over 1 m

Fig. 6.98: Data modulation of a green Nichia-LED

In 2006, using new LEDs - set up by DieMount with optimized coupling - even greater data rates could be attained. 210 Mbit/s were able to be transmitted at 470 nm error free over 50 m of SI-POF. For a back-to-back measurement a modulation of over 1 Gbit/s (eye diagram in Fig. 6.99) could be achieved for the first time. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

PMMA SI-POF, AN = 0.51 50 m 210 Mbit/s max. of 1,090 Mbit/s back-to-back 470 nm LED, DieMount 800 μm Si-pin-PD [Ziem06h] POF-AC

470


Fig. 6.99: Eye diagram 1,000 Mbit/s, blue LED, 1 m POF

All experiments in this section are summarized in Table 6.10 and Fig. 6.100. These systems are of particular interest for distances up to 100 m. Table 6.10: Overview of the POF systems at short wavelengths Ref.

Institute

Fiber

[Kit92] [Fuk93] [Ziem00c] [Daum01a] [Daum01a] [Yago99] [Mat00a] [Lam01] [Lam02] [Lam03a] [Blu02] [Kat02] [Kat04] [Kat04] [Gau04b] [Gau04b] [Gau04a] [Gau04a] [Nes06a]

Mitsubishi Toshiba T-Nova T-Nova T-Nova Optowave NTT Firecomms Firecomms Firecomms POF-AC Toyota Toyota Toyota ISMB ISMB ISMB ISMB ISMB

SI-POF SI-POF SI-POF DSI-POF DSI-POF SI-POF SI-POF SI-POF SI-POF DSI-POF SI-POF DSI-POF DSI-POF DSI-POF SI-POF SI-POF SI-POF SI-POF SI-POF

Length Bit rate Remarks [m] [Mbit/s] 100 10 P = -12.4 dBm 30 100 InGaAlP-LED, -20,5 dBm 500 0.192 ISDN, S0-bus 50 125 Nichia LED 100 155 Nichia LED 50 125 -17.9 dBm, 50m POF 100 30 -1.0 dBm 100 100 Agetha project 100 200 Agetha project 100 60 HSG-LED, DSI-POF 400 6 MHz video system 60 125 Toyoda E1L53-3G 20 250 25 mA modulation 20 250 +1.1 dBm 100 50 4-level coding 200 50 4-level coding 50 150 8-level coding 100 100 4-level coding 200 100 8-level coding

OLED [nm] 596 573 560 520 520 520 520 495 510 520 525 510 515 490 520 520 520 520 520


471

Table 6.10: Overview of the POF systems at short wavelengths, cont. Ref.

Institute

Fiber

[Blu07] [Blu07] [Luc04a] [Luc04c] [Gau04a] [Gau06] [Thi04] [Wip05] [Witt03] [Kra03] [Ziem03e] [Ziem03e] non publ. [Ino99] [Yago99] [Yago99] [Kra04a] [Ziem06h] [Ziem06h]

Teleconnect Teleconnect Luceat Luceat Luceat Luceat Ratioplast Astri Infineon DieMount POF-AC POF-AC POF-AC’03 Optowave Optowave Optowave DieMount POF-AC POF-AC

SI-POF SI-POF SI-POF SI-POF SI-POF SI-POF SI-POF SI-POF DSI-POF DSI-POF SI-POF SI-POF SI-POF SI-POF SI-POF SI-POF SI-POF SI-POF SI-POF

Length Bit rate Remarks [m] [Mbit/s] 200 300 200 250 350 425 200 40 100 100 1 10 100 100 50 100 150 1 50

107 40 10 6 MHz 10 10 10 20 125 125 380 250 145 125 125 125 125 1000 210

OLED [nm]

DieMount-LED, VDSL2 DieMount-LED, VDSL2 media converter video system +3 dBm with FEC media converter media converter 200 μW -3.5 dBm Nichia sample Nichia sample Nichia sample P = 1.24 mW -5.1 dBm Preceiv. = -22.1 dBm +3.5 dBm in POF, SI-POF with equalizer with equalizer

520 520 520 520 520 520 520 520 510 520 525 525 510 470 475 475 470 470 470

1000 bit rate [Mbit/s] 300 100 blue LED green LED

30

yellow LED analog video

10 1

3

10

30

100

Fig. 6.100: POF systems with short wavelength transmitters

300 1000 length [m]

472


6.3.3 Systems with SI-POF at Wavelengths in the Near Infrared Range

In the previous sections we have shown that you can also use SI-POF for data rates of over 1,000 Mbit/s, however, for short distances only. Consequently, transmitters in the near infrared range could obviously be used. The attenuation of the PMMA fiber in this range is indeed considerably greater, however, distances of up to 10 m as generally used in vehicle networks, can be bridged. The PMMA attenuation curve indicating losses in 10 m lengths is shown in Fig. 6.101.

10,000

3,000

attenuation [dB/km]

application windows for the PMMA-POF

1,000

300

100

|20 dB /10 m

|1.5 dB /10 m |0.7 dB |0.8 dB /10 m /10 m |0.6 dB /10 m

|7 dB /10 m

50 350 400 450 500 550 600 650 700 750 800 850 900 wavelength [nm] Fig. 6.101: Losses in PMMA-POF

Of particular interest is the window around 770 nm for short distance use. As opposed to a wavelength of 650 nm the powerful VCSELs available are reasonably priced and can also be employed in a wide range of temperatures. Furthermore, lasers at 780 nm are normally faster and the Si-PDs have a better sensitivity. 6.3.3.1 PMMA Fiber Systems for Infrared

The ability to use 780/850 nm VCSELs for the short-distance transmission on SI-POF was investigated at the University of Ulm in 1998. [Schn98]. For this purpose, SI-POF with a core diameter of 125 μm and cladding diameter of 250 μm of the type Toray PGR-FB 125 with NA = 0.48 was used. A 775 nm GaAs VCSEL with 4 μm aperture diameter was used as the light source. It has a 1.9 mA threshold current and emits a maximum power of 1.1 mW at 5 mA. The fast Ge-APD detector has a sensitivity of -24.8 dBm at BER = 10-11. At first, a 1 m transmission was established at 2.5 Gbit/s with this source.


473

With an additional 835 nm GaAs VCSEL with 0.6 mA of threshold current, further transmission experiments were conducted. Through modulation with bias current it was possible to increase the transmission distance considerably. The following parameters were attained (see Fig. 6.102): ¾1.0 m-transmission at 1.0 Gbit/s, bias-free, -26 dBm sensitivity ¾1.0 m-transmission at 2.5 Gbit/s, bias-free, -22 dBm sensitivity ¾2.5 m-transmission at 1.0 Gbit/s, 3 mA bias, -26 dBm sensitivity ¾2.5 m-transmission at 2.5 Gbit/s, 3 mA bias, -23.5 dBm sensitivity


1 Gbit/s 2.5 Gbit/s

125 μm SI-POF, Toray PGR-FB 125 1 m; 2.5 m 1,000 Mbit/s; 2,500 Mbit/s 775 nm GaAs-VCSEL; 835 nm GaAs-VCSEL Ge-APD [Schn98] University of Ulm

Bias

VCSEL 835 nm

1 m/2.5 m 125/250 μm SI-POF Toray PGR-FB 125

Ge-APD

Fig. 6.102: POF for short haul data transmission according to [Schn98]

In a Diploma thesis [Kich99] submitted to the University of Ulm, the use of VCSEL for POF systems was also examined. 1 Gbit/s over 15 m of SI-PMMA 1 mm POF with a 780 nm VCSEL (single mode) was achieved. The source was powered by 2.93 mA bias current and ±0.5 V modulation. Standard NA, DSI-POF and glass fibers were used. The following individual experiments were performed: ¾ 900 Mbit/s over 15 m Hoechst EP51, AN = 0.46, -22 dBm for BER = 10-11 ¾1,000 Mbit/s over 15 m Mitsubishi MH4001, AN =0.32, -27.5dBm: BER=10-11 ¾1,000 Mbit/s over multimode glass fiber, -30.5 dBm for BER = 10-11

Due to the small launch angle (lens with AN = 0.156), there was no equilibrium mode distribution in the POF. The receiver was a Si-APD with 1 mm diameter. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

SI-POF Hoechst EP51, DSI-POF MH4001 15 m 1,000 Mbit/s 780 nm VCSEL Si-APD with : 1 mm [Kich99] University of Ulm

474


BIAS

VCSEL 780 nm

15 m SI-POF 1 mm PMMA

Si-APD

Fig. 6.103: Gbit/s POF system at the University Ulm

Fig. 6.104: Eyes for 900 Mbit/s over 15 m SI-POF and 1,000 Mbit/s over 15 m DSI-POF

In 2004, Infineon Technologies presented a transmission of 3,200 Mbit/s over short distances of 0.5 mm and 1 mm of SI-POF, so far the highest data rate over 1 mm fibers which has ever been published. A VCSEL with a wavelength of 850 nm served as the transmitter. Fiber type: Length: Bit rate: Transmitter: Receiver: References: Company:

SI-POF, 1 mm and 500 μm 2m 3,200 Mbit/s 850 nm VCSEL GaAs-PD (small area) [Hurt04], [Schu04] Infineon Technologies

Fig. 6.105: Transmission of 3,200 Mbit/s over 2 m of a 0.5 mm SI-POF


475

6.3.3.2 PC Fiber Systems in Infrared

Step index profile POF on the basis of polycarbonate shows similar characteristics as the PMMA-based fibers, can, however, be employed with high temperatures at least in the absence of high humidity. Such systems could be of special interest for future vehicle networks in which short distances, high data rates and high temperatures are combined. Furukawa introduced new POF in 1998. The core material was partially fluorinated polycarbonate PC(AF). In [Hatt98] the transmission of a 125 Mbit/s data rate is described over 85 m of POF, and a data rate of 156 Mbit/s over 80 m, and finally 250 Mbit/s over 58 m of fiber medium. The fiber has an attenuation window between 730 nm and 820 nm, whereby the least losses are at 780 nm with 300 dB/km. A laser diode was used in the experiment for this wavelength. The fiber has a numerical aperture of approximately NA = 0.30 and thus a bandwidth of 20 MHz km (cf. [Nish98]). A special advantage of the PC(AF)-POF is its high resistance to temperature of up to +145°C (PMMA-POF to +85°C). Figure 6.106 illustrates the principle test set-up. The receiver sensitivity for the bit rates was: -32.35 dBm (125 Mbit/s), -31.50 dBm (156 Mbit/s) and -26.60 dBm for 250 Mbit/s for at a BER of about 10-12. The fiber launched power was approximately -8 dBm. Fiber type: Length: Bit rate: Transmitter: Receiver: References: Company:

500 μm SI-PC(AF)-POF, 300 dB/km at 780 nm, AN = 0.30 bandwidth 20 MHz km 58 m to 85 m 125 Mbit/s, 250 Mbit/s laser diode commercial receiver (HP, NEC) [Hatt98], [Nish98] Furukawa

780 nm LD 125 Mbit/s 156 Mbit/s 250 Mbit/s

receiver -32.35 dBm -31.50 dBm -26.60 dBm

85 m/ 80m /58 m SI-PC(AF)-POF 300 dB/km at 780 nm

Fig. 6.106: POF system at 780 nm with PC(AF)-POF

6.3.3.3 System Experiments at the POF-AC

Since 2002, different transmission systems based on laser diodes in the near infrared range have been set up at the POF-AC Nürnberg. An edge-emitting 780 nm laser for barcode lasers from Rohm (Laser Components) and an 850 nm VCSEL were used. In the first experiment a data rate of 1,700 Mbit/s over a standard POF Toray PFU CD 1001 (10 m) was transmitted.

476


The maximum data rate over 2 m PMMA POF amounted to 2,000 Mbit/s. Fiber type: Length: Bit rate: Transmitter: Receiver: References: Company:

PMMA SI-POF, PFU-CD 1000, 1,670 dB/km 10 m 1,700 Mbit/s 780 nm LD, POF coupled power: +4.7 dBm received power: -12.0 dBm (10 m) 800 μm Si-pin-PD Hamamatsu S5052 [Vin02b], [Ziem03f] POF-AC Nürnberg

BIAS LD 780 nm Laser Comp.

10 m St.-PMMA-POF Toray PFU-CD 1000 1,670 dB/km

Si pin PD S 5052

Fig. 6.107: Transmission experiment with a 780 nm Laser (PMMA SI-POF)

Later experiments with an improved laser transmitter even resulted in a transmission data rate of 1,800 Mbit/s over 10 m of PMMA SI-POF (see [Ziem02j], [Ziem02k]). Using a new receiver (TIA), a data rate of 2.200 Mbit/s was eventually reached (in [Vin05b], [Ziem05j]). The highest data rate so far amounted to 2,270 Mbit/s over 5 m with Toray PFU-CD 1001 ([Ziem06d]). The eye diagram can be seen in Fig. 6.108.

Fig. 6.108: Transmission of 2,270 Mbit/s over 5 m of a 1 mm SI-POF

In the following year data transmission over a polycarbonate fiber was then carried out (Mitsubishi PC-POF, core diameter of 1 mm). The attenuation of this fiber of 900 dB/km at a wavelength of 780 nm clearly lies below the value of the previously used PMMA fiber. Thanks to the greater mixing of modes data rates of 1,800 Mbit/s or 1,000 Mbit/s respectively over 10 m or 20 m of PC-POF can be transmitted error free. The experimental setup is shown in Figure 6.109. The received power (fiber) was -4.3 dBm and -14.8 dBm.



477

1 mm PC-POF Mitsubishi FH4001-TM, AN = 0.75 900 dB/km at 780 nm 10 m, 20 m 1,800 Mbit/s, 1,000 Mbit/s 780 nm LD, +4.7 dBm 800 μm Si-pin-PD, S5052 [Ziem03e] POF-AC Nürnberg


PC-SI-POF Mitsubishi, 900 dB/km

Si pin PD S 5052

Fig. 6.109: Transmission experiment with a 780 nm Laser (PC-SI-POF)

In order to be able to use POF at high temperatures, a POF based on a modified PMMA, which can be employed at up to +130°C, was developed at the RPC (Institute of Microelectronics and Informatics, Russian Academy, Research and Production Complex in Tver). The numerical aperture corresponds to that of a standard POF. The bandwidth, however, is greater due to the stronger mode mixing. At 780 nm the attenuation of the fiber is below 1 dB/m. Different samples from 10 m to 23 m in length were tested. In 2003, using a 15 m long fiber there was a successful transmission for the first time of 2.5 Gbit/s over a 1 mm thick fiber. The eye diagram of a transmission of 1,000 Mbit/s over 23 m can be seen in Fig. 6.111. It is almost completely open. Fiber type: Length: Bit rate: Transmitter: Receiver: References: Company:

SI-POF, modified PMMA (Tver) 23 m 1,200 Mbit/s 780 nm LD, 5 mW, POF coupled power: +4.7 dBm received power: -12.0 dBm (10 m) 800 μm Si-pin-PD Hamamatsu S5052 [Vin02b], [Ziem03f], [Vin04a], [Ziem04b], [Vin05c] POF-AC Nürnberg 1,200 Mbit/s over 23 m (2002) 2,560 Mbit/s over 11 m (2005) 2,500 Mbit/s over 15 m (2003)


14.88 m mod. PMMAPOF 1 mm (TVER)

Si pin PD S 5052

Fig. 6.110: Transmission experiment with a 780 nm Laser (SI-mod. PMMA-POF)

478


Fig. 6.111: Eye diagram for 1,000 Mbit/s over 23 m (SI-mod. PMMA-POF)

For a long time a SI-POF based on modified PMMA from Toray was commercially available (PHKS CD1001 22P, usable up to +115°C). As was shown in [Ziem03e], 1,600 Mbit/s at 780 nm over 10 m could be transmitted. This fiber’s attenuation was 1,950 dB/km, the received power was -14.8 dB at the PD after 10 m of fiber. Another fiber which was tested at the POF-AC was the HPOF-S from Hitachi. This fiber consists of a silicone material and has a cladding diameter of 1.5 mm. 2,200 Mbit/s can be transmitted over 10 m of the fiber; at 13.5 m it was still 1,700 Mbit/s. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:


HPOF-S (Hitachi), 1.5 mm 10 m, 13.5 m 2,200 Mbit/s, 1,700 Mbit/s 780 nm LD, +3.4 dBm in the POF received power: -10.6 dBm (10 m); -15.4 dBm (13.5 m) 800 μm Si-pin-PD Hamamatsu S5052 [Vin04b] POF-AC Nürnberg

10 m SI-POF 1.5 mm Hitachi HPOF-S

Si pin PD S 5052

Fig. 6.112: Transmission experiment with a 780 nm Laser (SI-EOF)

Fujifilm reports a transmission of DVI data at 1.65 Gbit/s over 15 m of a newly developed GI-POF with a reduced attenuation at 780 nm. This POF most likely was made from partially or completely deuterated polymer material. The VCSEL used contains four active zones. Because of this split of power the life span increases more than ten-fold.


Fiber type: Length: Bit rate:

479

mod. PMMA POF Lumistar, 300 μm 15 m 5,000 Mbit/s (back-to-back) 1,650 Mbit/s (DVI) over 15 m 780 nm VCSEL, 2 mW at 6 mA (up to +60°C) [Nak03a] Xerox, Fujifilm

Transmitter: Reference: Company:

The bit rate and transmission range parameters for all NIR POF systems are summarized once again in Fig. 6.113 Data rates above 1 Gbit/s over some 10 m could be achieved in many experiments. We shall see later on that with this development many new possible applications for POF will open up. The points at the lengths of 50 m to 100 m represent special cases since they were achieved with a special POF, the development of which is no longer being carried on.

5000

bit rate [Mbit/s]

2000 1000 500

200 100 1

2

5

10

20

length [m] 50

100

Fig. 6.113: Transmission experiments at 780 nm and 850 nm (overview)

6.3.4 Systems with PMMA GI-POF, MSI-POF and MC-POF

It is not without reason that the multimode, graded index and multi-core POFs are treated together in one section. These three types of fibers are linked by the idea of a definitely higher bandwidth than with SI-POFs. The difficulties in the production of different index profiles have already been discussed in detail in Chapter 2. As was to be expected, the system experiments concentrated on particularly high data rates. Short-wave transmitters are not used because they are too slow and the attenuation of the three POF types in this area is also generally too great. Wavelengths over 650 nm do not play any role either. In this case lengths are limited to less than 10 m, for which the bandwidth of SI-POF is still enough for Gbit/s data rates. Thus, all systems subsequently described work with 650 nm lasers.

480


6.3.4.1 PMMA GI-POF System Experiments before 2000

In [Tan94b] a data rate of 700 Mbit/s was transmitted over 50 m of a GI-POF. The PMMA GI-POF was manufactured by Nippon Petrochemicals Co., Ltd. The attenuation for the wavelength used at 650 nm was 400 dB/km. The core diameter of the POF was 0.6 mm with an AN = 0.20. A 0.5 mm APD with broadband amplifier served as the receiver. The bit rate was limited through the Anritsu ME 522A BER measuring test set. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

600 μm PMMA GI-POF 400 dB/km at 650 nm; NA = 0.20 50 m 700 Mbit/s 650 nm laser 500 μm APD [Tan94b] Nippon Petrochemicals Co. Ltd.

As early as 1994, a system with a bit rate of up to 1 Gbit/s was introduced by Kuchta (IBM, together with Keio University [Kuch94]). Two different GI-POFs from Keio University were examined, each with a core diameter of 550 μm. They differed in their NA from 0.24 or else 0.30. At the samples examined that were 90 m long, a bandwidth of 4,350 MHz was measured. A Toshiba 654 nm LD TOLD9421 was used as a transmitter in the first experiments. At a max. power of +4 dBm the highest possible modulation rate was 950 Mbit/s, reached with a simple pre-emphasis. For both samples, the receiver power was approximately -20 dBm after 90 m (corresponding to 267 dB/km attenuation). Also used was a 670 nm VCSEL which, however, allowed a modulation of up to 1.5 Gbit/s but with only -10 dBm of launched optical power. With this source, only 30 m GI-POF could be covered. The receiver used was a Hamamatsu S4753 400 μm Si-pin photodiode with GRIN lens for optimal coupling. The sensitivity attained was -23.3 dBm at 1 Gbit/s (at a BER = 1.5 · 10-9 ). Figure 6.114 illustrates the test scheme. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

550 μm PMMA GI-POF, 267 dB/km; AN = 0.24 / 0.30 90 m 950 Mbit/s, 622 Mbit/s Toshiba 654 nm-LD TOLD9421, +4 dBm 400 μm Si-pin-PD Hamamatsu S4753 sensitivity: -23.3 dBm [Kuch94] IBM, Keio University 1,062 Mbit/s over 30 m with 670 nm VCSEL


90 m PMMA-GI-POF Ø: 550 μm; NA = 0.24/0.30 Keio University

654 nm LD +4 dBm 950 Mbit/s

670 nm VCSEL -10 dBm 1,062 Mbit/s

30 m PMMA-GI-POF

GRIN

GRIN

481

Si-pin-PD Ø: 400 μm S4753

Si-pin-PD Ø: 400 μm S4753

Fig. 6.114: PMMA GI-POF system with LD and VCSEL according to [Kuch94]

In 1994, Prof. Koike also presented the first POF transmission system with a data rate of 2,500 Mbit/s at a range of 100 m ([Koi94], [Yam94], [Koi96c], [Yam96b], [Ish95b]). A PMMA GI-POF with 200 dB/km attenuation at 647 nm was used. The bandwidth is specified as 0.5 - 2 GHz · 100 m. The POF-NA was 0.21; the core diameter of the fiber was 420 μm. A NEC laser diode with a 647 nm wavelength served as the source. When coupled to the fiber with a GRIN lens, it was possible to achieve a launched power of +6.1 dBm in the POF. A Si-pin PD with a diameter of 400 μm coupled to a GRIN lens and FET amplifier served as the receiver with which a sensitivity of -16.9 dBm at BER = 10-9 was attained. At a fiber length of 100 m, the result was a deterioration in the sensitivity (penalty) of 0.6 dB through mode dispersion. Figure 6.115 illustrates the system principle. Fiber type: Length: Bit rate: Transmitter: Receiver: References: Company:

647 nm LD

PMMA GI-POF; 200 dB/km at 647 nm; AN = 0.21 100 m 2,500 Mbit/s NEC LD 647 nm; +6.1 dBm 400 μm Si-pin-PD; -16.9 dBm at 2.5 Gbit/s [Koi94], [Yam94], [Koi96c], [Yam96], [Ish95] Keio University

GRIN

100 m PMMA GI-POF Keio University

GRIN Si-pin-PD Ø: 400 μm

Fig. 6.115: First 2.5 Gbit/s GI-POF system at the Keio University

PMMA GI-POF was investigated by Boeing as part of the HSPN project for an optical network in planes ([Krug95]). It was intended for use in the Boeing 777. An optical network based on 100 μm/140 μm glass fibers will be used for the first

482


time commercially in this plane. In the tests, GI-POF with 750 μm diameter (600 μm core) at wavelengths of 650 m were used. Data rates of 10 Mbit/s and 100 Mbit/s were transmitted over a maximum of 30 m. Figure 6.116 shows the architecture of such an onboard network using optical connections based on POF. The system was planned with two different transmitters. The available LED are able to launch -8.5 dBm max. power into the POF; with VCSEL it should be able to reach 0 dBm. The receiver developed by Honeywell had a minimum sensitivity of -31 dBm. The PMMA GI-POF used had a typical fiber attenuation of 145 ± 5 dB/km at 650 nm. The typical connector attenuation was 1.5 ± 0.5 dB. At a permissible temperature range of -40°C to +85°C, the max. power must not exceed 1 mW in order to guarantee eye safety. A service life of 20 years should be reached. Fiber type: Length: Bit rate: Transmitter: Reference: Company:

600 μm PMMA GI-POF, 145 ± 5 dB/km at 650 nm 30 m 10 and 100 Mbit/s 650 nm LED, -8.5 dBm (VCSEL planned) [Krug95] Boeing

CMF or MAT PC with FDDI-interface card 100 Mb/s over 600/750 μm GI-POF at 0.65 μm FDDI to HSPN interface card

Avionics Brouter FDDI to Ethernet converter Hub 10 Base-F 10 Mb/s over 600/750 μm GI-POF at 0.65 μm

10 Base-F to HSPN interface card

PMAT 2 PC with 10 Base-F-interface card Fig. 6.116: Boeing POF test for avionic networks according to [Krug95]

In [Mor98] the combination of the RC-LED, developed by Mitel, and the GI-POF was introduced. The attenuation of the PMMA GI-POF was specified as 180 dB/km. A Si-pin-PD with a diameter of 800 μm was used as the receiver. For the 250 Mbit/s experiment, it was possible to couple -12.3 dBm in GI-POF at 30 mA diode current without lens. The fiber length was 50 m. At -23.7 dBm sensitivity, a BER of 10-12 was achieved.


483

In a second 500 Mbit/s experiment, a sensitivity of -17.6 dBm was achieved. With a lens coupling, the optical power was increased to -4.2 dBm at 30 mA in the GI-POF. In this case, system deterioration through dispersion was 0.9 dB. To drive the RC-LED, a predistortion filter was used. Presumably, a Mitsubishi 750 μm MSI-POF was used in both tests, which is - strictly speaking - is not a GI-POF. Figure 6.117 illustrates the set-up. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

Mitsubishi MSI-POF; 180 dB/km 50 m 250 Mbit/s; 500 Mbit/s 650 nm RC-LED; -4.2 dBm in the Fiber 800 μm Si-pin-PD [Mor98a] Matsushita -12.3 dBm

650 nm RC-LED 250 Mbit/s

50 m GI-POF, Mitsubishi 180 dB/km

Si-PD -23.7 dBm

-4.2 dBm


50 m GI-POF, Mitsubishi 180 dB/km

Si-PD -18.7 dBm

Fig. 6.117: GI-POF system with RC-LED according to [Mor98]

Further experiments by Matsushita using RC-LEDs and VCSELs are described in [Fur99] and [Num99]. Lasers are adversely affected during operation by reflections. If there are very large amounts of light fed back, a coherence collapse may occur, which can cause extremely strong optical power fluctuations. This is why the influence of reflections from the POF was taken up in [Fur99]. With a laser diode, a -20 dB reflection can still be tolerated. The RC-LED still functions to a reflection of as much as -10 dB. A further advantage of RC-LED is the low temperature dependency of the wavelength with 0.07 nm/K compared to 0.2 nm/K for the LD. With the source, the transmission of 500 Mbit/s over 50 m of MSI-POF (700 μm diameter from Mitsubishi) was possible (Fig. 6.118). At 30 mA of current, the source emits 2.26 mW of optical power. At a sensitivity of -20.1 dBm, the error probability BER was < 10-12. Furthermore, the temperature dependence of the output power was compared. Between -10°C and 70°C laser current must be increased from 50 mA to 130 mA in order to retain 2 mW of optical power. For the RC-LED, the current must only increase from 20 mA to 50 mA.

484




700 μm MSI-POF, Mitsubishi 50 m 500 Mbit/s RC-LED 650 nm, 2.26 mW 800 μm Si-pin-PD -20.1 dBm sensitivity [Fur99], [Num99] Matsushita

50 m MSI-POF 700 μm Mitsubishi 210 dB/km at 660 nm

Si-pin-PD 800 μm

Fig. 6.118: IEEE 1394 system experiment from Matsushita

In [Sak98] Sony presented a number of such systems. A 650 nm LD is coupled directly to the test fiber. A Si-pin PD with a lens coupled to the POF was used as the receiver. During the test, 500 Mbit/s were transmitted over 50 m, as is, for example, necessary for IEEE 1394 systems of the S400 level. Two different fiber types were examined: ¾750 μm PMMA GI-POF (Mitsubishi Rayon prototype, fact a MSI-POF) ¾1 mm MC-POF with 37 cores (Asahi Chemical)

The duplex transceiver designed by Sony only has dimensions of 14 u 8 u 36 mm³. In the experiment, the sensitivity was -21.4 dBm at a BER of 10-12 in simplex mode with the MSI-POF (0.3 dB penalty). In duplex mode, the sensitivity was still -15.7 dBm at a BER of 10-12 with the MSI-POF (1.4 dB penalty). The cause for the deterioration was not mentioned. Whether duplex or simplex mode was used was also not mentioned as well as the measures used to suppress NEXT (system setup in Fig. 6.119). Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

750 μm MSI-POF (Mitsubishi Rayon) 1 mm MC-POF, 37 cores (Asahi Chemical) 50 m 500 Mbit/s 650 nm LD Si-pin-PD, -21.4 dBm [Sak98] Sony



50 m MC-POF, 1 mm Asahi (37 cores) 50 m MSI-POF, 750 μm Mitsubishi

485

Si-pin-PD

Fig. 6.119: IEEE 1394 system experiment by Sony

Teshima, one of the leaders in the development of MC-POF, presents various transmission experiments in [Tesh98]. A data rate of 500 Mbit/s was transferred over 50 m of DSI-MC-POF. The fiber consisted of 37 cores. The numerical aperture was AN = 0.19 and the attenuation was 155 dB/km at 650 nm. A Sony 650 nm LD served as the transmitter. A data rate of 156 Mbit/s was transferred over 50 m of SI-MC-POF. It also had 37 cores but an AN = 0.33. In both experiments the BER was < 10-14 . Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:


37 core DSI-MC-POF, AN = 0.19; 155 dB/km at 650 nm 37 core SI-MC-POF, AN = 0.33 50 m 500 Mbit/s (DSI-MC-POF), 156 Mbit/s (SI-MC-POF) Sony 650 nm LD photodiode coupled by a lens [Tesh98] Asahi Chemical

50 m MC-POF 1 mm Asahi (37 cores)

Si-pin PD

Fig. 6.120: 500 Mbit/s system experiment with multi core POF

The transmission of 2.5 Gbit/s over 200 m was also successfully demonstrated at the University of Eindhoven in 1998 ([Khoe99]). The Mitsubishi PMMA GI-POF had an attenuation of 164 dB/km at 650 nm. A 645 nm NEC laser diode served as the source and had a spectral width of 0.4 nm and a maximum optical power of +6.8 dBm (4.8 mW). A Si-APD was used for the receiver which made it possible to attain -29 dBm of sensitivity at a BER of 10-9 (shown schematically in Fig. 6.121). Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

500 μm PMMA GI-POF; Mitsubishi; 164 dB/km at 650 nm 200 m 2,500 Mbit/s 645 nm NEC LD; +6.8 dBm Si-APD; -29 dBm [Khoe99] University of Eindhoven

486


645 nm LD

200 m PMMA-GI-POF, Mitsubishi 164 dB/km, 0.5 mm

Si-APD

Fig. 6.121: 2.5 Gbit/s GI-POF system at the University of Eindhoven

6.3.4.2 Recent PMMA GI-POF Systems

In the last few years there has once again been a continued increase in the development of PMMA GI-POFs. The present goals are primarily simple systems for use in building networks. At the Keio University a PMMA GI-POF with an optimized index profile has been developed which permits the transmission of a Gigabit Ethernet over 100 m. Fiber type: Length: Bit rate: Transmitter: Reference: Company:

PMMA GI-POF, optimized index profile, 4.5 GHz 100 m index profile coefficient g = 2.4 100 m 1,250 Mbit/s 650 nm LD [Mak03] Keio University

As was described in the chapter on Fibers, Optimedia currently produces the best PMMA GI-POFs. In [Park06a] the transmission of 1,500 Mbit/s over 100 m of the 900 μm fiber was demonstrated. Fiber type: Length: Bit rate: Transmitter: Reference: Company:

900 μm PMMA GI-POF, OM-Giga 100 m 1,500 Mbit/s 650 nm LD [Park06a] Optimedia also 1,000 Mbit/s over 40 m with Firecomms-VCSEL 655 nm

Fig. 6.122: 1.5 Gbit/s over 100 m GI-POF


487

With the same fiber the Fraunhofer Institute in Erlangen realized a transmission of a Gigabit Ethernet over 50 m. The transceiver used is so designed that it can be integrated into a SC-RJ connector (Fig. 6.123). Transmission is also possible with 15 m standard SI-POF. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

900 μm, PMMA GI-POF, Optimedia 50 m 1.250 Mbit/s 652 nm LD, 5 mW 800 μm Si-pin-PD with commercial TIA -12.5 dBm sensitivity (BER = 10-12) [Off05] Fraunhofer IIS Erlangen

Fig. 6.123: Transceiver for 1.25 Gbit/s over 50 m GI-POF and eye after 50 m OM-Giga

6.3.4.3 System Experiments by Telekom and POF-AC

Multi-core fibers were also tested in Telekom’s Technology Center after the experiments with PMMA SI-POFs. With the aid of a 657 nm laser and a wideband receiver a bit rate of 800 Mbit/s could be transmitted error free over 50 m with MC-POF (37 cores, Asahi Chemical). Fiber type: Length: Bit rate: Transmitter: Receiver: References: Company:

Asahi DSI-MC-POF, AN = 0.19 50 m 800 Mbit/s Sony LD SLD 1133VL, 657 nm, 7 mW Hamamatsu S9052, low impedance receiver [Ziem00a], [Stei00a] Deutsche Telekom

BIAS peaking LD 650 nm filter SLD 1133VL

DSI-MC-PMMA-POF 50 m with 800 Mbit/s

Fig. 6.124: Data transmission on MC-POF

Si pin PD S 5052

488


From 2003 on, these tests at the POF-AC Nürnberg were continued with improved components. At first a data rate of 630 Mbit/s was transmitted over a 100 m long MSI fiber from Mitsubishi. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

657 nm LD +3 dBm

700 μm MSI-POF, Mitsubishi ESKA-MIU 100 m 630 Mbit/s 657 nm LD, Sony SLD 1133VL 800 μm Si-pin-PD Hamamatsu S5052 [Vin04b] POF-AC

100 m MSI-POF , 700 μm Mitsubishi ESKA-MIU

800 μm Si-pin-PD

Fig. 6.125: Data transmission over MSI-POF

The data transmission with multi-core fibers was tested at the POF-AC with two different versions. First, the 217 core fiber with a simple step index profile was used. Three fiber samples, 21 m, 44 m, and 90 m in length, were available. The measurements were carried out with higher power Sanyo lasers and a transimpedance receiver. Fiber type: Length: Bit rate: Transmitter: Receiver:

Reference: Company:

1 mm Asahi MC-POF, 217 cores 21 m, 44 m and 90 m 900 Mbit/s, 750 Mbit/s, 590 Mbit/s DL-4147-162 Sanyo, +8.6 dBm 800 μm Si-pin-PD Hamamatsu S5052, transimpedance receiver with HEMT received power: +4.5 dBm, -0.75 dBm and -10.2 dBm [Vin04b] POF-AC Nürnberg

The second type of multi-core fiber tested was a POF with 37 cores and double step index profile. Over distances between 30 m to 100 m data rates of 1,400 Mbit/s to 800 Mbit/s could be transmitted, whereby the capacity lay in the same range as with the MSI-POF. However, the MC POF offers the additional advantage of having very small bending radii. Fiber type: Length: Bit rate: Transmitter: Receiver:

1 mm Asahi MC-POF, MSC-1000 30 m, 50 m, 64 m and 100 m 1,400 Mbit/s, 1,300 Mbit/s, 1,200 Mbit/s, 800 Mbit/s DL-4147-162 Sanyo, POF coupled power: +8.6 dBm 800 μm Si-pin-PD Hamamatsu S5052 transimpedance receiver with HEMT



BIAS LD 650 nm Sanyo

489

[Ziem03g], [Vin05c] POF-AC Nürnberg

MC-DSI-PMMA-POF MSC-1000 (Asahi) 37 cores, AN = 0.19

Si-pin-PD equalizer S 5052 (optional)

Fig. 6.126: Transmission experiments on MC-POF

Later the experiments were repeated on both large fiber lengths using an additional passive compensation filter (RC high pass), whereby over 1 Gbit/s over 100 m of MC POF could be attained for the first time. Fiber type: Length: Bit rate: Transmitter: Receiver: References: Company:

Asahi MC-POF, 1 mm 64 m, 100 m 1,270 Mbit/s, 1,150 Mbit/s 1,170 Mbit/s over 100 m with optimized equalizer 650 nm LD, 5 mW 800 μm Si-pin-PD S5052 with transimpedance receiver [Vin05a], [Vin05c] POF-AC Nürnberg

The latest measurements for two different fibers from [Was07] resulted in maximum bit rates of 725 Mbit/s over 90 m for the 217-core fiber and 1,170 Mbit/s over 100 m for the 37-core fiber (eye diagram in Fig. 6.127).

Fig. 6.127: Error free transmission of 1,170 Mbit/s over 100 m MC-POF at 650 nm

Since 2005, the PMMA GI-POFs of the Korean manufacturer Optimedia have been tested. By using red laser diodes, the maximum data rate has been limited to approximately 1,600 Mbit/s. Even after 100 m with an error-free transmission of 1,550 Mbit/s no serious influence on mode dispersion could be made out. With an optimized equalizing filter and a new laser 2 Gbit/s were able to be transmitted over 50 m fiber (Fig. 6.128).

490



PMMA GI-POF, OM-Giga Optimedia 100 m 1,550 Mbit/s, also 2,250 Mbit/s over 50 m 650 nm LD, 5 mW 800 μm Si-pin-PD S5052 with transimpedance receiver [Vin05b], [Ziem05f], [Vin05c] POF-AC Nürnberg

Fig. 6.128: Eye diagram for 2,000 Mbit/s over 50 m

The bit rate has also been measured with these new components during the department’s internship work. An example for the measurement results at different lengths is shown in Fig. 6.129. Since there are already 9 plug-in connectors in the link over 100 m, the bit rate is limited by the receiving level necessitating use of a narrower low pass filters for noise suppression. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company: 3,000

OM-GIGA, 900 μm PMMA GI-POF 20 m to 100 m (one connection every 10 m) up to 2,250 Mbit/s 650 nm LD 800 μm Si-pin-PD [Gort06] POF-AC Nürnberg

bit rate [Mbit/s]

1,000 600

PMMA GI-POF AN = 0.35 Optimedia 650 nm LD

fiber length [m] 100 10

20

Fig. 6.129: Highest bit rates for PMMA GI-POF

50

100


491

In current measurements with optimized components and fiber samples without plug-in connectors a maximum bit rate of 1,880 Mbit/s over 100 m could be attained for the 1 mm OM Giga (Fig. 6.130). Two fiber samples with bend-optimized fibers both 100 m long (each with a core diameter of 700 μm) allowed maximum data rates of 1,600 Mbit/s or 1,630 Mbit/s respectively.

Fig. 6.130: Transmission of 1,880 Mbit/s over 100 m OM-Giga ([Was07])

POF ribbon cables with OM Giga were used also for transmitting the data, as is described later on. Each of four parallel channels can transmit 1.6 Gbit/s over 50 m whereby the specification for the mask is met quite well in the eye diagram. We can summarize the potential of the different index profiles as follows: ¾Multi-core fibers permit transmission of 500 Mbit/s to 1,000 Mbit/s over up to 100 m of fiber, especially when using double step index profile. Furthermore, they have the advantage that materials and production are similar to standard POF. MC-POF allows extremely small bending radii which is important for installation. ¾Multi-step index fibers are easier to produce than GI-POF (multiple extruder). At present, only one type from Mitsubishi is available which allows up to about 500 Mbit/s over 100 m (comparable to DSI-POF). ¾Graded index PMMA POFs allow 2,500 Mbit/s over 100 m and more. The fibers most readily available at present are those from Optimedia. The greatest remaining problem is that of limited bending radius which can, however, be reduced though an improved primary coating. 6.3.5 Systems with Fluorinated POF

As described in Chapter 2, increases in the transmission lengths at high data rates clearly over 100 m can only be attained with fluorinated polymers. Since there is no suitable cladding material for these polymers, all PF-POFs are automatically graded index profile fibers. In principle all of the following experiments presented

492


have been realized with fibers from Asahi Glass. It was no until the past few years that fibers from Nexans (Lyon) have also been used. The third manufacturer is Chromis Fiberoptics whose fibers, however, have not yet been used in published system experiments (except Prof. Ralph). Today almost all PF-GI-POF systems are manufactured with a core diameter of 120 μm and NA of 0.22 to 0.25. At the beginning of development greater values for both parameters were also partially used. The essential developments are in regard to the optimization of the index profile and the continued lowering of the attenuation. 6.3.5.1 First Systems with PF-GI-POF

In [Kan98] a system is introduced for the first time that uses the newly developed CYTOP® fiber made by Asahi Glass. The GI-POF consists of completely fluorinated polymer and has a significantly reduced attenuation in the near infrared range. The core diameter is 120 μm. With a 850 nm source it was possible to transmit 1 Gbit/s over 100 m with a BER of 10-12. The detector with 1 GHz bandwidth is a New Focus Model 1601 featuring a sensitivity of -18.6 dBm. A further GI-POF with 200 μm core diameter and AN = 0.175 was used for tolerance tests. Figure 6.131 illustrates the testing principle. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

850 nm VCSEL

120 μm PF-GI-POF CYTOP®, AN = 0.175 100 m 1,000 Mbit/s 850 nm VCSEL New Focus Model 1601; -18.6 dBm [Kan98] Seiko Epson Corp.

100 m PF-GI-POF 120/200 μm CYTOP£

Si-PD

Fig. 6.131: 1 Gbit/s transmission with CYTOP® fiber according to [Kan98]

A so-called space division multiplex system with 865 nm VCSEL is introduced in the same paper [Kan98]. Two laser chips spaced 200 μm away from each other are used as sources. The duplex POF has a core/cladding diameter of 120/250 μm (500 μm protective shielding). A GaAs-pin PD with a diameter of 200 μm was used a the receiver. 400 Mbit/s were transmitted over more than 50 m of fiber medium (see Fig. 6.132). Fiber type: Length:

120 μm PF-GI-POF CYTOP®, Duplex 50 m


Bit rate: Transmitter: Receiver: Reference: Company:

493

2 u 400 Mbit/s 865 nm VCSEL GaAs-PD [Kan98] Seiko Epson Corp. GaAs PD GaAs PD

865 nm VCSEL


Fig. 6.132: Parallel data transmission over GI-POF according to [Kan98]

In [Imai97] 200 m of fluorinated GI-POF was used to transmit 2.5 Gbit/s. The fiber has an attenuation of 120 dB/km at 850 nm and 56 dB/km at 1,300 nm (see also [Khoe99]). A 1,310 nm laser was used as the source (see Fig. 6.133). Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

1,310 nm LD

120 μm PF-GI-POF CYTOP®, 56 dB/km at 850 nm 200 m 2,500 Mbit/s 1,310 nm LD 30 μm APD with lens coupling -25.7 dBm sensitivity (BER = 10-10) [Imai97] Fujitsu Laboratories Inc.

200 m PF-GI-POF 56 dB/km

PD

Fig. 6.133: 2.5 Gbit/s PF-GI-POF system from Fujitsu

A test of different GI-POF is demonstrated by Watanabe in [Wat99a]. In the test, various PF-GI-POF with 83 μm, 99 μm, 147 μm and 221 μm core diameters were tested with commercial glass fiber multimode transceivers (1,250 Mbit/s). These use 850 nm VCSEL as a transmitter and PD with a 100 μm diameter in the receiver. Each test was carried out with 100 m of GI-POF. For the fiber with 109 μm core diameter (78 dB/km attenuation) at 850 nm, a sensitivity of -15,54 dBm was determined at room temperature and +50°C. The authors come to the conclusion that the core diameter should be smaller than 100 μm in order to function together with GOF components (see Fig. 6.134).

494


The result is nonetheless not surprising. VCSEL is characterized by a relatively small emission surface and emission angle. The input power was likely the same for all fibers. The relatively short transmission length should also not result in a bandwidth limitation of the data rate to be used. Thus, the limiting factor is the coupling of the fibers to the photodiode. The diode is over illuminated for larger fibers, i.e., larger coupling losses occur. By using inexpensive components, polymer fibers have great potential. Given the restriction that only existing glass fiber system components are to be used, the question must be posed as to why the glass fibers are also not retained as a medium. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

PF-GI-POF core = 83 μm, 99 μm, 147 μm, 221 μm 100 m 1,250 Mbit/s 850 nm VCSEL 100 μm Si-pin-PD [Wat99] Asahi Glass

100 μm PD

VCSEL 850 nm

100 m PF-GI-POF Ø: 83 μm, 99 μm, 147 μm, 221 μm

Fig. 6.134: Test of different GI-POF with glass fiber components ([Wat99a])

The use of commercial fiber glass transceivers with a 850 nm VCSEL was tested in [Lin01], whereby fiber lengths up to 300 m and bit rates up to 3.2 Gbit/s were used. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

PF-GI-POF 300 m 3,200 Mbit/s 850 nm VCSEL 120 μm GaAs-pin-PD [Lin01a] True-Light Corporation also 1,250 Mbit/s over 100 m, 200 m and 300 m


850 nm VCSEL 3.2 Gbit/s

300 m PF-GI-POF 40 dB/km

495

120 μm GaAs-pin-PD

Fig. 6.135: POF test system with commercial 850 nm components

In Belgium the transmission of Gbit Ethernet on 300 m of GI-POF has been tested. From these transmission experiments and extensive broadband measurements with different launchings the authors have come to the conclusion that PF-GI-POFs are better suited for Gbit systems than OM1 fiber. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

120 μm PF-GI-POF, A4g 300 m 1,250 Mbit/s 850 nm VCSEL 125 u 125 μm² pin-PD [Gof05] Royal Military Academy, Belgium

Infineon Technologies also set up a system in 2003 with a transmission of 1.5 Gbit/s over 300 m of PF GI-POF using a fiber from Nexans. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

120 μm PF-GI-POF (Nexans) 300 m 1,500 Mbit/s 850 nm VCSEL GaAs-PD [Schu04] Infineon Technologies

6.3.5.2 Experiments at the Technical University of Eindhoven

Professor Khoe and his group at the University of Eindhoven succeeded at the end of the 1990’s in further improving the parameters of the GI-POF transmission system. In [Li98] the transmission of 2.5 Gbit/s over 300 m of PF-GI-POF (CYTOP®, 170/340 μm, 110 dB/km) has already been successfully demonstrated. The 230 μm-diameter Si-APD receiver attains -29 dBm at BER = 10-9 and with only a 0.3 dB coupling loss from the fiber into the photodiode. A 645 nm laser from NEC served as the source with a spectral width of 0.4 nm, +6.2 dBm maximum output power and a 0.3 dB coupling loss when coupling into the POF. This laser had already been used for example in [Koi94] and [Khoe99]. Anti-reflection coated lenses were used for the couplings. Disturbing reflections were avoided by a 4º inclined cutting of the fiber ends. In order to improve efficiency, a NA adaptation on the laser from 0.55 to 0.16 and from 0.25 to 0.55 on the receiver (each with 2 lenses) was carried out. Figure 6.136 shows the system.

496


Three fiber pieces each with a length of 100 m were available and connected with plugs. The total link attenuation at the laser wavelength came to 32.6 dB. A 1 dB penalty was measured through mode dispersion. In addition, there were the 0.6 dB coupling losses at the transmitter and receiver which required a transmission power budget of 34.2 dB. This was possible thanks to the 35.2 dB difference between the transmitter power and the sensitivity. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

645 nm LD, NEC

170 μm PF-GI-POF CYTOP®, 32.6 dB/300 m at 645 nm 300 m (2 connectors) 2,500 Mbit/s 645 nm Laser NEC; 'O = 0.4 nm, +6.2 dBm 230 μm Si-APD; -29 dBm [Li98] University of Eindhoven

3 u 100 m PF-GI-POF 170/340 μm, Asahi Glass

Si-APD

Fig. 6.136: 2.5 Gbit/s-System according to [Li98] with 300 m reach

A new distance record of 2.5 Gbit/s over 450 m was presented by the same authors also in 1998 ([Li98]). A 1,310 nm LD was used. The fiber pieces used were 4 u 100 m and 1 u 50 m GI-POF. The laser was coupled to a 62.5 μm GI glass fiber. An optical amplifier (SOA, Semiconductor Optical Amplifier) increased the power to the required level, which is shown in Fig. 6.137. The transmitting power attained were not specified, but must have been approximately 10 mW. With this setup, 5 Gbit/s were transmitted over 140 m and later 200 m, whereby the bandwidth of the receiver was cited as the limiting factor. Furthermore, a transmission length of 300 m with 2.5 Gbit/s was achieved without using the SOA. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

170 μm PF-GI-POF CYTOP®, 31 dB/km 450 m (4 connectors) 2,500 Mbit/s 1,310 nm LD, with SOA amplified 80 μm APD [Li98] University of Eindhoven also 5,000 Mbit/s over 140 m and 200 m


497

GOF, 62.5 μm

1,310 nm LD

SOA

4 u 100 m, 1 u 50 m 170/340 μm PF-GI-POF

APD

Fig. 6.137: 2.5 Gbit/s system according to [Li98] with 450 m reach

The group finally improved the transmission length to 550 m in 1999 (see [Khoe99], [Li99]) at 2.5 Gbit/s data rate. This was made possible by providing a 550 m GI-POF fiber piece with a core diameter of 170 μm without any connectors (Fig. 6.138). Experiments with various sources were carried out. The measured attenuation for the wavelengths was as follows: ¾110 dB/km at 650 nm (LD as source) ¾43.6 dB/km at 840 nm (VCSEL as source) ¾31 dB/km at 1,310 nm (LD as source)

The VCSEL supplies 1.3 dBm of power at a spectral width of 1 nm. It was possible to couple it directly to the POF (< 1 dB loss). A passive filter for the VCSEL frequency response compensation was used. A Si-APD with 230 μm diameter was used for the receiver at 840 nm. It reached -28.6 dBm sensitivity with a BER = 10-9, whereby a budget of 29.9 dB was available. The experiments resulted in 4.5 dB penalty through mode noise and dispersion and 24.0 dB attenuation through the 550 m POF link (24.0 + 1.0 + 0.3 + 4.5 dB yields 29.8 dB). Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

170 μm PF-GI-POF CYTOP®; 43.6 dB/km 550 m 2,500 Mbit/s 840 nm VCSEL, 'O = 1 nm, 1.3 dBm 230 μm Si-APD [Li99] University of Eindhoven

840 nm VCSEL

550 m 170 μm PF-GI-POF

Si-APD 230 μm Fig. 6.138: POF system with record transmission distance according to [Li99]

498


The transmission of 2.5 Gbit/s over 550 m at 1.3 μm was also described in [Li99]. The used 1310 nm DFB laser had a modulation bandwidth of 5 GHz, a spectral width of 0.1 nm and max. 0.4 dBm of optical output power (1.1 mW). The laser is a standard transmitter element for singlemode fiber systems and is equipped with a corresponding fiber pigtail for singlemode fiber systems. The singlemode fiber was also used for direct coupling to the GI-POF (< 0.1 dB loss). With this method, only a small part of the mode field is excited, which increases the bandwidth considerably. The receiver used for this wavelength was a InGaAs-APD with a diameter of 80 μm. The POF was imaged with a dual lens while changing the NA from 0.25 to AN = 0.55 (< 0.3 dB loss). The sensitivity was -28.4 dBm with a BER of 10-9. Thus, a transmission budget of 28.8 dB was available by a loss of 16.3 dB. A measured penalty of 4.4 dB through mode noise and dispersion resulted in the required budget of 16.3 + 0.1 + 0.3 + 4.4 = 21.1 dB. The remaining system margin of 7.7 dB would make a transmission length of up to 750 m possible. Figure 6.139 illustrates the system scheme. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

170 μm PF-GI-POF CYTOP®, 31 dB/km 550 m 2,500 Mbit/s 1,310 nm LD, 'O = 0.1 nm, 0.4 dBm 80 μm GaAsP-APD, -28.4 dBm [Li99] University of Eindhoven

SM-GOF

1,310 nm DFB-LD

InGaAs-APD 80 μm

550 m 170 μm PF-GI-POF

Fig. 6.139: 550 m GI-POF system at 1,310 nm according to [Li99]

The power budgets of both the 550 m experiments and 1,310 nm are compared in Fig. 6.140. The clear advantage lies with the 1,310 nm laser diode, since the attenuation of the POF is essentially lower here. However, the components used are not in keeping with the “low-cost” philosophy of polymer fibers.


499

loss by: POF attenuation, LD - POF - coupling, POF - PD - coupling, penalty

Pout VCSEL

sensitivity Si-APD

Pout DFB-LD

sensitivity InGaAs-APD

0

-5

-10

-15

-20

-25

-30 dBm

Fig. 6.140: Comparison of power budgets for 840 nm and 1,310 nm

In 2001 and 2002, the group at the University of Eindhoven showed that PF-GI-POFs are also suitable for transmission lengths of up to 1 km. In one of the first experiments an 840 nm VCSEL was used. The fiber consisted of three cascaded, 330 m pieces (system setup in Fig. 6.141). Fiber type: Length: Bit rate: Transmitter: Receiver: References: Company:

PF-GI-POF, 27 dB/km at 840 nm 990 m 1,250 Mbit/s 840 nm VCSEL with +1.1 dBm average opt. power (1.3 mW) 230 μm Si-APD, sensitivity -31.3 dBm (BER = 10-9) penalty 1.2 dB by mode dispersion [Boo01a], [Nar01] University of Eindhoven

840 nm VCSEL Ethernet, 1.25 Gbit/s

3 u 330 m PF-GI-POF 120/250 μm, LucinaTM, Asahi Glass

Si-APD 230 μm

Fig. 6.141: Gigabit Ethernet transmission over 990 m PF-GI-POF

In the following year a 1 km-long piece of fiber was available. This time a 1,300 nm edge emitter, coupled to a single mode fiber, was used, the output power of which was enlarged by means of an optical semiconductor amplifier. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

120 μm PF-GI-POF 1,006 m 1,250 Mbit/s 1,300 nm LD with semiconductor optical amplifier 80 μm InGaAs-APD [Khoe02] University of Eindhoven

500


SOA

ext. Mod

1,300 nm tunable LD Ethernet 1.25 Gbit/s

1006 m PF-GI-POF 120/250 μm, LucinaTM, Asahi Glass

InGaAsAPD 80 μm

Fig. 6.142: Gigabit-Ethernet transmission over 1.006 m PF-GI-POF

6.3.5.3 Data Rates over 5 Gbit/s with GI-POF

Transmission experiments with CYTOP® PF-GI-POF involving high data rates were also conducted at the University of Ulm. An experiment with 7 Gbit/s at 80 m of GI-POF length is described in [Schn99]. The POF used had a core diameter of 155 μm. A 930 nm VCSEL with a max. 4.5 mW of power at 10 mA diode current served as the source. A bias current of 7 mA and ±0.75 V modulation amplitude was used in the experiment. The VCSEL was connected directly to the GI-POF with butt coupling. Figure 6.143 illustrates the test set-up. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

BIAS

VCSEL 930 nm

155 μm PF-GI-POF CYTOP® 80 m 7,000 Mbit/s 930 nm VCSEL, 4.5 mW InGaAs pin-PD [Schn99] University of Ulm

80 m GI-POF 155 μm CYTOP

InGaAs pin-PD

Fig. 6.143: 7 Gbit/s experiment at the University Ulm

The highest data rate of yet for a POF system with 11 Gbit/s over 100 m PF-GIPOF was demonstrated by Lucent Technologies in [Gia99a]. The source was a 1,300 nm Fabry-Perot laser diode. Via the singlemode fiber coupled to the laser it was possible to couple 1 mW of power into the GI-POF through a lens. The attenuation of the GI-POF with a core/cladding diameter of 130 μm/300 μm was 44 dB/km at 830 nm and 33 dB/km at 1,300 nm. The receiver was a pin photodiode with fixed glass fiber pigtail (62.5 μm multimode). A lens provides the coupling between the POF and receiver glass fiber with a loss of 4.8 dB. The error probability is less than BER = 10-10 with -8.6 dBm received power. Figure 6.144 illustrates the test set-up.


501

The laser used fulfilled class 1 with less than +8 dBm of output power. The penalty due to dispersion amounted to 2.5 dB. This system should also be regarded as a technology test for performance of the GI-POF, since the test setup by no means met the demand for inexpensive components. Fiber type: Length: Bit rate: Transmitter: Receiver: References: Company:

130 μm PF-GI-POF CYTOP®; 33 dB/km 100 m 11,000 Mbit/s 1,300 nm LD, +8 dBm pin-PD (with a 62.5 μm GI-GOF pigtail) [Gia99a] Lucent Technologies 33 dB/km

FP-LD 10 μm 1,300 nm SM-GOF

100 m PF-GI-POF 130/300 μm, Asahi

62.5 μm pin-PD GI-GOF

Fig. 6.144: Up to that time highest bit rate for POF systems at Lucent Technologies (2000)

In further publications [Gia99b], [Gia99c], a 830 nm VCSEL is used in addition to the 1,300 nm Fabry-Perot laser. It was possible to modulate it with 9 Gbit/s. The system deterioration by 4 dB was caused by the limited extinction ratio value. The POF was coupled to a Picometrix pin-PD with a diameter of 70 μm and a bandwidth of 9 GHz (2 dB loss at a 2:1 spot size reduction). Figure 5.62 demonstrates the modified setup. Fiber type: Length: Bit rate: Transmitter: Receiver: References: Company:

VCSEL 830 nm

130 μm PF-GI-POF CYTOP®; 44 dB/km 100 m 9,000 Mbit/s 830 nm VCSEL 70 μm pin-PD Picometrix [Gia99b], [Gia99c] Lucent Technologies

100 m PF-GI-POF 130/300 μm, Asahi 44 dB/km at 830 nm

70 μm pin-PD

Fig. 6.145: 9 Gbit/s system with VCSEL at Lucent Technologies

Nexans also demonstrated the transmission of around 10 Gbit/s over 100 m of PF-GI-POF. An 850 nm laser and a pin-PD, each in a very compact housing, were used as active elements. The parameters of the system were:

502


¾850 nm VCSEL (50 :) with pin monitor diode and SiGe driver ¾TIA receiver with pin-diode ¾5 mA BIAS current, 7.5 mA Imod,p-p ¾f3 dB: 5.5 GHz (f6 dB: 8.0 GHz) with mode filter ¾850 nm VCSEL ¾10.7 Gbit/s, PRBS 1023-1 ¾coupling with ball lenses ¾BER < 10-12, BER < 10-10 with optimized launch

10.7 VCSEL Gbit/s 850 nm

100 m Lucina PF-GI-POF

pin-PD

Fig. 6.146: 10.7 Gbit/s-System by Nexans

The active components are shown in Fig. 6.147. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

120 μm PF-GI-POF 100 m 10,700 Mbit/s 850 nm VCSEL pin-PD with transimpedance receiver [Wid02b] Nexans Lyon

Fig. 6.147: 10.7 Gbit/s system components

A comparable 10 Gbit/s system was also set up at the University of Ulm. A somewhat thicker GI-POF was used. The 850 nm VCSEL is characterized by particularly high power and bandwidth. Fiber type: Length: Bit rate: Transmitter:

155 μm PF-GI-POF, AN = 0.25 80 m 10,400 Mbit/s 850 nm VCSEL, 7.1 mW at 18.3 mA


Receiver: Reference: Company:

503

9.4 GHz bandwidth at 12 mA made at the Ferdinand-Braun-Institute Berlin InGaAs pin-PD with lens coupling [Sta03] University of Ulm

In 2004 a 10 Gbit/s system was presented at Keio University for the first time, in which a 850 nm laser was also used. Fiber type: Length: Bit rate: Transmitter: Reference: Company:

PF-GI-POF, Keio University 100 m 10,000 Mbit/s 850 nm laser [Ish04b] Keio University

In 2005, the transmission of 12 Gbit/s was presented for the first time. The basis was a GI-POF with improved index profile. It is particularly interesting that this fiber made the high bit rate possible, at a wavelength of 850 nm as well as at 1300 nm. The eye diagram for the 1,300 nm experiment is shown in Fig. 6.148. Fiber type: Length: Bit rate: Transmitter: Reference: Company:

PF-GI-POF, Keio University, new optimized profile 100 m 12,000 Mbit/s 850 nm, 1,300 nm laser [Ish05a] Keio University

Fig. 6.148: Eye diagram at 12 Gbit/s (1.3 μm, after 100 m fiber)

A significant increase in the range of 10 Gbit/s POF systems was achieved by Dr. Randel’s group at Siemens in Munich [Lee07a]. The set-up of the experimental system with a 1300 nm Laser is shown in Fig. 6.149.

504


1300 nm DFB laser

9 -120 μm butt coupling 9 μm SM fiber

VOA

9 μm SM fiber

120 μm MM fiber

mode mixer

120 μm MM fiber

120 -120 μm butt coupling 50 -120 μm butt coupling BER tester

pin-PD receiver

MLSE

50 μm MM fiber

120 μm PF-GI-POF

Fig. 6.149: 10 Gbit/s system according to [Lee07a]

Approximately 90 m of fiber can be jumpered with direct detection. By using an MLSE equalizer for the dispersion compensation (Maximum Likelihood Sequence Estimation) the transmission link can be extended to 220 m, assuming a FEC limit of BER = 10-4. The system parameters were: Fiber type: Length: Bit rate: Transmitter: Launch: Receiver: Reference: Company:

120 μm PF-GI-POF (40 dB/km, NA = 0.185) up to 220 m 10,000 Mbit/s 1,300 nm DFB laser Over Filled Launch (mode mixer: 10 loops around a 20 mm cylinder) 50 μm GI-GOF receiver with MLSE/error correction [Lee07a] Siemens Munich/University of Eindhoven

The improvement through the MLSE receiver is shown in Fig. 6.150. About 6 dB in sensitivity are gained which corresponds approximately to an extension in the range from 90 to 220 m.

10-1 10-2 BER 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 60

fiber length [m] 100

140

Fig. 6.150: System improvement by MLSE ([Lee07a])

180

220


505

In principle, this procedure can be used with most dispersion-limited POF systems in order to gain some additional dB in the system. With PMMA POF this only corresponds to an increased length of some 10 m. The greatest bit rates for POF systems realized so far were achieved at the Georgia Institute of Technology [Ral06], [Ral07] and [Poll07]. A PF-GI-POF from Chromis Fiberoptics served as the medium. Since transmitters with 1,300 nm are not sufficiently fast, a 1,550 nm source and a correspondingly fast receiver were used. As the attenuation of PF-POF at 1.55 μm is already relatively large (» 100 dB/km) the transmission length was limited to 30 m. A BER < 10-12 could be achieved for up to 30 Gbit/s. At 40 Gbit/s you can transmit with an error probability of 1.45 10-3 (receiver-limited). The fiber diameter permits an offset of r10 μm when coupling-in. The eye diagrams for 10 Gbit/s and 30 Gbit/s are shown in Fig. 6.151. Fiber type: Length: Bit rate: Transmitter: Receiver: References: Company:

50 μm PF-GI-POF (Chromis Fiberoptics) 30 m 10,000 - 40,000 Mbit/s 1,550 nm fiber laser with external modulator 50 μm multimode fiber detector Newfocus 1454 POF output power: 3.87 dBm [Ral06], [Ral07], [Poll07] Georgia Institute of Technology

Fig. 6.151: Eye diagrams for 10 Gbit/s and 30 Gbit/s ([Ral06])

In addition to the bit rate, the impulse response is also determined. A comparison with a measurement on a 50 μm glass fiber (GI) is shown in Fig. 6.152. Thanks to the strong mode mixing in the POF visibly less modal dispersion occurs. A comprehensive explanation of the effects of mode mixing on the bandwidth of PF-GI fibers can be found in Section 10.3.

506


Fig. 6.152: Pulse responses for GI-POF and GI-GOF ([Ral06])

All of the system experiments mentioned above with PF-GI-POF are summarized in Fig. 6.153. The fiber limit lay currently in the range of 1,250 Mbit/s · km. The performance is practically identical at 850 and 1,300 nm - a definite advantage over glass fibers. Both the range (higher fiber attenuation) as well as the data rate (slower lasers) are clearly limited. Mode multiplex systems are introduced in Section 6.3.7.4. The transmission of 2 u 10.7 Gbit/s over 10 m of PF-GI-POF at a wavelength of 1,550 nm was achieved by [Schö06]. 50,000 bit rate [Mbit/s] 30,000 10,000 3,000 1,000

1,550 nm 1,300 nm

300

850 nm

length [m]

650 nm

100 10

20

50

100

Fig. 6.153: Summary of the PF-GI-POF systems

200

500

1000


507

6.3.6 POF Multiplex

The wavelength division multiplexing method for increasing channel capacity as well as the bi-directional data transmission is well known from glass fiber technology. In experimental systems more than 100 wavelengths have been combined and the 1 Tbit/s limit has been clearly surpassed in terms of the total capacity. At the ECOC‘2000 Alcatel, Siemens and NEC presented systems with data rates of 6 to 7 Tbit/s. At the OFC’2007 there was a paper on the transmission of 25.4 Tbit/s over a glass fiber. In only a few years, commercial systems will be available with several 100 wavelengths. The demands of the POF system designers have been considerably more modest. On the other hand, inexpensive and powerful LED are available ranging from blue to the near infrared. This allows the full use of attenuation windows of the PMMA and the PF-POF. Principal POF problems are the large diameter and the numeric aperture. In glass singlemode fiber technology, various components can be used as multiplexer and filters based on fiber gratings, interference filters, and interferometers. These elements cannot be used for POF. Even using an interference filter leads to the problems depicted in Fig. 6.154:

transmission

POF

2

POF

1

light pathes

3 filter

1 2

3

wavelength

Fig. 6.154: Problems with interference filter for POF

Interference filters based on transparent layers have considerable angular dependence of the transmission. In a standard NA-POF, the propagation angle in the fibers can deviate by up to 20° from the axis. In this case, a filter's transmissions range can shift by more than 6%. The same applies to reflection and transmission gratings. Interference devices such as fiber gratings and Mach-Zehnder interferometers cannot be used at all for multimode fibers since each mode fulfils its own interference condition. When all modes overlap, the interferences vanish. Generally, there are several solutions for wavelength-selective elements: ¾The use of absorption filters which have practically no angular dependency ¾Use of very wide interference filters that can tolerate the spectral shift. ¾Expansion of the light beam from the POF and reduction of the NA in order to be able to use normal elements.

508


All of these methods will be described in the following sections. The problem is far less critical for GI-POF with its small core diameters and NA common today compared with 1 mm SI-POF. WDM systems on PMMA fibers will first be described. In the following part solutions with PF-GI-POF will then be compiled and finally the components for bi-directional transmission on fiber will round off the presentation. 6.3.6.1 Wavelength Multiplex Systems with PMMA POF

The use of wavelength multiplex on PMMA POF will be limited to a few channels. Laser diodes are available only for 650 nm; in the other transmission windows we have so far had to use LEDs which show a high spectral width as well as limited bit rates. On the other hand, the large fiber diameter allows for very simple multiplex constructions without active adjustment. Bi-directional transmission with a system with a 830 nm source for a 6 MHz video channel and a 660 nm LED for a 10 kHz control signal was already demonstrated in [Tak94]. No further information on the test setup is available. Fiber type: Bit rate: Transmitter: Demux: Reference: Company:

PMMA POF analog 6 MHz / 10 kHz 830 nm (6 MHz video signal) 660 nm LED (10 kHz control signal) splitter [Tak94] Hitachi research center

In [Ziem97a] and [Ziem97b] a method was proposed that markedly improves the performance of the system. The fact that the diameter of the photodiode is markedly larger than that of a typical LED was exploited. By simply "stacking" the elements, an WDM system for bi-directional transmission is created. This is show in Fig. 6.155. filter for NEXTsuppression

photo diode Ø 1 mm

1 mm POF

LED; 200 u 200 μm²

Fig. 6.155: WDM transceiver without optical elements and couplers


509

At the oft quoted institute at the University of Eindhoven directed by Prof. Khoe, a demultiplexer for POF systems based on a reflection grating was developed ([Hun96], [Khoe97]) with which a WDM system was later built. The demultiplexer is shown in Fig. 6.156. The transmission medium is a GI-POF with a core diameter of 750 μm and AN = 0.29. The outputs of the multiplexers are formed by SI-POF with a 1 mm diameter and AN = 0.46. The entire arrangement consists of a lens for focusing and a reflection grating. A slightly angled position maps the input POF to both output POF.

1 mm SI-POF

0.75 mm GI-POF

grating

Fig. 6.156: POF-WDM demultiplexer according to [Hun96]

The wavelengths used are 645 nm and 675 nm, dependent on the available laser diodes. A grating with 1,200 lines/mm and 500 nm Blaze wavelength was selected for the setup. The collimator lens has a focal length of 25.4 mm which results in a theoretical linear separation of the wavelengths by 995.8 μm. This corresponds very precisely to the 1 mm spacing between the output fibers. The lens diameter is 25.4 mm which is large enough to capture the complete far field of the GI-POF. Figure 5.74 illustrates the transmission function for both multiplexer outputs, measured with a white-light source before the GI-POF and with a resolution of 0.1 nm.

-50

rel. power [dB]

-55 -60 -65 -70 -75 -80 -85

wavelength [nm] 610

620

630

640

650

660

670

680

690

Fig. 6.157: Demultiplexer spectral transmission according to [Hun96]

700

710

510


The attenuation at the used wavelengths is less than 5 dB. The suppression of the alternate channel is better than 55 dB which should make error-free operation possible using a small band laser. A WDM system with 84 m PMMA GI-POF was set up with this demultiplexer in [Khoe97]. Both transmitters were a NEC 645 nm laser with a data rate of 2,500 Mbit/s and a Philips 675 nm CD laser with 620 Mbit/s. The sensitivities of both receivers were -26 dBm and -31 dBm respectively. Fiber type: Length: Bit rate: Transmitter: Demux: References: Company:

750 μm PMMA GI-POF 84 m 2 u 2,500 Mbit/s 645 nm LD/675 nm LD grating demultiplexer [Hun96], [Khoe97] University of Eindhoven

A Tunisian group proposed a further arrangement of the demultiplexer for POF-WDM systems in [Att96]. The goal was to achieve as compact a form for the multiplexer as possible. For this purpose, a POF was coupled to a glass fiber bundle with a round cross-section. It consisted of 61 single fibers with a core diameter of 100 μm with AN = 0.28. Approximately 60 % of the 1 mm core diameter of the POF is utilized. The individual fibers are arranged linearly at the focusing lenses side. This results in a slit of 6.1 u 0.1 mm. The advantage of the arrangement is that a considerably smaller linear separation of the channels is required compared with a round 1 mm input, as demonstrated in Fig. 6.158. Smaller lenses and gratings thus can be used.

O1

0.1 mm

O1

1 mm

O2

O2

input O1 + O2 input O1 + O2

Fig. 6.158: POF demultiplexer

A grating with 1,800 lines /mm and a 8 mm u 8 mm size is used for the realized demultiplexer. Both focusing lens have a 3.6 mm focal width. The wavelengths: O1 = 632.8 nm, O2 = 650 nm, and O3 = 670 nm are used. The complete additional loss of the arrangement with these wavelengths is specified as 7 dB. A specific transmission system has not yet been realized.


Fiber type: Transmitter: Multiplexer: Reference: Company:

511

PMMA SI-POF, AN = 0.50 632.8 nm, 650 nm, 670 nm LD Grating with cross area conversion [Att96a] Ecole Nationale d’Ingénieurs de Tunis

At the POF-AC Nürnberg WDM systems for demonstration purposes have been set up as part of two diploma theses. The first system worked with 4 LEDs. As shown in Fig. 6.159, the spectra of these four LEDs visibly overlap. In order to present crosstalk, relatively narrow optical filters had to be inserted. 1.0 P [a.U.] 0.8 0.6 0.4 0.2 O [nm] 0.0 400 450 500 550 600 650 700 750

Fig. 6.159: 4-LED-WDM system and spectra of the LED

The de-multiplexer was optically realized by expanding with lenses and using interference filters. The crosstalk was reduced electrically by means of an analog compensation circuit. To do this, coupling coefficients between the channels were first measured and afterwards adjusted by tunable controllers (Fig. 6.160). The system was tested at a bit rate of 10 Mbit/s. In order to demonstrate the crosstalk compensation, Fig. 6.160 shows pulse trains with and without compensation circuits.

R1 = a11S1 + a12S2

+ -

R2 = a22S2 + a21S1

+ -

Fig. 6.160: Near end cross talk compensation in the POF-WDM system

512



St.-POF 20 m 4 u10 Mbit/s 4 LED SFH250 with pre amplifier [App02b], [App02c] POF-AC

A second POF WDM system was set up to transfer an analog VGA signal (see [Bar03b]). A red, green and blue LED with a modulation bandwidth of 30 MHz to 60 MHz (Fig. 6.161) served as sources. Beam splitter cubes served as multiplexers and de-multiplexers respectively, as they are also used in LCD projectors. rel. amplitude [dB] 0

-3

-6 NSPB 500S NSPG 500S SFH 757

f [MHz]

-9 1

3

10

30

100

300

Fig. 6.161: Modulation bandwidth of the three LED

A similar demonstration system was also set up at the Fraunhofer Institute in Nuremberg in which an analog signal and an additional data channel were transmitted in opposite directions (Fig. 6.162). Multiplexers were realized with gratings and interference filters. Fiber type: Length: Bit rate: Transmitter: Multiplexer: Receiver: References: Company:

St.-SI-POF 50 m 1 u analog video, CD signal (2.8 Mbit/s) 650 nm LD, 520 nm and 465 nm LED reflective blazed grating (2 wavelengths) lenses and interference filter (3 wavelengths) commercial components [Jun02a], [Jun02b] Fraunhofer IIS Nuremberg


Video

LD 650 nm

Audio

LED 520 nm

Data

I1

I1

I2

I2

Video

receiver

Audio

LED 465 nm

Data

fiber I3

receiver

receiver

I3

O-MUX/DEMUX

513

O-MUX/DEMUX

Fig. 6.162: 3-channel-POF-WDM system of the Fraunhofer IIS Nuremberg

An example of the transmission of the 3-channel multiplexer and the component are illustrated in Figs. 6.163 and 6.164. 0 transmission [dB] -5 -10 -15 -20 -25 -30 400

O [nm] 450

500

550

600

650

700

Fig. 6.163: Transmission of the 3 channel demultiplexer (FhG IIS)

Fig. 6.164: 3-channel demultiplexer (FhG IIS)

514


A WDM system for use in the lessons has been developed by the University of Applied Sciences Harz (Hochschule Harz) as part of their Optomux project ([Fis06a], Fig. 6.165). Fiber type: Length: Bit rate: Transmitter: Multiplexer: Reference: Company:

St.-SI-POF 25 m 3 u 60 Mbit/s 470 nm, 530 nm and 660 nm LED Prism [Fis06] University of Applied Sciences Harz, Harz-Optics

Fig. 6.165: Optoteach POF-WDM system of the HS Harz

6.3.6.2 Wavelength Multiplex Systems with PF-GI-POF

There are good reasons why fluorinated GI polymer fiber offers several advantages for using wavelength multiplexing. The small core diameter and the small NA simplify the setting up of multiplexers. Furthermore, the PF-GI-POF offers a very wide transmission window with low attenuation and almost disappearing chromatic dispersion. A multitude of different sources are available from glass fiber technology since they use the same spectral range. In [Kan98] a WDM-system is described with 790 nm/860 nm VCSEL. The transmitting elements only have a distance of 75 μm from each other and are modulated with 400 Mbit/s. Due to the large core diameter of the GI-POF of 120 μm, both sources can be directly coupled with a lens (Fig. 6.166). GaAs PD are used as receivers. A lens is used for receiver coupling. A filter was used as demultiplexer in a splitter cube. The large wavelength distance made a simple selection possible.


Fiber type: Length: Bit rate: Transmitter: Receiver: Demux: Reference: Company:

515

120 μm PF-GI-POF 50 m 2 u 400 Mbit/s 790 nm/860 nm VCSEL GaAs-PD Interference filter with lenses [Kan98] Seiko Epson Corp. selective mirror


GaAs-PD

790 nm/860 nm VCSEL

GaAs-PD

Fig. 6.166: 790 nm/860 nm POF WDM system according to [Kan98]

A further proposal for a POF-WDM system was introduced by NTT in [Miz99]. In this proposal, 4 wavelengths were to be transmitted over a 250 μm GI-POF. The four transmitters are arranged in a rectangle directly on the fiber input spaced at 125 μm, obviating the need for a separate multiplexer. The photodiodes are also arranged in a rectangle at the fiber output. Channel separation is achieved through dielectric filters. The principle is demonstrated in Fig. 6.167.

laser diodes

dielectrical interference filters

photo diodes

250 μm GI-POF 125 μm

125 μm

Fig. 6.167: 4 wavelength multiplex according to [Miz99]

The possible distance of the lasers is specified as 25 nm so that interference filters can be used. Through direct coupling, there is no multiplexer loss. For the demultiplexing, a loss of approximately 7 to 9 dB is unavoidable as each photodiode receives the entire signal, but only uses one wavelength. It was assumed here that the photodiodes are round, have a diameter of 90 μm, and that ideal filters were used. Figure 5.72 illustrates the size ratios of POF and PD. For photodiodes with 90 μm diameter, the loss is 8.8 dB (left picture). For a maximum diode diameter of 125 μm, the loss is still a little over 7.0 dB (see right picture).

516


4 PD with 125 μm Ø

4 PD with 90 μm Ø

125 μm

113 μm

250 μm

90 μm

125 μm

250 μm

Fig. 6.168: Arrangement of 4 PD with each 125 μm distance at a 250 μm POF

Fiber type: Transmitter: Demux: Reference: Company:

250 μm GI-POF 4 direct coupled lasers Color filter before direct coupled PD [Miz99] NTT Advances Technology Corp.

Uehara described a WDM system for transmitting videos in various publications (for example, [Ueh98], [Ueh99]). Figure 6.169 illustrates the schematic set-up. Three different wavelengths were used in the experimental implementation. The signals are combined and separated on the receiver end via multiplexers with dielectric mirrors. The PF-GI-POF used has an attenuation of < 100 dB/km in the wavelength range of 650 nm to 1,300 nm and < 50 dB/km in the range from 850 to 1,300 nm. At a core/cladding diameter of 150/250 μm, the AN is 0.20. The bandwidth is 300 MHz km to 500 MHz km over the wavelength range. The wavelength was selected according to the proposals made by the “Eight / Forum”. Figure 6.170 illustrates the proposal made by this committee (see also [Miz00]).

O3 O2 O4 camera

e/o transmitter

MUX/DEMUX

o/e receiver

monitor

Fig. 6.169: Proposal for a POF-WDM-System in the 1,200 - 1,600 nm region


517

channels according to the “Eight / Forum”

1150

1200

1250

1300

1350

1400

1450


Fig. 6.170: Wavelength channels according to the “Eight-/-Forum”

Each of the 8 wavelength channels are 10 nm wide and are oriented according to the available lasers. The large width of the channels and the minimum spacing of 20 nm make it possible to use non-stabilized sources and relatively simple filters in the multiplexers. The transmitters operate with LD that transmit the signal with pulse frequency modulation (PFM-IM) at a carrier frequency of 80 MHz. pin-PD were used as receivers for wavelengths of 1,200 to 1,600 nm. Figure 6.171 shows the multiplexers that have been created with planar waveguides. A groove was made in the substrate in which the interference band-pass filters have been inserted. The component can be used to add or drop a specific wavelength.

band pass filter

O1

O1andO2 O2

waveguides Fig. 6.171: Multiplexer/Demultiplexer in planar wave guide structure

Due to the relatively small NA of the fibers used (0.2), the angle differences of the various light paths are not very large. The distance between the wavelengths used in the experiment was 40 nm, i.e., approximately 3 %. This makes the use of interference filters possible. At first, the system setup was successfully operated with 3 wavelengths over 100 m and with 4 wavelengths over 50 m. The more multiplexers are inserted, the shorter the distance to be covered since the power budget worsens. The author's estimates [Ueh98] show the following transmission lengths: ¾with 2 wavelengths, 250 m is possible ¾with 3 wavelengths, 150 m is possible ¾with 4 wavelengths, 100 m is possible ¾with 5 wavelengths, 50 m is possible

According to [Ueh99] the test wavelengths were O2 = 1,265 nm, O3 = 1,305 nm, and O4 = 1,345 nm. For Point-to-Point transmission 500 Mbit/s and 1 Gbit/s over 100 m are possible with these sources and the GI-POF. The maximum emitted

518


power was +3.8 dBm; the sensitivity was -33.5 dBm. If cross-talk suppression is greater than 36.9 dB, error-free video transmission is possible. Fiber type: Length: Bit rate: Transmitter: Demux: References: Company:

120 μm PF-GI-POF, < 100 dB/km 50 m to 250 m Video signals on 80 MHz-carriers Laser diodes acc. to the “Eight-/-Forum”, max. +3.8 dBm Interference filter in waveguide structures [Ueh98], [Ueh99], [Miz00] NTT Multimedia-Laboratory

In 1999, Prof. Khoe from the Eindhoven University also presented a proposal for a 2.5 Gbit/s WDM system at 645 nm, 840 nm and 1,310 nm in ([Khoe99], see Fig. 6.172). filter 1

filter 2

PD

GI-POF

1,310 nm filter 3

PD

645 nm

filter 4

PD

840 nm

Fig. 6.172: 3 wavelength WDM demultiplexer according to [Khoe99]

The transmitter and receiver for this system correspond to the components for the 2.5 Gbit/s Point-to-Point transmissions (see above). The measured insertion loss of the demultiplexer was < 1.6 dB with a cross-talk of < -35 dB. In [Khoe00] practical experiments were shown with this demultiplexer. The 3 lasers described above at 645 nm, 840 nm and 1,300 nm served as transmitters. Figure 6.173 shows the experiments that were performed. 645/840/1,310 nm 645 nm LD

2 u 100 m (Ø: 170/340 μm) PF-GI-POF 840/1,310 nm

840 nm VCSEL 840/1,310 nm 1.310 nm LD

328 m (Ø: 110 μm) PF-GI-POF

328 m (Ø: 110 μm) + 128 m (Ø: 140 μm) PF-GI-POF

Fig. 6.173: WDM-POF system at the University Eindhoven


519

All three channels were transmitted over a distance of 200 m simultaneously. The attenuation at 645 nm (110 dB/km) had a limiting effect. A transmission length of 328 m (GI fiber with a core diameter of 110 μm) and later 456 m was achieved for the combination of both longer wavelength sources. Table 6.11 summarizes the data of the three channels. Table 6.11: 3 channel WDM system Channel source max. power GI-POF attenuation WDM loss demultiplexer loss sensitivity

645 nm LD +6.8 dBm 110 dB/km 4.0 dB 1.4 dB -29.0 dBm

840 nm VCSEL +1.3 dBm 43.6 dB/km 6.8 dB 1.6 dB -28.6 dBm

1,310 nm FP-LD +3.0 dBm 31 dB/km 6.6 dB 1.6 dB -28.4 dBm

6.3.6.3 Bi-Directional Systems with POF

The bi-directional transmission of signals on a fiber is equally interesting for both access networks and in-house networks. One aspect is that they save fibers and plugs as opposed to the 2-fiber solution. Another point is that they reduce the amount of space needed and that the user cannot accidentally insert the plug in the wrong way. You can divide the different systems into WDM systems which use different wavelengths for both directions and single-wavelength systems in which both directions work with identical transmitters. The latter variation is very strongly limited by near end crosstalk. First of all, we would like to introduce a WDM system which was set up between the University Ulm and the Technology Center of Deutsche Telekom in 1997 for the bi-directional transmission of data ([Ziem97b] and [Som98b]). Figure 6.174 illustrates the principle system set-up. Simple LED at 520 nm and 650 nm were used as sources. These are coupled to the fiber link with the receivers (Hewlett Packard HFBR2526) using commercially available connectors. Colored, printed films placed between the plug connectors have the function of suppressing the near end crosstalk (transmission in Fig. 6.175). Table 6.12 below calculates the power budget for both directions. A 10 Mbit/s data stream was transferred over 63 m in a test setup in the Future Lab of Deutsche Telekom AG. Fiber type: Length: Bit rate: Transmitter: Receiver: Demux: References: Company:

SI-POF 63 m 10 Mbit/s 520 nm LED/650 nm LED HFBR2526 Splitter with color filters [Ziem97b], [Som98b] Deutsche Telekom, University of Ulm

520


650 nm LED

520 nm LED Y splitter

Y splitter In

In

Out

transmission line

Out Si-PD

WDM filter

WDM filter

Si-PD

Fig. 6.174: 500 nm/650 nm POF WDM system for bi-directional transmission Table 6.12: Power budget for the 520 nm/650 nm POF system Source Loss YPower Splitter O: 650 nm 0 dBm O: 500 nm 0 dBm

transmission [dB] 0

50 m POF

Loss YSplitter

WDM Four Con- Received Filter nectors Power

5 dB

9 dB

5 dB

2 dB

6 dB

-27 dBm

5 dB

7 dB

5 dB

4 dB

6 dB

-27 dBm

operating wavelength 500/650 nm

-5 -10 -15 -20

multiplex filter with BJC-600e printer

-25 -30 -35 460

480 500

520 540

560 580

600 620


Fig. 6.175: Transmission of the NEXT suppression filter foils

In 1998, Sony introduced a module for bi-directional data transfer in [Hor98]. At a data rate of 125 Mbit/s, 50 m of DSI-POF could be covered. In the experiment, a BER of 1.9 10-10 was achieved. Duplex operation was, however, only checked by means of a computer simulation. A 650 nm LD with a maximum of 1.6 mW output power at 55 mA served as the source. The low-NA-PMMA POF used had a AN = 0.32. Transmitter and receiver are attached to a common mount in the module, as shown in Fig. 6.176.


521

mirror

laser diode

POF

prism

photodiode

Fig. 6.176: Bi-directional transceiver from Sony

The laser beam strikes the angled surface of the prism. Thanks to the alignment of the laser polarization, reflection is nearly complete. The laser light is then launched into the POF by means of a lens with deflection mirror. The coupling efficiency of the LD into the POF is specified as 91.4 % (0.26 dB loss). The small emitting surface and the small emission angle of the LD are, of course, also utilized. The incoming light from the remote transmitter is unpolarized. This is why a part of the light focused by the lens is refracted through the prism onto the photodiode. The coupling percentage from the POF into the PD is 24.0 %, which is a corresponding loss of 6.2 dB. The degree of polarization of the LD is > 150 at over 1 mW optical power (0.7 % in the second polarization state). The limiting factor in this system is the NEXT (Near End Cross Talk), in other words, the received power of its own transmitter. NEXT was calculated in the study. For a transmitting power of 1.6 mW (55 mA), this comes to: ¾only for the LD-PD unit: ¾for the transceiver without POF: ¾for the transceiver with POF:

2 μW (0.13 %) 5 μW (0.32 %) 8 μW (0.49 %)

A 125 Mbit/s test with 1 mW of average input optical power ran successfully. The computer simulation yielded a signal-to-noise ratio (SNR) of 22 dB at a NEXT of 0.49 % This should allow duplex operation. One problem are connectors positioned near behind the transceiver. Through the index difference of air and PMMA, two reflections are created. For shorter lengths, the POF attenuation does not make much of an impact, the light is also for the most part polarized. Errorfree duplex operation is hence not possible under worst case circumstances. The authors specified approximately 5 m as the minimum distance for the first plug. Also not taken into account is the effect that both transceivers of a single line can have different transmission levels (for example, brought about by different temperatures). This causes the SNR to deteriorate even more. An active echo compensation could correct the situation if the reflections only occur at a few points that are constant in time. In [Kure00] and [Tak00] the calculations for the signal-to-noise ratio is presented for this type of bi-directional transmission. The

522


fact is taken into consideration that the interferences caused by near end cross-talk are not to be confused with white noise; they are determined by the transmission level and strength of the reflections. According to the simulations presented, cross-talk (relative to the transmission level) for a BER < 10-12 of up to 20 % is tolerable. Fiber type: Length: Bit rate: Transmitter: Transceiver: References: Company:

Low-NA-POF, AN = 0.32 up to 10 m 125 Mbit/s, bi-directional red laser diode LD and PD with polarization sensitive mirror [Hor98], [Kure00], [Tak00] Sony

650 nm LED

Si-PD

POF

Y-splitter

Y-splitter

650 nm LED

Si-PD

Fig. 6.177: Bi-directional single wavelength transmission system by Sony

Bi-directional transmission with one wavelength is also described in [Gar99]. The transmitters and receivers are coupled with Y-splitters (see Fig. 6.178). The authors calculated a transmission budget of 19 dB for the Point-to-Point transmission. For a 10 Mbit/s transmission (Ethernet), a range of 110 m is possible (180 dB/km). The splitters have 4 dB insertion loss, plus an additional 2 dB attenuation for the added plug-in connection. This reduces the transmission budget to 7 dB, which corresponds to a range of 40 m. Using better POF (140 dB/km), the range could be increased to 50 m. The authors estimate the full-duplex option from the isolation of the couplers to be approximately 21 dB. Plugs behind the couplings do not worsen the values; however, a mirror spaced at 2 mm was able to cause system failure. An increase in the range can be achieved by using a more powerful 650 nm LD. Nevertheless, the isolation of the couplings would have to be improved. A slight improvement was achieved by using an index matching gel. Fiber type: Length: Bit rate: Transmitter: Receiver: Multiplexer:

PMMA SI-POF 40 m (numerically) 10 Mbit/s bi-directional 650 nm LED, HFBR 1527 Si-pin PD HFBR 2526 Y-splitter with 21 dB isolation


Reference: Company:

523

[Gar99] Centro Politécnico Superior Zaragoza

650 nm LED HFBR 1527

650 nm LED HFBR 1527

Y-splitter

Y-splitter

transmission line Si photo diode HFBR 2526

Si photo diode HFBR 2526 Fig. 6.178: Bi-directional transmission according to [Gar99]

The bi-directional transmission of IEEE 1394 data was introduced by Sharp in 2002. The plug used in this system is the OMJ plug-in connector (2.5 mm or 3.5 mm) which includes the electrical contacts as well as a 1 mm POF. The multiplexer has been built as a special optical block (PMMA, injection-molded part, Fig. 6.179) and enables the passive adjustment of PDs and LDs. The data rates possible range from S100 to S400. Fiber type: Length: Bit rate: Transmitter: Receiver: Multiplexer: References: Company:

DSI-POF, AN = 0.25 .. 0.32 10 m 125 Mbit/s, 250 Mbit/s, 500 Mbit/s bi-directional 638 nm - 666 nm LD, -6.0 to -6.5 dBm -17.5 dBm (S100) up to -13.6 dBm (S400) special compact optics [Fuji02], [Miz03] Sharp Co. PD lens

optical block

single fiber duplex up to 10 m 665 nm laser S100 to S400

mirror

lens LD prism

Fig. 6.179: Optical multiplexer for bi-directional transmission

A very interesting concept for the bi-directional transmission of data has been developed by Toyota. In this case the WDM principle made use of a red and green LED. An interference filter was used as the multiplexer. The setup of the complete system is illustrated in Fig. 6.180.

524


Fig. 6.180: Bi-directional WDM system by Toyota

The maximum data rate amounts to 250 Mbit/s for both channels at a transmission length of 10 m of DSI-POF. The eye diagrams of both channels can be seen in Fig. 6.181. Fiber type: Length: Bit rate: Transmitter: Receiver:

Multiplexer:


DSI-POF, Mitsubishi Eska-Mega 10 m with connector 250 Mbit/s bi-directional 495 nm (own development); -5.7 dBm in the POF and 650 nm LED (Hamamatsu L7726); -1.5 dBm in the POF Si-PD sensitivity at 495 nm: -17.4 dBm sensitivity at 650 nm: -20.6 dBm self written waveguides with interference filters 85% transmission at 495 nm and 96% reflection at 650 nm Module: 6 u 7 u 9 mm³ [Kag03], [Yon04], [Yon05] Toyota

Fig. 6.181: Eye diagrams of the two channels for the Toyota WDM systems


525

One particular feature is the beam guiding in the multiplexer. Instead of a system of lenses a waveguide is used which is written self-employed into a polymer block by UV light. All adjustment steps are thus superfluous. The already written splitter with filter and coupled POF are shown in Fig. 6.182.

Fig. 6.182: Splitter of the WDM system by Toyota

A very economical proposal for bi-directional transmission over POF was made by an English group in [Kat98]. The authors studied the possibility of using a LED as a transmitter and receiver simultaneously. It is well known that semiconductors can also be used as detectors. The maximum emission is produced at shorter wavelengths. By means of this shift, the authors were able to determine a system loss of 5 dB. In comparison to a typical photodiode (Siemens SFH 250), the sensitivity is approximately 7 dB lower. This makes the system 12 dB worse than a conventional Point-to-Point system. In addition, it can only be used in semi-duplex mode since the diode operation must be switched over. For shorter distances up to 20 m, this could be an interesting solution for cost reasons. Fiber type: Length: Bit rate: Transmitter: Reference: Company:

PMMA SI-POF up to 20 m (numerically) Half duplex operation LED in photodiode operation [Kat98] University of North London

A similar approach is also described in [Ing06]. A 1.25 Gbit/s half-duplex transmission with a VCSEL as transmitter and receiver is realized over 500 m of GI-GOF (50 μm). The sensitivity of the VCSEL when operating as a photodiode is about 0.1 mA/mW (at 850 nm, 0.9 nA dark current) and a receiving bandwidth of 933 MHz is attained. The sensitivity is -12.3 dBm, as shown in Fig. 6.183.

526 10-2 10


BER

1.25 Gbit/s NRZ PRBS 27-1

-4

10-6 10-8 10

back to back 500 m 50 μm GI-GOF

-10

10-12 10-14 -16

average received power in the fiber [dBm] -15

-14

-13

-12

-11

-10

-9

-8

Fig. 6.183: Sensitivity of a VCSEL in photodiode operation

Infineon developed the transceiver SFH800 for the automotive industry (see [Schö99b]). By using chip-on-chip technology, the LED transmitter is mounted directly on the photodiode, as shown in Fig. 6.184. The component is intended for use in passive star-type networks for data rates up to 10 Mbit/s. Fiber type: Bit rate: Transmitter: Receiver: References: Company:

SI-POF 10 Mbit/s, half duplex operation Photodiode with LED „on chip“ >300 μW at 30 mA, 650 nm -23 dBm sensitivity [Schö99b], [Schö00b], [Gri00] Infineon Technologies

LED

approx. 1 mm

photo diode

Fig. 6.184: SFH 800 from Infineon for bi-directional POF operation

In [Bau02] concepts are presented for also using this principle for replacement systems with significantly higher data rates. By using a RC-LED with below 1 ns switching time up to 200 Mbit/s can be transmitted. The sensitivities of the photodiode are (BER = 10-9):


527

¾-23 dBm (up to 50 Mbit/s) ¾-22 dBm (up to 100 Mbit/s) ¾-17 dBm (up to 200 Mbit/s)

Over the past few years the DieMount company has developed various systems for bi-directional transmission with SI-POF. With the aid of couplers particularly low in reflection which the company developed crosstalk in one-wave systems was able to be reduced considerably. Furthermore, the use of special specular optics can increase performance coupled into the fiber significantly above 0 dBm. As opposed to normal systems you can work with greater emitted power with a bidirectional system with integrated couplers since the limit for eye safety is only valid at the coupler output. At a wavelength of 470 nm a maximum fiber length of 100 m can attain a data rate of 125 Mbit/s. At 650 nm with an LED the range of 95 m is comparably large. A range of 50 m is guaranteed for this product. Fiber type: Length: Bit rate: Transmitter: Receiver: Multiplexer: Reference: Company:

PMMA SI-POF 100 m 125 Mbit/s bi-directional 470 nm LED Si-pin-PD Splitter with low NEXT [Kra04a] DieMount

The second system introduced by DieMount is a WDM setup at 470 nm and 657 nm with LED, thus enabling a fiber length of 50 m. Figures 6.185 and 6.186 show the principle of the micro-mirror coupling and the transmission characteristics of both color filters for suppression of cross talk. Fiber type: Length: Bit rate: Transmitter: Receiver: Multiplexer: Reference: Company:

PMMA SI-POF, 165 dB/km @ 647 nm; 76 dB/km @ 470 nm 50 m 125 Mbit/s bi-directional 647 nm LED, -3 dBm; 470 nm LED, -1 dBm Si-pin-PD special low reflection splitter with color filters [Kra04c] DieMount

Fig. 6.185: Special coupling of the LED by micro mirrors

528

100


transmission [%]

90

red filter

80

blue filter

70 60 50 40 30 20 10 0 350

400

450

500

550

600

650

700

750

wavelength [nm]

Fig. 6.186: Filter function of NEXT filter in a WDM system

6.3.7 Special Systems, for Example, with Analog Signals

In the examples mentioned above we have always been dealing with digital signal transmission, the domain of optical telecommunications engineering. In some cases, however, the transmission of analog signal makes sense. In the following segments we will present some ideas for the analog transmission of video signals at first. Thereafter, we will present some special experiments such as how analogmodulated digital signals can be transmitted. One great advantage of POFs in comparison to glass multimode fibers lies in the large number of modes. As a rule, mode distribution noise does not play any significant role, unlike, for example, for 50 μm GI-GOF. One disadvantage is that the lasers used are not usually linear like the 1.3 μm DFB laser, for example, which is employed in glass fiber systems. The parameters of singlemode glass fiber systems cannot, of course, in any way be reached with POFs. POF systems therefore make sense in applications over short distances in which the main aspects are simple installation and robust systems. 6.3.7.1 Video Transmission with POF

The fact that POF is also suitable for transmitting broadband analog signals was already proven in [Fan98]. In the experiment described, a 60-channel video signal was used as a source. Each amplitude-modulated channel used up 6 MHz of bandwidth. Channel 10 that is located at 145.25 MHz was removed and replaced with a digital 2 Mbit/s channel. BPSK was used as the modulation method (binary PSK). Figure 6.187 illustrates the principle test set-up.


529

59 video channels AM

channel 10 BPSKmodulated 2 Mbit/s

LD 659 nm

200 m GI-POF Ø: 500 μm

SiMSM-PD

Fig. 6.187: Hybrid POF system for video transmission

The GI-POF (200 m) used has a diameter of approximately 500 μm. A 659 nm laser diode with a maximum input power of 1.5 mW (+1.8 dBm) was used as the source. For the receiver, a Si-MSM photodiode was used (MSM: Metal Semiconductor Metal). A modulation index of 3 % was selected for the BER measurements for the analog channels. A BER < 10-9 as achieved for a modulation index starting at 2.2 %. In order to characterize the laser non-linearity, the value of the CTB (Composite Triple Beat) and CSO (Composite Second Order) was determined, i.e., the sum of the mixed products of the second and third order for the carrier frequencies transmitted. In the study, the values: CTB | 64 dBc and CSO | 63 dBc were found for all the channels. Thus, error free video transmission should be possible, although the authors fail to mention anything about the quality of the analog channels after the transmission. Since a PMMA fiber appears to be the subject of discussion for the POF, the respective attenuation was probably around 200 dB/km at 659 nm. The reception level was presumably relatively small so that the analog channels were noticeably disturbed by the receiver noise. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

500 μm PMMA GI-POF 200 m 60 u 6 MHz video BPSK signal with 2 Mbit/s at 145.25 MHz 659 nm LD, 1.5 mW Si-MSM-PD [Fan98] University of Connecticut

At the POF-AC Nürnberg a system for the analog transmission of a video channel using the baseband was set up. The transmitter was a particularly powerful green LED from DieMount (Fig. 6.188). The receiver consisted of the Si-pin-PD SFH250 from Siemens and a particularly low-noise amplifier (receiver circuit and system in Fig. 6.189). Fiber type: Length: Bit rate: Transmitter:

PMMA SI-POF, 99 dB/km 400 m 6 MHz analog video 520 nm LED, DieMount

530


Receiver: References: Company: video input

SFH250 with operational amplifier [Blu01], [Blu02] POF-AC Nürnberg LED 520 nm

BIAS

(DieMount)

Si-PD low noise video SFH250 amplifier output

1 mm POF 90 dB/km| 520 nm

Fig. 6.188: System setup for baseband video transmission

Fig. 6.189: Receiver and the complete system for baseband video transmission

Up to a fiber length of 350 m there was no noticeable decline in the signal quality. At 400 m noise was visible, but the image would still have been usable for control purposes. The pictures for distances from 300 m to 400 m are shown in Fig. 6.190.

Fig. 6.190: Picture quality after 300 m, 350 m and 400 m standard POF

System quality is being improved step by step in different versions by using better components. LEDs and photodiodes still remain, however, moderately priced, basic components. The (simulated) current noise density at the receiver input and the emission spectrum of the LEDs are represented in Fig. 6.191.


2.0

input noise current density [pA/ Hz]

16

531

P [a.u.]

14 12

1.5

ver. 1

10 8

1.0

6

ver. 2 0.5

4 ver. 3

0.0 102

103

104

105

2 106 107 108 frequency [Hz]

0 440 480 520 560 600 640 wavelength [nm]

Fig. 6.191: Noise current density at the receiver input and emission spectrum of the LED

In the meantime a commercial product with a comparable concept is available from the Italian manufacturer Luceat, whereby a range of 200 m is guaranteed (in the high-end version between 50 m and 250 m). The system contains an audio transmission and a gain control. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

PMMA SI-POF 50 m to 250 m analog video, baseband up to 6 MHz 520 nm LD Si-pin-PD [Luc04c] Luceat

The error-free transmission of 37 analog and 16 digital television channels was realized at the Fraunhofer Institute in Nuremberg in 2003. A conventional 655 nm laser was used as a transmitter while the receiver was again based on a Hamamatsu Si-PD. The transmitter and receiver module are shown in Fig. 6.192. Fiber type: Length: Bit rate: Transmitter: Receiver: References: Company:

900 μm, PMMA GI-POF, Optimedia 25 m analog TV, 47 - 695 MHz 655 nm LD 800 μm Si-pin-PD [Web03a], [Jun04b], [Jun05a] Fraunhofer IIS also 30 m SI-POF and 100 m PF-GI-POF

532


Fig. 6.192: Video transmitter and receiver for up to 470 MHz (FhG IIS)

The complete spectrum of transmitted signals can be seen in Fig. 6.193. In one of the first experiments in 2003 only two channels (325 MHz and 380 MHz) were transmitted over 50 m. In later experiments (2004) the complete band up to 470 MHz was transmitted over 35 m of SI-POF.

Fig. 6.193: Video transmission up to 470 MHz (FhG IIS), complete spectrum

The signal quality was analyzed for the transmission of the band up to 470 MHz over 30 m of SI-POF (channels from 147.25 MHz to 335.25 MHz). A deterioration in CNR of 46 dB (input) to 43 dB (output) was established. The CSO remained unchanged at 53 dB. The transmission over 100 m of PF-GI-POF (LucinaTM) was successfully demonstrated with this system. The values for CNR and CSO performed as with SI-POF, only the signal level was a few dB smaller because of the inferior laser coupling. The transmission of 37 analog and 16 digital channels over 25 m of PMMA GI-POF was demonstrated in 2005, also with insignificant changes in CNR, CSO and CTB.


533

At the 2006 POF Conference in Seoul a system for transmitting the BK band on POF was introduced [Kim06b]. A generator with 60 analog video signals (NTSC format, 55.25 MHz up to 439.25 MHz) serves as a source. A 1.31 μm DFB Laser (10 mW) was modulated with an index of 3.4% per channel. The transmission link was a 25 m long PF-GI-POF with a 50 μm core diameter of Asahi glass. A pinphotodiode served as the receiver. Only very slight deterioration was determined for CSO and CNR (Fig. 6.194). Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

50 μm PF-GI-POF (Asahi Glass) 25 m 60 NTSC channels analog TV 55.25 MHz to 439.25 MHz 1,310 nm DFB laser, +10 dBm Si-pin-PD [Kim06b] National University Kyungpook, South Korea

CNR [dB]

CSO [dB]

48

62

47

61 0m

46

0m

60

45

59

44

58

43

25 m

57

42

56

41

55

40

measurements

54

25 m

measurements

Fig. 6.194: Quality of the analog video transmission (25 m PF-GI-POF)

6.3.7.2 Transmission of Analog Modulated Digital Signals

Conventional copper cables are only of limited use for transmitting broadband digital lines. The most important limitations lie in the increase in attenuation with the root of the frequency (skin effect) and the crosstalk between neighboring wires. Consequently, in order to make maximum use of the channel characteristics, special modulation procedures are employed in which then a quasi-analog signal results. One example is the transmission of DSL. In this case the signal is modulated on several subcarriers which in turn contain QAM modulation with varying degrees of modulation. For optical transmission such signals can either go back directly to the digital level or the optical system is used as a transparent analog channel.

534


At the University of Applied Sciences Gelsenkirchen a system for transmitting a VDSL data stream over SI-POF was set up ([Flex99], [Poll01]). The VDSL signal has a bandwidth of roughly 10 MHz. Standard components from Hewlett Packard (HFBR series) were used as the transmitter and receiver. The transmission length was 50 m. The signal from the forward and backward direction was modulated directly on the LED that was operated by approximately 20 mA of bias current. Band passes were used on the receivers according to the VDSL transmission bands as a means of reducing noise. Possible applications of such a system are extensions of VDSL lines within buildings, should the existing copper cabling be insufficient. Figure 6.195 illustrates the system that was set up. Fiber type: Length: Bit rate: Transmitter: Receiver: References: Company:

SI-POF 50 m 50 Mbit/s VDSL signal (approx. 10 MHz bandwidth) SLED 650 nm (HFBR) Si-pin-PD (HFBR) [Flex99], [Poll01] University of Applied Sciences Gelsenkirchen Si-PD

SLED, 650 nm VDSL modem

VDSL modem Si-PD

50 m SI-POF Ø: 980 μm AN = 0.47

SLED 650 nm

Fig. 6.195: VDSL transmission over POF according to [Flex99]

In the meantime DSL technology has continued to make noticeable advances and there are better components for polymer fibers available. The Teleconnect Company from Dresden as part of the POF-ALL project has developed a system for transmitting Fast Ethernet over 1 mm of SI-POF in which multicarrier technology from VDSL2 is employed. The advantage of VDSL2 is that the frequency range can be split up into practically as many frequencies as desired up to 30 MHz. Each individual carrier, with about 4 kHz bandwidth, can be modulated differently and adapted in its level. If this technology is used for POF, then both directions can be used with the identical frequency range, since there is no crosstalk on duplex fibers. The available frequency range of 30 MHz corresponds very well to the usable range of a SI-POF from 200 m to 300 m in length and the modulation bandwidth of conventional green LEDs. The set-up of the test system is shown in Fig. 6.196. A total of 3,474 carriers are available in the frequency range from 8 kHz to 30 MHz for typical band plans. For example, carriers 1 to 1,739 can be used for upstream (data to the network provider) and carriers 1,740 to 3,474 for downstream (data to the customer).


Ethernet signal generator

VDSL2 TX evaluation testset RX

535

Ethernet signal analyzer opt. transm. (DieMount, 520 nm LED)

SI-POF

optical receiver (SFH250 PD + transimp.-ampl.)

optical receiver

SI-POF

opt. transmitter (DieMount)

RX

VDSL2 evaluation testset TX

Fig. 6.196: VDSL2 over POF, test setup at Teleconnect Dresden

Figure 6.197 shows the SNR for downstream and upstream after 300 m of SIPOF (high-quality fiber from Luceat). The bit rate downstream amounts to approx. 40 Mbit/s. The transmission of 107.42 Mbit/s is possible for over 200 m. SNR [dB]

32 28 24

DS

20 16 12 8 US

4

US frequency [MHz]

0 0

2

4

6

8

10

12

14

16

18

Fig. 6.197: Signal to noise ratio per carrier after 300 m POF

9 8 7 6 5 4 3 2 1 0

bit/symbol

DS

US 0

2

US 4

6

8

10

12

Fig. 6.198: Bit per symbol per carrier after 300 m POF

14 16 18 frequency [MHz]

536


In the final system the frequency range would practically be used without any gaps since you would not have to worry about interference from other services. The attainable modulation depth per carrier in bit/symbol is shown in Fig. 6.198 (the 8 bit/symbol means QAM 256, 7 bit/symbol QAM 128, etc.). The transmission of existing data formats over polymer fibers was also the goal of a test setup conducted by T-Nova in the year 2000 ([Ziem00c]), as shown in Figs. 6.199 and 6.200. The modules are for transmitting the 192 kbit/s BRI (S0-bus), as is used between ISDN terminal equipment and the ISDN-NTBA.

560 nm LED, Si-pin-PD

RJ-45 inductive coupler

v-pin

FSK-Mod FSK-Demod duplex standard PMMA-POF

40 V/5 V converter Fig. 6.199: ISDN-POF module for the NT side

v-pin

560 nm LED, Si-pin-PD FSK-Demod

RJ-45 inductive coupler

FSK-Mod Duplex Standard PMMA-POF

40 V/5 V converter

220 V/40 V converter

Fig. 6.200: ISDN POF module for the equipment side

The medium in this system is a PMMA SI-POF with a AN = 0.47. At a wavelength of approximately 570 nm, this fiber has its absolute minimum attenuation. On a 500 m coil of the type GH 4000, an attenuation of 80 dB/km was measured with a 560 nm Nichia LED. With the LED, about -5 dBm can be launched into the fiber at an average current of 20 mA. At a sensitivity of -45 dBm, up to 500 m can be bridged under laboratory conditions (no plugs, constant temperature). While using a photo multiplier, the sensitivity was approximately -51 dBm. The good sensitivity can be achieved through frequency modulation of the LED. The FM is also well suited, because the BRI has a three-level code, the voltage swing of which is used to recognize the active (±750 mV; 0 mV) and the passive (±600 mV; 0 mV) state. In the modulator, the signal is converted into a proportional frequency shift. In the receiver, the carrier is at first set to a uniform


537

level using a limiter amplifier, then filtered and reconverted to the original signal on a discriminator. The bandwidth of the POF is wide enough to transmit a second channel, for example, bi-directional communication over a single fiber or for coupling both interfaces on the ISDN-NTBA. Figure 6.201 shows the possible choice of carrier frequencies.

„-1“

„0“

„+1“

„-1“

„0“

„+1“

channel 1: 3.263 MHz 3.647 MHz 4.031 MHz

3 MHz

4 MHz

channel 2: 5.470 MHz 5.854 MHz 6.238 MHz

5 MHz

6 MHz

complete band < 1 octave Fig. 6.201: 2 channel transmission of S0 bus over POF

By selecting all frequencies within an octave, the interference is reduced through non-linearity in the transmitter and amplifier. This system can be expanded through a WDM arrangement by reducing the possible ranges. It is then possible, for instance, to transmit two BRI (4 channels) in both directions over only one POF.

S0-Bus 1

conversion to frequencies f1, f2, f3

POF

+

S0-Bus 2 conversion to frequencies f4, f5, f6

LED conversion from the frequencies f1, f2, f3

S0-Bus 1

PD POF

Fig. 6.202: Electrical multiplexing of two S0 busses

conversion from the frequencies f4, f5, f6

S0-Bus 2

538


S0-bus 1

S0-bus 3

electr. Mux 1+2

LED O1

LED O2 POF

POF optical splitter

S0-bus 2

S0-bus 3 electr. Demux 3+4

electr. Mux 3+4 S0-bus 4

S0-bus 1

POF

PD

PD POF

POF

electr. Demux 1+2

S0-bus 4

S0-bus 2

Fig. 6.203: Combination of WDM for bi-directional transmission

At first sight, the transmission of an ISDN signals over POF instead of copper wires has no advantage in quality. The connection costs also cannot be lowered. However, a major benefit is being able to dispense with an electrically conducting connection. The NTBA and terminal device are usually already connected over the power supply. The second electrical connection over the BRI produces a loop that could destroy the components in the event lightening strikes. The POF would simply eliminate this problem. The better electromagnetic compatibility of POF should also not be overlooked. This is an attractive alternative for local exchanges with high security requirements. Yet another advantage offered by POF-ISDN cabling is the possibility of migrating to a faster system in the future that offers high data rates without having to lay new cables. DSI-POF, MC-POF or, wherever available, GI-POF can be used immediately for the BRI. You can also change over later to Fast-Ethernet, IEEE 1394 or even Giga-Ethernet. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

St.-NA-POF GH 4000, 80 dB/km at 560 nm 500 m 0.192 Mbit/s (ISDN S0 bus), frequency modulated 560 nm LED (Nichia, sample) -45 dBm [Ziem00c] Deutsche Telekom

Multicarrier procedures can also be used for transmitting higher data rates. One advantage is that each carrier can be modulated with different quantization. Only a few bits per symbol are used with frequencies having a poor SNR; with a high


539

SNR there are however high modulation stages. This splitting up even occurs dynamically on copper cables (DSL) or in radio networks. The principle of adapting the modulation to the SNR is shown in Fig. 6.204, assuming that the limiting effect is the low-pass characteristic of the fiber. Alternatively, you can also vary the power of the carrier so that a constant CNR arises. 21

rel. power [dB] POF-frequency response 3 dB bandwidth: 200 MHz total bit rate: 1,650 Mbit/s

18 15

64QAM 300 Mbps

12

64QAM 300 Mbps

64QAM 300 Mbps

9

32QAM 250 Mbps

16QAM 200 Mbps

6

8QAM 150 Mbps

3

QPSK 100 Mbps

0

DPSK 50 Mbps

-3 -6 -9

noise level 0

50

100

150

200

250

300

350


Fig. 6.204: Schematically operation of multiple carrier transmission on POF (1,650 Mbit/s)

In principle, the VDSL-over-POF system described above represents exactly this procedure. The disadvantage is that you can only obtain advantages when a range clearly above the 3 dB bandwidth can be utilized. Here a SNR is required that clearly lies above that of a conventional NRZ system. This procedure could therefore be of interest with short fiber lengths. However, for purely noise-limited systems the attainable gain remains modest. The first practical cases realized were presented in [Zeng06] und [Ran06a]. The first work describes the transmission of 1,250 Mbit/s over a PF-GI-POF and the signal is modulated (BPSK or ASK) on a carrier at 3 GHz. Further tests with carrier frequencies at 2.5 GHz, 3.5 GHz und 4.0 GHz show the potential for multicarrier transmission. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

50 μm PF-GI-POF, NA: 0.17, 50 dB/km 100 m 1,250 Mbit/s (carrier 3 GHz), ASK or BPSK 1,310 nm DFB-LD 25 GHz receiver with 50 μm GI-POF pigtail sensitivity -13.2 dBm/-9.8 dBm (BPSK/ASK) [Zeng06] University of Eindhoven

540


A 1 mm SI-POF was used for transmission in [Ran06a]. A 658 nm edge emitter was used as the laser. Forty carriers were used in the frequency range between 11 MHz und 89 MHz. A 256-QAM modulation with 1.8 MS/s was used on each carrier (8 u 40 u 1.8 MS/s = 576 Mbit/s). Minus the share for the error correction (Reed-Solomon code 255,239) the result is a user data rate of 540 Mbit/s. Later on with additional carriers they were able to attain a total of 1,000 Mbit/s. The authors do not provide information about the complete measurement of the BER, but do prove however the reliability performance on the basis of measurements of the error vector magnitude which lies in the entire frequency range below the limiting value of the FEC (about 3.2%). As shown above, a comparable data rate (910 Mbit/s and 1390 Mbit/s, POF-AC) can be transmitted using a simple passive equalizer. Even higher data rates can be attained with an optimal receiver. Fiber type: Length: Bit rate:

1 mm SI-POF, NA: 0.50 100 m 40 u 1.8 MS/s (256-QAM) = 576 Mbit/s FEC: RS (255,239), effective 540 Mbit/s later also 945 Mbit/s effective 658 nm LD 200 μm Si-pin-PD, 1 mm Si-pin-PD [Ran06a], [Ran06c] Siemens AG

Transmitter: Receiver: Reference: Company:

6.3.7.3 Radio over Fiber

A very specialized transmission procedure is presently being developed at the University of Eindhoven ([Ng02a]). The goal is to transmit signals for radio base stations without signal conversion, i.e. directly in the radio band, at least the ISM band at 2.5 GHz. The fundamental principle is shown in Fig. 6.205. head end

radio access unit

intensity modulator

tunable laser

O0

WDM

WDM

O0

f=kfsw

periodically optical filter O0

BPF

O1 PD

data (downlink) O1 fsw

MS

LD O1

LPF data (uplink)

PD

Fig. 6.205: Principle of „Radio over Fiber“ according to [Ng02a]

LPF

mobile station


541

The tunable DFB laser is frequency modulated. The data are then added to the signal in a Mach-Zehnder intensity modulator. The modulation frequency of the laser lies within the bandwidth limits of the fiber. The frequency is converted upward by the interaction of the modulated signal with the periodic characteristic curve of an optical filter, e.g. a Fabry-Perot filter (FP). The back channel can be realized by means of a WDM procedure on the same fiber and the receiver itself supplies the frequency for the upward conversion so that a separate oscillator is not needed. In the experiment a 1,310 nm laser with 10 mW was used. The triangular frequency modulation - here with fsw = 800 MHz and a maximum of 28.8 GHz deviation - was realized by an external phase modulator. The FP filter had a period of 9.6 GHz resulting in a frequency conversion of 5.4 GHz, 10.8 GHz, etc. A carrier frequency of 225 MHz was given on the modulator which in turn was either BPSK or QAM modulated (up to 56 Mbit/s error-free transmission). The uplink supports up to and over 1 Gbit/s. This procedure enables the use of both glass multimode fibers, demonstrated with over 4 km, and POF. Other descriptions can be found in [Gie03], [Koo04b], [Lar06b], [Lar06c], [Ng04a] and [Ng04b]. 6.3.7.4 Mode Multiplex

Another group of transmission procedures makes use of the fact that the propagation angle remains relatively constant over limited lengths of POF. If one succeeds in coupling light into the fiber at different angles, and these angles are detected again separately, then these angle ranges can be used as independent transmission channels (Mode Group Division Multiplex: MDGM, see also [Koo03B], [Boo05] and [Schö06]). The principle of MGDM (from [Koo03b]) is shown in Fig. 6.206 together with an example for mode distribution (after transmission over 25 m of SI-POF, [Boo05]). Experimental results for bit rates of 5 Gbit/s, using glass MM fibers, however, can be found in [Schö06a]. The disadvantages of this procedure are the increasing crosstalk over greater distances and the need for ring-shaped detectors. The cross section of higher modes increases with the increase in the propagation angle. Consequently, the capacity of the individual channels can vary greatly. Multiplexer: References: Company:

Mode multiplex [Koo02a], [Koo03b], [Boo05], [Schö06], [Ziem06g] University of Eindhoven

After modal distributions on SI-POF and PMMA GI-POF with lengths up to 100 m were determined in different works and experiments with low data rates (2 u 1 Mbit/s) demonstrated the principal practicability, [Lee06b] presented a complete system.

542


Fig. 6.206: Principle of MGDM and example for a mode distribution ([Koo03b])

Two channels each with 500 Mbit/s were transmitted over 25 m of a 1 mm SI-POF. Both transmitters were lasers which were coupled in at different angles. The decoupling and separation of the mode groups was affected with the aid of tilted mirrors (see Fig. 6.207). Fiber: Length: Bit rate: Transmitter: Multiplexer: Reference: Company:

1 mm PMMA SI-POF (NA: 0.50) 25 m 2 u 500 Mbit/s (simultaneous transmitted and received) 635 nm and 658 nm LD (launched at 0° and 20° angle) Mode multiplex separation of the mode groups by angled mirrors [Lee06b] University of Eindhoven

POF angled mirror

lens lens detector array

Fig. 6.207: Demultiplexer for MGDM according to [Lee06b]

The first experimental results for mode multiplex on PF-GI-POF were presented by [Schö06] at the POF Conference in Seoul. A 62.5 μm PF-GI-POF was used. The signal in different mode groups was coupled into two fibers over a correspondingly positioned singlemode fiber. These mode groups were then brought together over a coupler maintaining one mode. At the end of the test fiber the signal was re-detected with a freely positionable singlemode fiber. Both channels could - separately - be transmitted errors free.


Fiber: Length: Bit rate: Transmitter: Multiplexer: Reference: Company:

543

62.5 μm PF-GI-POF 10 m 2 u 10.7 Gbit/s (simultaneous transmitted, separately received) 1,540 nm LD Mode multiplex, launch central and with a 20 μm offset separation of the mode groups by a micro positioner [Schö06] University of Kiel

The following illustrations show the mode field measurements of diverse POFs with different launchings. These measurements were not specifically made for MGDM applications, but they do show, however, that the mode families can remain stable for quite some time under optimum conditions. Work was carried out in [Ohd04] and [Ish05b] to show what influence the fiber NA has on the mode coupling in PMMA GI fiber. Diverse fibers with NAs ranging from 0.15 to 0.30 and core diameters of 400 μm or 600 μm respectively were investigated.. The near field after 100 m of fiber (AN = 0.30) with central coupling and launching with offset is shown in Fig. 6.208 (left from [Ohd04], right from [Ish05b]).

Fig. 6.208: Near field after 100 m PMMA GI-POF (every left: central launch, every right: launch with offset)

Fig. 6.209: Far fields after 10 m PMMA SI-POF

The mode behavior of SI-POF was investigated in [Jan04]. The far fields after 10 m of POF (Mitsubishi CK-40) when coupling in collimated light (6°, 15° and 24° relative to the fiber axis) are shown in Fig. 6.209.

544


That MGDM can also function very well in glass fibers was demonstrated in [Kra00] and [Klu02] (University of Mannheim). Here a 200 μm SI-PCS was used (AN = 0.39). A 632 nm HeNe low divergence laser served as the source. Up to 13 different mode groups can be differentiated after a short piece of fiber (40 cm). Figure 6.210 shows a picture with every second mode group.

Fig. 6.210: Mode groups in a 200 μm PCS ([Kra00], [Klu02])

The capacity of the method is described more detailed in [Kra00]. The crosstalk between the channels is analyzed. A data transmission is investigated in [Klu02] than. Using PCS length of up to 20 m has been measured. A 200 μm POF was available too. The principal option for data communication was found based on the measured crosstalk attenuation of more than 10 dB. 6.3.7.5 Fiber Ribbon Systems

The last group of POF transmission systems dealt with here represents the simplest form of multiplexing, i.e. parallel transmission on several fibers. The principle has long been known in glass fiber technology and is also employed over short distances up to and including access networks. In most other applications WDM has gained acceptance since only one fiber is needed. The ribbon solutions have a much greater potential for POF. On the one hand the active components are generally very reasonably priced so that the use of many elements can still be cheaper, for example, than switching over to a single mode glass fiber solution. Furthermore, the distances are often very short so that a greater need for fibers does not play that big a role. Finally, the POFs are so thick and you can work with them so easily that ribbon plugs can be produced and installed quite economically, which is not the case with glass fibers. The first commercial system was introduced by Honda Cable. Up to 500 Mbit/s can be transmitted on each of four SI-POFs. The plugs correspond to an electrical RJ 45.


Length: Bit rate: Reference: Company:

545

10 m 4 u 500 Mbit/s, ribbon [Hon05] Honda-Cable ¾PMMA SI-POF ¾core/cladding diameter: 980/1,000 μm ¾Uni-directional transmission ¾100 - 500 Mbit/s per fiber ¾LED: 650 nm ¾pin-photodiode ¾electrical interface: LVPECL (Low Voltage Positive Emitter Coupled Logic) ¾operation temperature range: 0 to +60°C

Fig. 6.211: 4-fiber parallel transmission by Honda-Cable

As part of the Oval Project supported by the Bavarian Research Foundation a system for the transmission of HDMI signals has been set up at the POF-AC. On each of four parallel channels 1,600 Mbit/s over 50 m of fiber (500 μm PMMA GI-POF from Optimedia) can be transmitted.

Fig. 6.212: HDMI data transmission over 50 m PMMA GI-POF

The POF ribbon cables used have already been presented in Chapter 2. The system had the following parameters: Fiber type: Length: Bit rate: Transmitter: Receiver: Multiplexer: References: Company:

500 μm PMMA SI-POF and PMMA GI-POF, 8-fiber ribbon 50 m 4 u 1,600 Mbit/s 650 nm LD pin-PD 8-fiber ribbon [Jun06], [Ziem06g] OVAL project (Loewe, SGT, FhG IIS, POF-AC)

546

6.4 Other Optical Transmission Systems

6.4 Other Optical Transmission Systems with Fibers One aim of this book is to supplement the first edition by also including information on other fibers so that the development of optical short-range communication, in which other optical fibers are increasingly used, is taken into account. Polymer fibers which are suitable for high temperatures will be dealt with first. (The fiber characteristics have already been described in Chapter 2. Some transmission experiments with 780 nm transmitters have already been presented in Section 6.2.4.3). A description of multiple parallel POF connections then follows. Finally, systems with PCS and fiber glass bundles are discussed. 6.4.1 Data Transmission on High-Temperature POF Polycarbonate fibers were viewed for a long time as the most likely candidates for high-temperature POF in ranges up to +130ºC. The main user for such systems is the automobile industry since temperatures in certain areas of cars can go up to over +100ºC. System tests on PMMA SI-POF with high data rates have been presented in Section 6.3.1.3. A test for transmitting high data rates on PC-POF was carried out as part of the joint experiments by T-Nova and Nexans Autoelectric. Figure 6.213 shows the measurement results at 800 Mbit/s and a pseudo-random sequence at 500 Mbit/s when transmitting over a 10 m fiber. A 657 nm component from Sony was used as the laser, the attenuation of which lay at about 12 dB. For the experiment at 800 Mbit/s, a receiver with GPD amplifiers and a SFH75P photodiode were used. (The maximum data rate of the receiver was 1,200 Mbits.) Thanks to the alternate symbols, the transmission occurred without errors despite the limited bandwidth of the POF. The received power at the photodiode was -11.2 dBm. In a second experiment a pseudo-random sequence of the length 27 - 1 was used. The maximum possible data rate here was only 500 Mbit/s because of the limited bandwidth of the POF. The laser was operated without predistortion and the receiver setup was unchanged at a maximum of 1,200 Mbit/s. The received power was also unchanged at -11.2 dBm. Consequently, the PC-POF can unhesitatingly be employed for applications such as IEEE 1394 S400 in the event that increased demands on temperature levels are made, for example, in the engine compartment of vehicles. Higher bandwidths can be achieved by using modified cladding or with PC-MC-POF. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

1 mm PC-POF, 1,200 dB/km at 650 nm 10 m 500 Mbit/s 657 nm LD, Sony SLD 1133 VL SFH75P with amplifier [Stei00a] Deutsche Telekom


547

500 receiver signal [mV] 400 300 200 100 0 -100 -200 -300 -400 23.01.2001 © Giehmann -500 0 20 40

time [ns] 60

80

100

Fig. 6.213: PC-POF system at 500 Mbit/s (PRBS-sequence) over 10 m

New experiments with the PC-POF from Mitsubishi have been undertaken at the POF-AC using an improved test system. Initially, 950 Mbit/s could be transmitted error-free over a 10 m fiber ([Ziem03g]). Later on, 1,000 Mbit/s over 10 m at a received power of -13 dBm were attained ([Vin04b]). In comparison, the data rates for PMMA SI-POF under otherwise equal conditions lay at around 1,500 Mbit/s. the reason for this is the greater NA of the PC POF. Furthermore, the lower receiving level only permits a small penalty.

BIAS

LD 650 nm

PC-SI-POF Mitsubischi FH4001 1,500 dB/km @ 650 nm

Si pin PD S 5052

Fig. 6.214: PC-POF system, 950 Mbit/s (PRBS sequence) over 10 m

Other types of fibers used with high temperatures are fibers made of modified (cross-linked) PMMA and elastomers, both of which were used in various transmission experiments conducted at the POF-AC. ¾H-POF (Tver): transmission of 620 Mbit/s over 15 m at 650 nm (received power is -6.5 dBm), Sony laser ([Ziem03g]). ¾H-POF (Tver): 1,200 Mbit/s over 11 m and 990 Mbit/s over 23 m of POF at 650 nm (Sanyo laser diode, [Ziem03g], [Vin04b]), see Fig. 6.215. ¾PHKS CD1001 (modified PMMA POF from Toray), transmission of 1,050 Mbit/s over 10 m and 830 Mbit/s over 20 m at 650 nm (received power +0.5 dBm and -2.6 dBm, [Ziem02j], [Ziem02k]). ¾HPOF (modified PMMA, sample from Hitachi): 850 Mbit/s over 24 m at 650 nm ([Vin04b]). ¾HPOF-S (elastomer), tested only at 780 nm, see above.

548


BIAS LD 650 nm

H-POF TVER 800 dB/km @ 650 nm 11 m, 15 m and 23 m

Si pin PD S 5052

Fig. 6.215: Modified PMMA-POF system at 650 nm

The results of the tests show that all high-temperature fibers investigated permitted similar data rates as the PMMA SI-POFs, however, with clearly higher losses. There are still numerous problems in the details which have to be solved before they are actually used in vehicle networks. 6.4.2 Multi-Parallel POF Connections POFs offer many advantages for use in optical bus systems as multiple parallel connections over very short distances. First of all, the fibers have a very good ratio of cladding to core diameter which makes adjustment easier. Furthermore, the flexible material and the great NA permit smaller bending radii than for comparable glass fibers. The most important aspect is probably that they are simple to work on. A POF bundle can simply be cut off with a hot blade or smoothed very quickly by polishing it. Extensive developments have been conducted in this area at the University of Dortmund. In order to realize many channels in a small space, 1/8 mm POF was used. Since only a maximum of 50 cm were to be bridged, a 850 nm VCSEL could be used without any problems. In Figs. 6.216 to 6.218 the principle of the parallel link is first illustrated and then photos of the plug and a PC board with the POF link. In the principle figure you can see that the fibers are plugged vertically to a VCSEL array and then diverted in the plug by 90º. Thanks to the bending radius below 2 mm there are no problems with the POF used. 2,500 Mbit/s can be transmitted over each of the 64 channels. Fiber type: Attenuation:

Length: Bit rate: Transmitter: Receiver: Multiplexer: References: Company:

125 μm SI-POF, AN = 0.50 1.7 dB at 660 nm (50 cm): 4.5 dB at 870 nm 8.5 dB at 980 nm 0.5 m 2,500 Mbit/s 850 nm VCSEL array (8 u 8) Si-pin-PD array, -23 dBm at 2.5 Gbit/s and BER = 10-11 SDM up to 128 fibers; module size 3.5 u 10 u 10 mm³ [Witt98], [Jöh98], [Ney01], [Ney02] University of Dortmund


549

Fig. 6.216: Principle of parallel optical links with POF

Fig. 6.217: Parallel optical link with POF, connector and the complete link

POF bundle

alignement pins VCSEL array processor

Fig. 6.218: Coupling of the fiber bundle

Similar experiments with thin POF have also been carried out at the University of Ulm, whereby red VCSELs were used as transmitters. 2,000 Mbit/s could be transmitted over 1 m of fiber. The laser itself can be modulated up to 5 Gbit/s. These data rates can also be transmitted over 100 m of graded index glass fibers. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

125 μm PMMA SI-POF, AN = 0.50, 500 dB/km 670 MHz bandwidth over 10 m 1m 2,000 Mbit/s, also 5 Gbit/s over 100 m MM-GOF 650 nm VCSEL, 0.79 mW InGaAs pin-PD with lens coupling [Sta03] University of Ulm

550


6.4.3 Systems with 200 μm PCS and Semi-GI PCS Glass fibers with polymer cladding (PCS) have been used successfully in automation engineering for many years. The data rates used up to a few years ago lay at a maximum of 12 Mbit/s, the bandwidth of the fiber playing no role whatsoever. Only recently has the potential of PCS for much higher data rates been investigated, e.g., for use in future vehicle networks. The advantages of the PCS lie in their small bending radius, high resistance to heat and useability in the near infrared range where better VCSELs are available. Transceivers for data rates of up to 155 Mbit/s are available from Hewlett Packard (Agilent and Avagotech later) which can be used with POF as well as with 200 μm PCS. Approximately 6 dB less power can be coupled into the PCSs. Thanks to the very low fiber attenuation this disadvantage is compensated for lengths of 50 m and more so that longer distances can be bridged. The comparison of bit rates and ranges for POF and PCS is shown in Fig. 6.219. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company: 200

200 μm SI-PCS 10 m to 700 m 20 Mbit/s to 125 Mbit/s 650 nm SLED, -16.2 dBm in the fiber Si-pin-PD [HP07] Hewlett Packard, Agilent, Avagotech

bit rate [Mbit/s]

100 50

typical values

typical values application range with PCS

application range with POF

20 10 10

20

40

60 80 100 10 fiber length [m]

30

100


Fig. 6.219: Transmission distance and bit rates of HFBR components on POF and PCS

For several years now fast components for MM glass fiber systems have been produced at the Astri Research Centre in Hong Kong. 850 nm components have been developed for 200 μm PCS which permit a data transmission of up to 1,250 Mbit/s over 10 m of PCS. Eye diagrams for three different bit rates are shown in Fig. 6.220.



551

200 μm SI-PCS 10 m 300 Mbit/s; 600 Mbit/s; 1,250 Mbit/s 850 nm VCSEL, min. -0.6 dBm GaAs MSM-PD [Wip05] Astri Hong Kong usable from -40°C o +105°C

Fig. 6.220: Eye diagrams after 10 m for 300 Mbit/s, 600 Mbit/s and 1,250 Mbit/s

At the Fraunhofer Institute in Nuremberg a WDM system was realized on a PCS basis with which wavelengths of 650 nm and 850 nm could each be transmitted at 800 Mbit/s over 30 m. A very slow control channel (700 kbit/s) was transmitted bi-directionally over 500 m using LED sources. The spectra of the LEDs used are shown in Fig. 6.221. Popt [log. a.U.]

550

600

650

700

750

800

850

O [nm] 900

Fig. 6.221: Spectra of the used LED

Fiber type: Length: Bit rate: Transmitter: Receiver: Multiplexer: Reference: Company:

200 μm SI-PCS, AN = 0.37 30 m 2 u 800 Mbit/s 650 nm/850 nm LED, LD 400 μm PD with TIA and limiting amplifier Lens-interference filter combination housing with FSMA connections 45 u 25 mm² [Tsch04b] Fraunhofer IIS

552


The POF-AC and the FhG IIS cooperated in setting up an experimental system for transmitting analog VGA signals on PCS. Conventional red laser diodes served as transmitters. Up to 100 m of fiber could be bridged with a medium screen resolution. Fig. 6.222 shows the experimental setup. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

SI-PCS, AN = 0.37 100 m VGA signal, 1,280 u 1,024 pixel 650 nm LD, 2.5 mW 400 μm PD S5973, Hamamatsu [Fac04] POF-AC and Fraunhofer IIS

Fig. 6.222: VGA test system with 200 μm PCS

Transmission experiments with SI-PCS have also been conducted at POF-AC as part of different projects, including one with BMW, whereby lengths between 5 m and 100 m were used. The laser wavelengths were 650 nm and 780 nm and the same large-area detector was used as with the 1 mm fibers. Fiber type: Length: Bit rate:

Transmitter: Receiver: Reference: Company:

SI-PCS, AN = 0.37 5 m, 25 m, 50 m, 75 m and 100 m 340 Mbit/s, 500 Mbit/s, 600 Mbit/s, 900 Mbit/s and 1,850 Mbit/s (650 nm) 800 Mbit/s, 1,200 Mbit/s and 2,500 Mbit/s (780 nm) 650 nm LD, -6 dBm to -15 dBm received power 780 nm LD, -11 dBm to -14 dBm received power 800 μm PD S5052, Hamamatsu [Vin04b] POF-AC


553

In later experiments greater transmission lengths with different PCS types were then investigated. Compensation filters for equalizing mode dispersion were used, whereby the following results were attained at a wavelength of 780 nm: ¾2,230 Mbit/s over 10 m ¾1,040 Mbit/s over 50 m ¾ 500 Mbit/s over 100 m ¾ 260 Mbit/s over 200 m Fig. 6.223 shows as an example the eye diagram for the transmission of 350 Mbit/s over 100 m.

Fig. 6.223: Eye diagram for a 200 μm PCS

The bit rates and ranges of the SI-PCS systems are summarized once again in Fig. 6.224. Distances of many hundred meters can, of course, be bridged because the attenuation is much smaller than with PMMA POF. However, the bandwidth of the SI-PCS only lies approximately in the area of DSI-POF. 3,000

bit rate [Mbit/s]

1,000

300 100

30 10 1

length [m] 10

Fig. 6.224: Overview 200 μm PCS systems

100

1,000

554


In order to be able to utilize the PCS’s lower attenuation even at higher bit rates, PCS with a semi-graded index profile is manufactured by Sumitomo and OFS ([Sum03]). A work from 1995 ([Kos95]) presents a system of data transmission with the then new fibers. A 850 nm VCSEL was used as transmitter and a small-surface InGaAs-APD as receiver. 3,000 Mbit/s could be transmitted over 100 m; over 1,000 m it was still 1,500 Mbit/s. The semi-GI-PCS thus approximately attained the performance of PF-GI-POF - is, however, considerably more expensive. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

Sumitomo semi-GI-PCS 100 m, 500 m, 1,000 m 3,000 Mbit/s; 2,000 Mbit/s; 1,500 Mbit/s 850 nm VCSEL InGaAs-APD [Kos95] Sumitomo

At the POF-AC the data transmission of Sumitomo semi-GI PCS with a 650 nm and 780 nm laser respectively was tested over 500 m. The maximum possible data rates were 600 Mbit/s and 1,000 Mbit/s (eye diagram in Fig. 6.225). An 800 μm Si-pin detector was used that was actually much too big. Consequently, improvements can still be made in the system. Fiber type: Length: Bit rate: Transmitter: Receiver: Reference: Company:

Sumitomo semi-GI-PCS 500 m 1,000 Mbit/s, 600 Mbit/s 780 nm LD, 650 nm LD 800 μm Si-pin-PD Hamamatsu S5052 [Ziem06i] POF-AC Nürnberg

Fig. 6.225: Eye diagram for 1,000 Mbit/s over 500 m Semi-GI-PCS at 780 nm

In a later experiment different samples from two manufacturers were tested. Using wavelengths of 650 μm, 780 μm and 850 μm (VCSEL) at 300 m data rates


555

of 1,650 Mbit/s, 2,200 Mbit/s and 1,900 Mbit/s respectively could be transmitted. Fig. 6.226 shows the eye diagrams each for 1,000 Mbit/s. They are opened and show large system margins.

650 nm

780 nm

850 nm

Fig. 6.226: 1,000 Mbit/s transmission on 300 m semi-GI-PCS

6.4.4 Systems with Glass Fiber Bundles The last class of fibers to be presented is a development by Schott Glass. These fibers are called MC-GOF and consist of a glass core and cladding, the glass being “normal” mineral glass which is much cheaper and not quartz glass. Each one of the approximately 400 cores is 53 μm thick and has a cladding thickness of only a few μm. Attenuation and NA are approximately comparable with standard POF. The advantages lie in the heat resistance and the extremely small bending radius. In addition to the work done at the POF-AC there has not yet been any published data about attainable bit rates and bandwidth measurements. In two different series of measurements fiber lengths between 5 m and 20 m were tested. Fiber type: Length: Bit rate: Transmitter: Receiver: References: Company:

MC-GOF, 375 cores, 1 mm, Schott 5 m; 10 m; 20 m 1,800 Mbit/s; 1,300 Mbit/s; 900 Mbit/s (650 nm) 2,100 Mbit/s; 1,800 Mbit/s; 1,400 Mbit/s (780 nm) 650 nm LD (-1.4 dBm to -5.1 dBm received power) 780 nm LD (-4.7 dBm to -6.8 dBm received power) 800 μm Si-pin-PD S5052 [Vin04a], [Vin04b] POF-AC Nürnberg

Si pin PD S 5052

BIAS LD 780 nm LD 650 nm

MC-GOF (375 cores) Schott, approx. 200 dB/km

Fig. 6.227: Data transmission on MC-GOF

556


The MC GOFs will also be used in internship experiments at the Nuremberg University of Applied Sciences for which Schott Glass will make available fiber lengths from 1 m to 50 m. In Fig. 6.229 the results of two internship groups working with a 850 nm VCSEL as a source are shown. Please note that the fibers are coupled via an adapter POF to the active elements so that there are two to four plug-in connectors per length. Fiber type: Length: Bit rate: Transmitter: Receiver: References: Company:

MC-GOF, 375 cores, 1 mm, Schott 2 m to 70 m up to 2,610 Mbit/s 850 nm VCSEL 800 μm Si-pin-PD S5052 [Kön06], [Has06], [Was07] POF-AC Nürnberg 260 Mbit/s over 100 m with 780 nm LD

Fig. 6.228: Eye diagrams for 5 m (2,610 Mbit/s) and 30 m (1,840 Mbit/s) from [Was07]

3,000

1,000

bit rate [Mbit/s]

1 mm MC-GOF AN = 0.50 Schott Glass 780 nm / 850 nm

300 fiber length [m] 100 1

3

10

30

100

Fig. 6.229: Data transmission on MC-GOF (intership experiment, green: max. values)

A comparison of the capacity of this fiber at wavelengths of 650 nm, 780 nm and 850 nm is shown in Fig. 6.230 from [Was07]. The slight differences between the curves are rather due to the different power of the transmitting diodes. The eye diagram for a bit rate of 1,200 Mbit/s with a 50 m long fiber at 650 nm is subsequently shown in Fig. 6.231.

6.5 Overview of Multiplex Techniques

557

10,000 bit rate [Mbit/s] 3,000 1,000 650 nm 780 nm

300

850 nm

fiber length [m]

100 1

2

5

10

20

50

100

Fig. 6.230: Data transmission on MC-GOF at 3 wavelengths ([Was07])

Fig. 6.231: Data transmission on 50 m MC-GOF with 1,200 Mbit/s ([Was07])

6.5 Overview and Comparison of Multiplex Techniques The first edition of this book contained a chart showing the development of the capacity of POF systems (Fig. 6.232). The best values at the time was the transmission of 2 u 2.5 Gbit/s over 458 m (University of Eindhoven). This value has endured to today as a system capacity. Nevertheless, this does not mean that the development of POF systems would not have made further progress. A large part of the current developments does not refer to the improvement in the parameters of PF GI POF, but lies more in the area of reasonably priced PMMA fibers. Consequently, it was possible in the laboratory to increase the data rates for 1 mm POF to 2.3 Gbit/s. With a 1 mm PMMA GI-POF 2 Gbit/s over 100 m are possible and with green LEDs distances of several 100 m of PMMA POF can be possible.

558


At the SOFM 2006 in Boulder the transmission of data rates of 10 Gbit/s up to 40 Gbit/s over short lengths (30 m) of PF-GI-POF with a 50 μm core diameter was presented for the first time. These results are of primary interest for parallel data connections. 100

capacity in Gbit/s100 m

10

1

0.1 year 0.01 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 Fig. 6.232: Development of POF system capacity

The following list shows the present highest capacity of different polymer and glass fibers (estimations). ¾up to 2,500 Mbit/s over short lengths PMMA-POF ¾up to 40,000 Mbit/s over PF-GI-POF (30 m) ¾500 m transmission distance for low data rates with PMMA SI-POF ¾550 m transmission distance for 2.5 Gbit/s with PF-GI-POF ¾1000 m transmission distance for 1.25 Gbit/s with PF-GI-POF ¾100 Mbit/s · km bit capacity for SI- and DSI-POF ¾50 Mbit/s bit capacity on 1 mm MC-GOF ¾50 Mbit/s bit capacity on 200 μm SI-PCS ¾1.500 Mbit/s bit capacity on 200 μm Semi-GI-PCS ¾100 Mbit/s · km bit capacity for MSI- und MC-POF ¾500 Mbit/s · km bit capacity for PMMA GI-POF ¾2.280 Mbit/s · km bit capacity for PF-GI-POF It has always been difficult to assess the future development of PF-GI-POF, but the successes above all at Chromis Fiberoptics are a cause for optimism. Their use in home networks could be of increasing interest especially for PMMA GI-POF. Further improvements in system capacities can be expected with the development of new multiplex technologies. Different multiplex systems relevant for POF are compared in Table 6.13 with typical values according to current publications.


559

Table 6.13: Overview of multiplex methods for POF SDM

WDM

MGDM

SCM

N

1

1

1

possible bit rate (for a fiber bandwidth of B)

| 2BN

| 2BN

| 6..8B

| 4..6B

required number of wavelengths

1

N

1

1

available transmitters for PMMA-POF

650 nm LD

650 nm LD all O LED

650 nm LD

650 nm LD

available transmitters for PF-POF and SiO2 fibers

850 nm 1300 nm LD

850 nm 1300 nm LD

850 nm 1300 nm LD

850 nm 1300 nm LD

required special components

ribbons LD/PD arrays

Mux/ Demux

small TX mode sel. RX

linear LD low noise RX

use of available components

simple setup optical filters

only one wavelength required

only 1 TX/RX required avail. compon.

thicker cables

large MUX for PMMA-POF

unknown influence of the mode coupling

non stable frequency response.?

number of fibers

advantages

disadvantages

The table does not show any clear favorite. In the medium-term the wavelength multiplex may emerge as the clear winner for PF-GI fibers. Various exceptional features speak for it: ¾particularly wide band with low attenuation and dispersion (600 nm to 1300 nm) ¾relatively small multiplexer/demultiplexer ¾large number of available laser diodes at different wavelengths For PMMA fibers the number of possible channels are relatively limited with MGDM and WDM. Furthermore, the large core diameter and the large NA lead to relatively voluminous optical components. Multi carrier procedures are already widely developed in radio and DSL technologies and can be adapted quite easily for POF in order to better utilize the limited capacity. However, the development of better GI POF, faster transmitters and adaptive equalizers offer for the present even much more potential for very simple solutions. Above all the development of SDM systems is of interest for many applications with POF. The extremely simple and reasonably priced cables, uncomplicated end face treatment and adjustment as well as the availability of very cheap active components will make multiple parallel systems extremely attractive especially over short distances. Here PMMA SI-POF can transport straight away many Gbit/s at distances up to a few meters and thus surpass copper conductors. In addition, the power requirements go down and available VCSEL in the near infrared range can be used.

7. Standards

In order to have unified interfaces available, producers of optical fiber components agreed on standards which are continuously being further developed in response to demands and technical progress. This chapter deals with the relevant specifications for the use of polymer optical fibers. Figure 7.1 provides an overview of the most important POF areas of application for data communication, for the existing standards as well as the recommendations for the method of measurement. ATM-Forum IEEE 1394 Ethernet

D2B, MOST IDB 1394 Byteflight, Flexray

IEEE 1394 Ethernet

consumer elektronics computer

building networks

JIS IEC VDE/VDI

measurement methods

automotive applications

SERCOS Interbus Profibus Ethernet automation machine control

Fig. 7.1: Standards for POF applications

As early as the beginning of the 1990’s recommendations were made in the Japanese Industrial Standard (JIS) for the method of measurement of POF. Definitions for applications of POF in building networks, PC, entertainment electronics, automobiles and in machine tool control followed. Figure 7.2 shows the time development of standards for polymer optical fibers. Parameters for the different POFs in IEC 60793-2-40 have recently been established.

562

7.1 Standards for Polymer and Glass Fibers

1975

1980

1985

1990

1995

standard SI-POF

2000

2005

2010

Low NA or DSI-POF

Japanese Industry Standard (JIS)

GI/MSI-POF

IEC ATM Forum

IEC 60793-2-40 IEEE 1394

D2B

MOST

Fig. 7.2: Development of standards for POF

7.1 Standards for Polymer and Glass Fibers 7.1.1 Polymer Fibers The first standards for polymer fibers were established in Japan. The Japanese Industrial standard JIS-C-6837 set the parameters which were later taken over in IEC 60793-2. Table 7.1 according to >Wei98@ shows the values. Table 7.1: Specification of SI-POF according to JIS-C-6837 Parameter

Unit

core

[μm]

485

735

980

cladding

[μm]

500r30

750r45

1,000r60

jacket core non circularity

[mm]

1.5 r 0.1

2.2 r 0.1

2.2 r 0.1

[%]

d6

d6

d6

loss at 650 nm

[dB/km]

d 400

d 400

d 400

with EMD launch

[dB/km]

d 300 -

d 300 -

d 300 -

d 0.5

d 0.5

bandwidth* bending loss numerical aperture

[MHz100m] [dB/10 bends] -

½ mm SI-POF

¾ mm SI-POF

0.50 r 0.15 0.50 r 0.15 * > 10 MHz 100 m defined by the IEC

1 mm SI-POF

d 0.5 0.50 r 0.15

These values have essentially been defined according to the needs of industrial applications. For example, all fibers with a NA between 0.35 and 0.65 are in this class even if it does not make much sense to couple them to each other. The theoretical coupling loss can amount to as much as 5.4 dB because of the difference in NA.


563

In the meantime Standard 60793-2-40 has been completely revised and now contains 8 different classes of polymer fibers (A4a to A4h, see >IEC04@). Tables 7.2 and 7.3 provide the specific parameters. The first three classes are standard step index profile fibers made of PMMA with unchanged values Class A4d describes PMMA DSI-POF. MSI- and GI-POFs made of PMMA fall into Class A4e whereas the last three classes describe GI-POF made of perfluorinated fibers. Table 7.2: Specification of SI-POF according to the IEC 60793-2-40 Parameter

Unit

Class Class A4a A4b [μm] n. a. n. a. core [μm] cladding 1,000 r 60 750 r 45 [mm] jacket 2.2 r 0.1 2.2 r 0.1 [%] core non circularity d6 d6 loss at 650 nm [dB/km] d 400 d 400 with EMD launch [dB/km] d 300 d 300 bandwidth [MHz100m] t 10 t 10 bending loss [dB/10 bends] d 0.5 d 0.5 num. aperture 0.50r0.15 0.50r0.15

Class Class A4c A4d n. a. n. a. 500 r 30 1,000 r 60 1.5 r 0.1 2.2 r 0.1 d6 d6 d 400 d 400 d 300 d 180 t 10 t 100 d 0.5 d 0.5 0.50r0.15 0.30r0.05

Table 7.3: Specification of GI/MSI-POF according to the IEC 60793-2-40 Parameter

Unit

[μm] [μm] [mm] [%] core non circularity loss at 650 nm [dB/km] loss at 850 nm [dB/km] loss at 1300 nm [dB/km] bandwidth 650 nm [MHz100m] bandwidth 850 nm [MHz100m] bandwidth 1300 nm [MHz100m] bending loss [dB/10 bends] num. aperture core cladding jacket

Class Class Class Class A4e A4f A4g A4h t 500 200 r 10 120 r 10 62.5 r 5 750 r 20 490 r 10 490 r 10 245 r 5 n. a. n. a. n. a. 2.2 r 0.1 d6 d4 d4 d2 n.d. d 180 d 100 d 100 n. a. d 40 d 33 d 33 n. a. d 40 d 33 d 33 n. a. t 200 t 800 t 800 n. a. 1500-4000 1880-5000 1880-5000 n. a. 1500-4000 1880-5000 1880-5000 d 0.5 d 1.25 d 0.6 d 0.25 0.25r0.07 0.19r0.015 0.19r0.015 0.19r0.015

The parameters of this standard are not always comprehensible as to why upper limits for the bandwidth are given for fibers f to g. This is unnecessary, of course. Moreover all manufacturers continue to agree that a PF-POF with a 200 μm core diameter will not be produced because it is much too expensive. Core diameter tolerances of 60 μm are much too big for data-grade quality fibers. More realistic are maximum deviations of 10 μm. Furthermore, specifying a bandwidth from 10 MHz 100 m for A4a is much too pessimistic. Even under negative conditions

564


artificially created there are no values attainable below 30 MHz 100 m. We must assume here that the fiber manufacturers managed to push through parameters they established and not those of the users and thus selected a kind of lowest common denominator. In order to avoid such problems, the parameters of the fibers to be used are specified more narrowly, e.g. with MOST, in most standards for applications. This is actually quite the opposite of the definition of standardization. 7.1.2 Plastic Clad Glass Fibers The definition of the parameters for glass fibers with polymer cladding is stated in the standard IEC 60793-2-30 (>IEC06@). The current values are shown in Table 7.4. Table 7.4: Specification of SI-PCS according to the IEC 60793-2-30 Parameter core cladding jacket

core non circularity loss at 850 nm bandwidth num. aperture

A3a A3b A3c [μm] 200 r 8 200 r 8 200 r 8 [μm] 300 r 30 380 r 30 230 r 10 [μm] 900 r 50 600 r 50 500 r 50 [%] d6 d6 d6 [dB/km] d 10 d 10 d 10 [MHz100m] t 50 t 50 t 50 0.40r0.04 0.40r0.04 0.40r0.04

Unit

A3d 200 r 8 230 r 10 500 r 50 d6 d 10 t 100 0.35r0.02

In regard to the definition and measurement of the bandwidths of PCS fibers not all considerations have definitely been settled. The difference in the specified bandwidth values of classes a to c and d do not correspond to the differences in the numeric apertures. The semi-GI-PCS which have gained increasing attention over the past few years do not even appear in category A3. This will surely be addressed in the next revisions. Normally, the most important test methods are summarized in the different standards, including references to other specific standards. The following table names specific tests for category A3. Table 7.5: Specified tests for A3 fibers Test Condition damp heat

dry heat

temperature cycle

Standard Test Parameters IEC-60793-1-50 A: +85°C, 85% RH, 3000/240 h (long/short time)

B: +75°C, 85% RH, 3000/240 h (long/short time) C: +70°C, RH: 85%, 750 h IEC-60793-1-51 A: +125°C, 3000/240 h (long/short time) B: +85°C, 3000/240 h (long/short time) C: +70°C, 720 h IEC-60793-1-52 A: change between -40°C and +125°C B: change between -40°C and +85°C C: change between -20°C and +70°C


565

7.1.3 Fibers in General The overview of Standard IEC 60793-2 describes the following types of fibers: ¾Part 2-10: Category A1 multimode fibers (GI-GOF) ¾Part 2-20: Category A2 multimode fibers (SI-GOF) ¾Part 2-30: Category A3 multimode fibers (PCS) ¾Part 2-40: Category A4 multimode fibers (POF) ¾Part 2-50: Category B singlemode fibers (SMF) ¾Part 2-60: Category C singlemode fibers for intra connection The IEC differentiates between step index and graded index on the basis of the index coefficients, whereby the following is true: ¾A1: graded index fibers: ¾A2: step or quasi step index fibers: ¾A3: step index fibers: ¾A4: step, multi step, graded index fibers:

1dgKüs04@, >Ziem03a@, >Blo03@ and >Blo04@ are summarized in Table 7.19. The big advantage of using green LEDs lies in the very flat course of the spectral attenuation curve in addition to the generally lower POF attenuation so that even great deviations from the center wavelength are possible without problems. In addition, you have a very much lower temperature-dependence of the output power of GaN LEDs compared with red LEDs.

7.2 Application Standards

577

Table 7.19: Specification for Fast-Ethernet transmitter (proposal) Parameter

Unit

Green LED

Red LED

Ocenter

nm

510 r 20

650 r 10

max. 'O

nm

60

30

'O/'T

nm/K

-

0.08

max. average Popt

dBm

0

0

min. average Popt

dBm

-8

-8

tr (10% - 90%)

ns

3

3

tf (10% - 90%)

ns

3

3

maximum NA

-

0.30

0.30

max. overshot

%

25

25

The latest outline of Standard IEC 24702 “Industry Cabling” contains two different fiber versions and four different link classes with POF. Both fiber versions are compared in Table 7.20. Table 7.20: Fiber types in the standard „Industrial Cabling“ Parameter

Unit

OP1

OP2

index type

-

DSI

PF-GI

fiber class in IEC60793-2-40

-

A4d

A4g

μm

980

120

-

0.30 r 0.05

0.19 r 0.015

dB/km

100 200 -

100 40

MHz100 m

ffs 100 -

800 1,880

core diameter numerical aperture loss

bandwidth

at 520 nm at 650 nm at 850 nm/1300 nm at 520 nm at 650 nm at 850 nm/1300 nm

Class A4a, i.e. the standard POF, also does not come into question for 50 m since its bandwidth in fiber Standard IEC 60793 is only specified with 10 MHz 100 m. In reality, of course, this fiber has in any case enough bandwidth for Fast Ethernet over 50 m and diverse products are sold for it. The standard defines another four different connection classes with maximum distances between 25 m and 200 m. Even greater lengths can then be attained with glass fibers - singlemode or multimode not listed here. The permissible attenuation values for the links have been compiled in Table 7.21.

578


Table 7.21: Link classes for „Industry Cabling“ Link class OF25

OF50

OF100

OF200

with OP1

with OP2

5.5 dB (at 520 nm) 8.0 dB (at 650 nm) 8.0 dB (at 520 nm) 13.0 dB (at 650 nm) 13.0 dB (at 520 nm) 23.0 dB (at 650 nm) -

4.0 dB (at 850 nm) 6.0 dB (at 850 nm) 7.0 dB (at 850 nm) 23.0 dB (at 650 nm) 11.0 dB (at 850 nm)

The PF-GI-POF can, of course, be used for short connections at 650 nm. One disadvantage compared with the 1 mm POF is that you have to use a laser because of the smaller core diameter. 7.2.7 D2B (Domestic Digital Bus) System D2B is actually not a standard but rather represents a specific business solution. The D2B Standard specifies a ring system which connects different devices in vehicles such as navigation computers, car radios, CD changers, telephones, etc. using POF (Fig. 7.8).

central interrface unit phone

car radio

active speakers

CD changer

navigation system Fig. 7.8: Ring structure according to the D2B standard

579


Table 7.22 summarizes the most important data for the D2B standard. A detailed description can be found in [Pet98]. A special plug system is used that permits plugging at 90° or 180°. If necessary, the ring can be separated and equipped with a coupler for hooking up another device. Table 7.22: Properties of the D2B system Parameter

Value

minimum LED power

-15 dBm

maximum POF attenuation

400 dB/km

range

8m

system margin

5 dB

coupling loss

1.3 dB

decoupling loss

0.3 dB

add-on coupler

1.2 dB

receiver sensitivity

-26 dBm

user data rate

5.6 Mbit/s

temperature range

-40°C to +85°C

The D2B has a maximum transmission length of 8 m. If one or two couplers are used in the line, then the permissible line length is reduced to 7.0 m or 3.6 m respectively (D2B02). The components are booted up and the diagnosis is carried out with D2B over separate copper conductors. The polymer fiber used with D2B has a 980 μm thick core and a 2.2 nm thick protective sheath. The minimum bending radius is specified at 25 mm >Her02@. The protective sheath is in two parts. The inner sheath is 1.5 mm and black to prevent the possible coupling-in of light.

D2B:

MOST

2.3 mm

1.51 mm 1.0 mm

2.2 mm

1.5 mm 1.0 mm

fiber: 980 μm PMMA / 10 μm cladding inner jacket: PA 12 black outer jacket: PA 12 orange

fiber: 980 μm PMMA / 10 μm cladding inner jacket: PA 12 black outer jacket: PA 12 elastomer, colored

Fig. 7.9: Comparison of POF for D2B and MOST ([Her02])

580


The sheaths cannot be separated. The POF for MOST (see following section) also has a two-part sheath. However, the inner sheath is solidly attached to the fiber and the outer sheath can easily be cut back. The purpose of this arrangement is that the connector with MOST is not crimped on the fiber, but on the inner sheath or is welded with a laser (fiber set-up in Fig. 7.9).

Fig. 7.10: View on the POFs for D2B and MOST ([Her02])

In addition to the somewhat different deviations the MOST POF is flame-retardant (test according to ISO 6722). Furthermore, it is somewhat more flexible in order to facilitate standardized production by machines. 7.2.8 MOST (Media Oriented System Transport) The MOST Specification ([MOST01]) gives recommendations for multimedia-capable networks in automobiles of the future. Since the founding of the MOST initiative in 1998, 14 international automobile manufacturers together with 50 key component suppliers have been working on the MOST technology. Figure 7.11 shows the specific service points SP1 to SP4. The optical service points are SP2 at the electro-optical converter and SP3 at the opto-electrical converter. Polymer optical fibers are used as the transmission medium. Tables 7.22 and 7.23 summarize the specifications of points SP2 and SP3. -10 dBm E/O electrical interface converter

2.5 dB

-24 dBm 2.5 dB

O/E electrical converter interface

SI-POF B = 22.5 Mbit/s

SP 1

SP 4

D = 9 dB MOST device

SP 2

Fig. 7.11: Connection of two MOST devices with POF

SP 3

MOST device

581

7.2 Application Standards Table 7.23: Specification at the reference point SP2

peak wavelength spectral width (FWHM)

Unit

min.

typ.

max.

nm

630

650

685

nm

optical output power

dBm

output power at „light off“

dBm

30 -10

-1.5 -50

extinction ratio

dB

rise time (20% - 80%)

ns

10 5.97

fall time (80% - 20%)

ns

5.97

pulse width

ns

20.88

24.4

mean pulse width distortion

ns

-0.51

+1.51

positive overshoot (within 2/3 UI*)

%

-20

+25

negative overshoot (within 2/3 UI)

%

-10

+20

* UI: Unit Interval = 22.14 ns Table 7.24: Specification at the reference point SP3 Unit

min.

max.

detectable optical power

dBm

-24

-2

detectable optical power „light off“

-24

dBm

-40

extinction ratio

dB

10

rise time (20% - 80%)

ns

6.86

fall time (80% - 20%)

ns

6.86

pulse width

ns

20.88

24.4

mean pulse width distortion

ns

-0.51

+1.51

The sampling frequency of a CD player at 44.1 kHz forms the basis of the bus clock pulse. The formation of blocks each with 512 bits results in a gross baud rate of 22.6 Mbit/s. Principally, all components are designed for a frame clock-pulse rate of between 30 and 50 kHz. Up to 64 nodes can be connected in a MOST link. A differentiation is made between synchronous data (with permanently assigned channels), asynchronous data (using available channels) and control data with permanent assignment within a time frame. Because of the frame clock-pulse rates the delay times are at a maximum of 25 μs. The time frame for the transmission of the different data is shown in Fig. 7.12. As can be seen, a division between synchronous and asynchronous data is possible and can be varied. Although it was first conceived as a pure ring architecture, the MOST system can assume other topologies, e.g. combined rings, star, etc., by adding system master units.

582


control data

synchronous data

asynchronous data

64 bytes 30 to 50 frames per second / 20 μs to 33 μs Fig. 7.12: MOST frame structure

The data structure of the MOST system is oriented extensively toward the demands of the connectable multimedia terminal devices. Table 7.25 indicates the various possible data formats with their respective bit rates. Table 7.25: Data structure for MOST Data Format synchronous data: minimum12 channels á 16 bit maximum 30 channels á 16 bit audio (2 channels stereo; 16 bit) DVD data (depending on picture quality 4, 8 or 16 channels) TV data (with MPEG coding) still pictures (JPEG coded) asynchronous data (maximal) car computer, navigation system control data (2,700 messages per second)

Bit Rate

8.467 Mbit/s 21.168 Mbit/s 1.411 Mbit/s 2.822 Mbit/s, 5.645 Mbit/s 11.290 Mbit/s 1.411 Mbit/s 2.822 Mbit/s 0.1 Mbit/s 12.7 Mbit/s 0.1..11 Mbit/s 0.7 Mbit/s

7.2.9 IDB 1394 The Standard IEEE 1394 was primarily developed for the networking of devices in entertainment electronics in apartments (see above). Over the last few years the applicability of this standard in vehicle networks has been discussed within this group. In Sections 6.2.2 (the power budget) and 8.1.1.4 (applications) further information is provided. Except for information on the optical parameters of the active components hardly any details have been published. We can expect a continuing development of the MOST standard toward higher bit rates and that transmitters and receivers will be used for both systems. Up till now IDB 1394 has provided for the use of the SMI connector which, however, does not completely meet the requirements of automobile technology in its present design, e.g. in regard to the connection cycles. Since then there has also been talk in the meantime of the specification of a POF with a considerably enlarged NA in order to reduce the bending sensitivity.


583

However, this contradicts the greater demands on bandwidth. A foreseeable solution is the use of multicore fibers for all high-bit rate vehicle networks. This version appears to make a great deal of sense considering the most recent progress in the production of this type of fiber. We wish to summarize a comparison between the different existing bus systems in automobiles from >Wan04@. The coexistence of optical and electrical solutions should continue to exist for quite some time. Finally, the active components must above all be able to compete in price and reliability with electrical systems in order to be able to make use of the diverse advantages of optical data transmission. Table 7.26: Comparison of automotive busses according to [Wan04]

application

control transmission access bit rate medium

LIN low-level communication singlemaster synchronous

CAN soft-realtimesystems multimaster asynchronous

FlexRay hard-realtime-systems (X-By-Wire) multimaster synchronous asynchronous polling CSMA/CA TDMA FTDMA 20 kbit/s 500 kbit/s 10 Mbit/s electrical electrical optical (single wire) (twisted pair) electrical

TTP/C hard-realtime-systems (X-By-Wire) multimaster synchronous asynchronous TDMA

25 Mbit/s optical electrical

MOST multimedia telematics

multimaster synchronous asynchronous TDN CSMA/CA 25/50 Mbit/s optical (electrical)

7.2.10 EN 50173 Standard EN 50173 “Generic Cabling Systems” is one of the most important standards at all. It describes the set-up of structured data and telecommunication networks. Up till now this standard was of primary interest for public buildings and companies. Because of the rapid development of broadband connections there will be structured cabling of residential buildings in the future. Today’s telephone and coaxial cable networks are only oriented toward one specific service and it is not „generic“. The structured cabling in office buildings should make it possible to combine existing and future applications with one infrastructure. The basis of this approach has been the use of balanced copper cables. This cabling has, of course, never really been generic. In commercial networks there are actually only two relevant services: great amounts of data are transmitted over Ethernet connections and then there are telephone connections on a 64 kbit/s basis. This low data rate does not place any demands whatsoever on the cable quality; only two twisted wires are needed. Consequently, the original EN 50173 is essentially a standard shaped by Ethernet applications. Cables in Category 5 have been developed for the transmission of Fast Ethernet. Thanks to the 4B5B coding a bandwidth of 62.5 MHz is

584


used. In order also to be able to use the same infrastructure for 1,000 Mbit/s, the transmission procedure has been completely revised. A multi level code is now used instead of the binary coding. The separate transmission in both directions with a pair of conductors in each direction has now become a bi-directional transmission on all four twisted pairs (Fig. 7.13).

Fast-Ethernet: 100 Mbit/s 4B5B 125 Mbit/s binary: 62.5 MHz

Gigabit-Ethernet 1,000 Mbit/s: 2 bit/symbol; splitting into 4 channels 125 MB/s: 62.5 MHz

Fig. 7.13: Idea for using the same channel for 100 and 1,000 Mbit/s

Not every Category 5 cable, however, can be used for Gbit. Ethernet, especially because of the lack of return loss. That is why the intermediate Class 5e has been introduced. Nowadays cables from Categories 6 and 7 are primarily used which are capable of 1 Gbit/s without any problems. On the other hand, the transition to 10 Gbit/s will not take place “generically” since completely new connectors are necessary and the range of 100 m will presumably not be possible. The IEC is presently endeavoring to expand the standard to other applications. The different areas are: ¾50173-1: general requirements ¾50173-2: office ¾50173-3: industry ¾50173-4: home ¾50173-5: data centers The DKE (Deutsche Kommission für Elektrotechnik/German Committee for Electrical Engineering) was requested to work out a recommendation for the use of polymer fibers in the home. A joint work sub-circle (GUK) 715.3 was created for this purpose. The situation in the home environment is that much more complicated than in the commercial area. In addition to telephones and the Ethernet you have to take analog television as a service into consideration. If you include the satellite intermediate frequency band, then you have to have a cable with a bandwidth above 2 GHz with a considerably higher crosstalk attenuation. Only balanced


585

copper cables in Category 8, which is planned, or good coaxial cables could meet these requirements. Silica glass fibers - only singlemode fibers – offer sufficient capacity, but are unsuitable for private customers. The goal of a generic infrastructure which is supposed to meet home demands in the next couple of decades is extremely ambitious and actually cannot be fulfilled. For polymer fibers the first problem already arises with the inadequate directives of the fiber standard IEC 60796-2-40. The capacity of standard POF can be viewed from three different angles: ¾According to IEC 60796-2-40 Class 4a has a bandwidth of 10 MHz 100 m. Therefore, Fast Ethernet can just be transmitted over 15 m. ¾In reality all commercial 1 mm standard POFs have a bandwidth of about 40 MHz 100 m. Fast Ethernet over 50 m is always possible, with some effort also 100 m and more, is really not a problem. Copper cables are also strongly equalized. A Category 5 cable only has a bandwidth of 3 MHz 100 m. ¾Up to 1000 Mbit/s over 100 m are even possible on standard POF with methods which correspond to the gigabit transmission on copper cables (DMT, Siemens 2006, see Chap. 6). Taking the actual capacity of POF into consideration, the group has recommended three different link classes. A range of 25 m should be guaranteed within apartments. For medium-sized and large residential buildings connections up to 50 m or 100 m respectively are planned. Connection lengths over 100 m are practically never the case in residential buildings. In Germany there are only very few buildings with more than 15 stories. Typically, 4 to 8 apartments are grouped around an elevator shaft. Very large residential buildings consist of segments which are separated by firewalls through which cables may practically never be pulled. Table 7.27 illustrates the proposed link classes with the possible corresponding applications and the fiber classes declared according to IEC 60796-2-40. Table 7.27: Proposal of the GUK 715.3 for link classes in building networks Class

Application

Fiber

OF-25

100 Mbit/s

A4a (at 40 MHz100 m) and all others

OF-50

OF-100

1000 Mbit/s

A4d and higher

CATV

A4e

100 Mbit/s

A4d and higher

1000 Mbit/s

A4e and higher

CATV

A4e

100 Mbit/s

A4d (at 520 nm), A4g and higher

1000 Mbit/s

A4g and higher

CATV

A4g, h

586


Under certain circumstances fibers with a lower category, e.g. with equalization, can be used. Since the standard is still being debated, we will not go into a description of the power budget calculations. It is becoming apparent that only fibers A4d (DSI-POF) and A4g (PF-GI-POF) will be considered in the final version, that link class 25 m will be eliminated and that the use of blue and green sources is not planned. In effect, this standard is practically useless. Even today diverse products for Fast Ethernet over 50 m to 100 m on standard POF are available in the market. Fiber A4d does indeed offer nominally sufficient bandwidth for Fast Ethernet over 100 m. However, the argument against it is that moderately priced fibers in Category A4e (MC and GI) are already available today which can also be used for Gbit/s. The recommendation to use Class A4g (120 μm PF-GI-POF) surely makes sense. Now we will have to wait and see how successful this type of fiber is when introduced into the market. Even today it is apparent that the actual development of building installations is passing by the concept of this standard: ¾Standard POF (A4a) is already established in many applications and is used predominately in home networks in the beginning. Fast Ethernet over 100 m and Gigabit Ethernet up to 25 m are possible without any problems. ¾From the editor’s point of view primarily MC-POF and PMMA GI-POF when the bending problem is solved - are becoming apparent candidates for higher data rates. Both allow the use of existing connectors and connector less installation respectively. ¾The PF-GI-POF mainly makes sense for the transmission of CATV signals. This fiber version is to be recommended in any event even with distances over 100 m. Bit rates up to 10 Gbit/s have already been realized. If PMMA POF is considered the first generation, then PF-GI-POF can dominate the succeeding generations. As far as the transmission of CATV signals is concerned, we will have to wait and see as to whether any respective systems will be developed at all. The technological basis has been available for years. Today, however, a rapid transition toward a general transmission of all television signals over the internet protocol (IP TV) is evident. Together with VoIP (Voice over IP) there is again a return to generic networks because only one network is needed, as a rule Ethernet, for socalled triple play applications. In summing up, we can say that different standards apply when dealing with copper cables and POF as regards standardization. When copper cables move up to higher bit rates they are granted comprehensive changes in the transmission procedures which are not granted to POF. There are a number of different versions (Categories 3, 5, 5e, 6, 7, shielded and unshielded, with 100 : in Europe and 150 : in the USA) whereas only a few versions are allowed for POF. In addition, the inclusion of analog TV signals - with a questionable remaining propagation time - tightens up the requirements dramatically. The goal of a long-term “generic” approach as opposed to a market-oriented one may possibly be too greatly overemphasized.

7.3 Standards for Measurement Techniques

587

7.3 Standards for Measurement Techniques In comparison with multimode or singlemode glass fibers there are only a few standardized methods of measurement for POF. They have been set down in the Japanese Industrial Standard (JIS) and in various IEC standards. Table 7.28 summarizes the standards for measurements of POF. Table 7.28: Standards for measurement methods on POF Measured Value far field, NA attenuation dispersion, bandwidth connector losses operation temperature minimum bend radius tension force (connector) impact strength optical parameters flammability toxic gases climatic cycle cladding diameter cladding non circularity core diameter jacket diameter fiber length core-cladding concentricity core non circularity attenuation bandwidth theoretical numerical aperture numerical aperture change of the optical transmission

Standard IEC 61793-1-C6, JIS C 6863 IEC 61793-1-C1A, VDE 5570 IEC 61793-1-C2A EN 18600-1 IEC 794-1-F1 IEC 794-1-E11-B IEC 794-1-E1-E1 IEC 794-1-E3 IEC 60793-2-40 IEC 332-1 (CEI 20-35) IEC 60754-1/IEC 61034-1 IEC 60794-1-F1 IEC 60793-1-20 IEC 60793-1-20 IEC 60793-1-20 IEC 60793-1-21 IEC 60793-1-22 IEC 60793-1-20 IEC 60793-1-20 IEC 60793-1-40 IEC 60793-1-41 IEC 60793-1-20 IEC 60793-1-43 IEC 60793-1-46

Examples of many of the measurement methods described are demonstrated in Chapter 9. There are still no international standards whatsoever for a number of fibers and parameters, for example, for the bandwidth. In Germany over the past few years a group of POF experts has concerned itself with working out its own standards for characterizing polymer fibers and POF cables which will subsequently be described. Some of these recommendations here have in the meantime been included in the European standards.

588


7.3.1 The VDE / VDI Guideline 5570 After the series production of the MOST standard had begun, it was soon discovered that the measurement results for attenuation in prefabricated cables could deviate considerably. The result of this knowledge was the formation of a working group which was to first work out unified rules for the measurement of attenuation in fibers and cables. Further steps then included working out recommendations for measuring the mechanical reliability and the influence of climate and chemicals. The document has appeared as VDE/VDI Recommendation 5570: “Testing of prepared and unprepared plastic optical fiber (POF)”. It consists of four parts and contains: ¾Part 1: Terms and Definitions ¾Part 2: Test Procedures for Optical Characteristic Values ¾Part 3: Test Procedures for Mechanical and Environmental Characteristic Values ¾Part 4: Power Budget The working out of part 5 for the measurement of transmission characteristics, i.e. bandwidth, pulse broadening, etc., is planned. Here we simply wish to point out the main aspects of this recommendation since many of the procedures are described in Chapters 2 and 9. The recommendation begins with a definition of the fundamental terms used. Surprisingly, their use is by no means the same everywhere. The designations for fiber and vables are shown in Fig. 7.14.

optical core (PMMA) optical cladding (Fluorpolymer) jacket (PA, PE, PVC...)

1.0 mm

optional secondary jacket

2.2 mm Fig. 7.14: Definition of terms in the VDE/VDI 5570

fiber

cable


589

Further combinations of the cables with additional jackets or other conductors result in a mor complex structure, also called cable. The definitions used come for the most part from the fields of automation and vehicle networks in which POF has long since been established. An example of the definition of practical measurement procedures in the recommendation is subsequently described. In practice LED sources are often used for measuring the POF attenuation of a certain wavelength. Much too great an attenuation value - based on the attenuation minimum selected, e.g. at 650 nm - is determined by the large spectral width. The recommendation describes how an exact measurement is possible by the formation of a correction factor. The course of the procedure is represented in Fig. 7.185. First, the spectrum of the LEDs used and the attenuation spectrum of the fiber type to be tested are measured. The tabulated attenuation curve from >Wei98@ can be used for PMMA SI-POF. The next step is determining the theoretical LED spectrum after the course of a defined length of POF. An effective attenuation is determined from the relationship of the integrated spectra. The correction factor results from the difference between the effective attenuation and the attenuation at the desired wavelength. DPOF(O)

PLED(O)

P(l)

DPOF(O)

6=P0

6 = P`

D Deff

Deff

DPOF(ORef)

O step 1: LED spectrum measured

O s te p 2 : spectral POF attenuation measured

O

0m

l

O

s te p 3 : s te p 4 : step 5: LED spectrum Deff calculated KF calculated is multiplied Deff Deff = = D (O ) with the loss POF Ref 10lg(P0/P´)/l curve

Fig. 7.15: Measurement of the attenuation with spectral correction

As can be demonstrated, this correction factor can also be used when the actual spectral attenuation curve deviates from the typical curve accepted in the first step. However, the difference should for the most part be wavelength independent and not too large. In the subsequent Table 7.29 the steps for the formation of the correction factor in formulas are given.

590


Table 7.29: Calculation of the spectral correction factor for loss measurements with LED Parameter normalized spectrum of the LED total LED power

Formula / Calculation PLED (O)

1

f

P0 =

³

Unit 1/nm

PLED (O ) dO = 1

O 0

spectral POF loss coefficient attenuation fiber length LED spectrum after the POF LED power after the POF

D(O)

dB/km

a = D(O) l l

dB

PLED´(O)

1

f

Pc(l) =

³

km 1/nm

PLED (O ) 10( D( O ) / 10l) dO

O 0

effective loss coefficient effective excess loss coefficient in relation to the reference value correction factor

Deff = 10 log (P0/P´)/l

dB/km

Dexcess = Deff - Dref

dB/km

KF = 10 (lDexcess/10)

1

In any event, you must keep in mind that the correction factor varies in a nonlinear way with the length of the fiber. One example should describe the method in which the measurement of the POF attenuation was made at 650 nm with a laser. ¾According to the table the PMMA POF has an attenuation coefficient of 132 dB/km at 650 nm. ¾A red LED with a width of 40 nm, a center wavelength of 650 nm and a Gaussian-shaped spectrum is used. An effective attenuation coefficient of 185 dB/km, i.e. an additional attenuation coefficient of 53 dB/km, is the result for a length of 10 m, corresponding to a correction factor of KF = 1.13. ¾With a reference value of 30 μW the result is a measurement value of Pmeas = 18.5 PW with a 10 m long test fiber (you have to keep in mind that the correction factor is always valid only for a certain length). ¾Consequently, the attenuation coefficient measured is: ¾Dmeas = 10 log (30/18.5)/0.01 km = 210 dB/km. ¾The reference attenuation quantity per unit length with a correction of 53 dB/km is: 157 dB/km. With a reference value of 132 dB/km the difference is only 25 dB/km for 0.01 km = 0.25 dB so that the use of the correction factor is permissible.


591

The editor recommends a test set up as described in Section 9.4.5.4 for the exact measurements of the spectral attenuation under laboratory conditions. Other parts of the recommendation describe the measurement of the vicinity of the emission characteristics of a source for distributing the equilibrium mode distribution of a fiber, the so-called EMDicity. The different methods of measuring the numerical aperture are also introduced which include: ¾Far field method ¾Reflection method ¾Inverse far field method The production and application of reference fibers are described very extensively, followed by definitions of sources of error which are taken into account. Finally, measurement procedures are described on part 3 for mechanical, climatic and chemical environmental influences. The recommendations here essentially come from BAM and are described in part in great detail in Sections 9.6 and 9.7 which has not changed since the first edition. Test Procedures for mechanical parameters: ¾Tensile strength ¾Resistance to transverse compression ¾Impact strength ¾Alternating bending ¾Torsion ¾Alternating roller bending ¾Static bending ¾Wringing fit of the protective layer ¾Wringing fit of the ferrule Test Procedures for environmental parameters: ¾Thermal stability ¾Resistance to high temperatures and humidity ¾Resistance to climatic changes ¾Pistoning ¾Coupling-in of extraneous light ¾Resistance to chemicals

8. Application of Polymer Optical and Glass Fibers

In hardly any other area has the number of applications so rapidly developed as in the area of optical short-range communication. Applications of polymer fibers, glass fiber bundles in the field of lighting technology as well as in automation have been established for many years. Since the end of the 1990s POFs have also been used in various mobile networks. At the time when our first edition was published there had hardly been any information published on MOST applications. This second edition now contains a detailed description. The use of POF and other thick optical fibers in the fields of sensor technology and home networks are just about to be used on a large scale. Even greater perspectives for optical technologies are becoming apparent in the fields of interconnection. This area will also be covers in the following chapter more detailed. Those areas of use which lie outside the field of data communications and beyond the scope of this book will only be treated briefly. For these areas we refer to existing publications ([Wei98], [FOP97]).

8.1 Data Transmission with POF The most important media for transmitting high data rates today are electrical lines, mostly copper, optical fibers and radio. Each one of these channels has its own special characteristics: ¾Electrical lines connect the transmitter and receiver directly. Contacts between lines can be made easily. The range and data rates are primarily limited by the skin effect, i.e. the attenuation increases with f1/2. ¾Optical systems work with light as carrier frequency. The bandwidth is mostly limited by the effects of dispersion, whereby the transmission behavior is 2 almost Gaussian-shaped (e-(f/f0) , Fig. 8.1). The distance is limited by the attenuation of the optical path. In most cases connections inside the link require great precision. ¾The special feature of radio is that all users within a cell (the range of the transmitter) have to share the capacity. Because of the multi-path propagation and the resulting interferences and external sources of disturbance extremely complicated channel behavior results which has to be compensated for through adaptive procedures.

594

8.1 Data Transmission with POF

attenuation [dB/km] 1000

SI

GI

SM

100

copper cable 10

1 optical fiber 0.1 105

106

107

108

109

1010 1011 1012 modulation frequency [Hz]

Fig. 8.1: Comparison of the frequency responses for copper cable and different optical fibers (typical values)

Compared with all other media, optical systems offer by far the greatest capacity - which can be multiplied at will through parallel lines - the least disturbances and the greatest reach. The low need for space and the lower power input ensue with raising bit rates. In such a case optical lines have to be installed and opto-electrical converters have to be used at the ends of the transmission path. Optical data transmission will only then be used when conventional procedures reach their limits. Up till now this has been the case for glass fiber systems especially in telecommunications networks and in large corporation networks. The low cost of polymer fiber technology now opens up entirely new fields for optics, especially in short-range transmission up to some 100 m. Sometimes conventional copper wire solutions are substituted, but for the most part applications are being employed, the realization of which so far had not made much sense. Data communication with POF can be divided into the essential areas indicated in Table 8.1. The use of POF in buildings and apartments can be quite problematical. In contrast to the automobile industry complete systems are not installed just once; the networks are constantly being expanded and improved by installing faster components. The fiber infrastructure thus has to be dimensioned not only for current use, but also have the potential for use in new kinds of systems in the future demands.


595

Table 8.1: Applications and requirements for data transmission with POF Application Typical Parameters mobile networks ¾lengths between 10 m ¾ cars (car) and 200 m (ships, ¾ trains/ships airplanes) ¾ airplanes ¾data rates up to »1 Gbit/s LAN ¾lengths 25 - 100 m ¾ office ¾data rates 100 Mbit/s to ¾ home 1 Gbit/s ¾ condominium Interconnection ¾many parallel channels ¾ on board ¾data rates to 10 Gbit/s ¾ intra board ¾centimeters to meters

Specific Requirements ¾complete systems ¾critical environmental conditions ¾extreme high reliability and long live time ¾simple installation ¾different data formats ¾mix of different components ¾very small ¾low power operation ¾automatic equipment

In addition, the building networks can combine the components of many different manufacturers. In an automobile, for example, one manufacturer supplies the entire cable harness. Connecting extraneous components is avoided when possible. The following sections will provide the reader with an overview of data communication with POF in the various areas of application. Please refer to the chapter on standards for the definition of technical details. 8.1.1 POF in the Automotive Field In Europe, the use of polymer optical fibers for the entertainment networks in DaimlerChrysler vehicles since 1998 represents the first comprehensive application of POF in data communications. The following arguments speak for the use of POF in vehicles (e.g. [Zam00a]): ¾low cable weight ¾small cross-section ¾insensitivity to electromagnetic interferences The different standards for car networks with POF were described in greater detail in Chapter 7. The most important representative areas are: ¾CAN (Controller Area Network) ¾D2B (Digital Domestic Bus) ¾MOST (Media Oriented System Transport) ¾IEEE 1394 (presently not yet specified for the automotive field, but conceivable as a future system) ¾Byteflight (passive star system for vehicular control, [Pan00]) In vehicles, airplanes and rail transportation more and more digital communications connections are being utilized. As a result, increased demands on the architecture of the data connections as well as the transmission media are being made.

596


In the area of driver information and entertainment systems, less relevant in regard to safety requirements, serial bus systems are being increasingly used. The individual devices are connected in series by means of high-rate connections. The advantage here is the saving of cables. The disadvantage is the breakdown of an entire series of devices when a transceiver subassembly is defective. Figure 8.2 shows the number of cables in a mid-sized car according to [Zam00a]. Some years ago, the cables for power supply had the bigger portion, whereas today the strongly increasing number of data connections dominates.

1,000

number of cables total power supply data connections

500

0 1980

year 1985

1990

1995

2000

2005

Fig. 8.2: Number of cables in cars

Since 1997, optical components for use in automobiles are available from Harman/Becker Automotive Systems ([Schö01]). Since 1998, such components are standard features e.g. in the DaimlerChrysler Vaneo as of 2001, Fig. 8.3.

Fig. 8.3: Vaneo (DaimlerChrysler 2001)


597

Figures 8.4 and 8.5 from [Schö01] show the development of different multimedia terminal devices in the automotive field. At the beginning of the 1990’s CD changers (CDC) were first installed as a complementary unit to the car radio. Later on, digital amplifiers (Amp) were added. Combinations of car radios and mobile telephones (Tel.), in part with separate voice data systems (Voice IO), completed the features offered. In the meantime, vehicles are equipped with additional devices such as navigation systems (Navi), traffic guidance systems (telematics), mobile internet access and DVD players.

in c r eas ing c om plex it y of v ehic le equipm ent 2001 MMI CDC phone

radio amp radio

1990 CDC

amp

CDC

TV DVD

voice IO telematics internet ??? navi

1998

1994

amp

phone

voice IO

CDC

radio

Fig. 8.4: Development of digital devices in automobiles

Figure 8.5 (also from [Schö01]) illustrates the development of optical bus systems. The devices listed above are optically connected to diverse input systems such as monitors located in different parts of the vehicle. Up until recently the electronic media in the vehicle have been primarily allotted to the driver and thus essentially served the purpose of vehicle control support, whereas now the focus is more and more on the entertainment of the passengers. The first commercially available products are back seat monitors on which television programs can be received or DVD can be played.

598


MOST - the development

DAB

Display

TV Telematics

NAV Voice RADIO

DSP/AMP

GSM

CDC

DSP/AMP

RADIO

CDC RADIO

NAV

Voice GSM DSP/AMP

CDC RADIO

Telematics

GSM

NAV

Voice

Voice

GSM

DSP

DSP/AMP

CDW RADIO

GSM DSP/AMP

CDC

CDC

RADIO

DVD Display

TV Telematics

NAV

Voice

Computer

DVD

Display

TV Telematics

NAV

Voice GSM/UMTS

DSP/AMP

CDC

RADIO

RADIO D2B Optical: 4.9 Mbps 1997

MOST-Net: 22.5 Mbps 2000

1998

50/150 Mbps MOST future

Fig. 8.5: Development of the bus cross-linking of digital devices in automobiles

8.1.1.1 D2B The D2B system was developed by DaimlerChrysler in 1998. The primary goal was the transmission of audio signals between the different components in entertainment electronics. The audio systems were considered to be the most important application. Figure 8.6 from >D2B02@ shows the configuration in the M Class.

Radio or MCS D2B master

Phone

Sound System

D2B Component 5

D2B Component 1

Voice Control D2B Component 4

Diagnostic Wake-up

Tele Aid D2B Component 3 Fig. 8.6: D2B configuration

CD Changer D2B Component 2


599

According to >Sco04@ over 6 million nodes were installed in Mercedes and Jaguar vehicles in 2004. D2B is used in Jaguar’s X-type, S-type and XJ-Series. The D2B in vehicles connects the CD-changers, car telephone, back seat systems with screens, DVD-player, selector and microphone, voice recognition, sound system, and the navigation computer. The data rate of the bus amounts to 5.6 Mbit/s and the data is transmitted by means of a LED at 650 nm and with 1 mm / 2.2 mm SI-POF cables (see Chap. 7). The maximum length is 8 m and the D2B connector is a special development. The connector was supposed to enable good optical contacts relatively independent of the surface quality of the POF. A cap was developed containing index gel in which the cut-off POF is placed. The actual contact is the smooth front surface of the cap (Fig. 8.7).

coupling

POF

POF

PMMA insert

PMMA insert

index gel Fig. 8.7: Connector for D2B

The spacing between the fiber end faces is less than 1 mm so that the losses per connection remain below 2 dB. 8.1.1.2 MOST The use of D2B is limited to only one manufacturer which is why a drop in price could not be achieved despite large production quantities. This may have been one of the main reasons for the development of MOST. German automobile manufacturers in particular took the lead in this consortium. The MOST Corporation was founded in 1998 by BMW, DaimlerChrysler, Becker Radio and OASIS Silicon Systems. In 2001, the 7-series BMW was the first series production model worldwide equipped with this data bus. The first version of the MOST-bus can transport up to 25 Mbit/s. According to >Thi03a@ the prices for a MOST-link dropped from about € 10 to € 5 between 2002 and 2003 (the optical components were two-thirds of the costs). An overview of the development of MOST technology can be found in >Muy05a@, >Muy05b@ and >Thi03b@. The figures mentioned in the different references on the use of MOST technology are:

600


¾6 million MOST nodes were installed in September 2003 ¾70 different terminal units with a MOST interface were on the market in 2003 ¾More than 10 million MOST nodes per year as of 2004 ¾More than 20 million installed nodes in 2005. The number of vehicle models with MOST: ¾1. vehicle model in 2001 ¾10 models by the end of 2003 ¾15 models in September 2004 ¾36 models by the end of 2005 (see Fig. 8.8)

Fig. 8.8: Car series with MOST bus (Sept. 2005, [Muy05b])

In May 2005, the MOST Consortium comprised the following members (>Muy05b@): official partners Associated partners: Carmakers Associated partners: Suppliers

Audi, BMW, DaimlerChrysler, Harman/Becker, Oasis SiliconSystems Aston Martin, Ford, Honda, Hyundai/Kia, Jaguar, Land Rover, Nissan, Porsche, PSA, Renault, Toyota, Volvo, VW Advanced Optical Components, Agilent Technologies, Alpine, Analog Devices, ASK Industries, Audiovox Electronics, AWTCE, Bosch, Bose, C&CE, c&s group, Citizen Electronics, Clarion, Delco, Dension Audio, DENSO, FCI, Firecomms, Fujitsu TEN, Furukawa, GADV, Hamamatsu, Hirschmann, Hosiden, Hyundai Autonet, HYUNDAI MOBIS, IAV, IMC, Infineon, Iriso, Johnson Controls, K2L, Kenwood, Korea, Electric Terminal, Kostal, Lear, LINEAS Automotive, Matsushita Communication, Matsushita Electric, Melexis, Mitsubishi Electric, Mitsubishi Rayon, Mitsumi Newtec, Motorola, Nokia, Ontorix GmbH, OPTITAS, Philips, Pioneer, Renesas Technology, RUETZ Technologies, Sanyo, SEWS-CE, SHARP, Siemens VDO, SMSC, Softing, STMicroelectronics, TYCO AMP, Vector, Visteon, Yazaki


601

One great advantage of the MOST technology is the use of standardized transceivers, fibers and connectors. The transmitters and receivers for MOST are shown in Fig. 8.9.

Fig. 8.9: MOST transmitters/receivers by Infineon and Hamamatsu ([Fre04b], [Thi03b])

The development of different connector versions is particularly important. Hybrid connectors are used with MOST in which POF and copper cables can be combined. In addition to device connectors, in-line couplings are also customary. Figure 8.10 shows some examples.

Fig. 8.10: MOST connectors and in-line couplings (AMP, [AMP00])

602


Fig. 8.11: Versions for MOST connectors ([Thi03b])

The necessary transmission data rates increase parallel to the number of devices connected. Even now the introduction of 50 Mbit/s and later 150 Mbit/s is being prepared in the MOST Consortium. At least in the long-term data rates of 400 Mbit/s and more can be expected. Polymer fibers themselves offer sufficient bandwidth even for these speeds. The development of transmitting diodes makes such systems appear realizable. The design of correspondingly more sensitive and faster detectors is still somewhat problematical. Even just a few years ago it was thought that higher data rates could only be realized with smaller receivers since the photodiode capacity was otherwise the limiting factor. In the meantime, this hypothesis has since been refuted. Even with 1 mm thick fibers data rates far above 1 Gbit/s can be realized. The following current versions have already been realized and included in the MOST standard: ¾LED/POF solution for 50 Mbit/s ¾RC-LED/POF products for 150 Mbit/s have been certified ¾Electrical transmission on twisted pair copper cables with 50 Mbit/s (Fig. 8.12) ¾Transmission of 150 Mbit/s with PCS and 850 nm VCSEL

Fig. 8.12: Electrical transmission in the MOST system

One essential advantage of the current MOST version on twisted-pair insulated conductors is being able to insert 8 plug-in connections per link (>SMCS06@). This also enables the use of MOST for those vehicle manufacturers which products are comprised of numerous individual modules.


603

The use of Gigastar links was proposed in >Kra02b@ as a solution for high data rates. Up to 1,300 Mbit/s can be transmitted over unshielded twin conductor cables as a differential CML signal or one-pair STP cables (3 mm). The system allows a maximum link length of 30 m. 8.1.1.3 Byteflight The Byteflight system has only been used so far by BMW. It connects airbag systems with other control components in the Intelligent Safety Integration System (>Gri00@). The connectors and fibers used correspond to the MOST standard. However, the data is transmitted bidirectionally on one fiber. The topology is an active star. The concept of a passive star did not work out because of the large insertion loss of the central coupler. A BMW of the 6-Series displayed at the POF’2004 in Nuremberg equipped with MOST and Byteflight can be seen in Fig. 8.13. According to >Fre04c@ a BMW contains 12 sensors for speed, acceleration, and pressure for the airbag system.

Fig. 8.13: 6-series BMW with MOST and Byteflight (POF’2004 Nuremberg)

The components were developed in cooperation with Siemens, Motorola and Elmos. The data rate of Byteflight amounts to 10 Mbit/s. One of the components in the bus is configured as a synchronization master and sets the clock speed for all other users (every 250 μs) which can send their data between the sync pulses. There are two different priorities, whereby the transmission with a maximum waiting period is guaranteed for high-prioritized data and the remaining capacity is utilized for the less time-critical messages (Fig. 8.14).

604


1 2 3 ... 10

35

high priority messages

1 2 3 ... 10

38

75

time for low priority messages e.g. comfort functions, diagnostics

Fig. 8.14: Data structure for Byteflight

The bidirectional rata is transmitted on POF at half-duplex. The transceivers have been developed by Infineon (Fig. 8.15). The same components are used for the components and the active star.

Fig. 8.15: Byteflight transceiver (Infineon) and IC (Elmos, [Gri00])

The specified data of the transceivers according to >Schö00b@ and >BFT03@ are: ¾optical output power at 30 mA: ¾rise time and fall time: ¾peak wavelength at 25º C: ¾peak wavelength from -40º C to +85º C: ¾receiver sensitivity: ¾maximum receiving power: ¾power consumption in standby mode: ¾operating temperature:

-5.2 dB < 35 ns 650 ± 10 nm 650 ± 20 nm < -23 dBm -1.0 dBm (800 μW) < 10 μA -40ºC to +85ºC

At present there are still several directions the on-going development of the Byteflight system could take. It seems to be certain that the concept of the active star is being primarily expanded for critical safety applications. The most probable successor to Byteflight is Flexray with a system bit rate of at least 100 Mbit/s. So far, Flexray has only been planned as an electrical version although technically both POF as well as PCS are usable. 8.1.1.4 IDB 1394 The possibility of using Standard IEEE 1394 in automobiles has been discussed for years; in home applications it is already a widespread standard. The ulterior


605

motive is primarily the desire to transmit uncompressed video data and the possibility to connect external devices to the vehicle system via a Customer Convenience Port (CCP). In order to meet the greater requirements in vehicles, the standard version IDB 1394 has been developed. The carmakers Renault (Espace model) and Nissan (Fig. 8.16) have demonstrated prototypes with IDB 1394 in the past few years. Renault uses LED-based 200 Mbit/s connections which can transmit 3 simultaneous video signals for a DVD player, digital television and a rearview camera. In the Nissan vehicle seven different cameras can be retrieved over the 400 Mbit/s network. The driver can simultaneously view up to four pictures on one screen.

Fig. 8.16: Nissan demonstration car with IDB 1394 ([New04])

A description of the IDB 1394 draft is given in >Tee01@ and >Lit03@. Symmetrical twisted copper cables as well as POF are planned as media for longer links. The maximum transmission lengths should be 18 m - 10 m with two plug-in connections. One great advantage of the 1394 specification lies in the free choice of topology. Tree, mesh, star or ring network topologies are possible. A connector based on the well-known SMI connector is planned for the CCP. The data structure of 1394 has been optimized for multimedia applications. Real time applications such as video transmission as well as data connections with variable bit rates have been realized. Components for POF-based IDB 1394 are already available. Firecomms introduced new RC-LEDs in >Lam05@ which can be used for 200 Mbit/s under automobile conditions. The operating temperature ranges from -40ºC to +95ºC with a POF-coupled power of more than -5 dBm. 8.1.1.5 MOST with PCS One of the decisive disadvantages of the use of POF in mobile networks is the limited temperature range. Currently, only up to +85ºC are allowed for MOST POF. Newer fiber versions permit the use up to +105ºC. Actually, there are indeed even hotter areas as is shown in Fig. 8.17.

606


Fig. 8.17: Temperatures in a car according to [GMM02]

An increase in the temperature range to +125ºC would at least make possible the use of optical networks in the engine compartment, in the entire dashboard and under the roof. Temperature-resistant polymers could definitely be used up to +170ºC. At present, however, there is no fiber development taking place in this area. This is the reason for DaimlerChrysler’s use of polymer-clad glass fibers (PCS) as an alternative. The core diameter of these fibers amounts to 200 μm with a NA of 0.37. The advantages of PCS over POF have been mentioned in several papers: ¾use in higher temperature ranges up to +125ºC ¾better power budget through use of VCSEL, enables among other things the setting-up of passive star networks ¾higher bandwidths - in the meantime no longer valid A system which has been set up is described in >Zeeb02@ and >Zeeb03@. The center of the passive star network is a recently developed 13-port coupler with 11.1 dB up to 15 dB insertion loss. Fibers up to 15 m in length can be connected to every port. The data rate in the network is 622 Mbit/s, whereby up to 270 Mbit/s could be transmitted per channel. After 2½ years of driving experiments with 26 equipped S-Class vehicles there was no decline in the system parameters. The PCS cables with an outer diameter of 1.5 mm allow a bending radius of 5 mm and do not exhibit an increase in attenuation either after 100,000 bending cycles or after thermal loads between -55ºC and +95ºC. A service life of 10,000 hours at +125ºC at prices of 50 cents are claimed for the VCSEL used. The costs for PCS suitable for MOST are given in >Fre04c@ at around 50 cents/m which is considerably more expensive than POF. The costs for connectors and especially for the central coupler have not yet been published.


607

A comparison between PCS and POF in regard to the possible power budget is given in Table 8.2 in >Zeeb02@. Table 8.2: Comparison of the power budgets for POF and PCS Parameter

Unit

POF/LED

PCS/VCSEL

wavelength

[nm]

650

850

minimum emitted power

[dBm]

-9.6

-1.0*

receiver sensitivity

[dBm]

-25.0

-27.0

[dB]

15.4

26.0

[dB/km]

300

< 10

dynamic range fiber attenuation connector loss

[dB]

2.0

< 2.0

coupling loss at the transceiver

[dB]

2.5

2.5

core diameter of the fiber

[μm]

980

200

bending radius

[mm]

25

10

* with active power control or with passive compensation

At first, the PCS seems to come off much better. The advantages of the system are primarily based on three characteristics: ¾The fiber attenuation of the PCS with 10 dB/km is practically negligible compared with that of POF (300 dB/km). With a maximum connection length of 10 m, however, this only amounts to a few dB. ¾The guaranteed output power of the VCSEL is much higher than that of MOST LED. The reasons for this are a more efficient coupling-in and especially the output power control. These measures were dropped with the MOST LED for reasons of cost. An adjusted LED with optimized coupling, e.g. with micro-mirrors, could also guarantee a very much higher fibercoupled power. ¾Very much better values can be achieved for the attenuation of PCS connectors than are specified for the MOST POF connector. However, much lower tolerances (about 10 μm) have to be met and the surface preparation of PCS requires much more effort, e.g. cutting with a CO2 laser. The attenuation for POF connectors could also be reduced considerably if the tolerances of the connectors and fibers were reduced to the level of PCS. In any case, it is advantageous that the 850 nm VCSEL can be modulated with high data rates without any problems. It certainly remains questionable whether the increased demands on adjustment tolerances and transmitter stabilization can be realized with the comparably lower POF manufacturing costs. A possibility for setting up the required central coupler in a passive star is shown in >Bäu00@. The different fibers are coupled to a mixing cylinder with a diameter of 1,000 μm (Fig. 8.18). Insertion losses between 17.10 dB and 18.53 dB were established in three measurements (Fig. 8.19).

608


200 μm PCS fibers

mixing cylinder

reflective surface

configuration with 16 PCS

Fig. 8.18: Construction of reflective star couplers ([Bäu00])

insertion loss [dB] 18.6 18.4 18.2 18.0 17.8 17.6 17.4 17.2 17.0

1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 port number

Fig. 8.19: Insertion loss of three 16-port star couplers according to [Bäu00]

For bi-directional data transmission >Bäu00@ proposes the use of VCSEL modules in which light is combined between a photodiode and the transmitter via a micro-mirror. Up to 250 Mbit/s are possible in half-duplex operation (transmission power -2.5 dB and the receiver sensitivity -20 dB). The PCS solution is specified in the MOST standard as Advanced Optical Physical Layer (aoPHY, >Pof06@). A passive star is no longer planned; point-to-point connections will now be set up. The specified parameters of the links are: ¾fiber: ¾transmitter: ¾power budget: ¾minimum bending radius: ¾data rate: ¾connector:

200 μm/230 μm SI-PCS 850 nm VCSEL 20 dB 9 mm >150 Mbit/s MOST compatible


609

Sample components for fibers, connectors and active elements are available from OFS, Polymicro, Infineon, Yazaki, Advanced Optical Components, Delphi, Leoni and Tyco. There is varying information on the set-up of the PCS lines but there is uniformity as regards use of 200 μm/230 μm with a NA of 0.37. The outer jackets consists of PA12 and has a diameter of 1.5 mm or 2.3 mm respectively. An example of commercially available VCSEL is given in >Pof02@. Here a 850 nm VCSEL Honeywell 4085-321 was tested. Between -55ºC and +125ºC the output power without adjustment would fluctuate between 10 μW and 150 μW (Fig. 8.20).

Fig. 8.20: Current-power-characteristics of a 850 nm VCSEL

A relatively constant output power can be adjusted (0.90 ± 0.25 mW) with the aid of a temperature-dependent resistor network in the VCSEL control, i.e. a combination of NTC and PTC. An error-free transmission of data with 500 Mbit/s could then be realized. 8.1.1.6 Outlook for the Automobile Networks The continuing development of optical vehicle networks seemed to be predetermined with the introduction of MOST in more and more vehicles, the introduction of an additional bus system with Byteflight and the successful demonstration of the technical feasibility of ever faster solutions (MOST with 150 Mbit/s, IDB 1394 with 400 Mbit/s). A timeline for possible development is shown in Fig. 8.21 (from the perspective of 2001). Sometime around 2003 things stagnated when DaimlerChrysler first announced it would use 200 μm PCS in combination with 850 nm VCSEL in the future instead of POF. In 2004, DaimlerChrysler then announced it would not be using any optical fibers at all in the future (>Wol04@). This statement was in fact quickly retracted, but since then a great deal of uncertainty has prevailed among the many suppliers and other carmakers.

610


1,000

bit rate [Mbit/s]

IEEE 1394 ?

100

MOST2 MOST

10 D2B 1 CAN 0.1 1996

year 1998

2000

2002

2004

2006

2008

Fig. 8.21: Potential bit rate development in automobiles (prognosis from 2001)

Today it seems to be clear that there is no pressing reason for replacing POF. In the meantime, there are RC-LEDs among the active components which also attain 150 Mbit/s and clearly improve the power budget and temperature range. SI-POFs have shown that without qualification they allow up to 2.5 Gbit/s over 10 m. A PMMA POF with a temperature range up to +105ºC is available on the market which has only a slightly higher attenuation compared to standard POF. On the other hand many experiments have demonstrated copper-based solutions for high data rates. Both shielded twisted cables as well as coaxial cables have been tested. An example is described in [Beer05]. Here the data is transmitted at a maximum of 800 Mbit/s with symmetrical shielded cables. The maximum operating temperature is +125ºC and the minimum bending radius of the cable is 10 mm. FCI has developed a corresponding plug-in connector.

Fig. 8.22: Proposal for a MOST copper connector (for up to 800 Mbit/s, [Beer05])

At least under laboratory conditions the newer copper solutions are also conform to the limiting values for electromagnetic compatibility (one of the main arguments for using optics was the protection from disturbances). A very decisive step for the continued expansion of POF in vehicle networks could be the development of very fast (1 Gbit/s) modulable LEDs which have already been achieved in the laboratory. Only when the link costs in optics come close to those of copper technology, will they be successful.


611

8.1.1.7 Corrugated Micro Tube POF Cable in the Car A very interesting use for POF cables with a corrugated micro tube (CMT, see also Chapter 3) could result with the introduction of a new operating voltage level in vehicles, possibly 42 V. Figure 8.23 shows the connection of a digital device using POF with corrugated micro tube which serves the double purpose of mechanical protection and power supply (the second terminal is as usual the frame of the vehicle).

digital device

data connection with CMT-POF 42 V power supply over the metal sheath Fig. 8.23: Connection of devices with power supply via the protective cladding

Space requirements and the weight of the cable harnesses can be limited in spite of the increasing number of data connections. Even in the MOST specifications different hybrid plug-and-socket connectors are planned so that the power supply and the data transmission can be installed at the same time. 8.1.1.8 Optical Camera Links for Trucks Nexans Autoelectric in cooperation with DaimlerChrysler has developed a new kind of truck camera parking system. Several different cameras are mounted on a trailer which cover the entire surrounding area without any gaps. The analysis electronics are located in the tractor-trailer, e.g. in an Actros, Fig. 8.24. The images taken are equalized so that the driver gets a complete picture and can safely park in narrow parking spaces or heavily trafficked loading areas. A new solution is being pursued here by combining the different cameras with the central electronics. Conventional cables reach their limits in regard to length up to 30 m - and the required bandwidth. Wireless transmission is difficult to realize because of the great amounts of data and the danger of superimposition in areas with many trucks. Optical fibers permit error-free transmission with high capacities.

612


Fig. 8.24: Actros truck (DaimlerChrysler)

Solutions for the required robust connectors have already been developed for trains. The 200 μm PCS as well as 1 mm POF (SI-POF with MOST specifications and multicore fibers) are currently being tested. The camera equipment - digital or analog - with optical fibers is shown in the following sketch. camera 1

camera 3

curled hybrid cable fiber optical connection

control unit

camera 2

connector coupling

camera 4 trailer

tractor trailer Fig. 8.25: Trucker trailer with camera parking system

A flexible connection between the tractor and the trailer naturally constitutes a great challenge. So far relatively thick coiled cables have been used which are about 7 m long (cable length), contain around a dozen electrical conductors and have an outer diameter of 15.5 mm. The spiral diameter amounts to 80 mm (Fig. 8.26).


613

Fig. 8.26: Curled cable and cross area (Nexans Autoelectric)

Tests are being conducted in cooperation with the POF-AC Nürnberg as to which optical fibers can meet the demands when making coiled cables with shorttime high temperatures and operating with tight, continuously changing bends. The connectors are produced by Ratioplast/Lübbecke. Figure 8.27 shows the maximum connection lengths which may be used.

2m

5m

steering box fiber

7m

20 m

1m

curl / hybrid cable coupling

Fig. 8.27: Maximum connection length

All in all a connection length of up to 35 m may result. There is enough bandwidth available (about 8 MHz) with standard MOST POF for an analog camera signal in PAL quality. Even several channels can simultaneously be transmitted by frequency multiplexing. The bandwidth is just sufficient enough for a complete digital signal (uncompressed). It is then advantageous to use a MC-POF or SemiGI-PCS. One of the first experiments showed that the attenuation of the MC-POF only increases by about 0.15 dB/m by installing it in the coiled cable - a total of about 1 dB for the coiled conductor. The attenuation of the POF in the coiled cable does not measurably change when stretching the coiled cable. With a POF attenuation of 0.4 dB/m including the influences through aging and temperature fluctuations and a loss of 2 dB per plug-in connection the maximum link loss amounts to: Dlink

llink DPOF N D connector D coil 35 m 0.4 dB m 4 2 dB 1 dB

23 dB

614


Corrugated sheath cables are used for the fixed cable connections in the tractor and the trailer (Fig. 8.28) in order to assure optimum protection of the light waveguide. Ratioplast’s plug-in connectors guarantee a low insertion loss. Light is collimated with the aid of a lens so that a secure connection is guaranteed contact-less and even at relatively great distances. The necessary angular exactness is ensured by the guide of the hybrid connector.

Fig. 8.28: Lens connector for camera link with corrugated micro tube cable

A complete box for the multi-pin hybrid cable with protective lid and an integrated optical fiber is shown in Fig. 8.29. The optical fiber in the corrugated sheath tube can be seen in the cable bundle in the side view.

Fig. 8.29: Hybrid plug and connector with CMT

8.1.2 Data Networks in Apartments and Buildings The use of POF in vehicle networks represented the first large-scale application and has pushed the development of polymer fibers far ahead. An even greater potential consists of apartments and building networks which requires differentiating between different fields of application. In office buildings the connection of


615

the workplace computers, access to the internet and telecommunication networks and data transfer to a storage system are important. In the private realm of apartments the dominant services are now as before video applications such as TV and video on demand. Consequently, the demands on the network architecture differ. For a long time now there have been standards for service independent, structured networks. In residential buildings the networks are as always service-related and not set up to be expanded (for cost reasons only the absolute minimum is almost always installed). The author lives for example in a new building built in 2001 in which after only six years the coaxial cable network installed as a tree network has to be replaced. An enormous demand for the refitting of apartments and buildings will come about with the rapid expansion of broadband networks in Germany. 8.1.2.1 Use of POF in LAN Applications At present, data cables based on symmetrical copper cables are dominant in LAN applications, but first of all glass fibers predominate in networks. Whereas just a few years ago 10 Mbit/s Ethernet (10BaseT) had the main share of interfaces in star or tree structures, today pure star networks are predominantly set up on the basis of 100 Mbit/s connections. The basis of modern LAN topologies are the standards for structured cabling, e.g. EN 11801. With structured cabling the LAN is divided into different segments for which there are corresponding recommendations. Within the buildings the vertical cabling, i.e. between cellar and upper floors, and the horizontal cabling on the individual floors are separated. For the different bit rates varied categories are established for the quality of the copper cables. The length of the copper cables is always 100 m (90 m for a permanently installed cable plus 2 lengths of 5 m each for the patch cables in the distribution room and the office (see Fig. 8.30). An illustration of the individual elements: 1: 2: 3: 4: 5: 6: 7: 8: 9:

Individual building server with connection to the telecom networks Vertical cabling, e.g. 1000BaseSX Floor switch Patch cable (maximum of 5 m) Distribution room for the floor Horizontal cabling Plug-and-socket connection in the office Connecting cable (maximum of 5 m) Terminal device, e.g. the computer

The following arguments could be used for the use of polymer optical fibers in LAN applications: ¾less space required for the cables ¾lower susceptibility for disturbances ¾galvanic separation of the components

616


Data networks in office buildings as a rule are planned and set up very carefully. The use of shielded cables rather than unshielded cables dominates in Germany - in contrast to the U.S.A. Paying careful attention to a unified ground potential throughout the entire building makes an optimal use of the advantages of shielded cables possible. Consequently, electromagnetic disturbances do not play a major role in data networks, at least when properly installed.

5

8 9

6 4

7

3 2 1

Fig. 8.30: Components in structured LAN cabling

The data cables in office buildings are usually laid on grids below the respective floor ceilings. The high space requirements for the data cables does not play a significant role. Connecting electronic devices to the electric circuit and through data networks always produces loops which can act as antennas or even create undesired current paths. In commercial use these problems should always be taken into consideration. Above all, the problem of induction, e.g. caused by lightning striking, has to be solved by means of appropriate protective grounding. In such a case POF would be an interesting alternative, which could surely be used in special applications. Practicable and proven solutions do exist for copper cables, too. 8.1.2.2 Use of POF in Private Networks Today’s apartments are mostly equipped with three different cable-based networks: the telephone network, the connection to the broadband coaxial cable network or an antenna system and the 230 V electrical power supply. Each of these networks is adapted for its own specific, albeit very different purpose. Figure 8.31 shows a typical network structure in an apartment.


617

phone

TV

electrical power

Fig. 8.31: Typical apartment cabling (3-room apartment)

As can be seen, only the electrical power supply effectively connects all rooms. The telephone and broadband networks do in fact provide a connection to the access networks, but not the possibility of networking different terminal devices within an apartment as illustrated in Fig. 8.32. house surveillance

office room PC with internet phone, fax

monitored washing machine

bed room with TV and radio

telephone outlet

TV outlet TV with DVD-recorder intelligent refrigerator

guest room with TV

children´s room with TV and computer

Fig. 8.32: Examples of cross-linked devices in a household

The list of possible devices requiring networking could be expanded at will. Surveillance and control systems for heat, windows and doors have increasingly gained in importance. The author personally experienced apartments still being planned and built in 2001 without any kind of system for networking. The tenant is thus confronted with the problem of establishing data connections between devices with the lowest possible expenditure of time and money. Two possibilities for completely overcoming such a situation without installing cables is to use Power-Line technology or to set up a radio system. Both options are technically

618


advanced and thoroughly affordable. However, the possible bit rates and the attainable quality are subject to definite limitations. Cable-based systems are preferable when transmitting high-quality moving pictures in real time or with a broadband connection of computers, for example, when working at home. Different copper cables as well as optic fibers can be considered. Table 8.3 summarizes some possible technologies for use in private surroundings. Table 8.3: Technologies for home networks Technology Capacity radio technologies UMTS 2 Mbit/s over 70 m 300 kbit/s » 100 m 14 Mbit/s (HSDPA) Bluetooth 1 Mbit/s over 10 m 50 Mbit/s (802.15.3) wireless ATM 25 Mbit/s over 30 m wireless LAN UWB / 802.11n copper cable PNA

54 Mbit/s over 30 m IEEE 802.11g ..1 Gbit/s over 10 m some Mbit/s

coaxial cable

some 100 Mbit/s

data cable (twisted pair) PLC

1 Gbit/s over 100 m

optical cable glass SM fiber glass MM fiber PMMA POF

up to approx. 45 Mbit/s (Home-Plug AV) nearly unlimited 2.5 Gbit/s 100..1000 Mbit/s over 100 m

Advantages/Disadvantages no local networking

extremely simple networking, limited capacity support a wide area of services, still relatively expensive widely used shared medium under development requires existing phone line, EMI sensitive requires existing coaxial cables relatively complex converters large cables (approx. 7-8 mm): highest developed LAN technology easy to install, critical EMI sensitivity and radio emission, shared medium

extremely expensive installation only limited effort for installation still a new technology extremely easy installation

abbreviations: EMI: Electromagnetic Interference PNA: Phone Network Association, uses existing copper cables PLC: Power Line Communication, uses electrical power supply UMTS: Universal Mobile Telecommunication Service, mobile phone standard, 3rd generation

As can be seen in the table, the PMMA POF lies in the mid-range of performance characteristics for the various transmission media. As regards the simplicity of installation, radio systems and PLC, of course, cannot be surpassed. Among the cable-based systems, POF is distinguished as having the easiest cable


619

setup and the most reasonably priced connection technology. A size comparison of different cables is illustrated in Fig. 8.33, clearly demonstrating that POF can be integrated very well into existing cable duct systems. coaxial cable : 7 mm

4 pair data cable : 7.5 mm duplex POF 2.2 mm u 4.4 mm

15 u 15 mm² cable duct filled with 2 coaxial cables 2 duplex POF subsequently installed

Fig. 8.33: Size comparison of different cables

Fig. 8.34: The red one or the black one? Among the media for inhouse cabling, the customer has the choice between symmetrical copper cables or POF mainly (Photo: I. Männl, University of Applied Sciences Nuremberg)

620


Besides the question of transmission media, the point of greatest interest is the interface to the consumer. A system can only gain general acceptance when terminal devices are equipped with appropriate connectors, the services desired can be supported with sufficient quality and the components for setting up the network are available at reasonable prices. Table 8.4 lists some of the interesting interfaces. Table 8.4: Interfaces for home networks Interface

Bit Rates

Advantages/Disadvantages

ATM Forum

25 Mbit/s, 155 Mbit/s, 622 Mbit/s, 2.5 Gbit/s

supports high-quality services and is already employed in long-distance networks, up until now too expensive for home use

Ethernet

10 Mbit/s, 100 Mbit/s 1,000 Mbit/s

used above all for IP applications, wide-spread and good value, dominant in LAN field difficult with video transmission

USB

12 Mbit/s (new 480 Mbit/s)

wide-spread standard for PCs very simple operation requires running PC up until now data rates too low

IEEE 1394

100 Mbit/s, 200 Mbit/s 400 Mbit/s, 800 Mbit/s up to 3.2 Gbit/s planned

universal system for all applications (incl. video) multi master network with extremely easy operation

POF systems have already been created for all 4 interfaces mentioned. The ATM forum has already specified the use of PMMA POF for 155 Mbit/s. Of particular interest is the inclusion of POF in the IEEE 1394 specification (up until now 100 Mbit/s and 200 Mbit/s over 50 m; 400 Mbit/s over 100 m is in preparation). In contrast to Ethernet, this interface could gain acceptance not only with computers, but also in diverse multimedia devices such as game consoles, cameras and video cameras, televisions and DVD players and with computer peripherals. The IEEE 1394 standard is intentionally not fixed to a medium, but provides the user with the option of selecting his own cable. Therein lies great application potential especially for POF as illustrated in the overview above. In addition to the question of possible interfaces, the general building network market in Germany should be considered. In contrast to other countries such as Japan or the USA most people in Germany live in houses for several families. Figure 8.35 shows the distribution in building size based on the last apartment and house count in Berlin. On the left is shown the number of buildings in Berlin (approx. 200,000) in different sizes (A/B: apartments per building). The right side of the diagram compares the number of apartments (1,200,000) within the various building size classes.


buildings

apartments

621

1 A/B 2 A/B 3 to 6 A/B 7 to 12 A/B 13 to 20 A/B >20 A/B A/B: apartments/building

Fig. 8.35: Building size distribution in Berlin

Although almost half of the buildings are single-family houses, they only represent about 10% of all Berlin apartments. Approximately ¼ of the apartments are in buildings with 7-12, 13-20 or >20 apartments respectively. In Germany around 70% of all apartments are nevertheless in houses for several families. In conclusion, Fig. 8.36 shows the accumulated frequency of cable lengths in buildings - calculated for the building size distribution in Berlin. Practically all cable lengths - measured between the access point in the house and the terminal device - are under 100 m, typical lengths being 30 m to 40 m. It can also be seen here that POF fits in well with the requirements not only for networks in apartments, but also in the buildings (see[Kra98]).

1.0

cumulated frequency

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1

cable length

0.0 0m

20 m

40 m

60 m

80 m

100 m

Fig. 8.36: Cable length distribution in Berlin

A special feature in some European countries, Germany included, is a very high share of apartments in multi-family houses. In the USA this share is about 20% and in Germany about 70% (Fig. 8.37). Thus, the development of new solutions for building networks must proceed especially in Germany.

622


60-70%: Spain Germany Italy 50-60%: Schweden Austria 40-50%: Denmark France Finnland 30-40%: Netherland Portugal 20-30%: Belgium 10-20%: Great Britain Sha04@ shows the relationship between reach (without walls) and the attainable bit rate for the wireless systems in different generations. Although the maximum capacities differentiate greatly, there is still a clear relationship between capacity and reach. One has to keep in mind that the capacity will drop strongly once again when several walls have to be penetrated, especially with higher frequencies. Reinforced concrete walls and ceilings are almost impenetrable. throuput [Mbit/s]

1000

UWB 100 802.11a, wireless LAN 802.11b, wireless LAN

10

1 802.15.4, Bluetooth 0.1 0

10

20

30

40

50

60

70

80

90 100 distance [m]

Fig. 8.42: Reach of radio networks ([Sha04])

Of course, there have also been “real” improvements in capacity with wireless systems, e.g. by means of more efficient error correction algorithms and better multiple-access procedures. The multiple input - multiple output (MIMO) technique is particularly effective, although each device has to have several antennas. Dramatic increases in capacity by orders of magnitude are only possible in wireless technology when the frequency band is expanded to several GHz - with other services being switched off, which is illusory - or when the transmission power is increased immensely which under certain circumstances can be unhealthy. Bit rates of several 100 Mbit/s, as can be transmitted by SI-POF without any problems, are actually only possible for wireless within rooms. All cables form a point-to-point connection. They thus guarantee the capacity independently of what

628


the other devices are doing at the moment. Nevertheless, in order to be able to profit from the mobility of wireless networks with broadband applications, e.g. a HDTV device, POF and wireless can be combined as in Fig. 8.43. The building shown has a broadband connection, initially maybe some VDSL lines, later a 2.5 Gbit/s glass fiber connection. A POF-based star network distributes the data into the apartments per duplex fiber. There is another switch in every apartment and from here on the data are transported on simplex fibers in order to additionally facilitate the installation. The entire system can be set up with Fast Ethernet with components available on the market today. A later expansion to 1,000 Mbit/s per line is conceivable. In addition, broadband wireless base stations have been installed in a number of rooms. Since they only have to cover one room each, they can operate with low transmission power and at high frequencies which would reduce the disturbances in the neighboring rooms. A handover via the central building node is possible so that full mobility is given.

broadband radio cell

optical switch

apartment network e.g.: 25 m at 650 nm simplex fiber building network e.g.: 60 m at 470 nm duplex fiber DVB-X receiver

active node

broadband access: ADSL2+, VDSL, HFC, WiMax, FTTB..

Fig. 8.43: Optical apartment and building network with POF

The advantages of broadband wireless solutions and fixed POF installations can be ideally combined in this proposal. The result for the end customer is a desirable minimization of pollution through radio waves.

629


8.1.2.5 POF Topologies Possible topologies are of fundamental importance for the use of POF. Today there are practically no terminal devices available with POF interfaces. The user has to use the electrical interfaces available on his devices and the signal has to be converted to the POF by means of a medium converter (Fig. 8.44). The corresponding power supplies are naturally also necessary. power supply

power supply

POF

DSL modem

patch cable

to PC

patch cable POF-media converter

POF-media converter

Fig. 8.44: Connection of a PC to the DSL modem using POF media converters

The disadvantage of this solution lies in the many necessary components. In principle, four adequate electrical Ethernet interfaces, which are in effect superfluous, are available in the system. Under certain circumstances the end user will prefer this variant to copper cabling. On the one hand POF is much thinner than the necessary copper cable and on the other both sides are completely separated electrically. In the next few years these connections will be simplified little by little. Even today PC plug-in cards with a POF connection are available from some manufacturers. There are already Ethernet switches or hubs respectively, e.g. from Luceat. The next alternative step could be that DSL modems are directly equipped with a POF connection (the first such device, equipped with transceivers from Firecomms, was presented by Netopia in >OTS06c@. The user can then set up his network entirely without external medium converters, patch cables and additional power supplies (Fig. 8.45).

POF

POF DSL modem with POF

PC-POF plug in card

POF ethernetswitch

Fig. 8.45: Connection of multiple PCs to one DSL modem with POF interface (fictively)

630


Further steps will then be to have special terminal devices equipped with POF interfaces. A necessary prerequisite for this is of course a stable standard. As described above, especially wireless base stations with POF connections, would make sense. In addition to the general networking of apartments and buildings there could be a number of applications in which POF is installed in specific point-to-point connections. One example is the transmission of uncompromised video data between receiver and screen. Other possible POF connections could connect sensors which could then be optically powered from outside. As part of the Bavarian Research Foundation’s sponsored project Loewe Opto, the Fraunhofer Institute for Integrated Circuits Erlangen, the SGT Weidenberg Company and the POF-AC Nürnberg as partners have developed a system for transmitting HDMI video data (3 u 1.6 Gbit/s) over a POF ribbon cable. About 15 m can be reached with SI-POF and about 50 m with GI-POF. Figures 8.46 and 8.47 show the experimental set-up and the ribbon cable used with a prototype connector.

Fig. 8.46: Demonstrator for HDMI over POF (project OVAL, see [Jun06])

Fig. 8.47: POF ribbon cable with connector prototype


631

8.1.3 Interconnection Systems with POF Chapter 5 described what diverse applications optical bus systems could have in the near future in the field of parallel data transmission. Fiber solutions could be an alternative to the waveguides integrated into the PC boards. The advantage of fiber-based versions is that the materials are not subjected to high temperatures when the PC boards are produced and assembled. Connections between different plug-in boards present no problems since the fibers or fiber bundles can be bent almost at will. An example of a POF-based system was shown in Section 6.4.2. In principle, all conventional fibers can be utilized for these applications. Since many channels parallel to high data rates are generally necessary, VCSEL arrays are the ideal sources. Fibers with a relatively small core diameter (125 μm or 250 μm) are used for an optimum coupling and at the same time with small bending radii. Compared to standard multimode glass fibers the latter are still relatively thick, but do allow greater tolerances. The most important difference, however, is surely the extremely easy processing. Thanks to the short transmission lengths PMMA POF can also be used at wavelengths of 780 nm or 850 nm. The use of step index fibers is also possible up to a few meters with data rates up to 10 Gbit/s per channel. A comprehensive overview of the details of a POF interconnection solution is given in >Witt04@ (see also >Jöhn98@, >Witt98@ and >Ney02@). 8.1.3.1 Parallel Date Transmission with Glass Fibers Commercial systems are already available on the market for different glass fibers (MM-GI-GOF and PCS). Infineon, for example, offers the Paroli System with 12 channels each of which can transport 1 Gbit/s over distances up to 300 m. A MT connector connects the active components. 8.1.3.2 Parallel Data Transmission with POF As part of the EU Optical Interconnected Integrated Circuits (OIIC) Project the University of Dortmund >Witt04@ has developed a multi-parallel optical solution with polymer fibers. One particular requirement was a maximum overall height of 5 mm (The principle is illustrated in Fig. 8.48).

Fig. 8.48: Parallel POF-connection ([Witt04])

632


980 nm VCSEL arrays each with a spacing of 250 μm were used as emitters (1 mW output power per laser with a maximum data rate of 1.5 Gbit/s), produced at the University of Ulm. Arrays of 4 u 8 diodes were produced. The active surface of the VSEL has a diameter of only 13 μm. The receivers were produced by the ETH Zurich. The InGaAs/InP photodiodes with a diameter of 150 μm of the active surface attained a switching time of 300 ps (at 50 :, diode capacity: 1.4 - 1.5 pF. The fibers used were 120 μm /125 μm SI-POF from Toray (NA: 0.48). The fiber bundles were formed by sticking the fibers into pre-drilled, thin plastic discs. The attenuation spectrum measured is shown in Fig. 8.49. 105

attenuation [dB/km]

104 103

125 μm POF 1 mm POF

wavelength [nm]

2

10

500

600

700

800

900

1000

Fig. 8.49: Attenuation spectrum of a 125 μm diameter POF

Very good VCSELs are available with wavelengths of 780 nm and 850 nm. The attenuation values here lie clearly above the minimum losses around 650 nm, but do permit however a few meters of a transmission link. Even the use of 980 nm components is possible in the range of several decimeters which corresponds to the dimensions of computer main boards. The increased attenuation in the short-wave range of the 125 μm POF does not play any role in this application in contrast to standard 1 mm POF. The fiber is guided and positioned in regard to the emitting and receiving components with the pre-drilled acrylic glass plate shown in Fig. 8.50. Production takes place with tolerances clearly below 10 μm so that efficient passive coupling is easily possible.

Fig. 8.50: Acrylic glass plate for holding the POF


633

Important for maintaining the slight overall height are the tight bending radii of the POF. The losses for a 360º bend at a wavelength of 650 nm are given as an example in Fig. 8.51. If an additional loss of 1 dB is permitted in the power budget, then radii less than 1 mm could be used. This would be unthinkable with glass fibers with the same diameter and also difficult with high frequency copper lines. 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

bend loss [dB]

bend radius [mm] 0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Fig. 8.51: Bending loss of a 125 μm POF (one 360° bend according to [Witt04])

Transmission experiments with the 950 nm VCSEL at 2.5 Gbit/s over a 50 cm link were conducted for testing transmission behavior of the fiber bundle. A BER Kas03@) especially for advertising pillar. A central light source supplies many fibers which in turn equally distribute the light via specially designed spotlights. This diploma thesis was awarded the N-ERGIE Nuremberg Prize since there was also a 75% savings in energy as quasi a side effect. Figure 8.57 shows the advertising pillar and the tapers used. The advantages of this recommended solution as opposed to conventional systems are: ¾The central light source can be placed in a position where it can be replaced very easily. Furthermore, it is vandal-proof. ¾The tapers directly illuminate the pillar’s surface so that only a little light is lost due to lateral light emission.

8.2 POF in Lighting Technology

637

¾By suitably shaping the tapers a particularly large amount of light is directed toward the lower part of the pillar through which a much more equal distribution of light is achieved. With conventional halogen lamps there are luminance differences of up to 1 : 10,000.

Fig. 8.57: Timo Reulein with advertising pillar, taper for the illumination

8.2.1.2 POF Starry Ceiling Lights A comprehensive range of POF lighting products is presented in >Fib02@. The advantages of fiber optic illumination mentioned are: ¾complete lack of ultraviolet (UV) and infrared radiation (IR) ¾electrical separation of light sources and the exit of light ¾no electrical potential at the exit of light ¾no extensive protective measures necessary at the exit of light ¾no temperature at the exit of light ¾high economic efficiency through low power consumption The components available include: ¾halogen projectors (10 W to 50 W) ¾color wheels ¾fiber bundles with 1 mm, 2 mm and 3 mm fibers ¾different lenses for coupling out light ¾sidelight fibers Figure 8.58 shows a particularly lavish example of a starry ceiling light with POF.

638


Fig. 8.58: POF starry ceiling from Brumberg ([Fib02])

In addition to the purely decorative elements the system can naturally also be used to display information, e.g. map routes could be marked with points of light. The entrance door of the POF-AC Nürnberg shows the institute’s logo as a sample of POF ends illuminated with different LEDs (Fig. 8.59). The logo of the University of Applied Sciences has also been accentuated in this way.

Fig. 8.59: Logos of the POF-AC and the Georg Simon Ohm University of Applied Sciences with POF illumination

The combined use of LEDs and POF in particular opens up almost unlimited possibilities. One conceivable application is two-way traffic signs which, however, have the problem of a limited temperature range with POF.


639

8.2.2 Side-Lighting Fibers Light can be decoupled laterally by using different procedures. The optical cladding of the POF is transparent and thin. One possibility to achieve lateral emission is to intentionally disturb the core-cladding interface through mechanical damage or damage by laser radiation. At the Nuremberg University of Applied Sciences experiments on light radiation have been carried out by cutting grooves into the sides (see Fig. 8.60 and 8.62). An equally practical method for decoupling light is the periodic bending of the fiber with small radii. Part of the light is then decoupled in the bends.

Fig. 8.60: Principle of a lateral light decoupling out of a POF ([Poi99])

If many of these fibers are brought together in a plastic tube and illuminated from one side, even better from both sides, then you get a flexible light element similar to a thin fluorescent strip lamp of more than 10 m in length. Since this bundle consists of plastic and does not conduct current it is much safer and has a greater load-bearing capacity than a fluorescent strip lamp, for example (Fig. 8.61).

Fig. 8.61:Use of sidelight fibers in lighting technology (LBM Lichtleit-Fasertechnik Berching)

640


Fig. 8.62: Illumination of an acrylic glass plate with the logo of the faculty NF at the Univ. of Appl. Sciences Nuremberg with POF having cut grooves ([Poi99a])

Figure 8.63 shows components for illuminating the gear shift of an automatic car transmission ([Nich00]). Here POF is also used to guide light as well as for direct illumination. LED are being increasingly used as a light source in automobiles as they are smaller, more efficient and have a longer life expectancy than light bulbs. However, one problem when using several LEDs in a device is maintaining both the exact same color during their service life and the temperature range.

Fig. 8.63: Components for detail illumination in automobiles ([Nich00])


641

One well-known user of optical fibers is the Hellux Company which prefers using plastic fibers (>Hell04@) since the color of light already changes with glass fiber after 3-4 m, with POF, however, only after 8 m. The average service life of POF for illumination purposes is 20 years as stated on the product page. The sheaths are halogen-free and flame-retardant according to Fire Protection Class II of VDE 0207 T24. An example of an application is shown in Fig. 8.64.

Fig. 8.64: Side lighting POF applications from Hellux ([Hell04])

Another vendor of POF illumination systems is the Stiers Company which also sells fibers with a diameter of 0.75 mm to 3 mm and fiber bundles as sidelight fibers. Diverse projectors and lenses for the exit of light complement their program (examples in Fig. 8.65).

Fig. 8.65: Side lighting fibers (Comp. Stiers)

642


>Spi05@ provides an overview of the different possible applications of sideemitting polymer fibers. The application areas for such materials are: ¾decorative lighting in air, water or ice ¾laser shows, displays luminous textiles, interior decoration and visible framing ¾safety equipment ¾emergency lighting ¾fiber optic sensors ¾dosimetry ¾medical light therapy The article also describes the different methods in order to have as even a light density as possible along the fiber: ¾coupling in light from both sides ¾attaching a reflector to the second fiber end ¾gradual changes in the scattering centers along the fiber - ideal theoretically, but most difficult to implement technically ¾producing luminescence in other ways, e.g. by external UV radiation

Fig. 8.66: Luminous clothing (Luminex) and side lighting POF according to [Spi05]

8.3 POF in Sensor Technology

643

8.3 POF in Sensor Technology The field of sensor technology in particular has been investigated intensively in the past 5 years as regards polymer fibers. In many sensor applications the distances between the measuring points and the evaluation electronics are relatively short and the measurements themselves are made at low speed. Both parameters accommodate the use of polymer fibers. In addition, the low price in large-scale applications, great flexibility and simple processability are important factors. Electrical sensors today still represent the predominant share of sensor applications. There is an entire series of application areas in which electrical connections present problems such as when there are strong electromagnetic fields. In the meantime sensors on the basis of singlemode glass fibers can be used in diverse applications. Selected systems are described at the end of Section 8.4. These glass singlemode fiber systems, however, are much too expensive for most application areas and this is where POF-based sensors can be used. The use of POF in sensor technology can be roughly divided into different areas: ¾A conventional electrical sensor is located at the measuring point and transmits its data via a POF or PCS. This happens mostly when the sensor point is to be fixed electrically isolated. One advantage of the use of thick fibers is that the power supply can also be affected optically. ¾The POF transmits and receives light. The measurement principle rests on the change of transmission between the sending and receiving fibers. In the most simple case the principle of the light gate is used. ¾The fiber itself serves as a sensitive element with which the transmission can be influenced by bends or links of the fiber. In this case the fiber combines data transport and sensor function. ¾The most diverse kinds of changes on the end face or lateral surface of the fiber serve as a sensitive element. These can be selective holes in the optic sheath as well as primary coatings which react to different chemicals. Either the total change in transmission is measured or at certain wavelengths. Some developments of POF sensors have been compiled in the subsequent sections. The list is by no means complete, but is intended to demonstrate the fundamental principles. One example of a sensor in which POF only guides the signal is in >Rib05b@. The temperature was measured here by means of a POF sensor system in the 30ºC to 70ºC range with 1 K resolution. The POF is connected to a ruby crystal. When stimulated with a blue or green LED fluorescence emission takes place at 694 nm. The duration of fluorescence depends on the temperature and is between 2 ms and 4 ms. The measuring signal is the time delay between the pump pulses and fluorescent light (4 ms to 5 ms).

644


8.3.1 Remote Powered Sensors An example of a simultaneous transmission of power and data is presented in >Böt06@. The aim of the set-up is to distant feed a digital camera via an optical fiber with the simultaneous transmission of the camera images over the same fiber. The camera supplies color images with 640 u 480 pixels. The principle is illustrated in Fig. 8.67. PVC DC-DC

810 nm TCP / IP

μC

MMF 1310 nm RX

FPGA Base Station

Camera

HPLD PS

TX CPLD Remote Unit

Fig. 8.67: Principle of the remote powered camera with data transmission ([Böt06])

The power source is an 810 nm edge-emitting laser with an optical power of 1 W, of which about 480 mW are coupled into the fiber. The loss over 200 m including the coupler amounts to 2.3 dB so that there are still about 280 mW available at the receiver. The data is transmitted with standard components at 1,310 nm. In order to be able to efficiently transmit both wavelengths, a 62.5 μm GOF was used. An approximately 1 mm large detector which converts light into a photocurrent in element operation is located on the side of the camera. The available voltage of about 0.7 V to 0.8 V is converted to the necessary level of 3.3 V and feeds all components. The entire unit only consumes 100 mW. Limited by the clock-frequency rate of the processor (4 MHz), one image per second can be transmitted. The converter is shown in Fig. 8.68. The user data are coupled in and out via optical wavelength multiplex couplers.

Fig. 8.68: Optical converter ([Böt06])


645

Such high power is as a rule not transmitted over polymer fibers. Up to about 10 mW can be coupled into the fiber when using normal LEDs. On the other hand, modern microprocessors can work at clock frequency of some MHz with currents below 1 mA. It would be very useful if one could manage without the DC-DC conversion. Segmented photo receivers connected in series can be added. Even better would be the efficient use of semiconductors with a high band gap, e.g. GaN, as a photo element. 8.3.2 Transmission and Reflection Sensors The following examples of POF sensors all work according to the same fundamental principle. A transmitter, generally a reasonably priced LED, couples light into a fiber. The light is first coupled out and is then coupled back in again either into the same fiber or into another POF. The quantity to be measured now alters the amount of the returning light so that the corresponding process can be detected. This principle can be applied with all fibers. However, POF has the advantage of having a large cross-sectional area so that the sensors can be produced relatively easily. Normally the measurement steps proceed very slowly (in seconds). The possible measurement speed is only limited by the bandwidths of the transmitter and receiver as well as mode dispersion in the optical path. If necessary, bandwidths into the GHz range would be possible. 8.3.2.1 POF as Distance Sensor The POF is very suitable as a distance or movement sensor in which the reflected light is measured. Figure 8.69 illustrates the corresponding POF types. Figure 8.70 shows the method of operation.

Fig. 8.69: POF applications for sensors ([Nich00])

646


transmitting fiber

receiving fiber object to be measured Fig. 8.70: Method of operation of a POF distance sensor

The use of such a sensor for measuring the rotational speed of a wind power generation rotor is described, for example, in [Zub99]. If typical fibers with a 1 mm core diameter are used then the working distances can amount to a few centimeters. Significantly greater distances into the meter range are possible if collimators are attached to the fiber or when the reflecting object is equipped with retro reflector foil. A reflection sensor on the basis of a fiber bundle (19 POF: PG-U-FB750), laid out hexagonally) is described in >Ber05@. A fiber emits light in the center and two rings from fibers around it detect the signal. The principle of detecting solid particles in flowing fluids is applied in this work. The advantage of this layout is that very much of the reflecting light is detected. By comparing the signals in both rings the effects of different backscattering behavior can be compensated for. Reflection is also used with the sensor described by >Zub00@. In this case not the distance of an object is to be measured, but the speed of rotation. For measuring the wind speed in wind power plants anemometers are normally used in which the revolutions per unit time are determined by an optical coupler. The electrical supply cables are problematical because electric currents can be induced by flashes of lighting. The concept proposed favors supply cables and outgoing cables of an optical signal via POF (490 μm POF). A cylinder with reflecting segments is mounted on the wind gauge (12 mm in diameter with 12 segments). With a 0.4 mm gap between the two fibers a coupling efficiency of about 5% is achieved. The evaluation electronics counts the pulses and can measure wind speeds from 10 to 100 km/h. A distance sensor using this principle is also described in >Per04@. The aim here was to determine cracks in concrete structures and POF was integrated into the corresponding constructional elements. An opening crack separates the fiber and generates an additional loss (Fig. 8.71).


647

Fig. 8.71: Crack detection due to the distance inside the fiber ([Per04])

8.3.2.2 POF Sensors for Concentration Another sensor application can be found in [Lom00]. In the latter case two POF, separated somewhat, are located in an acid. When the concentration of the acid is changed, the refractive index also changes and consequently the share of launched light, as illustrated in Fig. 8.72. transmitting fiber

receiving fiber

low acid concentration

high acid concentration

Fig. 8.72: Method of operation of a POF concentration sensor

8.3.2.3 Deformation and Pressure Sensors A special POF-based sensor under the trade name Kinotex has been developed for applications in automobiles (>Can02@, >Poi06b@ and >Poi05a@). Transmitting and receiving fibers are located inside a diffuse reflecting plastic foam (Fig. 8.73). If the foam is compressed, the optical density increases and more light can reach the receiver (Fig. 8.74).

648


Fig. 8.73: Principle of the Kinotex sensor

foam in the normal state

pressed foam more light will be reflected

Fig. 8.74: Change of the reflection by compression of the foam

This sensor can be used, for example, for detecting accidents. Another proposed application is establishing seat occupancy. Here it is not a question of whether someone is occupying a seat, but how heavy the passenger is or whether it may possibly be a piece of luggage. A matrix of corresponding sensors would be put on the seat. A computer would continuously measure the contact force distribution and determine the seating arrangement (Fig. 8.75).

Fig. 8.75: Seat occupancy detection with a sensor matrix

649


8.3.3 Sensors with Fibers as Sensitive Elements Optic fibers react sensitively to the most diverse external influences. Surely, the most well-known effect is the increase in attenuation when bending the fiber. An entire series of glass fiber sensors has utilized this principle and this procedure has been realized in practice with POF. The fundamental principle is shown in Fig. 8.76.

F

F

F

F

Fig. 8.76: Principle of an optical sensor using the bend loss

A fiber runs straight without external forces and the transmission is then at a maximum. When external forces arise, bends are produced in the fiber which reduce the transmission of the fiber. This effect comes about immediately and the speed of the sensor is only limited by mechanical inertia and the bandwidth of the transmitter and receiver. 8.3.3.1 The POF Scale A similar sensor principle has been developed at the POF-AC. A POF is wound into a tight coil. Should you compress this coil, a strong increase in attenuation takes place, whereby the actual bending attenuation is not utilized. If a POF is wound relatively tightly, then the attenuation increases less strongly as with the number of coils. The reason for this is that at the beginning of the winding mainly the high modes are emitted relatively quickly. After a few windings a new mode equilibrium has arisen. Substantial losses occur once again at the crossover point to the straight fiber. If the coil is now deformed, then the fiber has a different local bending radius at each point in its length. An equilibrium mode distribution cannot come about and modes are emitted over the entire length of the coil. The sensitivity of this sensor is much higher than when the actual bending attenuation is utilized. Furthermore, considerably greater radii can be used so that the fiber is not under a great stress. If the coil is combined with an elastic deformation element, e.g. a steel spring, then a force sensor can be constructed instead of a deformation sensor. High sensitivity can be achieved even with small deflections with biasing.

650


unstressed:

deformed

40

photo current [a.u.]

35 30 25 20

local bend radius

15

r

10

under pressure

0

unstressed position

5 0

force [N] 0

5

10 15 20 25 30 35

Fig. 8.77: Principle of the sensors and typical characteristic ([Poi05b])

Sizes of just a few mm3 can be achieved be using correspondingly thin fibers. The entire sensor can be cast in order to be able to protect the fiber from environmental influences. A weight scale with four such sensors was built as a demonstration model at the POF-AC (Fig. 8.78).

Fig. 8.78: Sensitive coil and combination with springs ([Poi05b])

8.3.3.2 POF Expansion Sensor Different methods for producing efficient and reasonably priced optical sensors are presently being developed as part of the For Photon Project supported by the Bavarian Research Foundation. One possible application is the measurement of the sagging of the propeller blades of a wind power plant which can be switched off more precisely when there is exact data about the wing deformation at high wind speeds so that more electrical power is generated. From an optical perspective this principle has been known for a long time and has been described in >Dör06@ and >Kie06@. A fiber is solidly attached to the expanding object to be measured. A modulated signal is applied to the fiber and the phase of the modulation signal now shifts because of the linear change which is determined by a phase comparator after being compared with a reference path.


651

The higher the modulation frequency is, the better is the resolution of the procedure. A schematic representation of the measurement principle is shown in Fig. 8.79. Figure 8.80 shows a typical measurement signal with which length changes of at least 10 μm can be recognized.

'L

S

f

PC

U ')

A/D

E

POF

sin(2S f t)

sin(2S f t + M)

Fig. 8.79: Measurement principle of the strain sensor

920

voltage [mV]

915 207 μm 910 905 231 μm

900

time [s] 895

0

10

20

30

40

50

60

Fig. 8.80: Measurement example of the strain sensor

If several of these sensors are mounted on the rotor wings, the sagging can then be measured very exactly. The advantage of POF lies primarily in the very easy processing and the low component costs. Furthermore, a POF can be expanded about ten-times more than a glass fiber.

Fig. 8.81: Model of the bending measurement system for wind power station wings

652


8.3.4 Sensors with Surface-Modified Fibers In many applications the sensitivity of normal fibers is insufficient for the measurement task at hand. By selectively changing something in or on the fiber the influence of the measured quantity on the fiber is intensified. For example, such changes can be mechanical damage to the cladding, changes in the refractive index and also a coating with chemically active materials. Here, too, only a few examples can subsequently be given. 8.3.4.1 Bending Sensors with Notched Fibers An important area of use for sensors is in automobiles. In cooperation with the POF-AC Nürnberg the company Siemens-VDO has developed a new sensor principle for detecting collisions with pedestrians (>Mie04@, >Mie05a@, >Mie05b@, >Tem05@ and >Djo03@). In accordance with EU Guideline 2003/102/EC every vehicle as of 2005 must have special protective measures for pedestrians which can either be structural measures such as soft impact zones or active systems. One of these systems is shown in Fig. 8.82.

Fig. 8.82: Pedestrian protection system ([Mie04])

An optical sensor which can determine local deflections at several points within milliseconds is located in the bumper of a vehicle. The time characteristics of these deflections are typical for a particular kind of accident. If the car computer registers a collision with a person, the hood opens up a few centimeters in order to soften the direct blow to the engine block.


653

Polymers fibers in which certain zones of the cladding have intentionally been damaged - so-called treatments - are used for measuring the deflection. These treatments have been applied to the side of the fiber. When the fiber is bent in the direction of the notches the coupling-out of light is diminished, when bent in the opposite direction the light emission is increased (Fig. 8.83). Not only the degree of the bend can be determined, but also the direction.

Fig. 8.83: Principle of the measurement of bending radius and direction

The general usability of single-sided treated fibers for sensor technology is described in >Djo03@. According to the author the advantages of the principle are that the fibers can be embedded easily in the laminate. The measurement of the bending radii is thereby independent of local stress. Figure 8.84 shows the principle according to >Djo03@.

Fig. 8.84: Sensitive zones in bending sensors

In order to achieve the spatial resolution, ribbon cables are used in which different sensitive zones are applied (Fig. 8.85). According to >Tem05@ the treatments, approximately 100 μm wide and 20 μm to 30 μm deep, are burned in with the aid of a 266 nm UV laser. The Intelligent Pedestrian Protection System (IPPS) was introduced in 2007.

654


ø 30 mm

interface electronics (FOSE)

500

1440

leads (insensitive)

176

sensor area

„turn around“ (insensitive)

e.g. 18 cm cell (bumper dependent)

Fig. 8.85: Setup of the whole sensor with fiber ribbon

The sensor from ACTS in >Alb05@ is intended for the same application. In the latter, however, the increasing bending attenuation is to be measured and there is no spatial resolution. impact absorber electronics bumper POF

sensor structure

foam material

Fig. 8.86: ACTS force sensor

8.3.4.2 POF Evanescence Field Sensors Leoni has also developed a sensor based on polymer fibers for use in vehicles (>Kodl03@, >Kodl04@, >Kodl05@ and >Poi05a@). A polymer fiber without optical cladding serves as the sensitive element. Following the theory of wave guiding in fibers, with total reflection the light penetrates a few micrometers at the core-cladding (or core-air) interface (Fig. 8.87). This area is called the evanescent field.

Fig. 8.87: Entering of the optical wave into the surrounding medium ([Poi05a])


655

The fiber core in the sensor proposed and tested by Leoni is surrounded by a roughly structured material which is attached to only a few points on the fiber core. Thus, there is almost always total reflection against air and the transmission is high. Should the surrounding material be compressed by force, the adjacent surface increases at the core and an increasing share of light is either absorbed or coupled out. In this way the sensor can be used, for example, to recognize objects caught in a car window when the window is being closed (Fig. 8.88).

Sensor

window plane

Fig. 8.88: Evanescence field sensor as a jam protection for car windows ([Kodl03])

A typical curve for the transmission of externally applied force is shown in Fig. 8.89. The sensor can be adjusted so fine that a robot’s arm can pick up an egg shell without crushing it. Other applications of this sensor principle are described in >Kodl05@. Here the sensor is used to recognize contaminants. Of course, optical touch panel switches can also be produced.

signal [dB]

0 2 4 6 8

force [N] 10

0

1

2

3

4

5

6

7

8

Fig. 8.89: Typical sensor characteristic and robot hand ([Poi05a])

656


8.3.4.3 Fill Level Sensors A sensor for measuring liquid fill levels, which also uses a surface effect, is proposed in articles >Lom05a@ and >Lom05c@. Strongly bent POF, which is partially ground (about 140 μm) in the bends, is put in a tank. Each one of these bends generates a loss of about 3.4 dB. If the sensor is submerged in water, the transmission changes by about 0.5 dB per sensor point. The more measuring points there are, the more exact the liquid level can be determined.

Fig. 8.90: POF as a liquid fill sensor [Lom05c]

A quite similar sensor was developed at the College of Weissenburg as part of a school project looked after by the POF-AC (Fig. 8.91, >Fei02@). This sensor uses fibers with an optical cladding, but without a protective sheath. If such a fiber is bent without resting against anything, then there is still almost total reflection at the cladding-air interface so that light can continue to be guided. The critical angle is diminished dramatically when submerged in the liquid so that the bending losses increase considerably. The liquid level is thus detected in combination with the threshold value switch.

Fig. 8.91: Liquid fill sensor from a school project ([Fei02])


657

8.3.4.3 POF Bragg Grating Sensors Bragg gratings have been known for a long time in singlemode fibers and are used as optical filters. After it was possible to produce singlemode polymer fibers, Bragg gratings could also be used in POF. A team from the University of Sydney presented the production and application of POF gratings in different articles (>Liu02b@, >Liu03@, >Liu04@, >Liu05a@ and >Liu05b@). The realization of a Bragg grating in POF with an isolation of 28 dB is described in >Liu04@. The thermal sensitivity is ten times greater than with silica glass fibers due to the high thermal coefficient of PMMA. Consequently, dispersion tuning can be carried out with the aid of chirped POF gratings. Furthermore, polymer fibers can be much more strongly expanded than glass fibers. The gratings can also be tuned through expansion, as the example in Fig. 8.92 shows.

Fig. 8.92: Tuning of a POF grating by strain ([Liu04])

A tuning range of 10 nm was reached through a temperature change of 55 K. A change in the dispersion from 2,400 ps/nm (0.02%) to 110 ps/nm (0.4%) was achieved in a chirped grating through expansion. The use of a POF grating as an expansion sensor is described in >Liu05b@. Changes in length up to 1.9% can be measured. The spectral shift of the grating wavelength is of 1.46 pm per millionth expansion (total of 27 nm). The basis of the grating is a singlemode POF with a diameter of 6/125 μm (ǻn = 0.86%; NA: 0.16). The expansion is measured by the detuning of a fiber ring laser by means of an expanded POF grating (Fig. 8.93). Theoretically, PMMA allows an expansion up to 13% which corresponds to a tuning range of 100 nm.

658


Fig. 8.93: Tuning of a fiber ring laser using a POF grating ([Liu05b])

The production of UV-sensitive singlemode POF from PMMA as needed for fiber gratings is described in >Yu05@. The first Bragg gratings in POF were produced in 1999. The SM-POF is drawn from a preform and has a core/cladding diameter of 10 μm/110 μm (Fig. 8.94).

Fig. 8.94: Cross area of a singlemode POF ([Yu05])

8.3.5 Sensors for Chemical Materials In order to be able to determine chemical or biological substances with optical fibers, correspondingly sensitive layers have to be applied which then vary the light propagation accordingly. One of the first examples was the measurement of an ozone concentration with POF according to >Kee05@. In this case the POF only serves the purpose of feeding in and removing the 603 nm measuring light. The gas is measured in a 5 cm long analyzer which lets the light through after expanding correspondingly. The measurement range lies between 27 to 127 mg/dm3 with a resolution of 5 mg/dm3.

659


8.3.5.1 Humidity A real chemical fiber sensor has been presented by >Mor04@. The goal was to measure humidity by taking advantage of the fact that certain molecules swell up when absorbing water and thus change their refractive indices. If such materials are used as the cladding of optical fibers then humidity can be detected through the change in light guiding (Fig. 8.95). swelling polymer

water molecules n2

n2

Pin

L

POF

core: n1

sensor

Pin

Pout

L

POF

POF

core: n1

Pout

sensor

POF

in the humid air: n2 < n1 guided structure

in the dry state: n2 > n1 leaky structure

Fig. 8.95: Principle of optical humidity measurement ([Mor04])

The authors used hydroxyethylcellulose (HEC) as cladding material. The refractive index was 1.51 under dry conditions and 1.487 in a humid atmosphere. By mixing a PVDV cladding with a HEC film you get a refractive index just above the core index. Thus, there is no light guiding. In humid air the refractive index drops so far in less than a second that light guiding takes place and transmission greatly increases (Fig. 8.96). 5.0

transmission [a.U.]

4.5 air 70% RH on

4.0 3.5 3.0 2.5 2.0 1.5

air 15% RH

time [s]

1.0 -1

0

1

2

3

4

5

6

7

Fig. 8.96: Sensor response for the measurement of humidity ([Mor04])

8

9

660


In the example shown the transmission increases right after a few tenths of a second after connecting to the humidity. Saturation was reached after a few seconds. The typical values of the sensor are a fiber diameter of 0.5 mm and a sensitive length of 5 cm. This principle can be used, for example, to monitor a person’s breathing. The transmission of a corresponding sensor for breathing with 20 breaths/minute and 74 breaths/minute is shown in Fig. 8.97. The signal can surely continue to be improved by correcting the system’s time response.

intensity [a.u.] 2.5 20 per min.

74 per min.

2.0

1.5

1.0 0

2

4

6

8

10

0

2

time [s]

4

6

8

10

time [s]

Fig. 8.97: Principle of optical humidity measurement ([Mor04])

8.3.5.2 Biosensors The detection of biological substances is gaining in importance. Here, too, polymer fibers can be widely used, especially in the field of one-time applications. The use of microstructured POF for detecting antibodies has been demonstrated in >Emi05@. The biosensitive layer is applied to the side walls of microstructured POF. The test liquid fills these holes. Two examples of the MPOF used are shown in Fig. 8.98. The authors of the articles describe the production of MPOF from a fiber preform with a diameter of 20 mm. The drawn POF then has an outer diameter of 300 μm with 60 μm thick holes. A liquid volume of only 3.4 μl fills the 20 cm long measuring pipe. The evaluation takes place by means of a fluorescence spectrum, i.e. a 50 μm glass fiber is coupled to the spectrometer.


661

Fig. 8.98: MPOF ([Emi05] and [Jen06] with 300 μm and 320 μm core diameter, respectively (holes: 60 μm and 55 μm diameter)

8.3.5.3 Liquids Microstructured POF can generally be used effectively for measuring liquids. A sensor based on so-called hollow-core MPOF (HC-MPOF) is described in >Cox06@ (see Fig. 8.99).

Fig. 8.99: Hollow Core-MPOF (68 μm inner hole)

The idea of the measurement is that the refractive index of the core filled with the liquid is higher than the structure of the cladding. As a result, the transmission ranges of the fiber change when the core hole is filled with liquid.

662


In the example presented above the approximately 50 cm long fiber sample is completely filled with water (approx. 10-6 l) in about 10 minutes. In the process the transmission peak is shifted from 1430 nm/1140 nm to 875 nm/700 nm after being filled with water. Furthermore, the measurement with polarized light at 589 nm of the optical rotation of the fructose solution is presented with the aid of a chiral fiber. 8.3.5.4 Corrosion An optical sensor for recognizing corrosion on aluminum structures, in this case military airplanes, is shown in >McA04@. Cations are formed when aluminum corrodes. They diffuse in the porous cladding of an optical fiber (the core diameter is 200 μm and no data is given on the core material). The cladding with PMMA as a carrier material is doped with 8-hydroxyquinoline (8-HQ). These molecules together with the aluminum cations form complexes which generate fluorescence at 516 nm with UV-irradiation (360 nm to 390 nm). The principle is shown in Fig. 8.100.

Fig. 8.100: Principle of the optical corrosion sensor ([McA04])

8.3.6 Glass Fiber Sensors Singlemode glass fibers still enable more sensor principles compared to multimode polymer fibers, whereby the filter characteristics of interferometer arrangements are usually utilized. Different examples of such arrangements are described in >Coo03@. Reasonably priced and reliable optical sensors for measuring temperature, pressure, rate of flow and sound waves in subterranean systems for oil production are being developed in a sponsored project.


663

Small Fabry-Perot interferometers (Ø 0.1 mm u 5 mm) are inserted. Typical ranges for the measured quantities are: ¾-40ºC to +200ºC ¾pressure to 150 bar ¾rate of flow up to 100,000 barrels per day The set-up of such interferometers is shown in Fig. 8.101.

glass alignment tube

input fiber

thermal bond

reflector fiber

Fig. 8.101: Example for FP interferometer for pressure measurement ([Coo03])

The F-P sensors produce a periodic transmission function for the reflected light. An edge of this transmission function is used for the measurement. Multimode fibers can be used for short distances and singlemode fibers for greater distances. LEDs and SLEDs serve as broadband light sources. The deformation of the small tube is used for detection in the pressure sensor shown. The thermal expansion of the air gap is utilized when measuring the temperature. Other sensors with glass fibers use Bragg gratings (see above), make use of bending attenuation or also work with special coatings. Measuring the temperature can be realized through the temperature-independent frequency shifts with Brillouin scattering. The great advantage of glass fibers is the possibility to measure at distances of many 10 km from the active technology as well as in the fiber’s excellent stability resulting in great exactness and resolution. The advantages of polymer fibers are primarily the extremely easy handling and in many cases the large cross-section. We wish to point out once again an article with a comprehensive overview including a look back at the first developments of POF sensors (>Bar00@). Here a POF sensor developed in 1997 by Niewisch (>Nie97@) is mentioned in which liquid nitrogen works at 77 K. Different sensors with fluorescing fibers are also mentioned.

9. Optical Measuring Methods

The measuring techniques for polymer fibers and other thick-core fibers differ in some essential parameters from those of conventional glass fibers. The main difference lies in the dominant mode-dependent effects. The first edition of this book provided a general overview of POF measurement techniques with just a few specific results. The POF-AC Nürnberg has now existed for five years and has adapted and reworked many methods of measurement. A number of measurement arrays have been set up and tried out. The following chapter has been supplemented by these new methods and results while also including the general presentation. The measurement results of fiber bandwidths will not be further dealt with here as comprehensive examples were discussed in Chapter 2. This is also valid for the measurement of bending losses.

9.1 Overview There are three main areas in which optical measuring methods are applied: ¾for manufacturing, control and product specification, ¾during and after installation, ¾for maintenance and for searching faults. The investigations cover individual components as well as overall transmission systems. This chapter covers optical measuring methods for polymer fibers relevant to the user; the main aspects are the measurement of fiber attenuation and dispersion as well as emission characteristics and connector loss, as illustrated schematically in Fig. 9.1. The different measured quantities can be determined by different procedures, for example, for determining the attenuation in transmitted light and back reflections. All optical characteristic quantities can be measured in dependence of the launch conditions and the wavelength (spectrally resolved). Furthermore, the measurements can be combined with variations of the external conditions such as mechanical or climatic loads. The basic parameters of the components of a transmission system will be measured by the manufacturer and should be included in the data sheets and in application notes. The system designer can use these available data for his information and the correct choice of materials and components. The end user, last but

666

9.2 Measuring Power

not least, will perform field tests in order to evaluate the system properties and to localize occurring faults. The requirement is, that the components have been produced and characterized in accordance to accepted standards. near field far field light source

numerical aperture

detector connector POF

connector loss

attenuation dispersion

Fig. 9.1: Important items to be measured in polymer fiber transmission

There are only a few specific standard for polymer fibers available. Measurements methods developed and specified for glass fibers will be used in most cases. Furthermore, there are a limited number of commercial available measurement devices, optimized for POF. That’s why, measurement devices and methods must be adapted to the POF’s requirements. Information in data sheets are often minimal. The rating and the reproducibility of the listed values are difficult, because the measurement conditions are not or not sufficient described. Under some circumstances however, these values strongly depends on the measurement conditions. For that reason, the following section shall give the user a tool to interpret and the values of a datasheet and to be able to repeat the measurements if required.

9.2 Measuring Power In optical communications, the optical power is stated either as a linear value in W (mW, μW or nW) or as a logarithmic value in dBm. The unit dBm refers to the absolute power level relative to 1 mW: x dBm 10 log

opt. power P >mW @ 1 mW

dBm can be converted to mW using the following relationship: P [mW ]

x [ dBm] 10 10

9.2 Measuring Power

667

Where the optical power is calculated on the basis of μW, the unit description for the logarithmic value is dBμ. Figure 9.2 shows a diagrammatic illustration of the relationship between mW and dBm. normalized logarithmic power (dBm) -30 0.001

-25

-20 0.01

-15

-10

-5

0.1

0 1

5

10 10

linear optical power (mW) Fig. 9.2: Comparison between the unit measures mW and dBm

Positive values for dBm refer to power which is greater than the reference value of 1 mW whereas negative values refer to power smaller than 1 mW. The difference of two power levels is stated in dB, for example, if the level is reduced by 3 dB, the linear power is reduced by 50 %. The advantage of the logarithmic method of notation can be seen in the fact that the difference in power level for 2 points in a link system represents the attenuation of the respective link in dB. The typical power levels in POF transmission systems lie between -2 dBm and -26 dBm, or respectively, 0.63 mW and 0.0025 mW. The power meters used in this range consist of semiconductor detectors. Silicon photodiodes are the most sensitive for the range of wavelengths having the smallest attenuation of the polymer optical fiber (approx. 500 nm - 700 nm). Since the responsivity of the detector depends on wavelength, the power measurement of these detectors is only valid for the wavelength indicated on the equipment. When measuring different wavelengths, a conversion factor must be applied. In practical applications the optical power measurement is the most important measurement task. It is used to check whether transmitters maintain the minimum specifications and whether the necessary minimum receiving power is achieved on installed transmission links. Various models of power-measuring devices are available from different manufacturers. Normally, they measure the light exiting from fibers with a large-surface Si photodiode. The display is possible in absolute values in mW or dB. A calibration function can be used to show the power difference to a reference value in dB. The various levels of sensitivity for standard wavelengths, mostly, 650 nm, 780 nm and 850 nm, can be programmed. Better devices can also measure the power of slowly modulated light in order to eliminate the effect of ambient illumination. In order to be able to use different connectors a number of adapters are as a rule available which can either be plugged in or screwed on. All customary devices have the disadvantage of being mode dependent to a certain degree since flat photodiodes are used. The devices can usually measure to exactly 0.1 dB for identical modal distributions and 0.5 dB for varying field

668

9.2 Measuring Power

distributions. The customary measuring range starts at -50 dB to -60 dB and ends at +3 dBm. An overview of different hand-held devices is shown in Fig. 9.3. The list is by no means complete and is quickly growing, more proof of the increasing importance of POF technology.

Fig. 9.3: Hand held power meters for POF (left to right) above: FO-Systems/Leoni, Tempo, Photom, Senko, OWL bottom: Scientech, Rifocs, Advanced Fiber Solutions, Fotec, Ratioplast

In addition, there is an entire series of special devices used for continuity tests and with numerous channels (examples in Fig. 9.4). The manufacturers also sell stabilized LED and/or laser diode transmitters for almost all power-measuring devices.

Fig. 9.4: Power meters from Bauer Engineering and Adaptronik

9.2 Measuring Power

669

Using transmitters it is often problematical in that the center wavelengths and spectral bandwidth are chosen quite arbitrarily. Some of the LED transmitters lie in an emission wavelength at about 665 nm. Here the PMMA POF already has about 100 dB/km more attenuation then at 650 nm. In addition, spectral and modal filter effects arise with LEDs (see chapter on system design). Exact attenuation measurements of POF can only be carried out to a limited extent with such combinations of devices. They are ideal, however, for field tests. Lengths up to 200 m of PMMA fiber can usually be measured with standard measuring devices without any problems whatsoever. In 1996, the Teleconnect Company introduced a measuring device in cooperation with Siemens Coburg, now Leoni Fiberoptics, with which up to 700 m of standard POF can be measured. The quality of the fibers can be measured in production since the lengths delivered normally amount to 500 m (>Ziem97c@). The essential components for attaining these extraordinarily great lengths are: ¾laser diode with a wavelength of almost exactly 650 nm (temperature stabilized) ¾+7 dBm fiber-coupled output power ¾112 dB dynamic range through an extremely sensitive photo-detector ¾at 125 dB/km a theoretical measurement length up to 896 m Figure 9.5 shows the device.

Fig. 9.5: Attenuation meter with 700 m measurement range

A large number of special devices have been developed for the measurement of attenuation in cables for vehicle networks. Besides accuracy what matters here are fully automatic operation and short measuring times. Thanks to the short cables sensitivity plays a subordinate role. The OptiTest 10 measuring device from Schleuniger is shown in Fig. 9.6 on the left. The attenuation measuring station of the IDC 9600 MS processing system from Komax can be seen on the right (www.komax.ch).

670

9.3 Dependence on the Launch Conditions

Fig. 9.6: Measurement setups for attenuation on POF (Schleuniger, Komax)

A “golden fiber”, i.e. a reference fiber produced and measured with great precision, is used in these systems for calibration. Together with active connector positioning of the fiber to be measured an accuracy of 0.02 dB can be attained (data from the website www.schleuniger.de).

9.3 Dependence on the Launch Conditions The measuring devices in the section above do not as a rule use specifically optimized launch conditions. The transmitter is simply positioned directly at the fiber input. We will subsequently show that this procedure does not suffice for exact measurements. The transmission characteristics of an optical fiber are determined by attenuation and dispersion. The measured value for attenuation and/or dispersion depends on the light introduced into the polymer optical fiber; for this reason it is necessary to create reproducible launching conditions. It means that the distribution of the optical power onto the modes excited in the optical fiber must be known. If the complete core area and numerical aperture are illuminated uniformly (full filled launching), all modes will carry initially the same power (UMD = Uniform Mode Distribution, blue curves in Figs. 9.7 and 9.8). During the further passing of the light through the fiber, the rays propagating at a larger angle to the axis of the fiber experience greater attenuation than the rays with the lower angle since they have to travel a longer path and are reflected more often on the interface between the core and the cladding. For example: in a fiber with AN = 0.5, ncore = 1.497 and a core radius of 0.5 mm, the core ray, which is still just about reflected completely, runs at an angle of 19.5°; over a length of 1 m the ray will be reflected approximately 350 times at the interface between the core and the cladding. Due to inhomogeneities at the core/cladding interface as well as in the core material, it is possible that power may propagate in two different directions (mode coupling). In addition mode conversion will cause power to be exchanged between the different propagation directions at bends within the optical fiber. These effects lead to a change in the mode distribution excited at the beginning of the fiber. After a cer-


671

tain distance, a steady state distribution is achieved and from thereon mode distribution remains constant (EMD = Equilibrium Mode Distribution, red curves in Figs. 9.7 and 9.8), provided no such faults occur that would again lead to mode coupling effects. If the excitation is with a small numerical aperture (green curves in Figs. 9.7 and 9.8) there will also be EMD after a certain length due to the fact that higher order modes are created. optical power

UMD: all mode guide the same optical power EMD: equilibrium mode distribution exitation with small numerical aperture far field angle

Fig. 9.7: Mode distribution for different types of excitation (schematic representation)

The length dependent attenuation in relation to the fiber position is shown in Fig. 9.8. In the case of an over filled launch, the attenuation curve is hyper linear up to the coupling length, for under filled launch it is sub linear. A real measurement result will be shown later. 600

attenuation [dB/km]

500 over filled launch

400

equilibrium mode exitation

300 200 100 under filled launch length [m]

0 0

50

100

150

200

Fig. 9.8: Length-dependent attenuation relative to the length of the fiber, for different mode distributions (schematic representation, no experimental results)

672


Figure 9.9 shows the difference in attenuation between the rays launched parallel to the fiber axis and those at the maximum possible angle of launching (core radius a = 0.5 mm, ncore = 1.497). Assuming equal distribution of the mode to the propagation angles, and considering modes at launching angles from 0° to 20° (half of the possible modes) one obtains an average attenuation increase of up to 2.5 dB/km, whilst with the inclusion of the higher order modes a change of up to 6 dB/km occurs - in both cases relative to the basic attenuation of 100 dB/km. Only that part of the attenuation attributable to the longer path (volume attenuation) has been taken into account here. Further effects which contribute to mode dependent attenuation (mode conversion and mode coupling) are described later. This simple example demonstrates the difficult situation that exists when trying to carry out measurements that are independent of launching conditions. 12 10

excess loss, related to a basic attenuation of 100 dB/km [dB/km]

8 6 4 2 launch angle [°] 0

0

5

10

15

20

25

30

35

40

Fig. 9.9: Relative change of attenuation in relation to the launching angle (only caused by the difference in the propagation path)

For real polymer fibers as well as for PCS, however, there is also the very much stronger effect of high losses for rays with a large propagation angle caused by the attenuation of the cladding material. This cladding attenuation with PMMA POF lies around some 10,000 dB/km. Measurement results for PCS were shown in Chapter 2. Figure 9.10 shows the number of modes in an SI-POF with the above mentioned parameters (650 nm wavelength) in relation to the maximum launching angle considered. In order to achieve reproducible results with measurements for attenuation and dispersion, the modes should be in equilibrium mode distribution. This can be achieved by using a launching fiber, although this is not very practical for polymer fibers since the required length of 30 m to 60 m would lead to high additional attenuation (6 dB to 12 dB for 200 dB/km attenuation). The dynamics of the measurement setup is reduced by this value. For comparison: for glass fibers, the length of the launching fiber is in the range of 1 km to 2 km producing an additional loss of 2 dB to 4 dB (for 2 dB/km of attenuation).


4.5

673

number of modes [u 106]

4.0 3.5 3.0 2.5 AN = 0.50

2.0 1.5 1.0 0.5 0.0

launch angle [°] 0 0

5

10 5

15

20

10

25

30

35

40

15 20 25 propagation angle in the fiber [°]

Fig. 9.10: Number of modes in relation to the launch angle

Another possibility is coupling the light through a suitable optical arrangement with full core illumination and a numerical aperture corresponding to equilibrium mode distribution. Figure 9.11 illustrates an optical launching arrangement, which allows the independent adjustment of the numerical aperture and the spot diameter. However, in this method it is necessary to have the knowledge of the conditions for equilibrium mode distribution, which are different from fiber to fiber. NA aperture light source

spot aperture POF under test

Fig. 9.11: Optical launching arrangement with independently adjustable numerical aperture and spot diameter

In practical applications, a mode mixer (Fig. 9.12, see Chap. 2) is frequently used, which meets the Japanese Industrial Standard JIS 6863 or IEC 60794-1-1 Annex A. It consists of two cylinders with a diameter of 42 mm each and a distance of 3 mm from each other, around which a standard polymer fiber of 3.50 m or 20 m length is wound in ten loops. The following effects occur: ¾Due to the bends, higher order modes are radiated (radiation modes). ¾Modes are converted into each other due to the bends (mode conversion). ¾Due to faults at the interface between core and cladding, it is possible that several modes can be created from one mode (mode coupling). This process is dominant and depends on the respective type of fiber. ¾The insertion loss is approximately 4 dB.

674

9.4 Measurement of the Optical Parameters

Fig. 9.12: Mode mixer in accordance with the Japanese Industrial Standard JIS 6863

Apart from the mode mixer described above, other set-ups are also in use, for example the roll mode mixer ([Fus96]). However, these have not been standardized. For DSI fibers such a mode mixer does not achieve the desired result, since these would require a much smaller bending radius. Analysis shows ([Pfl99]) that bending radii of less than 15 mm would have to be used, which would lead to a high attenuation and is therefore not practicable. Consequently, this type of mode mixer should not be used for DSI fibers, but instead one should only measure attenuation up to a certain minimum length. This length must be established individually for each type of fiber using, for example, the cut-back method (see below).

9.4 Measurement of the Optical Parameters The methods for measuring the different optic characteristic quantities will be described in the following sections together with some examples of measurements. Here the effects of various launch conditions will be particularly emphasized just as with the measurement of the bandwidth (Chap. 2). The different experimental results have diverse practical significance, e.g. for coupling attenuations, for determining system ranges and capacities, and for the qualitative characterization of fibers and active components. The following will be dealt with in detail: ¾Near field distribution ¾Far field distribution ¾Inverse far field ¾Index profile ¾Optical attenuation ¾Optical time domain reflectometry ¾Dispersion


675

9.4.1 Near Field The near field describes the power distribution of the light in the output face of the optical fiber. It can either be measured via an enlarged image or scanned with a suitable optical fiber. Figure 9.13 illustrates a suitable installation, in which it was possible to achieve a 55 dB dynamic ([Gie00]). In this installation, a singlemode fiber (core diameter 9 μm), which is driven by a step motor, is guided radially along the fiber surface. The signal is received by a highly sensitive detector. The test installation has a dynamic range of approximately 60 dB. Figures 9.14 and 9.15 show the near field patterns of a standard NA-POF and a multicore POF recorded with this arrangement. manual positioning

sampling fiber SM-GOF; 9 μm horizontal translation stage

fiber connector

step drive POF

Giehmann T-Nova 2001

manual positioning

Fig. 9.13: Setup for near field scan measurement

0

rel. optical power [dB]

-10 -20 -30 -40 -50 -60 0

Giehmann T-Nova 2001

250

500

fiber sensor position [μm] 750

1000

1250

1500

Fig. 9.14: Near field pattern of a standard NA-POF, illuminated with an LED (O = 560 nm)

676 0.08


rel. optical power (linear)

0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00

© Giehmann

0.00

fiber sensor position [mm] 0.25

0.50

0.75

1.00

1.25

1.50

Fig. 9.15: Near field pattern of a DSI-multicore POF

The scanning path in Fig. 9.15 runs across the center point of the MC-POF and captures seven individual fibers (see Fig. 2.47). However, it is not aligned to the center of the individual fibers, which is why some fibers have been only partially covered. Commercial devices for measuring the near field distribution have become available in the meantime, for POF and PCS as well. The LEPAS measuring system from the Japanese manufacture Hamamatsu is used at the POF-AC Nürnberg (see >Bach02@). Not only is the detection of the entire cross-section of the fiber is important for correct near field measurements, but also that all propagation angles occurring are taken into consideration. Image-forming systems with a small NA have been used in many published measurements of fibers. The consequences of such an error are shown in Fig. 9.16 for the measurement of a GI-POF.

GI fiber

measurement result with a sufficient NA

Irel.

measurement result with a to small NA T -Tmax

T

+Tmax

Fig. 9.16: Measurement of the near field of a GI-POF using a to small NA system


677

Modes with different angles occur in the center of GI fibers, almost only paraxial rays are at the edge. If the measuring system only picks up the small angles, then a much too small intensity is established in the center. The near field distribution apparently corresponds to that of a SI fiber. The opposite effect can come about if the optics cannot detect the necessary angular range over the entire crosssection. What can happen here is that all modes are measured in the center of the fiber and only part of them on the fiber edges. The near field would then look like that of a GI fiber. In order to minimize such effects, a special combination of lenses has been produced by the Sill-Optik Company for the LEPAS system which permits the correct measurement of 1 mm POF. The optics and the image-forming principle are shown in Fig. 9.17.

objective lens

relay lens

FOP + CCD-Chip

Fig. 9.17: Near field optics and operation principle

The optics display an image of the output end faces on a CCD chip, with an enlargement of about 5. The fiber optic plate (FOP) prevents interference patterns through the chip cover. The technical data of the system are: ¾Acceptance angle ±30° ¾Resolution > 2 μm (numerically) ¾Magnification approx. 5.5 ¾Wavelength range 400 nm - 1100 nm ¾Working distance 13.8 mm In order to test whether or not the optics corresponded to the requirements, a thin (0.2 mm) laser beam (divergence < 0.1º) was positioned in the measuring level of the system, on the sides as well as at different angles. Figure 9.18 shows the results for 7 different positions.

678

9.4 Measurement of the Optical Parameters 0°

-30° 1.0

0°

+30° -30°

0°

+30° -30°

0°

+30° -30°

+30°

0.8 0.6 0.4

x= 0.2 -0.9 mm

x= -0.3 mm

0,0 -30°

x= -0.6 mm

0°

x= +0.3 mm

+30° -30°

x= 0.0 mm

0°

x= +0.9 mm

+30° -30°

0°

+30°

x= +0.6 mm

Fig. 9.18: Test of the near field optics

At a distance of 0.9 mm to the system axis only a very slight part of the angle was detected. Within a range of ±0.3 mm practically the complete angular range is measured. The system can produce satisfactory results for fibers with a diameter of up to 1 mm and a NA up to 0.50.

Fig. 9.19: Examples for near field measurements (approx. 1 m and 30 m MC-POF)

Two examples of near field measurements are shown in Fig. 9.19. Here a 37core MC -POF was measured after a short stretch (on the left) and after a long link. The attenuation can be determined for each individual fiber. In particular, the higher attenuation at the fiber edges could clearly be seen in this measurement. If this measuring system is combined with a corresponding pattern recognition for identifying the cores, then the individual fiber attenuation can be determined automatically.


679

9.4.2 Far Field The far field is defined through the field distribution at the distance D » 2a from the output face of the fiber (Fig. 9.20). intensity B

4max a

light source with launching optics

D

screen

Fig. 9.20: Idealized far field of the step index profile fiber

The angle 4max is calculated from the relationship tan 4max = B/D. The numerical aperture NA is calculated from the far field angle 4max using the following formula: sin 4max

AN

ncore 2 ncladding 2

All modes carried in the core are excited in this case (full mode launch), i.e. meridional rays as well as skew rays. The far field represents the intensity distribution on a spherical surface, in the center of which the light output face is located. The measurement is carried out with a photo-detector, selecting the respective angle. The angle resolution depends on the angle range covered by the detector area. Figure 9.21 shows a possible test installation for capturing the far field in one plane.

light source with launching optics

4 detector fiber

Fig. 9.21: Principle of the far field measurement

680


In order to achieve a three-dimensional representation, it is necessary to scan the complete half-space, which requires very extensive measuring. As a rule, one can assume a symmetrical radiation pattern for fibers. Hence it is sufficient to record the intensity curve in the sagittal and meridional section. It is possible to obtain fast results by using the far field measurement setup ([Klo98]) shown in Fig. 9.22. This system uses a fiber-optical measuring head with an arced element enclosing a circular sector of r80°. On this element with a radius of 35 mm and an angle distance of 0.5 degrees 321 silica glass fibers are arranged with a core diameter of 100 μm; they are arranged in such a way that the optical axis of all fibers point to the center of the circle sector. The ends of the optical fibers are bundled together and point to a CCD camera system which records the radiation from the fibers. In order to suppress scattered light, this fiberoptical arrangement is completely enclosed. A signal processor integrated into the system processes the measured values. The serial interface (RS232) makes it possible to communicate with the signal processor. The measuring head is mounted on a precise rotating turntable and can be rotated by 90° so that it is possible in a few seconds to record the far field in the meridional and in the sagittal planes. Where the intention is to obtain a three-dimensional measurement of the far field, the arced element is turned around its optical axis from 0° to 180° with the help of a step motor. The step range of the motor is 0.9°. The recording of a 3D presentation takes approximately 10 minutes ([Klo98]). The instrument was offered commercially by the company GMS.

computer CCD camera fiber bundle

fiber array with 321 fibers

rotatable turntable (by 180°)

polymer fiber Fig. 9.22: Fast far field measuring setup with the Emitor

Four different 1 mm POFs were measured under the same conditions for a test (the parameters are in Table 9.1).


681

Table 9.1: Data of the fibers used in Fig. 9.23 Name of the Fiber Type characterization producer

PFU-CD1000

AC-1000W

MH-4000

NC-1000

standardSI fiber Toray

DSI fiber

DSI fiber

Ashahi Chemical 0.32

Mitsubishi Rayon 0.33

Low-NA fiber Ashahi Chemical 0.25

numerical apertue

0.46

Since it is difficult to determine the actual zero value of intensity, a convention exists to use the value at which the intensity has dropped to 5% of the maximum value (partially at 10% too). Figure 9.23 shows the far fields of different fiber types. The signal has been normalized to the maximum measured value in the far field. The fibers are excited with a numerical aperture AN = 0.5 ( 30°). Compared to the launching, the radiation angles are significantly lower for all fibers. After a fiber length of 50 m, different far field widths result for the different types of fiber. This is caused by a different development of the mode distribution up to equilibrium mode distribution through mode coupling and mode conversion, resulting in different coupling lengths.

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

norm. intensity PF-U-CD1000 AC-1000 NC-1000 MH-4000

-40

-30

-20

-10

0

10

20 30 40 far field angle [°]

Fig. 9.23: Far field patterns of different fibers, light source LED 650 nm, 50 m fiber, according to [Hen99]

The 10% far field width of the SI-POF PFU-CD1000, with a numerical aperture of 0.42, is conspicuously wide. This fiber features an NA of 0.46. [Bun99b] and [Pei00a] state a coupling length of 36 m for the PFU-CD1000. When using the length of 50 m with this fiber, the state of equilibrium mode distribution has already been reached. Also, compared to the other fibers, the intensity distribution

682


rises steeply from zero, whilst the other measured curves show a bell shaped increase. This indicates that the proportion of power of the leaky waves is small and hence makes only a negligible contribution to the widening of the far field. Figure 9.24 shows the 10% far field width in relation to the bending radius for various numbers of windings. Launching was carried out with AN = 0.5. This investigation is the basis for setting up a mode mixer, as described, for example, in the Japanese Industrial Standard JIS 6863 and in the IEC 60794-1-1 (Annex A). As expected, the numerical aperture reduces most with the bending radius at 10 windings. At a bending radius of 21 mm the numerical aperture is 0.42.

0.50 measured numerical aperture 0.48 26 mm 0.46 21 mm 0.44 15 mm 0.42 12 mm 0.40 2 turns 0.38 4 turns 0.36 6 turns 0.34 8 turns 0.32 inverse bending radius [1/mm] 10 turns 0.30 0.03 0.04 0.05 0.06 0.07 0.08 0.09 Fig. 9.24: Numerical aperture (10% far field width) in relation to the inverse bending radius for the PFU-CD-1001 fiber

Figure 2.147 shows the excess loss in relation to the inverse bending radius for different numbers of windings. For the conditions of the above mentioned standards (bending radius = 21 mm, 10 turns) the excess loss is approximately 2 dB. Figure 9.25 shows the change in the far field of the DSI fiber MH 4000 for different numbers of windings, starting at 0. With two windings, the far field is significantly narrower than without any winding. At this bending radius, the higher order modes are emitted to a larger degree and hence do not contribute to the far field. This effect occurs with a bending radius smaller than 15 mm [Hen99]. Due to the length of the fiber, any mode coupling must be largely precluded. For two windings, the numerical aperture is approximately 0.30 and for no winding it is 0.44. This is due to the double-step index profile of the fiber: without the winding, the light is also conducted within the inner cladding; the bends cause the angle of total reflection between the inner and outer cladding to be exceeded for a large proportion of the rays so that the light reaches the outer cladding and is lost through radiation. Now the propagation characteristics are determined by the refractive index difference between core and the inner cladding, corresponding to a numerical aperture of approximately 0.30. The equilibrium mode distribution must still be formed in the core.


1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

norm. intensity

-40

-30

-20

683

0 turns 2 turns 4 turns 6 turns 8 turns 10 turns

-10

0

10

40 20 30 far field angle [°]

Fig. 9.25: Far field patterns of the DSI fiber MH 4000 for different numbers of windings (bending radius r = 12 mm, light source LED at 650 nm, length of fiber 4 m)

[Hen99] investigates the use of a mode mixer for DSI fibers. As a condition for equilibrium mode distribution for the fiber MH4000, a bending radius of 9 mm results, leading to an additional loss of 18 dB for 10 windings. This high value is not acceptable for attenuation and dispersion measurements so that in DSI fibers it is only possible to achieve equilibrium mode distribution with the help of mode mixers at the expense of a high attenuation. There are also in the meantime a number of commercial devices for the twodimensional measurement of far field distributions. The POF-AC uses the LEPAS system here as well as for near field measurements. The corresponding optics and the ray trajectory are shown in Fig. 9.26.

FOP + CCD-Chip f-T lens

field lens

relay lens

Fig. 9.26: Far field optics of the LEPAS system with ray trajectories

684


The f-ș lens converts the different angles at the location of the intermediate image which is then projected onto the CCD chip with the following microscope array, attaining the following parameters: ¾Acceptance angle ±45° ¾Angle resolution 0.18° (numerically) ¾Wavelength range 400 nm - 1100 nm ¾Working distance 2.8 mm An example in Fig. 9.27 shows the far fields of a 1 mm standard POF with coupled collimated light each with an angle of 10º after 10 m and 100 m respectively. The ring structure can clearly be seen which after 100 m has for the most part been resolved through mode mixing.

Fig. 9.27: Far field of a SI-POF for excitation with 10° after 10 m and 100 m

9.4.3 Inverse Far Field With the inverse far field method described in [Gies98] one can obtain even more detailed information about the light propagation than with the method described above. In this method one not only measures the radiated light selectively but also launches it at selected angles into the fiber (Fig. 9.28).

fiber Tin Fig. 9.28: Principle of inverse far field measurement

Tout


685

The advantage of this process lies in the fact that the launching spot with a small diameter (some tens of μm) and a small numerical aperture (AN | 0.02) can be placed at the desired spot of the fiber's end face, thereby making it possible to excite certain specific mode groups. With this type of excitation only meridional rays are excited, no skew rays. Figure 9.29 shows the far field distribution of a standard NA-POF after 10 m and 50 m length of fiber at launch angles of 15°, 20° and 25°. The steep rise in the far field curve indicates that no leaky modes were excited. Whilst at an launch angle of 15°, the power is completely distributed to the smaller angles after 50 m; at 20° and 25° launch angle, the domination of these mode groups can still be clearly recognized.

l = 10 m; O = 594 nm, St.-POF

l = 50 m; O = 594 nm, St.-POF

norm power [%] 100

norm power [%]

15°

20°

100

25°

80

80

60

60

40

40

20

20

0 -40 -30 -20 -10 0 10 20 30 40

far field angle [°]

20

25°

15°

0 -40 -30 -20 -10 0

10 20 30 40

far field angle [°]

Fig. 9.29: Far field patterns of a standard fiber with different launch angles ([Kle98])

For the DSI fiber (Fig. 9.30) the situation is different. In this fiber, all of the power has gone over into the core from the inner cladding after 50 m so that at a 15° launch angle the far field width has nearly reached the numerical aperture corresponding to the refractive index difference between core and inner cladding. In Fig. 9.31, the launching angle is changed by 1° between -30° and +30° each time and the overall intensity of the emerging light is measured with a large area detector. In the case of standard NA fibers (diagram on the left), an almost rectangular far field profile for fiber lengths of 1 m and 10 m changes to a parabolic form for greater lengths; correspondingly, the far field angle reduces (for the shorter lengths this is approximately 26°) (AN = 0.44). This is caused by the stronger attenuation of the higher order modes. After 50 m, equilibrium mode distribution has been reached.

686


L = 10 m; O = 594 nm, DSI-POF

L = 50 m; O = 594 nm, DSI-POF

norm. power [%]

norm. power [%]

100

20°

15°

100

25°

80

80

60

60

40

40

20

20

20° 25°

15°

0 -40 -30 -20 -10

0

10

20

30

0 -40 -30 -20 -10

40

0

10

20

30

40

far field angle [°] far field angle [°] Fig. 9.30: Far field patterns of a DSI fiber under different launch angles [Kle98]

The diagram on the right in Fig. 9.31 shows the result for a DSI fiber. With increasing length, the rays propagating under larger angles are heavily attenuated so that at 50 m and at 90 m the far field angle is 18° (AN = 0.32).

O = 594 nm, St.-POF

O = 594 nm, DSI-POF norm. power [%]

norm. power [%] 100

1m

1m

80

10 m 60

10 m 50 m

40

50 m

90 m

20

90 m 0

-30

-20

-10

0

10

20

30

-30

-20

-10

launch angle [°]

0

10

20

30

launch angle [°]

Fig. 9.31: Far field patterns in relation to launch angle [Kle98]

The rays with a large angle are reflected at the interface between the inner and outer claddings. Since the inner cladding has a higher attenuation compared to the core, the rays propagating in this have been attenuated so strongly after 10 m that they can no longer be traced in the far field. The manufacturers call the DSI fibers ‘low-NA fibers’; the numerical aperture is stated as 0.30, without making reference to respective lengths.


687

9.4.4 Index Profile A very important measurement for polymer fibers is that of the index profile. The index curve has to be checked regularly in the core, especially with GI-POF which is produced by diffusion. For the measurement the end face is either directly scanned or the fiber is radiated through laterally. An overview of the different methods for measuring the refractive index profile of optical fibers with lateral radiation is given in >Bun04a@. One method particularly well-suited for POF is based on the scanning of the fiber end face with a beam as closely collimated as possible (determine the resolution). The NA of the light has to be adapted to the NA of the fiber investigated. Two detectors measure the light passing through the fiber as well as a reference beam. The refractive index dependant on location can then be calculated from the difference. This method only produces good results when the fiber end face has been prepared exactly smooth and flat. The next two figures, 9.32 and 9.33, show the measurement results on a multistep index POF and a Semi-GI-PCS. This method of measurement results in only relative differences in the refractive index unless there has been a calibration beforehand on exact samples which are known. 1.0

nrel.

0.8 ESKA-MIU sample 2003 Mitsubishi

0.6 0.4 0.2 0.0 -400

x [μm] -300

-200

-100

0

100

200

300

400

Fig. 9.32: Refractive index profile of a MSI-POF 1.0

nrel.

0.8 0.6 0.4 0.2 x [μm] 0.0 -120 -100 -80

-60

-40 -20

0

20

40

Fig. 9.33: Refractive index profile of a Semi-GI-PCS

60

80

100 120

688


9.4.5 Attenuation Optical attenuation describes the loss of light between the input and output of an optical component, i.e. also of a fiber. In principle, only two power measurements are necessary so that the explanations in Section 9.2 should suffice. The problematical nature of this aspect is actually very much more complicated. The measuring processes for attenuation provide as a result both the overall attenuation which is of considerable interest to the system designer as well as the individual contributions due to absorption and Rayleigh scattering, which provide the manufacturer with important information for the purpose of process optimization. Characterization can be carried out for one or more discrete wavelengths (for example, for the link wavelength) or continually for a larger spectral range. 9.4.5.1 Insertion and Substitution Methods Different methods can be employed for measuring attenuation: the insertion method (non-destructive), the substitution method (non-destructive) or the cutback method (destructive). Since it is not possible to determine the input power P0 exactly, one compares the output power with and without test fiber (Fig. 9.34). At first, the light power PL2 is measured at the end of the optical fiber (test fiber). Subsequently, the transmitter and receiver plugs are connected with each other and the power PL1 is determined. The attenuation is calculated as follows: Insertion loss =

§P · 10 log ¨¨ L1 ¸¸ L © PL 2 ¹

D fiber D connector

With this method it is possible to measure the attenuation of the inserted cable including connectors. In order to be able to determine the fiber attenuation, the excess loss of the connector must be known. fiber under test coupling

L

pigtail

R

T PL2 coupling pigtail

T Fig. 9.34: Measuring insertion losses

PL1

R


689

power [dB] connector loss

insertion loss

connector loss L

length [km]

Fig. 9.35: Determining losses with the insertion method

A similar process is followed in the substitution method: at first the power PL2 is determined. Subsequently, the fiber is replaced by a short reference length and the power PL1 is determined. For this purpose the arrangement and characteristics of test and reference fiber must be identical. In contrast to the insertion method however, the number of connectors remains constant, which is why it is possible to determine the fiber losses without connectors. However, it is a requirement that the attenuation between the used connectors is constant. This is only possible within in a certain tolerance, since, depending on the type of connector, the values can diverge by up to several dB. This can therefore lead to significant errors in kilometric fiber attenuation measurement when measuring short fiber lengths with this measuring method. reference fiber pigtail

coupling

L1

PL1

T L2

receiver

transmitter fiber under test Fig. 9.36: Example of test arrangement for substitution method

Attenuation is calculated as follows: D

10 P log L1 L 2 L1 PL 2

R

PL2

690


Level [dB]

PL1

L1

loss of the fiber under test

reference fiber

PL2 L2 - L1 L2

length [km]

Fig. 9.37: Using the substitution method to determine attenuation

9.4.5.2 Cut-Back Method The cut-back method provides more accurate results than the insertion and substitution methods. In this method, at first the output power PL2 of the fiber is measured for the length L2; subsequently, the fiber is cut after a length L1 (typically 1 m after the light source) and the power PL1 is determined. The advantage of this method lies in the fact that launching conditions remain unchanged. Attenuation is calculated in the same way as with the substitution method. The advantage of the substitution method is that no destruction takes place method.

fiber under test pigtail

L2

PL2

T

R

receiver transmitter

L1

PL1

Fig. 9.38: Using the cut-back method to determine attenuation

9.4.5.3 Measuring Attenuation for Discrete Wavelengths Where the intention is to ascertain only the attenuation for a certain wavelength, a semiconductor laser or an LED can be used. In both cases the emitting characteristic of the component needs to be taken into account in order to ensure suitable coupling to the fiber. If a LED is used as the light source, there are some differences compared to the measurement of attenuation with monochromatic light. Due to the wide spectral


691

width of the LED of approximately 20 nm and more, it is possible that a filter effect occurs in combination with the attenuation spectrum. This is particularly conspicuous in the window around 650 nm, since the edges of the PMMA fiber attenuation rise steeply in this range. This has the effect that the width of the LED spectrum is reduced while passing through the fiber and that the peak wavelength is shifted to the attenuation minimum of the fiber unless it is there already. Due to the fact that the light energy of the LED is distributed over a relatively large spectral range, the edges of the LED spectrum experience much stronger attenuation especially in the 650 nm window, leading to a significantly higher attenuation compared to monochromatic measurements. Figure 9.39 shows the filter effect of the PMMA fiber. The blue curve represents the spectrum of a LED with a FWHM spectral width of 21 nm and a peak wavelength of 646.7 nm. The green curve represents the LED spectrum after 50 m ESKA EH 4001; the spectral width has dropped here to 14.4 nm and the peak wavelength has shifted to 650.6 nm. The deviation is particularly large when the peak wavelength of the LED is not exactly 650 nm, i.e. an even larger proportion of the light power is in areas with significantly higher attenuation. Therefore a correction must be carried out. The steps described below can be used to ascertain a correction factor, with the help of which it is possible to correct the result of the measurement. 600

attenuation [dB/km] LED spectrum at the fiber input

LED spectrum at the fiber output

500

P/P0 1.0

400

0.8

300

0.6

200

0.4

100

0.2

0 550

600

650

0.0 700 wavelength [nm]

Fig. 9.39: Filter effect of the attenuation spectrum of a POF (ESKA EH 4001, 50 m)

The area under the bell shaped curve represents the total optical transmission power of the LED. The LED spectrum P(O) is required. This needs to be normalized so that the following applies: f

³ PO dO 0

1

692


In addition, one needs to know the attenuation spectrum D(O) of the POF. This is described, for example, in [Wei98]. However, it is important that the spectrum used is identical to that of the POF or diverges from this only by a constant attenuation coefficient. In order to determine the correction factor for the attenuation Dmeasured for a required wavelength O0 (for e.g. 650 nm) and a certain length of fiber L, one proceeds by forming the following integral: § D O L · ¨ ¸ PO ¨10 10 ¸dO ¨ ¸ 0 © ¹

f

Peff

³

The result obtained is the attenuation of the total spectrum of the LED in dB (DLED = 10 log (Peff)). The correction factor is then calculated as follows: (see Section 7.3.1 also): correction faktor K F

DLED DO 0

9.4.5.4 Measuring Attenuation over a Larger Spectral Range Where it is intended to measure the attenuation over a larger wavelength range, a monochromator is used. The principle means of splitting an optical spectrum are prisms and diffraction gratings. Generally, modern monochromators are grating monochromators, which is why we will describe these briefly below. Their schematic construction is shown in Fig. 9.40. The input light is focused on the entrance slit, changed into a parallel bundle by a concave mirror and reflected on to the grating, from there it is thrown on to another concave mirror from where it is focused on the exit slit (Czerny/Turner arrangement). The grating is mounted in such a way that it can be turned about its vertical center point axis. This causes the spectrum of the light diffracted at the grating to be directed along the output slit.

entrance slit concave mirror grating exit slit

concave mirror

focal length f Fig. 9.40: Schematic structure of a monochromator


693

The reflection grating consists of a glass substrate in which parallel grooves are ruled, either mechanically with a ruling engine or holographically by superimposing two coherent laser beams. Subsequently, the grating is coated with a highly reflective medium. There are always several diffraction orders so that, with the same setting of the grating, wavelengths of, for example, 400 nm and 800 nm appear at the output slit. In this case, the interfering light must be eliminated with an edge filter. By giving the grooves a specific shape and depth, one achieves that a maximum of light is reflected into the first diffraction order. The grating is optimized for a certain wavelength range and is highly efficient in that range. The wavelength at which the grating is at its most efficient is called blaze wavelength (Fig. 9.41). 90

grating efficieny [%]

80

blazed grating

70 60 50 40

holographic grating

30 20 10 0

400

600

800

1000

1200


Fig. 9.41: Application ranges of diffraction gratings

The most important parameters of a monochromator are: 1. 2.

The range of wavelengths at which the monochromator transmits light. The dispersion, which indicates to which degree the light appears spectrally split at the output slit. This is expressed by the linear reciprocal dispersion, Drec in wavelength differential 'O [nm] per slit width 'x [mm]: 'O/'x = (d cos E)/(f m), where d: reciprocal grating constant, E: angle at the diffraction grating for which the diffraction order m occurs, f: focal length of the monochromator. Example: assuming a grating with 600 lines/mm, f = 200 mm, m = 1, E = 16°, which will result in the linear reciprocal dispersion Drec 'O/'x = 8 nm/mm. This means that for a slit width of 1 mm the spectral width of the light emerging from the exit slit is 8 nm and for a slit width of 0.5 mm it is 4 nm.

694 3.

4. 5.


Resolution: the minimum achievable spectral width 'O, determined by the resolution of the grating, the focal length of the monochromator and the minimum adjustable width of the output slit. Aperture ratio: mirror diameter/focal length. Blaze wavelength of grating OB, spectral position of maximum of reflection.

One of two basic configurations can be used to carry out an attenuation measurement with a monochromator: 1. 2.

The light is introduced into the polymer optical fiber from the source, and the light emerging from the fiber is analyzed with the monochromator. The light that has been split spectrally in the monochromator is coupled into the fiber and measured at the end of the fiber with a detector.

The first configuration is illustrated in Fig. 9.42. The cone of light emerging from the POF has a larger aperture angle than the monochromator. Besides, the cross section of the emerging ray is circular whilst the input slit of the monochromator is rectangular. This mismatch of the numerical aperture and the area shapes lead to significant losses where the fiber is directly coupled to the monochromator (Fig.’s 9.43 and 9.44). fiber under test coupling optics connector

light source

mode converter

detector

cross area conversion monochromator

Fig. 9.42: Experimental setup for measuring attenuation

polymer fiber

diameter: 1 mm num. aperture: 0.50

Fig. 9.43: Example for mismatch of area shapes

spectrometer slit: 0.5 × 10 mm


polymer fiber core diameter: aum. aperture: aperture angle:

monochromator: grating size: num. aperture: aperture angle:

980 μm 0.50 60°

695

f = 10 cm 32 u 32 mm 0.16 18°

60° 18°

slit Fig. 9.44: Example for mismatch of numerical aperture

By guiding the light through a lens, it is possible to increase the light spot; however, the numerical aperture is decreased. This serves to adjust the numerical aperture. If, for example, the cross sectional area of the fiber is reproduced in 3-fold magnification, the NA is reduced from 0.60 to one third, i.e. 0.20. Figure 9.45 shows the estimated losses for different mismatches between area shapes and numerical apertures. The losses are calculated as follows: D Fl

10 log

D NA

12

area fiber overlappin g area monochroma tor

§ AN fiber 10 log ¨ ¨ AN monochroma tor ©

· ¸ ¸ ¹

2

loss [dB] total mismatch

10 8

area mismatch

6 4

mismatch of the numerical aperture

2 0 1.0

magnification 1.5

2.0

2.5

3.0

3.5

4.0

4.5

Fig. 9.45: Mismatch dependent on magnification (slit width 0.5 mm, fiber diameter 1 mm, NA of fiber 0.50, NA of monochromator 0.16)

5.0

696


When coupling the fiber directly to the monochromator slit, a minimum loss of 7 dB is achieved at the stated values of the set-up. Smaller losses result when using a cross section transformer, which transforms a circular area into a rectangle. It consists of a silica glass fiber bundle with thin glass fibers of approximately 1 m length, the ends of which are arranged in circular shape at one end and in rectangular shape at the opposite end (Fig. 9.46). Due to the numerical aperture of the bundle of 0.22 and a diameter of 3 mm, it is possible to achieve an optimum adaptation of the POF by using a lens. The end with the rectangular arrangement of fibers coincides with the width (0.5 mm) of the monochromator slit. The remaining loss due to the mismatch between the numerical aperture between the quartz glass fiber bundle and the monochromator is 2.8 dB.

Fig. 9.46: End formations of fiber bundle

The detector is either attached directly to the monochromator output slit or is illuminated via a suitable lens system. This test set up lends itself for measuring the attenuation with the insertion and the substitution method; the cut-back method is less suitable for this set-up as one has to repeat the preparation and positioning of the fiber's end face after each cut-back. This means, that the advantage of this method is lost since the conditions are changed at the input and output sides of the test fiber. The second basic configuration is illustrated in Fig. 9.47: the light of a white light source is split into its spectral components by a monochromator and launched into the fiber to be investigated via a cross section transformer and an optical adaptation arrangement (Fig. 9.48). Detection is carried out with an integrating sphere and a photodiode or a photomultiplier. Well reproducible results were achieved with the cut-back method. This set up was optimized for standard SI-POF with AN = 0.50 so that the use of a mode mixer did not result in any additional improvement but only caused additional attenuation and hence a reduction in the dynamic range. The measuring set up is also suitable for fibers with a different numerical aperture. However, it is in each case necessary to adapt the numerical aperture of the launching optics.


light source

glass fiber bundle

aperture and cross area conversion connector

697

integration sphere

monochromator fiber under test detector Fig. 9.47: Schematic setup for measuring attenuation in polymer optical fibers glass fiber bundle = 3 mm, AN = 0.17

lens polymer fiber, length: 20 cm 1 mm AN | 0.5

ray trajectories

Fig. 9.48: Adaptation of the numerical aperture and the cross section of the ray

The integrating sphere (Fig. 9.49) is a hollow sphere that is often coated with barium sulphate (BaSO4) on the inside. The coating causes multiple and diffuse reflections of the incoming light until it is evenly distributed over the sphere surface. After this integration, effects such as the angle of incidence, polarization, modes or shadow formation are eliminated.

fiber connector PMMA - fiber

shutters detector connection Fig. 9.49: Integration sphere

698


Hence all of the light entering the sphere from the fiber is captured by the detector. The two openings for the polymer optical fiber and the detector are orthogonal to each other. On the inside of the sphere there are shutters to prevent light from striking the photomultiplier directly from the fiber. The configuration described last offers the advantages that both insertion and substitution methods as well as the cut-back method can be used and when using the integrating sphere as a detector system, all of the radiated light is detected. This measuring system offers a dynamic range of 30 dB to 35 dB for a wavelength range of 480 nm to 700 nm with a slit width of 0.25 mm and using a photomultiplier as the detector. 9.4.5.5 Results of Measurements Measurements using the substitution method deliver results with a standard deviation of 10 dB/km, while the cut-back method delivers results with a standard deviation of 3 dB/km (Fig. 9.50). 15

standard deviation [dB/km] substitution method

10

cut back method 5

0 400

450

500

550

600


Fig. 9.50: Standard deviations obtained with the cut-back and substitution methods ([Pei00a])

When using the substitution method, particular care must be taken when preparing the fiber's end faces to ensure good surface quality. The selection of the connector also has an effect on the reproducibility of the measurement. We have used FSMA connectors for the tests carried out here. Figure 9.51 shows the result of an inter-laboratory test for measuring the attenuation in a standard NA-POF ([Kell98], [Krau98]). The measurements were carried out using the substitution method over a wide range of wavelengths, with LED or laser sources and different set up configurations. The attenuation values compared are in the range of 650 nm wavelength and for fibers of 20 m, 50 m and


699

100 m length. For the assumed attenuation of a “normal fiber” of 156 dB/km (ATM Forum specification, [ATM96a], [ATM96b], [ATM99]), 0.5 dB were added for launching changes for each length. The effects on the kilometric attenuation are very severe for the shorter lengths. One can easily recognize the large spread of measured values at 20 m, but even for 50 m or 100 m the spread of measured values is too large so that a reliable statement about the actual attenuation cannot be made. However, it is worth pointing out that the spread for a length of, for example, 20 m is approximately 47 dB/km, which - when related to the 20 m length - means less than 1 dB. It is an absolute necessity in all attenuation measurement tests to give detailed information about the test setup and procedure, in particular also about the launching conditions of the fiber. attenuation [dB/km] 190 180

156 dB/km +0.5 dB for launch NA

170 160 150

156 dB/km (acc. to ATMF specification)

140

white light LED

130

Laser

120 20

50

100 length [m]

Fig. 9.51: Attenuation in relation to the length of fiber; the measurements were carried out as part of an interlaboratory test with different measuring set ups ([Kell98])

The attenuation spectra presented below were recorded with the test set up described in Fig. 9.47. Figure 9.52 shows the effect of length on attenuation in short lengths of fiber, measured with the cut-back method. Light launch was carried out near to equilibrium mode distribution. After a length of approximately 10 m attenuation remains constant, which means that after approximately 10 m equilibrium mode distribution has been achieved.

700


180

attenuation [dB/km]

170 160 150

ESKA 4001 AN = 0.47

140 130

length: 3m 5m 110 7m 100 10 m 20 m 90 500 520 120

wavelength [nm] 540

560

580

600

Fig. 9.52: Attenuation of a standard NA-POF (EH-4001) in relation to length for short lengths of fibers, measured with the cut-back method ([Pei00a])

This is confirmed when testing fibers with a greater length (Fig. 9.53) where attenuation no longer depends on the length. 800

attenuation [dB/km]

700 600

sample length: 20 m 30 m 50 m 60 m

40 m

500 400 300

200 100 0 400

wavelength [nm] 450

500

550

600

650

700

Fig. 9.53: Attenuation of a standard NA-POF (EH-4001) in relation to length, measured with the cut-back method

Figure 9.54 shows the spectral attenuation curves for a double step index profile fiber (ESKA MH 4001). The fiber was launched with a numerical aperture of approximately 0.50 (Fig. 9.48) and the reference length was 1 m in each case [Pei00b]. The attenuation of this fiber was measured with the cut-back method. It is clearly discernible that the equilibrium mode distribution is not reached until after a larger distance of fiber (>40 m). Until then, the measured kilometric attenuation depends on length. For shorter lengths to achieve equilibrium mode distribution, the fiber would have to be light launched with a numerical aperture of


701

approximately 0.30. This would mean that the launching optics in Fig. 9.48 would need to be modified.

600

attenuation [dB/km]

length: 3m 5m 10 m 20 m 30 m 40 m 50 m 100 m

500 400 300 200 100 0

wavelength [nm]

400

450

500

550

600

650

700

Fig. 9.54: Attenuation of a DSI-POF (ESKA MH4001) in relation to length, measured with the cut-back method

Figures 9.55 and 9.56 show the attenuation spectra of a multicore POF (37 cores) at different launching conditions.

600

attenuation [dB/km]

length: 30 m

500

50 m 64 m

400 300

100 m

200 100 wavelength [nm] 0 400

450

500

550

600

650

700

Fig. 9.55: Attenuation of a multicore POF (Asahi PMC 1000, 37 cores, AN = 0.19) for different measured lengths; light launched with AN | 0.50

In both measurements, the reference length was 0.68 m; in Fig. 9.55 launching took place with a numerical aperture of 0.50, in Fig. 9.56 with 0.17 using a glass fiber bundle. Both measurements were carried out with the substitution method. The attenuation coefficient depends heavily on length; when light is launched with AN = 0.17 it was no longer possible to measure the 100 m length, since the dynamic range was approximately 10 dB lower due to the mismatch of areas between the fiber bundle ( 3 mm) and the MC-POF ( 1 mm). In addition, the

702


attenuation coefficient was approximately 50 dB/km lower; when overfilling with a numerical aperture of 0.50, no equilibrium mode distribution was achieved even after 100 m, whilst with launching conditions with values close to the equilibrium mode distribution, EMD was reached after approximately 60 m.

600

length:

attenuation [dB/km]

500

30 m 50 m 64 m

400 300 200 100 0 400

wavelength [nm] 450

500

550

600

650

700

Fig. 9.56: Attenuation of a multi core POF (Asahi PMC 1000, 37 cores, AN = 0.19) for different measured lengths; light launched with AN | 0.17 (fiber bundle)

Figure 9.57 shows a summary of the attenuation coefficients of different fibers in relation to the length of fiber. At launching conditions near equilibrium mode distribution, attenuation independent of length is already achieved after 10 m, whilst with an MC-POF this is only the case after 60 m. attenuation 300 [dB/k ] 250 200 150 100 50 0

EH 4001 AN = 0.47 AN launch = 0.50 (St.-NA) MH 4001 AN = 0.30 AN launch = 0.50 (DSI) PMC 1000 AN = 0.19 AN launch = 0.50 (MC) PMC 1000 AN = 0.19 AN launch = 0.17 (MC)

length [m]

0 20 40 60 80 Fig. 9.57: Attenuation for different types of POF in relation to length

100


703

Since the length at which equilibrium mode distribution is reached is different for each polymer fiber, this length must be known prior to any measurement in order to obtain an attenuation coefficient that is independent of length. Figures 9.58 and 9.59 show the attenuation spectrum of a POF (Eska GH 4001) measured with a monochromator (fiber length 30 m, solid curves) and with LED (at different fiber length, different points) with and without spectral correction. The results are definitely closer to the monochromator measurements with the spectral correction described in Section 9.4.5.3. The remaining errors are caused by the too short measuring lengths and the uneven mode fields. 450 400

attenuation [dB/km]

350

ESKA GH 4001 AN = 0.51 length: 30 m

HLMP K155

300 250

SHR SHR 525C5 525C3

HR 430

200

HLMA DL00

150 100 50

LED

Nichia 560nm

0 400

450

POF sample length:

550

500 10 m

20 m

600

30 m


Fig. 9.58: Measuring attenuation with LED and monochromator (without correction) 450

attenuation [dB/km]

400

HLMP K155

350 300 250

HR 430

200 150

SHR 525C5

SHR 525C3

HLMA DL00

100 50 0 400 450 POFsample length:

Nichia 560nm 500 10 m 20 m

550 30 m

600

wavelength [nm] 650

700

Fig. 9.59: Measuring attenuation with LED and monochromator (with correction)

704


9.4.6 Optical Backscattering Method 9.4.6.1 Principle of the ODTR Another method for measuring attenuation is the optical backscattering method (Optical Time Domain Reflectometer - OTDR): short light pulses are coupled at one end of the fiber. The light is scattered in all directions by Rayleigh scattering; a small proportion returns to the optical fiber and is detected (Figs. 9.60 and 9.61). With this method it is possible to make statements about the attenuation curve along the fiber and about local disturbances. Due to the high attenuation of the polymer optical fiber, the transmission power must be very high and the receiver sufficiently sensitive. Currently, most commercially available backscattering measuring instruments are only for glass fibers, which are only of limited use for investigations into polymer optical fibers due to the wavelength ranges (850 nm, 1,300 nm and 1,500 nm; one manufacturer offers the option of 670 nm) and the launching conditions (AN: 0.10 .. 0.25, with singlemode and multimode launching glass fiber). One manufacturer [Luciol] offers an OTDR specifically for POF.

pulsed laser

fiber under test splitter

detector

optical absorber

control unit signal processing

display

Fig. 9.60: Schematic arrangement of an optical backscattering measuring instrument

input signal

scattering center

back scattered signal

Fig. 9.61: Generating backscattering signals

705


A short light pulse is launched into the fiber at t1 and passes the length L2 - L1 at speed v (approximately 2 108 m/s); the light is reflected at the fiber end and returns to the beginning of the fiber at time t3; the optical time domain reflectomter measures the run time of the pulse 't = (t2 - t1) + (t3 - t2) = 2(t2 - t1) (Fig. 9.62) and converts it into the length. The following formula is used: L

t t v 2 1 2

c t 2 t1 nk 2

Because of the pulse propagation time is determined by the speed of light in the fiber core, for a precize location of an event, e.g. refections at the fiber input or output, the correct knowledge of the refractive index is required.

t2

t1 length L

t3 L1

L2

Fig. 9.62: Principle of the back scattering method, times and powers

In Fig. 9.63, the backscattered signal is shown in a logarithmic scale over the length. At the start of the fiber as well as at the end of it, there are reflections leading to strong backscattering signals. The backscattered power Pr(z) is calculated as follows: Pr ( z)

1 P0 S D´s ti v e 2D´z 2

with P0 being the introduced power, S the backscattering factor, D´s the attenuation coefficient due to Rayleigh scattering [km-1], ti the time width of the launched pulse, v the group speed, D´ the overall attenuation coefficient [km-1], and z the length of fiber ([Gri89]). The backscattering factor S indicates the degree of re-coupling, i.e. the proportion of light backscattered into the numerical aperture; only this will get to the beginning of the fiber and be available for measuring. For step-index fibers the factor is calculated as follows: S

3 § AN · ¸ ¨ 8 ¨© ncore ¸¹

2

706


A factor of 2 takes account of the fact that the light pulses have to pass the length of fiber twice. The attenuation coefficient through Rayleigh scattering is determined as follows ([Ebe00]): D s >dB@

10 log 0.5 D´s S t i

Whereas in glass fibers the main proportion of attenuation is caused by Rayleigh scattering, in polymer optical fibers absorption through molecular vibrations and impurities is dominant. For a POF with AN = 0.5, ncore = 1.497, ti = 1 ns z = 0.0001 km and D´s = 2.8 km-1 ([Kai81]) one obtains Ds = 52 dB, which means that the power of the backscattered signal is 52 dB less than the launch signal. To this we must add the insertion losses of the splitter or coupler of approximately 7 dB. In addition, there is twice the attenuation of 100 m POF with 30 dB at O = 650 nm. That means that the OTDR has to cover a dynamic range of approximately 90 dB in order to be able to measure the attenuation in 100 m POF. For O = 520 nm, the situation is somewhat more favorable. Since here the Rayleigh scattering attenuation is larger and the POF attenuation lower than with 650 nm, it is possible to analyze a longer fiber. log. back scattered signal reflection at the fiber input reflecting connector P1

reflection at the fiber output

fiber 1

P2

fiber 2

connector loss

splice or non reflecting connector

length fiber 1

L1 t1

L 2 t2

length

Fig. 9.63: Determining attenuation from the backscattering signal

The fiber attenuation coefficient D in Fig. 9.63 is calculated as follows: D

P1 >dBm@ P2 >dBm@ L 2 L1


707

A plug connection having an air gap between the two fiber ends shows a clear peak with a step in attenuation in the backscattering signal, similar to the fiber beginning and end; however, a plug connection without reflection only shows a step in attenuation. The strong backscattering signal at the beginning and end of the fiber leads to an overloading of the detector, which means that during a certain time interval, which is determined by the amount of pulse and reflection as well as the recovery time of the receiver, no signal can be analyzed. The minimal distance between a reflecting and a non-reflecting event that can still be resolved by the OTDR is called the attenuation dead zone, while the minimal distance between two reflecting events is called the event dead zone. The local resolution is given by the pulse width ti. An pulse width of 10 ns corresponds to a length of approximately 2 m, 1 ns corresponds to 20 cm; since double the path (there and back) must be taken into consideration, the spatial resolution at 10 ns is 1 m and at 1 ns is 10 cm. However, the reduction of pulse width leads to a reduction in backscattered power and therefore to a reduced dynamic range of the measuring system. In order to be able to analyze greater lengths one selects a longer pulse duration ti at the expense of spatial resolution. Figure 9.64 shows the backscattering signal of a standard NA polymer optical fiber. During the first 10 m one can clearly recognize a non-linear curve due to the launching process, since excitation did not take place under conditions of equilibrium mode distribution. These occur after approximately 40 m; after that distance the curve remains linear and the attenuation coefficient independent of the length. -25

back scattered signal [dB]

-30 -35 -40

attenuation 0.16 dB/m 13 dB

-45

80 m

-50

length [m] -55

0

20

40

60

80

100

120

140

160

Fig. 9.64: OTDR measurement of a standard NA-POF according to [Bre00]

[Now98] describes an OTDR arrangement in which it is possible to measure the backscattering signal of more than 150 m POF at a wavelength of 532 nm without mathematically filtering with a dynamic range for fiber attenuation of 20 dB. The spatial resolution is 20 cm, the dead zone less than 5 m.

708


This OTDR method offers the following advantages: 1. 2. 3. 4.

Only one fiber end is required for measuring (suitable for installed fibers). Determining the length of fiber. Measuring the attenuation curve by local resolution. The measured result is independent of the optical quality of fiber end faces (however, in the case of surface defects it is possible that the dynamic range is reduced). 5. The measuring method is non-destructive. Table 9.2 provides an overview of the many possible applications of the optical backscattering measuring method. Table 9.2: Scope of application for the optical backscattering measuring method

fiber production

homogeneity of the optical fiber

cable production

receiving control, control of individual production steps

installation

attenuation before and after installing the fiber, attenuation of fiber connectors (plugs)

acceptance procedure overall attenuation of system maintenance

localizing faults

9.4.6.2 Improvement in the Resolution by Deconvolution POF and PCS have the greatest mode dispersion today among all fibers used in commercial data communication. While the spatial resolution essentially depends on the pulse width used with singlemode glass fibers, pulse spreading on the fiber limits the resolution with step index fibers. In addition, the necessary large area receivers can only attain a limited bandwidth thus generating additional pulse broadening. In certain applications, e.g. in vehicle networks, it would be desirable to achieve very exact spatial resolution in order to be able to localize defects in the cable harness. Pulse broadening in the cable can only be changed when collimated light is coupled in and only detects the low modes. No realistic measurement results can be obtained in this manner. The only way to improve the spatial resolution consists in a subsequent compensation of the mode dispersion. An example of a mathematical re-working of OTDR data is shown in Fig. 9.65 from >Otto02@. The assumption here was that after a fiber length of 149 m two discreet reflections lying close to each other would be found. You can see on the left side of the picture that both reflexes overlap, caused by broadening through mode dispersion, and that they are no longer separable. If the pulse response of the fiber for this length is known, then the pulses can be re-separated by deconvolution as the diagram on the right in the figure verifies. In this case the distance between the two reflections was 80 cm.

709


1.2

amplitude

1.0 0.8 0.6 0.4 0.2 time [ns]

time [ns] 0.0 0

10

20

30

40

50 0

10

20

30

40

50

Fig. 9.65: Reflected double pulse without (left) and with deconvolution (right), 149 m

The difficulty in this procedure lies in the fact that you have to know the length-dependent pulse response of the respective fiber investigated and be able to present it as an analytical printout. The pulse as a superimposition of 4 Gaussian functions, the intensities and widths of which change depending on the length, was described in >Otto02@. An example of the approximation of an output pulse after 200 m of fiber (with AN = 0.19) is shown on the left in Fig. 9.66. On the right side you can see the simulated pulse broadening after 200 m compared with the pulse response of the system. 1.0

amplitude

1.0 amplitude

output pulse after 200 m POF

0.8

0.8

system without POF

0.6

0.6

system with 200 m POF

0.4

0.4

pulse parts 0.2

0.2 time [ns] 0.0 0

10

20

30

40

0.0 0 50

time [ns] 10

20

30

40

50

Fig. 9.66: Simulation of the pulse response by 4 Gaussian curves

9.4.6.3 Commercial POF OTDR An OTDR for use with POF and PCS was developed by the Swiss company Luciol at the end of the 1990s (see >Bre00@, >Bre01@, >Bre03@ and >Luciol@). The device can be seen in Fig. 9.67.

710


Fig. 9.67: POF-OTDR from Luciol

Luciol is the sole vendor who can equip the device with blue or green LEDs in order to be able to measure POF at these wavelengths. A laser is used for 650 nm for which the manufacturer gives the following parameters: ¾standard wavelength (POF): ¾sensitivity: ¾spatial accuracy: ¾spatial resolution: ¾dynamic range (loss): ¾detector: ¾time constant:

500 nm, 650 nm -110 dBm 5 mm 10 cm 35 dB APD, single photon counting Now98@, >Yago01@ and >LFW00@ which, however, are more oriented toward optimized basic research. Table 9.3 compares the data obtained with the OTDR from Luciol. Table 9.3: Comparison of different OTDR types Parameter transmitter dynamic range measurement range resolution detector

[Luciol00] 650 nm LD 40 dB 110 m 0.12 m APD

[Now98] 532 nm Nd:YAG 50 dB 180 m APD

[Yago01] 650 nm LD 65 dB 200 m 0.08 m PMT

712


The measured curves of both experimental OTDRs according to >LWF00@, >Now98@ and >Yago01@ are shown in Fig. 9.70 for measurements each on 200 mm PMMA POF. back scatter signal [dB] 0

back scatter signal [dB] 0 -10

-10

-20 -30

-20

-40 -30

-50 -60 -70

-40 -50 0

50

100

150

200 250 position [m]

-80 0

50

100

150


Fig. 9.70: Measurement curves using experimental OTDR at 200 m POF

In both cases the end reflex can still be seen clearly after 200 m. The dynamic range of the device according to >Yago01@ is considerably greater which is why up to 200 m the Rayleigh signal also remains above the noise. The fiber attenuation is indeed lower at 532 nm, but the frequency-doubled laser presumably has less coupled power and/or a too low repetition frequency. It is not known whether the works cited will lead to a commercial device. Another commercial OTDR from the manufacturer Scientex (OTDR-2000POF) is reported on in >Nak04b@. It is not clear whether this device is actually sold outside Japan. The parameters are similar to those of the Tempo device. The device is shown in Fig. 9.71 and a measurement example for the attenuation can be seen in Fig. 9.72.

wavelength: dynamic range: measurement range: resolution: Fig. 9.71: POF-OTDR

650 nm 18 dB 200 m 1 cm


713

Fig. 9.72: Measurement example for PMMA-POF attenuation ([Nak04b])

9.4.6.5 Measurement of the Connector Attenuation An extremely important advantage of OTDR is being able to measure real connector attenuations. According to a narrow definition the connector attenuation should be the value by which the loss of a link is increased compared with the value it would have had without the plug-in connection. Since you cannot “look into” the fiber with conventional power-measuring methods, there only remains the method described above to install a fiber between transmitter and receiver in order to calibrate the system and then mount the plug-in connector in the middle. The OTDR now gives a possibility to exactly determine the optical level in front of and behind the connector. The only prerequisite is that the Rayleigh coefficients of both fibers are identical, i.e. ideal when using the same type of fiber.

Fig. 9.73: Measurement of connector loss with OTDR ([Hut00] and [Bre00])

714


Corresponding measurements have been demonstrated in >Hut00@ and >Bre00@, as shown in Fig. 9.72. If you extrapolate the Rayleigh curves in front of and behind the connector exactly to the connecting point of both fibers, the vertical difference then yields the exact connector attenuation. This procedure is non destructive and fast. In addition to the connectors the losses with other disturbances, e.g. at tight bends or other deformations, can also be determined. 9.4.6.6 Bandwidth Measurements with OTDR Another measuring possibility of OTDR is determining the bandwidth. Since there is always a well localized discreet reflection with the end reflection, every OTDR measurement also supplies the pulse response on a not-too-long fiber (of course, for the double length of the fiber since the pulse goes forth and back). An example for a 70 m fiber is shown in Fig. 9.74, measured with a Tempo OTDR on a standard POF.

power [a.U.] 1.2 1.0 0.8 'W = 13 ns 4.74 MHzkm

0.6 0.4 0.2 0.0 0

1

2

3

4

5

rel. fiber position [m] Fig. 9.74: Pulse broadening after a 50 m long test fiber (+20 m pre test fiber)

Pulse broadening of 13 ns over 140 m of fiber results in a bandwidth of 47.4 MHz 100 m which concurs quite well with the measurement results in transmission procedures (frequency domain). The following Fig. 9.75 shows the measured end reflections for a SI-POF for measurement lengths up to 150 m (cut-back method).


715

normalized amplitude 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

0

20

40

60

80

100

120


Fig. 9.75: Pulse shapes for a St.-NA-POF up to 150 m (Tempo-OTDR)

The pulse spreading occurring can clearly be seen. At 150 m the pulse is still sufficiently over the noise level of the device. The results for a PMMA GI-POF are shown in Fig. 9.76. Because of the smaller NA much light is lost when coupling in so that the dynamic range is somewhat smaller. Furthermore, the fiber has a somewhat higher attenuation. normalized amplitude 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.0

0.5

20.0 20.5

40.0 40.5

60.0 60.5 80.0 80.5

Fig. 9.76: Pulse shapes for PMMA GI-POF up to 100 m (Tempo-OTDR)

100.0 100.5 position [m]

716


Changes in the pulse width are not optically recognizable in the picture. A more exact analysis, however, shows that the pulse widths increase significantly by some tenths of a nanosecond. The results for the bandwidths of both fibers calculated from this pulse broadening is shown in Fig. 9.77. bandwidth [MHz] 5000 2000 PMMA-GI-POF 1000 500 200 PMMA-SI-POF

100 50

POF length [m]

20 10

20

50

100

200

Fig. 9.77: Bandwidths of the SI-POF and GI-POF, measured with OTDR

The results tally quite well with the measurements in the frequency range up to a fiber length of 100 m. You also get realistic values for GI POF - albeit very faulty. The values over 100 m are too inexact. The reason for this may be less the noise than the faulty linearity of the method of measurement. All in all, however, the bandwidth measurements with OTDR are a very viable alternative to transmission procedures. 9.4.7 Dispersion When measuring mode dispersion, principally the same considerations for launching light apply as for measuring attenuation (please refer to Sec.9.4.5). It must be pointed out however, that in spite of uniform mode distribution the bandwidth is subject not to linear change but to sub-linear change with different lengths of fiber due to mode coupling and mode conversion. This subject is discussed in more detail in Chapter 2. Two methods are available for measuring dispersion: 1. Measurements in the time domain 2. Measurements in the frequency domain 9.4.7.1 Time Based Measurement When carrying out time based measurements, a short and, if possible, monochromatic pulse is introduced into the fiber via a suitable optical arrangement; the


717

pulse is detected at the end of the fiber using a fast receiver with a bandwidth larger than that of the fiber to be measured. This is then made visible with the help of an oscilloscope (Fig. 9.78). The pulse changes width and height over the transmission link. The pulse response is expressed as follows: g( t ) Pout ( t ) Pin ( t ) .

pulse light source

fiber under test

receiver

launching optics oszilloscope Fig. 9.78: Schematic illustration of time based dispersion measurement

From the input and output width of the pulse it is possible to calculate the broadening over time. When assuming Gaussian pulse curves, the following simple formula results: 't

2 2 t out t in ,

whereby tout and tin represents the full width at half maximum, at which the pulse height has dropped to 50 % (see section 2.5.2, Fig. 2.87). 't over the length of fiber L gives us the dispersion parameter D: D = 't/L [ns/km]. The proportionality of 't and L applies up to coupling length; for larger lengths, the following applies: 't v LN proportional, whereby N < 1 must be determined for each fiber configuration individually (see Chapter 1). As described in the section on measuring attenuation, dispersion is also measured with the cut-back and substitution methods in order to keep launching conditions constant. The transmission capacity of a fiber is derived from the bandwidth-length-product: B L [MHz km] |

0.44 L 't

0.44 D

An example of measurements in the time range are shown in Fig. 9.79. A pulse width of 5.1 ns is measured on a 50 m long POF which corresponds to a bandwidth of 43 MHz 100 m.

718


0.7

power [a.U.]

0.6 0.5 0.4

'W = 5.1 ns 4.3 MHzkm

0.3 0.2 0.1 0.0

t [ns] 0

5

10

15

20

25

30

35

40

Fig. 9.79: Pulse broadening after a 20 m test fiber for comparison

9.4.7.2 Frequency Based Measurement When carrying out frequency based measurements, the light source is modulated with a sinus shaped signal (cf. Fig. 9.80). The frequency of the modulation signal Sin(Z) is increased continually from 0 Hz to the desired frequency and the amplitude Sout is determined for all frequency values (Fig. 9.80). The frequency response is expressed as G(Z) = Sout(Z)/Sin(Z). From the spectrum one obtains the required transmission bandwidth at a frequency at which the transmission function has dropped by 6 dB (Fig. 9.81).

light sweep source generator

fiber under test

spectrum analyzer

| launching optics

receiver

Fig. 9.80: Schematic illustration of frequency based dispersion measurements

The measured curves in Figure 9.81 only serve as examples to show that this procedure leads to very analyzable and reproducible results over a wide range of fiber types, lengths and measured wavelengths. An entire series of different fibers was described in Chapter 2 in regard to bandwidth. These measurements have almost exclusively been conducted with the procedure described here.

9.5 Connector Measurement

0 -3

719

electr. power [dB]

-6

PFU-U-CD 1000; 10 m

-9

PFU-U-CD 1000; 20 m

-12 -15

PFU-U-CD 1000; 50 m

-18

PMU-CD 1000; 50 m NC-1000 50 m

-21 PFU-U-CD 1000; 100 m

-24 -27

PFU-U-CD 1000; 100 m

-30

MH 4002 50 m

-33 PMU-CD 1000; 100 m

-36 -39

MH 4002; 50 m

-42 -45 -48 0.1

0.2

0.5

1

2

5

10


Fig. 9.81: Frequency response for selected polymer optical fibers at O = 520 nm ([Rit98])

9.5 Connector Measurements When investigating connectors, the following items are of particular interest: ¾the attenuation of the connector, ¾the increase in attenuation after a certain number of connection cycles, ¾the increase in attenuation during changes of temperature and relative humidity. The measuring method for connectors is described in DIN EN 186000. In reliance on this recommendation, [Schw98b] investigates connectors for POF from different manufacturers. We will briefly describe the measuring methods here. The first step is to set up a test arrangement as shown in Fig. 9.82a and to measure the power P01. Subsequently the POF is cut at a distance of 60 cm from the transmitter (T), the connector set is fitted in accordance with the fitting instructions and the power P11 at the receiver (R) is measured (Fig. 9.82b).

720


a)

T

R

P01

L1 = 150 cm

b)

T

connector set L1 = 60 cm

R

A B

P11

c) A

T L1 = 60 cm

B

C

D

R

P21

L2 = 30 cm

Fig. 9.82 a-c: Measuring method for determining the insertion loss of connectors

The insertion loss DK of the connector set is calculated in dB as follows: DK

§P · 10 log ¨¨ 0 x ¸¸ , © P1x ¹

whereby x refers to the test setup. In the next step, the POF is cut at a distance of 30 cm from the first connector set, the second connector set is fitted and the power P21 measured (Fig. 9.82c). The insertion loss of the second connector set in dB is calculated as follows: DK

§P · 10 log ¨¨ 1X ¸¸ © P2 X ¹

The above steps are carried out a total of four times in parallel. The four patch cables made in this way are swapped cyclically and reversed. The test setup remains the same. That means, for example, that patch cable BC is tested in test set-up 2, the power P22 is measured and calculated with P12 DK , patch cable FG is tested in test setup 3, the power P23 is measured and calculated with P13 DK, etc. From the insertion attenuation values obtained, the mean, the standard deviation and the maximum and minimum values are determined.


3.5

721

insertion loss [dB]

3.0 2.5 2.0 1.5 1.0 0.5 0.0

polishing

hot plate

DSI-POF

minimum mean value - standard deviation mean value mean value + standard deviation maximum Fig. 9.83: Insertion loss values of different connectors ([Schw98b])

The connector sets for which the fiber end face is polished (HFBR 4501, HFBR 4531, F-SMA push fit connector, polishing type, and TOCP 155) on average have a lower insertion attenuation compared with the connectors produced in the hot-plate process (TCP connectors, F-SMA connectors type hot-plate, F07, AMP DNP). This results from the fact that in the hot-plate process the fiber core is expanded and therefore the wave guiding is disturbed for a length of approximately 0.5 mm (see Fig. 3.68). For a distance of this size the loss is calculated to be approximately 0.6 dB.

722

9.6 The Reliability of POF

9.6 The Reliability of POF 9.6.1 Environmental Influences on Polymer Optical Fibers Polymer optical fibers, like other technological products, are subjected throughout the whole of their service life to a great many kinds of stress from the environment mechanical, climatic, chemical, biological, and radiometric. As a result of these stress factors, physical and chemical alterations may arise in the materials used. These can in various ways have an effect on the functional behavior, suitability for use, and serviceable life expectancy (that is to say, the durability) of the POF. Environmental stress factors, then, have an influence that should not be underestimated on the quality and reliability of the fiber optic transmission system. When polymer optical fibers are used, it is thus imperative to understand and take into consideration the effects of environmental influences - particularly of industrial environmental influences - on those properties of POF which enable it to transmit optical signals. The most important property in this context is the optical transmission, that is to say, the optical attenuation. In judging the suitability of a polymer optical fiber for a particular area of application, it is not the absolute attenuation that is of primary interest: rather it is the relative changes it undergoes, as subject to the action of the various environmental influences. The systematic investigation and assessment of the optical transmission (or attenuation) of a polymer optical fiber when subjected to extreme environmental conditions can be broken down, as a rule, into these three steps: 1. analysis of the environmental stress factors, 2. simulation of environmental conditions in carrying out the appropriate test procedures, 3. quantitative determination of the effect of environmental influences on the optical transmission. In the light of the test results, we can then conclude what are the conditions suitable to the use of this material and what are the limits (e.g. the maximum temperature at which regular operation is possible). The basis of our environmental simulation must be the analysis of the environmental stress factors to which a polymer optical fiber is liable in any given conditions of use. For this purpose, the environmental influences that are to be expected have to be registered and defined. We must make distinctions here based on the type of influence, on its frequency, intensity, and the opportunity it has to cause damage. In every particular situation of use, there is a plethora of different types of influence and their possible combinations to be taken into account. We can distinguish in principle between mechanical, climatic, chemical, biological, and radiometric influences. Each one of these, as Table 9.4 shows, can occur in the most diverse forms.


723

Table 9.4: Classification of industrial environmental influences on POF Types of Influence mechanical

climatic

static bend repeated bending flexing crush impact torsion vibration tensile tension

high humidity extreme temperature change in climatic conditions thawing freezing

chemical and biological lubricants fuels brake fluid hydraulic oil acids and alkalis solvents oxygen ozone reactive gases micro-organisms

radiometric UV, X-ray, and nuclear radiation

also in various combinations

Every time we attempt to simulate an environmental influence in laboratory tests, our aim must be to reproduce the actual environmental stress in a way that is as close to reality as can be. Accordingly, when investigating the attenuation or optical transmission of a polymer optical fiber under the stress of industrial environmental influences, we must make use of testing techniques and strategies that are adapted to the matter in hand. The development and construction of test equipment for the various environmental influences can be modeled on those test standards that already exist, e.g. for electric cables and wiring, as well as those designed for fiber optic cables on a glass fiber base. The specific qualities of a POF when subjected to testing must, however, be given special consideration. It is important to ensure absolute consistency in the test procedures and conditions as well as in the preparation and measurements of the samples for only results that can be reproduced and compared in this way are worth striving for. Highly complex polymer materials like those used in POF are critical in this respect since relatively slight deviations in the factors mentioned above can lead to great variation in the values that are to be determined, for example optical transmission. The choice of a simulation or test strategy in any given case is to a great extent dependent on the temporal duration of the stress factor in the real world. Basically, a distinction is to be made between stress that is limited in terms of time, occurring only occasionally, and stress that is continuous or nearly so. Occasional stress, for example, occurs in a motor vehicle, where the vibrations act on the POF only at those times when the engine is running. In the test or simulation, the intervening times of rest are phased out, and the periods of stress follow on each other without a break. An acceleration effect is aimed for here. In this case the stress characteristics correspond in a general way to conditions in the real world. Stress factors that are continuous, or nearly so, have an impact on the polymer optical fibers during the entire period of their service life, or at least for a great part of it. To this class belong climatic influences, for example, or static mechanical stress factors. The strategy of

724


acceleration is based in this case on the intensification of the stress conditions during the simulation, as compared with the situation in the real world. This intensification can be achieved by various methods: ¾ the superimposition of extreme stress conditions, as an ongoing stress situation, ¾ cyclically alternating stress between two opposite extremes, ¾ raising the stress to a value beyond the extreme possible in reality, or ¾ an increase in the rate of change in a situation of alternating stress. The final choice of a strategy for simulation or testing, and of the parameters that go with it, is based as a rule on the results of environmental analysis, as also on prior knowledge and experience in dealing with similar problems, along with foregoing investigation into the polymer optical fibers that are to be tested. All the demonstrations that follow in this chapter are based, when not otherwise stated, on many years of experience in the investigation of polymer optical fibers and the assessment of their reliability under the auspices of the Federal Institute of Material Research and Testing (BAM), Berlin. The major part in these tests is played by the 1 mm SI-POF on a base of polymethylmethacrylate (PMMA), which is almost the only one to be used in practice. Polymer optical fibers based on other materials, e.g. polycarbonate (PC), or deuterated or fluorinated polymer (e.g. CYTOP®), if relevant in practice, will also be taken into account when it is appropriate to do so. The simulation and testing procedures that are described are basically suitable for all POF types, though of course the parameters must be accommodated in each case to the nature of the material and its distinctive structural characteristics. 9.6.2 The Effect of Environmental Influences on Optical Transmission 9.6.2.1 Attenuation Factors of Polymer Optical Fibers The understanding of the basic connection between the various environmental influences and the attenuation factors in the polymer optical fiber gives us a basis for forming a definitive judgment on the possibility of its being used. A distinction may be made in principle in dealing with POF between attenuation factors specific to the material and/or those that are caused by imperfections or impurities (Table 9.5, cf. also Chapter 2.7.3). Losses at imperfections or impurities arising from the effect of environmental influences are responsible for changes in the optical attenuation or transmission. New imperfections or impurities can arise and existing ones can be enlarged as a result of external stresses, thus leading to premature component failure. The detection of imperfections or impurities that arise and the related transmission losses demands the use of appropriate measuring procedures and equipment when investigating reliability and durability. According to the questions posed and the aim of the test various measuring procedures can be considered, ranging from the straightforward measurement of attenuation by the insertion method to the more involved measurement of backscattering by means of an OTDR (OTDR: Optical Time Domain Reflectometer).


725

Table 9.5: Attenuation factors in POF (according to [Kain85]) intrinsic loss factors

absorption

extrinsic loss factors

scattering absorption

scattering

- high harmonics of the C-H bondings - electron transitions - Rayleigh scattering - organic contaminants - water absorption - changes in the material through chemically active substances - microporosity - microcracks - microinclusions - fluctuations of the core diameter - core-cladding boundary imperfections

9.6.2.2 Detection by Measuring Optical Transmission With a view to detecting imperfections or zones of impurity that have been caused by absorption, the integral measurement of optical attenuation or transmission is a very important method. When investigating reliability the point of interest is generally not the absolute value arrived at, but the relative change compared with the unstressed state (starting value). Measurements of attenuation or transmission can be carried out in a number of ways. An important factor in the choice of procedure is the margin of uncertainty in measurement. This can be caused by temperature, mismatching, by the difficulty of reproducing the launching conditions and nonlinearity’s as well as other factors. In aiming for the utmost sensitivity in our investigation of changes in the quality of optical transmission, we have the task of limiting these influences as far as possible. Taking these factors into account and taking our cue from the imperfections or impurities that are to be expected, the general procedure in reliability investigations is to make use of a modified insertion method to carry out the tests. The insertion method is characterized by a clearly reduced margin of uncertainty in comparison to the simple optical power measurement. The measurement is carried out in two steps. First an optical power meter measures the output power of the light source directly. This measurement is taken as a reference value for the optical power that has been launched. In the second step the polymer optical fiber to be tested is inserted and its output power is measured. From these two values the attenuation or transmission of the POF can be calculated. The margin of uncertainty in this procedure depends in essence only on the stability of the measuring instruments between the two points in time when measurements are taken and on the possibility of precisely reproducing the launching and coupling conditions. Drift effects of the measuring instruments over a longer period of time have no influence here, a very positive factor with tests that are carried out over an extended period. The reproduction of the launching and coupling conditions can be made considerably more accurate if a modification of the procedure is tried, namely, by using a special optical multiplexer for polymer optical fibers with integrated sources of light and an integrated detector.

726

9.6 The Reliability of POF RS 232

step drive control unit

PC

light source unit

detector unit PIN photodiode

RS 232 A/D converter

position control RS 422 encoder signal aperture control

axis 1 axis 2

LED power supply

low-noise amplifier

connector light source

O 1 O2 O 3 aperture

spectrum analyzer connector

4x1 POF coupler

1x2 POF coupler incremental position measuring system

linear positioning system

input for POF samples

n

... 3 2 1

n

... 3 2 1

reference fiber

POF connections information flow electrical connections

Fig. 9.84: Multiplexer for POF reliability investigations ([Gün00])

The layout of a typical multiplexer for this purpose will be introduced in the following description ([Gün00]). The basic layout (Fig. 9.84) can be subdivided into three functional areas: the light source unit, the detector unit, and the positioning system. The light source unit consists of three LED that attain their maximum optical power at wavelengths of 525 nm, 590 nm, and 660 nm respectively. This gives us the possibility of carrying out transmission measurements at the wavelengths that are relevant to the test. The individual LED are launched into the polymer optical fiber to be investigated or the reference fiber, by means of a controllable aperture and of a 4 u 1 coupler. An additional optical input makes it possible to carry out transmission measurements with an external light source, e.g. laser diode or white light source. The detector unit consists of a pin-photodiode followed by a low-noise amplifier. To enable the use of additional detectors, e.g. an optical spectrum analyzer, an asymmetrical 1 u 2 coupler is incorporated into the optical path so that an additional external optical output is available. Both the detector and the light source units are set on the adjustable platform of the positioning system. The actual switching between the POF to be tested is executed by the linear positioning system. As can be seen in Fig. 9.84, the coupling of the POF sample is executed as a front surface coupling with an in-between gap in the order of 100 μm. The POF samples are inserted into the multiplexer from without and fixed in special receptacles, uniformly and on a single plane. Measurement of the optical transmission by the insertion method is then carried out in the following way. First the optical power of all the LED is measured by means of a short reference fiber. Thereafter the platform is positioned for the other POF samples and their transmission values are likewise measured.


727

The multiplexer is controlled by a PC, which simultaneously handles the whole procedure of registering the measurement data by means of an A/D converter card. With this multiplexer it is an easy matter to measure transmissions with a margin of uncertainty of d 1 %, even over considerable periods of time (6,000 hours). The multiplexer envisaged here is designed to hold a maximum of 20 POF samples. A similar multiplexer with the capacity to hold 48 samples which has been developed specifically for the investigation of POF components is described in [Krue00]. The principle of this multiplexer has been further developed at the POF-AC Nürnberg since 2001. In the meantime instead of one coupler several offset fibers are used with different transmitters for which several receivers are also necessary. They do not influence the accuracy of this method of measurement. There are versions available for 1 mm POF, 200 μm PCS and most recently also for 50 μm MM-GOF. The dynamic range as well as the software has been improved. Depending on one’s wishes up to 40 fibers can be measured. Normally, the multiplexers at the POF-AC are equipped with FSMA connectors. Two versions for POF and PCS are shown in Fig. 9.85.

Fig. 9.85: New generation of multiplexers for PCS and POF (POF-AC)

9.6.2.3 Detection by Measuring Backscattering In connection with the investigation of the mechanical reliability of polymer optical fibers, the local detection of imperfections occasioned by scattering, e.g. at microcracks, plays an especially significant part. Mechanical stress such as repeated bending or torsion can when certain values specific to the material are exceeded lead very quickly to the development of micro-cracks, to damage of the interface between the fiber core and the cladding, or even to lasting changes in the geometry of the optical fiber. In extreme cases the fiber may even break. With larger imperfections the resulting loss can easily be detected by means of a simple measurement of the transmission. With smaller imperfections, such as short microcracks, this becomes increasingly difficult. In addition, the measurement of the optical transmission gives us no information about the location and extent of the imperfection. In such a case we need the help of a high-resolution measurement of the backscattering, making use of the Fresnel reflection effect. This effect always arises when the refractive index along the length of the POF is not constant and discontinuities appear, e.g. polymer-air transition in the case of a crack.

728


pulse generator

beam splitter

laser diode

polymer fiber L

micro pore

t

fiber break micro crack

I

detector system

I

t t=f(L,n)

imperfection display

Fig. 9.86: Detection of imperfections by measuring backscattering

The basic principle is shown in Fig. 9.86. An optical pulse from a laser diode, e.g. with OP = 670 nm; FWHM = 4 nm; 't < 100 ps, is launched into the polymer optical fiber to be tested. The pulse passes through the fiber. If there are imperfections present that cause a Fresnel reflection, a part of the pulse is scattered and reflected in a backward direction. By recording the optical power of the reflected light for the duration of the test, we can arrive at the information we need on the location and extension of the imperfections, provided that the instrument has been properly calibrated at the outset (Fig. 9.87 and Fig. 9.88). Information gathered in this way plays a part not only in reliability investigations, but also in locating and analyzing damages within installed POF cables ([Zed98]). 0.8

rel. backscattered signal

0.6 0.4 0.2 0.0

4

8

12

16

20

24

28

32 36 cable length [m]

Fig. 9.87: Diagram showing the backscattering of a POF cable without imperfections

9.7 Reliability under Various Environmental Influences

0.8

729

rel. back scatter signal

0.6 0.4 0.2 0.0

4

6

8

10

12

14 16 cable length [m]

Fig. 9.88: Diagram showing the backscattering of a POF cable with several imperfections

9.7 Investigation of Reliability under Various Environmental Influences 9.7.1 Mechanical Stress 9.7.1.1 Repeated Bending In industrial applications repeated bending can be counted among the most commonly occurring kinds of stress. Consequently, it is of great importance in connection with the reliability of the optical transmission of signals. Repeated bending can arise, for example, in connection with automated controls when the cable is used on a robot arm or in other changes of position induced by moving mechanical parts. An installation with trailing chains in conjunction with a crane or in automatic handling systems are other typical cases. If the cable is used in vehicles, we may find stresses caused by repeated bending in the door area, for example. Stress that results from repeated bending generally shows a characteristic pattern: at the point of bending, especially at the periphery, the repeated stretching and compression of the polymer optical fiber constitutes a cycle. That is to say, three different states of mechanical stress occur alternatively: tensile stress caused by stretching, absence of stress, and compressive stress caused by compression. The degree of the stress is dependent here on the radius and angle of the bending. In a first approximation, the stress at the periphery varies in inverse proportion to the bending radius. If the stress is so extreme that it leads to linear-elastic or linearvisco-elastic deformation, there is the risk of micro-cracks arising in the peripheral zones of the fiber, which would immediately result in a deterioration of the optical transmission. If micro-cracks continue to develop, it can lead to a break in the fiber. An additional complication is that the durability of a polymer optical fiber is dependent on the temperature.

730


Taking all these preliminary considerations into account, it is particularly important in industrial practice to investigate the optical transmission of the fiber in different conditions of repeated bending stress and under varying climatic conditions with a view to determining the appropriate limiting values for the smallest allowable bending radius in an extreme state of bending stress. A test apparatus designed for this purpose is shown in Fig. 9.89. It consists of an appliance for testing repeated bending, which is incorporated into a climatic chamber with a driving unit attached. The driving unit is situated outside the chamber. It facilitates the simulation of repeated bending stress at two points of bending with a bending radius that can be varied between 5 mm and 40 mm. Relative to the midposition, the maximum angles of bending possible are ± 90°. climatic chamber +-90°

weight 200 g light source

bending radius

drive shaft

receiver

drive unit data recording system

Fig. 9.89: Test equipment for testing repeated bending

While subjecting the POF to repeated bending, the transmission is measured at intervals, after a specific number of repeated bending cycles have been executed. For this purpose the lever arm is brought into a vertical position so that the sample is not subject to any bending stress during the measurement of the transmission. After an appropriate time for relaxation, about 60 seconds, has been allowed to elapse, the optical power is measured. The result of the test consists in the determination of the relative optical transmission, which is calculated on the basis of the transmission as measured during the rising number of repeated bending cycles compared with the transmission in the unstressed state. The most important findings on the question of how stress caused by repeated bending has an effect on the functionality and serviceable life expectancy of a polymer optical fiber are shown in Fig. 9.90 and Fig. 9.81. The entries on the diagram represent the relative optical transmission over a number of cycles of repeated bending: in Fig. 9.90 with varying values for the bending radius and at room temperature; in Fig. 9.91 with a constant bending radius and with varying temperature/climatic conditions.


731

As regards the functionality of the fiber, it can be established that repeated bending stress with varying values for the bending radius, at room temperature and extremely low temperatures has no immediate effect on the transmission. The transmission shows no change in comparison with the unstressed state. It remains at 100 %. Relative to the durability of the cable it can be seen that - independently of the bending radius and of the temperature - after a certain number of repeated bendings there is a rapid deterioration in the optical transmission and definitive component failure occurs when the 50 % threshold is reached. As is to be expected, a smaller bending radius and lower temperature lead to a shortened durability. 110

transmission [%]

100 90 80 R = 5 mm R = 10 mm R = 20 mm R = 40 mm

70 60 50 10

100

1,000 10,000 100,000 number of repeated bending cycles

Fig. 9.90: Optical transmission of a 1 mm SI-POF with PE jacket subjected to repeated bending stress with varying bending radii and T = +23°C ([Daum93]) 110 transmission [%] 100 90 80 70 60 50 10 100 T = +23°C T = -40°C T = +85°C/85% r.H.

1,000 10,000 100,000 number of repeated bending cycles

Fig. 9.91: Optical transmission of a 1 mm SI-POF with PE jacket subjected to repeated bending stress under varying temperature/climatic conditions and with R = 10 mm ([Daum93])

732


A peculiar reaction of the fiber can be seen at high temperatures. In this case it is true that a longer durability can be observed, but as regards the functionality of the cable, it can be seen that after only 100 repeated bending cycles a rapidly increasing deterioration in the transmission already appears. This effect on the optical transmission can be explained by an increasing and irreversible geometric change in the vicinity of the bending point, the optical fiber being still relatively flexible at these temperatures (Fig. 9.92). If the constriction becomes too great for the stress at any time, the fiber will break. In practical applications this means that at high operating temperatures the polymer optical fiber should not be exposed to repeated bending stress or only to a very limited degree. At lower temperatures component failure is generally characterized by a clean break in the fiber (Fig. 9.93, [Daum93]).

Fig. 9.92: Fiber constriction caused by repeated bending at T= +85°C/85% RH ([Daum93])

The estimate of the smallest allowable bending radius with a predetermined number of repeated bending cycles is based on the following considerations. As mentioned already, it can be taken as a first approximation that in bending stress the mechanical stress at the periphery varies in inverse proportion to the bending radius. With a decrease in the stress, that is, with a greater bending radius, the number of cycles needed to cause component failure is higher. By extrapolating the results of the tests - the number of repeated bending cycles up to the point of component failure with various bending radii - to any specified number of repeated bending cycles, one can estimate the corresponding smallest allowable bending radius (Table 9.6).


733

Fig. 9.93: Fiber break caused by repeated bending at T = -40°C ([Daum93]) Table 9.6: Estimate of the smallest allowable bending radius in repeated bending

with a given number of cycles and T = +23°C Given Number of Cycles

Estimated Minimum Allowed Bending Radius for Repeated Bending for 1 mm SI-POF with PE Jacket

104

20 mm - 25 mm

5

40 mm - 55 mm

6

100 mm - 135 mm

10 10

9.7.1.2 Flexing It is characteristic of the use of POF in mechanical engineering that in many cases a transmission of signals takes place between the control unit and the moving components of the system. A typical case of this is the transmission of data between a stationary machine control unit and the various drive units in large automated manipulator systems. In systems like this both cables that supply energy and those that transmit signals are carried by means of trailing chains. The mechanical stress arising from this is characterized by a cyclically repeated unrolling movement where a certain length of the cable executes a U-shaped turn through 90°. When this occurs, the polymer optical fiber is subjected to an alternating bending stress, which extends over the whole length of the trailing chain. As with repeated bending, this means that the POF undergoes alternating states of

734


mechanical stress at its periphery over the entire length. In this case as well, the degree of the stress varies in inverse proportion to the bending radius. In order to simulate this characteristic stress condition and to test the ease of handling, we can make use of an established flexing test as a basis for our testing procedure. This test method has been effectively used for quite sometime as a means of testing cables and insulated lines. The basic principle of an apparatus for carrying out this simulation is shown in Fig. 9.94.

movable unit with pulleyes (radius R)

stressed fiber length 1115 mm

v weight 200 g

light source

receiver

drive shaft

data recording system drive unit

Fig. 9.94: Test equipment for flexing

The POF is installed around the two pulleys in the form of an S and by a predetermined movement of the pulley arrangement is subjected to a repeating cycle of bending. This subjects the polymer optical fiber to alternating states of stress at its periphery (tensile stress caused by stretching, absence of stress, and compression stress caused by compression). In contrast to repeated bending where stress is focused just at a single point in the fiber, a whole section of the polymer optical fiber is subjected to extreme mechanical stress in this test. In order to ensure that the polymer optical fiber adheres securely to the bending radii, the POF is loaded with a weight (typically 200 g) at both ends and subjected to tensile load. During the test the optical power is measured at intervals after the execution of a certain number of cycles of flexing. For this purpose, the movable carriage is first brought to a resting position. After a relaxation time of about 60 seconds has passed, the optical power can be measured and the relative transmission value determined. Figure 9.95 records typical test results for the measurement of transmission in fibers subjected to stress due to flexing with varying bending radii.


735

110 transmission [%] 100 90 80 70 R = 20 mm (PE) R = 40 mm (PE) R = 40 mm (PA)

60 50 1

10

100

1,000

10,000 100,000 1,000,000 number of flexing cycles

Fig. 9.95: Optical transmission (1 mm SI-POF with PE or PA jacket) under flexing with varying bending radii and T = +23°C

From the inception of the stress up to a specific threshold the transmission is unchanged, or is only very slightly affected. When a critical number of cycles specific to the sample is reached, the transmission falls drastically within a few cycles more and component failure results. The usual reason for this is a fiber break in the part of the sample that has been subjected to stress. 9.7.1.3 Torsion A twisting of the polymer optical fibers can occur, for example, in the manufacture of cables or directly during the installation of a POF. In this case, too, we must make a distinction between static and dynamic stress. Static torsion is found, for instance, when polymer optical fibers are fixed in position. This has no determining significance for the reliability of the fiber. Dynamic twisting on the other hand puts the polymer optical fiber under considerably greater stress. This kind of stress arises mostly from a cable’s being subjected to movement, which happens with industrial robots or automatic manipulation systems, for example. As in the example of repeated bending, dynamic torsion leads to a cyclical repetition of the build-up of mechanical stress and its release in the polymer optical fiber. As possible effects of this stress on the optical transmission we may mention here the damage of the interface between the optical cladding and the fiber core; development of micro-cracks in the fiber, possibly leading to a break; a crack in the jacket (Fig. 9.98), leading to the direct influence of humidity or other aggressive substances on the fiber. At high temperatures an irreversible geometric change is also possible in view of the softening of the fiber. For the investigation of optical transmission under torsion stress, possibly in combination with climatic stress, a test apparatus like that shown in Fig. 9.96 is

736


the most useful. This consists of an installation for the testing of torsion, which is incorporated in a climatic chamber, while the drive unit attached to it is situated outside the chamber. The actual torsion testing device consists of two mounts for holding the POF, one fixed and the other capable of rotation, both of which in the form of a guide pipe with a clamp fitting at the pipe’s end. The left guiding pipe is mounted on a moveable carriage, which is drawn in the direction of the POF axis with a weight of 200 g so as to subject the sample length under test to a certain amount of tensile stress.

drive shaft fixed clamp

rotating clamp

drive unit

light source 600 mm data recording system

weight 200 g

climatic chamber detector

Fig. 9.96: Test equipment for torsion

The following testing cycle has proved to be suitable for investigating the optical transmission under torsion stress. The POF sample is first turned in a clockwise direction for a specific number of revolutions. Then the sample is brought back to its starting state and turned through the same number of revolutions in a counterclockwise direction before finally being brought back again to the starting position. The measurement of the transmission always takes place in the unstressed state after a relaxation time of about 60 seconds. As a result of the test the relative change in the transmission is determined, that is to say, the change that has set in after a given number of torsion cycles have been carried out as compared with the starting value of the sample that has not yet been exposed to torsion stress. Typical effects on the reliability of polymer optical fibers under torsion stress are shown in Fig. 9.97. Starting from the unstressed condition, the transmission at first continues almost unchanged in spite of the increasing number of repetitions of the torsion cycle until a particular threshold is reached, after which the transmission falls sharply. Component failure (transmission < 50%) occurs at room temperature at some point between 2,000 and 3,000 cycles. The transmission curve at low temperature (-40°C) is in principle similar to that at room temperature, but here component failure occurs as early as somewhere between 400 and 500 cycles. The notably


737

shorter durability can be ascribed to the brittle condition of the polymer optical fiber at this temperature as compared with that at room temperature - temperature being a factor determining the modulus of elasticity and the mechanical stress properties. 110

transmission [%]

100 90 80 70 T = -40°C T = +23°C T = +85°C/85% r.H.

60

number of torsion cycles

50 10

100

1,000

10,000

100,000

Fig. 9.97: Optical transmission of a 1 mm SI-POF with PE jacket subjected to torsion stress (1 cycle: r 10 u 360°) under varying climatic conditions

The exceedingly better durability that can be observed at T = 85°C/85% RH can be explained by the increased flexibility of the fiber at temperatures of this order, thanks to which the polymer optical fiber can better adapt to the stress of torsion so that irreversible damage, such as the destruction of the core-cladding interface, micro-cracks, or fiber break, does not occur until later.

Fig. 9.98: Crack in the POF jacket caused by torsion stress

738


9.7.1.4 Tensile Strength Polymer optical fibers can be subject to stress caused by the traction force, more often than not during manufacturing and installation, but also during the time they are in use. We can distinguish in principle between short-term and long-term tensile stress. Short-term stress with relatively powerful traction force can occur for instance at the time of installation, whereas continuous tensile stress is generally a consequence of a wrong installation. The change in the optical transmission under tensile stress is generally connected with the general deformation of the fiber that occurs. This general deformation depends on the degree of the tensile stress and consists of elastic, linear visco-elastic, nonlinear visco-elastic and plastic deformations. Intensified tensile stress can also lead to the formation of micro-cracks in the core and to a damage of the corecladding interface. In extreme cases a fiber break cannot be ruled out. In connection with industrial use attention must also be paid to the role of temperature in determining the mechanical stress properties of the polymer optical fiber. Figure 9.99 depicts a typical test apparatus for investigating the optical transmission of the fiber under tensile stress. In order to make it possible to carry out a tensile test at extreme temperatures, the apparatus is incorporated in a climatic chamber. climatic chamber top view force sensor

R

R F

drive unit

100 mm

light source

detector

data recording system

top view

F R = 40 mm

R = 40 mm

Fig. 9.99: Test equipment for testing the tensile strength

The left chuck drum of the tensile testing device has a radius of R = 40 mm. It is fixed in place and connected with a firm framework structure situated outside the climatic chamber. During the tensile test the right chuck drum (R = 40 mm) is moved by a drive unit in the direction of the POF sample’s length, which results in


739

the sample being stretched between the two drums and is subject to tensile stress. While the tensile stress increases, the optical power and the traction force that is being applied are measured continuously. The result of the test consists in a calculation of the relative transmission during the increasingly intense tensile stress, correlated with the stress-strain curve. Typical results after the execution of tensile tests are seen in Fig. 9.100 to Fig. 9.101. Particularly noticeable, especially at room temperature is the behavior, specific to polymers, of the stress-strain curve. 300

force [N]

T = -40°C T = +23°C T = +85°C/85% RH

250 200 150 100 50

strain [mm] 0 0

50

100

150

200

250

Fig. 9.100: Stress-strain curve of a 1 mm SI-POF with PE jacket subjected to tensile stress under varying climatic conditions

110

transmission [%]

100 90 80 70 T = -40°C T = +23°C T = +85°C/85% RH

60

strain [mm]

50 0

50

100

150

200

250

Fig. 9.101: Optical transmission of a 1 mm SI-POF with PE jacket subjected to tensile stress under varying climatic conditions

740


In the first phase there is at once a sharp increase in the traction with only a slight increase in the strain. In this zone there is initially an almost linear ascent of force over strain. This zone comes to an end when the yield point is reached - visible as an overshooting (decrease in stress with increase in strain). Here the zone of plastic deformation begins. In this zone after a brief drop in force, a gradual increase in tensile force accompanied by an increase in the strain can be observed. The sample is pulled out of shape as a result of cold flow. When the deformation of the POF sample can go no further, the sample breaks. This is the point at which the maximum traction force for the respective POF sample is reached. During the tensile test a steady diminution in the optical transmission can generally be observed, up to the point where the sample breaks. Before the zone of plastic deformation is reached, the transmission is only insignificantly affected to the order of 2% to 3%. With regard to ensuring reliability in practice, it is absolutely essential that the limit of extension is not be reached if the POF is subjected to an intense short-term tensile stress. In regard to the various climatic conditions this means that the various limiting maximum values for short term traction force must not be exceeded, either in the installation or during the period of its use, to prevent irreversible deformation or breaks in the fiber. In addition, one must take care that the development of micro-cracks does not lead to a deterioration in the transmission quality. 110

transmission [%]

100 90 80 70 T = -40°C T = +23°C T = +85°C/85% r.H.

60 50 0

50

100

150

200

250

strain [mm] Fig. 9.102: Optical transmission of a 1 mm SI-POF with PA jacket subjected to tensile stress under varying climatic conditions


400

741

force [N] T= -40°C T= +23°C T= +85°C/85% r.H.

350 300 250 200 150 100 50 0 0

50

100

150

200

250 strain [mm]

Fig. 9.103: Stress-strain curve of a 1 mm SI-POF with PA jacket subjected to tensile stress under varying climatic conditions

Since risks such as tensile stress of varying magnitude and the development of cracks cannot fundamentally be excluded, we recommend in practical situations that the allowable short-term maximum tensile force Fmax be limited using a safety coefficient S, to Fmax/S, where S = 1.5. Because the mechanical stress properties are affected with an increase in temperature and with the risk of a loss of dimensional stability, Fmax should absolutely be avoided at higher temperatures, or at least considerably reduced. With continuous tensile stress it must be noted that on account of the properties of the material only substantially lower tensile forces are permissible. It is recommended in the literature ([Schmi92]) that the zone of linear visco-elastic deformation not be exceeded with continuous tensile stress. This zone corresponds to an extension of 0.1% to 0.5% with thermoplastics. There are ways of improving the resistance of the cable to tension load either by changing the material of the jacket or the composition of the cable, e.g. with an additional strength member. 9.7.1.5 Impact Strength In industrial situations polymer optical fibers may also be subjected to stress in the form of impact. An easily imagined scenario is one in which tools or other objects are accidentally dropped on the POF during the installation. And then it is not out of the question that tools or other objects may suddenly impact on a POF if it has been installed without protection. Also in dealing with vehicles and especially during installation or in the course of repair work, one must take into account the possibility of unintentional stress in the form of impact. In impact the polymer optical fiber is forced to absorb energy, giving rise to stress peaks both in the jacket and in the fiber.

742


In addition, it is possible that the optical fiber may suffer irreversible geometric change in shape. The burst of the jacket as a result of impact stress can enable humidity or other aggressive substances to act upon the optical fiber without impediment. An apparatus for the testing of impact effects to polymer optical fibers is shown in Fig. 9.104. The installation used basically consists of a free falling mass with a weight of 1 kg to create the energy of the impact. The drop weight is guided by a guiding rod, practically without any friction, for the duration of the free fall before making contact with an impact piece, which lies, on the POF sample. This impact piece then transmits the entire energy of the impact to the polymer optical fiber. To intensify the stress, the underside of the impact piece has a radius of 10 mm which is positioned straight across to the POF sample. After the impact has been effected, an electromagnet picks up the drop weight which is then brought back by means of a drive unit to the prescribed drop height and allowed to fall again. climatic chamber

drop weight 1 kg

impact piece Ø 20 mm

detector

drive shaft


drive unit

light source

Fig. 9.104: Test equipment for impact strength

If impact tests are carried out on POF, we find, as was to be expected, that when the drop height is greater, fewer impacts are needed to induce component failure, that is, a reduction of the transmission to less than 50% of the starting value. This is the case in all climatic conditions. At temperatures of +23°C and -40°C the optical transmission of the POF samples basically shows a similar pattern as the number of impacts rises. Starting from the unstressed state, the transmission remains almost constant or is reduced only by a minute gradient until the critical number of impacts is reached. From this point on the transmission is rapidly reduced, leading to component failure. A visual examination of the POF samples after the end of the test often shows that the jacket of all the samples has split open. In addition, it can be observed that the fiber shows signs of brittle splintering with zones of pronounced fiber-like cracks or it may even break entirely.


743

Fig. 9.105: Impact test in extreme temperature or climatic conditions

A somewhat different behavior on the part of the transmission curves can be observed in climatic conditions such as T = +85°C / 85% RH. As the number of impacts rises, the transmission declines before reaching the critical point at a distinctly more rapid pace than in the previously discussed cases. When the critical threshold is passed, the transmission then falls off steeply in these environmental conditions as well. This change in the optical transmission can be explained by the setting in of irreversible geometric alterations in the fiber core, which at these temperatures is relatively soft.

744


A summary of typical test results for impact is shown in Fig. 9.106 and Fig. 9.107. The illustrations represent the number of impacts needed at room temperature for a typical POF sample to reach the 50 % transmission critical point, the drops taking place from varying heights.

90

drop height [mm]

transmission < 50%

80

transmission > 95%

70

drop weight 1 kg

60 50 40 30 20 10

number of impacts

0 1

10

100

1,000

Fig. 9.106: Component failure of a 1 mm SI-POF with PE jacket subjected to impact stress by an object falling from various heights

90

drop height [mm] transmission < 50% transmission > 95%

80 70

drop weight 1 kg

60 50 40 30 20 10

number of impacts

0 1

10

100

1,000

Fig. 9.107: Component failure of a 1 mm SI-POF with PA jacket subjected to impact stress by a drop weight from various heights


745

9.7.1.6 Crushing Strength Dynamic stress caused by lateral pressure or crush can affect a polymer optical fiber in conditions of industrial use in a number of different ways. Typical examples of stress are an exposed and unprotected POF being inadvertently run over or trodden on during the installation or the exertion of crush when the cable is inappropriately installed in connection with moveable mechanical parts and manipulation systems, as in the case of vehicle doors. Since this mechanical stress with its effect on the POF is very similar to the stress due to impact described previously, the same damage factors basically arise, e.g. formation of micro-cracks, irreversible change of shape, burst of the jacket. An apparatus for testing the effect of crush on the POF is shown in Fig. 9.108. It consists of a fixed base plate made of steel, and a mobile, directed steel stamp with rounded edges, the contact surface being 100 mm long. To measure the pressure, the unit that supplies the force also contains a device that registers its amount. The POF sample to be tested is fixed on both sides of the base plate to the holding devices that we have seen before so that it cannot move laterally. The stress of crush is dynamically experienced through the steady application of force with prescribed times of pressure followed by a time of relaxation. Here one crush cycle represents a phase of pressure followed by a span of time when the pressure is removed. data recording system detector steel stamp

100 mm

force sensor

F

drive shaft

drive unit

light source climatic chamber

Fig. 9.108: Test equipment for crushing strength

The transmission values are repeatedly taken after the pressure has been released from the sample and after a suitable time of relaxation has been allowed to elapse. The result of the test consists of the relative transmission values in a rising number of lateral pressure cycles, taking the unstressed state at the start of the test as a refe-

746


rence point. As a rule the test is carried out until material damage to the sample sets in, e.g. burst of the jacket, or until transmission declines to a level of 50%. transmission [%] 110 100 90 damage of the jacket

80 70

F = 4500 N (PE) F = 1950 N (PE) F = 4500 N (PA)

60 50 0

200

400

600

800

1,000 1,200 1,400 number of crushing cycles

Fig. 9.109: Optical transmission of a 1 mm SI-POF subjected to crush with pressure at various degrees of strength and T= +23°C

Figure 9.109 shows typical developments in the optical transmission of various POF samples subjected to crush. It can plainly be seen that the properties of the jacket have an important influence on the reliability and durability of the polymer optical fiber. When the stress of crush is too high, damage to the jacket generally results. In spite of the splitting of the jacket, however, the functionality of the polymer optical fiber can still be maintained with a transmission of >80 %. But because of the possibility of the rapid penetration of humidity or aggressive chemical substances in the region of the damaged jacket, such POF must be viewed as no longer fit for use. 9.7.1.7 Vibration Stress caused by vibration has a particular relevance to automobiles but can also be a factor in industrial applications. Experience with glass fibers shows that this kind of vibratory stress in extreme cases can lead to a failure of the optical transmission following breaks in the fiber. In view of their greater flexibility, however, such behavior is not to be expected from polymer optical fibers. Tests have shown that in the example of automobiles [SAE78] the typical vibratory stress lies in a frequency band of 10 to 2,000 Hz. In order to simulate these stress conditions we can - following [IEC95] - fasten the POF samples in the shape of a ring on a vibrating table (shaker), taking into


747

account the permissible radius of bending in accordance with the type of installation intended, e.g. fastening in place by means of cable ties. While the transmission is concurrently measured, the samples are then subjected to the vibratory stress described above. Tests like these carried out in the frequency range of 10 to 2,000 Hz over 100 hours which correspond to 1636 frequency cycles at 100 sec per decade resulted in no alteration in the optical transmission, thereby confirming the optimal qualities of polymer optical fibers in this respect. 9.7.2 Stress due to Change in Climatic Conditions Changes in climatic conditions are characterized by changes in temperature and/or humidity. These can take place either rapidly or gradually. Particularly extreme coditions in this respect are to be found in automobiles, where for example in an extreme case the internal temperature may vary between -40°C and +85°C (at times even as far as 105°C), with humidity values up to 98% RH (at +38°C, [SAE78]). Extreme changes in climatic conditions, especially when they arise over a short period of time, can have an effect on the optical transmission of polymer optical fibers. Rapid changes of temperature lead to inner stress in the polymer fiber. In this respect we must also take into account the various thermal extension coefficients of polymer optical fibers (DPMMA= 7 · 105 K-1), and the jacket surrounding them (DHDPE = 16 · 105 K-1, DPA6 = 8 · 105 K-1). At higher temperatures, as well as thermal aging effects (see the following section), relaxation effects can arise in the deformations at the macromolecular level which have been frozen into the structure through extension or shearing or through the action of heat and cold after the manufacture and processing of the fiber. It is a well established fact in the literature ([Kain85], [Kain86], and [Kain89]) that when water is absorbed by polymer optical fibers a considerable decline in transmission can occur. This effect must also be taken into account when POF is used under extreme conditions of stress due to climatic change since a change in state caused by water absorption can lead to further inner mechanical stresses. The investigation of optical transmission under the stress of extreme climatic changes calls for a climatic chamber and a suitable measuring device capable of high resolution measurement of the transmission like the one described in section 9.6.2.2. The POF to be tested are placed in the climatic chamber and arranged in a loose ring on a grid (Fig. 9.110). Here care must be taken that the POF samples are equally exposed on all sides as far as possible to the climatic stress at any time and that no bending or bending beyond the permissible radius takes place. To get as complete a picture as possible of the optical transmission, it makes sense to carry out a more or less continuous measurement of the transmission in all samples throughout the experiment. As result of the test we can determine the relative transmission at any point in the procedure with reference to the respective starting value.

748


Fig. 9.110: Equipment for climatic testing

Typical test results for various POF samples are shown in Fig. 9.112. The alterations in temperature and humidity shown in Fig. 9.111 form the basis of these results. A complete cycle of temperature change lasts 8 hours, of which 2 hours are to be envisaged at each of Tmin and Tmax. The relative humidity (95% RH), being determined by the properties of the climatic chamber, can only be sustained with reasonable accuracy when the temperature is between +23°C and +90°C. temperature [°C] / rel. humidity [%]

120 100 80 60 40 20 0 -20

humidity temperature

-40 -60 0

2

4

6

8

10

12

14

16 time [h]

Fig. 9.111: Temperature and humidity curves (2 cycles) in the simulation of extreme stress caused by climatic change


749

Within a time span of approximately ten hours after the start of the experiment, both POF samples show a decline in transmission in the order of 10%. Here we are faced with a characteristic effect which can be observed in all investigations where high temperature and high relative humidity occur in conjunction (see also the following section). However, the transmission thereafter remains practically constant over the remaining time span of the test (125 cycles of change or 1,000 hours). For practical use this has the implication that in a strictly limited time-frame of exposure to stress caused by extreme changes of temperature coupled with high relative humidity, apart from a fixed falling-off in the transmission, no further change in the optical transmission is to be expected. When the stress of extreme climatic change continues over a longer period of time, however, we must take into account the accelerated aging of the polymer optical fiber as is explained in more detail in the following section.

optical transmission [%] 110 100 90 80 70 PE PA

60 50 0

200

400

600

800

1,000

1,200 time [h]

Fig. 9.112: Optical transmission of a 1 mm SI-POF (L = 10 m) with PE and PA jacket under extreme stress caused by climatic change (125 cycles as shown in Fig. 9.111)

9.7.3 Aging due to the Stress of High Temperature and Humidity It has been sufficiently established in the literature ([Kain85], [Kain86] and [Kain89]) that water absorption in polymer optical fibers can cause a significant decline in transmission. The absorption of water in PMMA can be seen in two distinct situations: in the one case water accumulates in the polymer matrix and leads to swelling, in the other its gets absorbed into the micropores ([Tur82], [Mas84]). Kaino comes to the conclusion that a considerable increase in attenuation, varying

750


according to the material and the wavelength, can occur when water is absorbed. This increase in attenuation is caused, essentially, by the O-H absorption of the light at 750 nm (third harmonic of the O-H stretching vibration mode) and at 850 nm (combination of the second harmonic of the O-H stretching vibration mode and the O-H deformation vibration mode). In connection with the absorption of humidity, the thermal aging that occurs at high temperatures is the essential determinant of the usability and durability of polymer optical fibers. Decomposition effects can be shown to occur with thermal aging which, as a consequence of transference of energy in the form of heat, lead to a degradation or splitting of the polymer chain. Every polymer has its own weak points where decomposition effects are most likely to occur. In this class belong lateral chains, for example, and substitutes that are connected to the main chains with a low level of binding energy. With PMMA there is particular risk of depolymerization. This term stands for the splitting off of end groups and the loosening of monomer components from the end of the chain. As the parts that break off have unattached valences - being known as free radicals - they try to form new combinations, for example with oxygen. This leads to oxidization resulting in the accumulation of submolecular debris. The consequence is brittleness and disintegration, with a direct effect on the mechanical and optical properties of the polymer optical fiber (see for instance [Stru66], [Bros89]). With perfluorinated, graded index profile polymer optical fibers, a thermally determined alteration in the dopant material can come about, leading to changes in the refractive index. New materials that have just recently become available, however, behave with admirable stability in this regard. As [Kog00] and [Oni99] show in relation to CYTOP, thermal aging at 70°C over 10,000 hours leads to no significant alteration of the refractive index or to any deterioration in terms of attenuation and bandwidth. It is characteristic of the thermal aging process that the relevant property, in this case the optical transmission, does not deteriorate continually, but stays approximately constant at first over a greater or lesser period of time. Only after this initial phase has come to an end, a steadily accelerating deterioration of the transmission does set in. In order to predict the durability of the fiber, we can make use of the principle of correspondence between time and temperature as it has been carried over into polymer testing, seeing that the aging processes, so far as is known, obey the laws of reaction kinetics. On this basis a mathematical extrapolation of the decline in transmission due to aging can be made ([McK94]). The procedure we are about to describe has proved effective for investigating the effects of absorbed humidity and thermal aging on the optical transmission. The conditions of the simulation or of the aging are determined in such a way that the aging process is accelerated, bringing about an artificial (accelerated) aging. This strategy of speeding-up is based on an intensification of the simulated stress in the relatively short time span of the test. Here we must be careful that no other aging factors that have anything to do with the aging effect we wish to produce should enter the picture. For this reason, in investigating the life expectancy of polymer optical fibers there is an experimental strategy, which has proved effective that takes extreme, but still allowable environmental conditions as its starting point. This means essentially that the temperatures chosen to produce the accelerated aging


751

must lie clearly below the glass transition temperature Tg (in the order of 115°C for PMMA SI-POF). As explained in the preceding section, the POF samples are positioned on a grid in a climatic chamber and loosely rolled up for a prescribed length of time to induce accelerated aging. The inputs and outputs of the fibers are taken out of the climatic chamber and connected to a suitable measuring device capable of high resolution measurement of the transmission (see also section 9.6.2.2). There then follows a relaxation phase, lasting perhaps several hours, in which samples and the measuring equipment can settle before going into operation. After this rest period the fibers are subjected to temperature and humidity stresses while the transmission is measured at the same time. To begin with, the room temperature is kept constant while the humidity is raised to the desired maximum value and then also kept constant at this level for the entire duration of the test. After the maximum value has been reached, the temperature is gradually raised over a period of 4 hours until it reaches the selected aging temperature. During the entire test the optical transmission of all POF samples is measured and the relative transmission in relation to the unstressed starting state is determined. Figure 9.113 shows the typical transmission pattern for three different wavelengths as shown by a 1 mm SI-POF under stress from high temperature and humidity. Developments in the optical transmission can be broken down into four phases. So far as current findings indicate, these phases can be correlated with certain definite time-dependent aging effects ([Ziem00b]): 1) Within the first 24 to 48 hours a definite deterioration in the transmission occurs following on the first assimilation of humidity. 2) In the following preliminary phase there is only a very slow deterioration in the transmission to be observed. The length of this period, and the gradient of the transmission’s falling-off, depend on the manufacturer, on the material of the jacket, and on the type of POF, as shown for example in Fig. 9.114. The aging effects that take place during this period have at this stage only a slight effect on the transmission. 3) This phase is characterized by a rapid deterioration in the transmission. The reason for this is thought to be the increase in the “free volume” and a heightened absorption of humidity following from that. 4) If the humidity is changed while the temperature is kept constant, the transmission responds to these modifications. When the humidity is reduced to normal values, the transmission almost returns to the values current at the end of the second phase. If the humidity is raised again, the transmission immediately deteriorates, with the humidity now being very quickly absorbed into the polymer optical fibers.

752 1.0


rel. transmission

POF sample No. 4

0.9 0.8

2

0.7 0.6

1

0.5

4 3

0.4 0.3 0.2

525 nm 590 nm 650 nm

0.1 0.0 0

500

1000

1500

2000

2500

3000

3500

4000 4500 aging time [h]

Fig. 9.113: Typical changes in the optical transmission of a 1 mm SI-POF (L = 10 m) subjected to stress from temperature and humidity [Ziem00b] 1.2

rel. transmission at 650 nm T= 92°C/95% r.H.

1.0

POF-No. 1 POF-No. 4 POF-No. 7 POF-No. 10 POF-No. 13

0.8 0.6 0.4 0.2 0.0

aging time [h] 0

1000

2000

3000

4000

5000

Fig. 9.114: Typical changes in the optical transmission of various 1 mm SI-POF (L = 10 m) subjected to stress from temperature and humidity [Ziem00b]

Figure 9.115 shows the way in which physical aging effects as a result of the action of temperature and humidity can impact on the entire attenuation/transmission spectrum of a polymer optical fiber. It can be clearly seen that the decline in transmission occasioned by aging is not consistent across the entire spectrum ([Daum97]). A sharper decline in transmission can be seen in the zone of the lower wavelengths in particular. This effect must be given particular consideration when polymer optical fibers are used in displays and variable traffic signs where strong requirements in terms of the long-term reliability of color transmission need to be met.


0.6

753

15 m POF before aging after aging T= +85°C/85% r.H.

rel. transmission 0.5 0.4 0.3 0.2 0.1 0.0 300

400

500

600


Fig. 9.115: Typical spectrum-related changes in the optical transmission of a 1 mm SI-POF before and after aging caused by high temperature and humidity ([Daum97])

The estimation of durability is based on a temporal extrapolation of the established time/temperature relation for a defined criterion of aging, e.g. reduction of the transmission to 50% of the starting value. In selecting an extrapolation method, the user has basically two possibilities available. They are closely connected with the underlying physical laws that have a determining effect on the aging process. If we follow the theory of „free volume“, the extrapolation can be based on the Williams-Landel-Ferry (WLF) theory, which has been widely used in the testing of polymeric materials. In this hypothesis the factor that accelerates the aging process can be defined according to the following formula ([Bros89]): log a T =

8.86 (T - Ts ) 101.6 + (T - Ts )

where aT = acceleration factor (time shift factor) T = selected aging temperature (K) Ts = reference temperature (K) The critical value in this connection is the reference temperature Ts. There is no single definition for this given in the literature. Generally the value is characterized in such a way that the reference temperature gives the value at which deliquescence of the polymer can first be observed. On the strength of the knowledge we have to date, this phenomenon can well be related to the glass transition temperature TG of the polymer fibers. Consequently TG is generally substituted for the reference temperature Ts. The maximum possible operating temperature Tmax for a given durability tL can be obtained by conversion from the same equation as:

754


Tmax = T s -

101.6 log a T,L 8.86 + log a T,L

with an acceleration factor, relevant to the durability, of

log a T,L = log a T + log tL t A

Here the aging time tA is the time taken to reach a predefined criterion of aging (e.g. decline in transmission to 50%) with the aging process being accelerated, T being the aging temperature in each case. On the basis of accelerated aging tests with various aging temperatures, we can, by substituting the appropriate aging times and temperatures in the WLF equations, obtain an estimate for the maximum permissible operating temperature for a given durability. Judging from the literature ([Ziem00b]) and from the authors’ own tests, the result is a mean maximum permissible temperature of between 72°C and 85°C depending on the manufacturer, the jacket, and the type of POF - for a durability of 20 years using the standard 1 mm SI-POF with PE and PA jacket. The other attempt at an estimation of durability is based on the Arrhenius theory and proceeds on the assumption that a chemical reaction R is the underlying cause of the aging of many polymeric materials, including POF. The time taken by this reaction can be defined by means of the reaction rate dR/dt, as follows: dR dt

A e

§ W· ¨¸ ¨ kT ¸ © ¹

where W k T A e

= thermal energy as an activation energy = the Boltzmann constant = aging temperature (K) = constants specific to the material = base of the natural logarithms

For the practical assessment of the thermal aging process, this equation is set out and applied in the following form (DIN ISO 2578:1994) [DIN94]: tA

A e B T

= aging time [h] up to the point of a given decline in transmission tA A, B = constants specific to the material With a simple conversion, this equation can be expressed as a linear function: log t A

log A (log e) B T

Thus in practice there is a linear connection between the logarithm of the aging time required to effect a given decline in optical transmission, and the conversion value of the absolute aging temperature associated with it. On the basis of this


755

connection, expected results for higher temperatures can be extrapolated on the time taken to cause breakdown at lower temperatures. The Arrhenius hypothesis has been used with success in predicting the durability of electronic components, for example, and has been called in to determine the time and temperature limits in cases of long-term effects of heat on polymeric materials (DIN ISO 2578:1994). 10,000

time [h]

1,000

maximum operation temperature of a given durability of 15 years: T = +80°C +/- 2 K

88°C 89°C

90°C

100 91°C 92°C 10 aging time linear interpolation

1 2.78·10-3

2.77·10-3

2.76·10-3

aging temperature 1/T [1/K] 2.75·10-3

2.74·10-3

2.73·10-3

Fig. 9.116: Estimation according to Arrhenius, of the maximum permissible operating temperature for a special 1 mm SI-POF (L = 10 m) without jacket for a given durability

As with the WLF hypothesis, so here the Arrhenius equation can be used to estimate either the durability for a given operating temperature, or the maximum permissible operating temperature for a given durability. Figure 9.116 shows a typical result of this, determined by following the procedure described in DIN ISO 2578:1994 for a special 1 mm SI-POF without jacket. What is depicted is the estimate of a maximum operating temperature for a given durability of 15 years for a special polymer optical fiber, which is to be used for lighting purposes. With reference to the envisaged method for a prediction of durability it must be stated as a matter of principle, that the results - as is the case with all extrapolatory procedures - are accompanied by a degree of uncertainty which we should not be oblivious to. The more unfavorable the relation between the time taken for the test and the envisaged durability, the more unreliable the prognosis. In dealing with cable technology one is usually limited with a prediction to a duration of 20,000 hours - 25,000 hours (about 3 years), on the assumption that the constant operating temperature so determined will either not be reached at all or else only fleetingly and for short periods. We must likewise take into consideration that the tests needed here take a great deal of time, calling for some thousand hours of laboratory use. More rapid testing procedures for POF such as the investigations based on chemoluminescence for example [Scha99] are only just in the process of being developed.

756


9.7.4 Resistance to Chemicals Alongside mechanical and climatic stress factors, stresses induced by chemicals or other aggressive substances can be counted among the critical influences in the area of industrial applications. For example, the action of chemicals on the jacket of a polymer optical fiber can lead to changes in the chemical properties and mechanical characteristic values of the material, and with further penetration may give rise to changes in the optical transmission of the fiber. The essential causes of this that need to be reckoned with are the chemical transformation and dissolution of the polymers, or the absorption of chemicals into the fiber. In addition, changes to the intrinsic mechanical stresses as a result of the material’s softening are a possibility. We must also take into consideration that higher temperatures in certain cases can lead to an acceleration in the action of the chemical agent. An test apparatus suitable for these specialized investigations is depicted in Fig. 9.117. The construction of the test is in principle similar to that designed to produce accelerated aging. Only here the climatic chamber is replaced by a heating chamber that is protected against explosions. The samples are introduced into the heating chamber by means of glass vessels and chemically neutral holders. Generally speaking, tests designed to test resistance to chemicals follow a course analogous to the climatic tests. To intensify the conditions simulated, the temperature of the chemical medium can be raised, in accordance with a predetermined scheme, to a temperature just below the flash point for the chosen chemical ([Strec94]).

optical multiplexer with light source and detector

1

n


glass vessel with chemical heating chamber Fig. 9.117: Test equipment for investigating resistance to chemicals


757

Table 9.7 shows an overview of typical test results in testing the resistance to chemicals of polymer optical fibers. Depending on the chemical and the jacket, we can specify three classes of substances: 1. harmless, 2. harmful, 3. less harmful. If a chemical is ranked under the heading “harmless”, a safe and reliable transmission of signals in normal operating conditions can be guaranteed. Stress caused by contact with a chemical classed as “harmful” must be avoided at all costs. Here further protective measures are absolutely necessary: for instance a protective tube made of metal, or an additional jacket or protective covering, or one with higher resistance to chemicals. With “less harmful” chemicals, contact over an extended period of time should be avoided. A short term contact (e.g. dripping, followed by immediate removal) would appear to cause no lasting damage. Table 9.7: Effect of chemicals on a 1 mm SI-POF with different jackets Jacket

Chemical

PE

PA

gasoline

harmful

harmless

diesel fuel

less harmful

harmless

jet fuel

less harmful

harmless

transmission oil

harmless

harmless

synthetic motor oil

harmless

harmless

brake fluid

less harmful

less harmful

carburetor cleaner

harmful

less harmful

hydraulic oil

harmless

harmless

cutting oil

less harmful

harmless

insulation oil

less harmful

harmless

H2SO4 10%

less harmful

less harmful

NaOH 10%

harmless

harmless

In addition to the material of the jacket, the material of the cable sheath also plays an important role in regard to the resistance to chemicals of optical fibers on a POF base. The following Table 9.8 gives an overview of the most commonly applied materials and their resistance to aggressive substances, cf. also Tables 2.26 to 2.30.

758


Table 9.8: Resistance to chemicals of different materials for sheaths ([Mair99]) Chemical

Abbreviation

Properties

Polyvinylchloride

PVC

resistant to oils, fats, diluted acids and alkalis up to 50°C; special mixtures are resistant to solvents and fuels

Polyethylene

PE

resistant to diluted acids and alkalis and to many solvents, moderately resistant to fuels and oils

Polyamide

PA

resistant to oils, fats, fuels and most solvents; moderately resistant to diluted acids and alkalis

Polypropylene

PP

resistant to diluted acids and alkalis, many solvents, fuels and oils

Polyurethane

PUR

resistant to oils, fats, and solvents; moderately resistant to diluted acids and alkalis

Polytetrafluoroethylene

PTFE

admirably resistant to almost all chemicals

In addition the cables can be better protected, not just against aggressive substances but against thermal and mechanical stress as well, through the use of special laser-welded corrugated micro tubes (CMT) as cable sheaths ([Schei00]). CMTs (Fig. 9.118) are immediately effective as primary protection, and can, depending on requirements, be made of the following materials: copper, aluminum, brass, bronze, steel, or special steel alloys.

Fig. 9.118: POF in a corrugated micro tube ([Schei00])


759

9.7.5 Stress Caused by Ultraviolet and High-Energy Radiation

A significant amount of stress due to radiation can occur when polymer optical fibers are installed in nuclear power plant, high-energy physics laboratories, linear accelerator or synchrotron facilities, or even in medical or industrial radiation equipment. Admittedly this form of stress has not been very comprehensively investigated hitherto; at any rate, only a few generally known findings and publications on this issue are to be found. Special aspects of the resistance to radiation of POF materials (PMMA, PFMA, P4FFA) have been written up in [Lev94]. Experiments by [Hen93] with polymer optical fibers made of PMMA, some unprotected, others with jacket, indicate a high resistance to radiation at a radiation of 60Co with energy doses < 100 krad. Measurements of the optical attenuation at 670 nm and 780 nm show only very slight changes in the attenuation when the polymer optical fibers are exposed to this type of radiation. Slightly higher changes in the attenuation result in combination with both high (+80°C) and low temperatures (-40°C). With similar tests using POF on a PC base a significantly higher sensitivity to radiation can be demonstrated.

Fig. 9.119: Surface of a PE jacket after exposure to UV radiation (left: non-

irradiated area; right: irradiated area) If polymer optical fibers with jacket are exposed over a long period of time to sunlight or artificial light with a high ultraviolet element, discoloration (a bleaching effect, or a change of color) is the result (Fig. 9.119), and in extreme cases it may even lead to micro-cracks on the surface. Discoloration is not on the whole connected with any essential changes in the properties of the material. With the formation of micro-cracks, however, it can lead to an impairment of the mechanical properties. As the depth to which the UV radiation can penetrate the jacket is strictly limited, the optical fiber will not be adversely affected. Unprotected optical fibers (e.g. without protective covering, or the unprotected fiber end surfaces) that are exposed to high UV radiation tend to go yellow. Their optical transmission deteriorates, the longer they are exposed to the radiation.

760


Consequently this kind of stress must be avoided as far as is possible. Particular care must be taken, where POF is used in illumination or variable traffic sign systems, to protect the light entry surface, e.g. against the UV radiation within the spectrum of a halogen lamp, and the output (light exit) surface, e.g. from sunlight. The use of suitable filters can provide an adequate protection against damaging UV radiation in these cases.

9.8 Standards and Specifications In 1982, Mitsubishi Rayon published a comprehensive documentation of reliability tests on polymer optical fibers. The current state of the standards and specifications available for reliability tests is comprehensively demonstrated in Table 9.9. In this table both national and international standards as well as the testing specifications put out by manufacturers and by the Federal Institute for the Materials Research and Testing (BAM) are represented. One must take into consideration that both these testing specifications and the Japanese standard (JIS) were developed for POF and are only to be used in this connection. The European standard (EN) is based on the IEC standard, though it is partially more detailed. Both standards (EN and IEC) describe testing procedures for all kinds of optical fibers including polymer fibers. The table shows that most of the mechanical tests have been included in all the standards and specifications for testing. Special reliability tests like those necessary for POF, however, are only mentioned in the testing specifications of the manufacturers and the BAM. Examples are the investigation of the aging behavior at high temperature and humidity or the investigation of the optical transmission under stress occasioned by chemicals or other aggressive substances. The fact is that up to the present time no consistent testing standards have been established for investigating the reliability of polymer optical fibers. Table 9.9: Classification of testing standards and specifications for POF Norms 1

Test tensile strength crushing strength impact strength torsion repeated bending static bend flexing kink abrasion change of temperature

Specifications for tests 3

4

Asahi IEC Mitsubishi Toray EN 2 JIS BAM 60793-1 187000 Chemical 5, 6, 7, 8 Rayon Co. Ind. C 6861 9, 10, 11 12 Inc. 60794-1 188000 Ind. Co. x x x x x x x x

x

x

x

x

x

x

x x x x x x

x x x x

x x x x x x

x x x x

x x x x x

x x x x

x x x x

x

x x

x

x

x

x

x

761

9.7 Reliability under Various Environmental Influences Norms 1

Test

Specifications for tests 3

4

IEC Mitsubishi Toray Asahi EN 2 JIS BAM 60793-1 187000 Chemical 5, 6, 7, 8 Rayon Co. Ind. C 6861 9, 10, 11 12 Inc. 60794-1 188000 Ind. Co.

high temperature (dry, wet)

x

x

x

low temperature

x

aging

x

x

chemicals

x

x

industrial atmosphere

x

x

flame retardance radiation

x x

x

x x nuclear

x nuclear

x UV

1)

International Standard IEC 60793-1-5:1995 - Optical fibers - Part 1: Generic specification - Section 5: Measuring methods for environmental characteristics International Standard IEC 60794-1-1:1999 - Optical fiber cables - Part 1-1: Generic specification - General International Standard IEC 60794-1-2:1999 - Optical fiber cables - Part 1-2: Generic specification - Basic optical cable test procedures 2) Japanese Industrial Standard JIS C 6861:1991 - Test methods for mechanical characteristics of all plastic multimode optical fibers 3) European Standard EN 187000:1992 - Generic Specific.: Optical Fiber Cables European Standard EN 188000:1992 - Generic Specification: Optical Fibers 4) Technical Information (Luminous T); Asahi Chemical Industry Co., Ref.-No. ´95.9.1 5) [Daum92] 6) [Daum93] 7) [Daum94] 8) [BAM95] 9) Technical Information - Chemical Exposure Test; Mitsubishi Rayon Co. Ltd., 1985 10) Technical Information - The Long-Term Durability of Optical Performance of ESKA Extra Fibers and Cables, Mitsubishi Rayon Co., Ltd. 11) Technical Information - Eska Cables; Mitsubishi Rayon Co. Ltd., 1982 12) Technical Bulletin - Toray Polymer Optical Fiber Cord; Toray Industries Inc., Ref.No. 9404-1 (PE0204-22)

A further problem arising from the plethora of different testing standards and specifications is the ability or inability to compare test results. A closer inspection shows that the testing procedures described are distinctly at variance. Typical inconsistencies are, for example, divergent testing parameters, the use of parameters not suited to POF, or variations in test equipment. Just for the testing of repeated bending five different possible tests are described, the results of which would not be directly comparable with one another [Daum99]. Groups interested in these matters, i.e. manufacturers, users, organizations responsible for industrial standards, are currently making intensive efforts aimed at producing consistent and internationally recognized testing standards for polymer optical fibers.

10. Simulation of Optical Waveguides

10.1 Modeling of Polymer Optical Fibers Optical waveguides and fibers have been used for many years to transmit signals. A number of models have been developed to describe their characteristics. In the field of optical fibers one differentiates between singlemode fibers (SMF), in which one eigenwave (mode) per polarization is capable of propagating, and multimode fibers in which the light can propagate in the shape of different eigenwaves at varying speeds. How many modes can propagate in a fiber depends on its diameter compared with the wavelength of the light and the numerical aperture. The greater both are, the more modes can propagate. The fiber parameter V describes the fundamental properties of a fiber: V

2S a AN O

2S

a 2 2 ncore - ncladding O

Here Ȝ stands for the wavelength of the light, a is the core radius of the fiber AN the numerical aperture, ncore and ncladding describe the refractive indices at the fiber axis and in the fiber cladding. The greater V is, the more modes are guided. The number of modes is approximately proportional to V2. Polymer optical fibers have a large diameter and a high numerical aperture so that an extremely large number of modes can propagate in them. In the case of a standard step index polymer fiber with a numerical aperture of AN = 0.5 and a diameter of about 1 mm several million modes can propagate. The optical signal is distorted and attenuated when it propagates over the fiber. These effects have to be modeled when describing the signal transmission. They behave quite differently in different types of fibers. Whereas signal distortions in singlemode fibers are primarily caused by chromatic dispersion, i.e. the different speeds of individual spectral parts, the description of dispersion in multimode fibers is considerably more complex. Not only does chromatic dispersion occur in them, but also has the generally much greater multimode dispersion. This effect comes about because the individual eigenwaves here propagate at different speeds. The signal is split up into the different modes which then arrive at the receiver at different points in time because of the varying speeds, thus transmitting a distorted signal. In contrast to chromatic dispersion multimode dispersion is influenced by different effects which do not exclusively depend on the fiber. The delay between the modes are determined by the refractive index profile of the fiber and depend exclusively on the fiber. How the signal power is split up into the individual

764

10.1 Modeling of Polymer Optical Fibers

modes, however, is determined by the coupling of the light into the fiber. For example, when the light is coupled very narrowly around the fiber axis and almost parallel into the fiber, then only one individual mode would be excited, i.e. the entire signal power would be coupled into this mode and would not even notice the time delay between the individual eigenwaves at all since only one mode is providing power. The other extreme would be a very wide coupling of the transmission angle and the surface. In this case the power would be divided evenly to all modes whereby the skews of all modes would play a role. This example clearly shows that when modeling multimode fibers you not only have to take the fiber itself into consideration but also the coupling in and out of the light. Furthermore, modes in polymer fibers do not spread independently of each other but are coupled to each other. A part of the power can be coupled over to other preferred, neighboring modes because of impurities and non-ideal interfaces between the core and the cladding. This mode coupling arises particularly strongly in polymer fibers which is why current models for the propagation of light in optical fibers have to be expanded >Whi99@ and >Shi97@. The most important types of fibers will subsequently be presented and fundamental boundary conditions for their modeling are given.

Fig. 10.1: Comparison of the size of different optical fibers


765

Finally, the different approaches of modeling for polymer fibers will be presented which will include a short description of the procedure, performance and complexity. Since mode coupling or mode mixing is a very specific effect for polymer fibers and influences the performance of the modeling, the most important approaches for describing them and their possibilities for integrating them into existing models for light propagation will be presented. 10.1.1 Types of Fibers Polymer fibers are generally multimode fibers with large core diameters and a high numerical apertures for the easy coupling in and out of the light. They have good mechanical characteristics as well. There are different types of multimode fibers which differ in their refractive indices and the rotation-symmetrical profile of the refractive index in a radial direction. The refractive index profile of a fiber determines the speed and the time delay of all guided modes. In general, one differentiates between step index profile fibers (SI fibers) which have a constant refractive index in the core and a somewhat lower, but also constant refractive index in the cladding and graded index profile fibers (GI fibers) in which the refractive index continuously decreases from the core to the cladding and thus reduces the mode delay. Graded index profiles can be produced in which the propagation times of the modes are practically the same. However, producing such fibers takes a lot of effort, especially with very thick fibers (>Yab00a@ and >Ish96@). Consequently, there are still in-between forms in which the optimal refractive index profile can be approximated through many small steps, so-called multistep index fibers (MSI fibers). The more steps used to approximate the optimal profile, the better the propagation times of the modes tally with each other. The complexity of the manufacturing process, however, is further increased. It is always a compromise between cost/effort and performance (>Lev99@ and >Irie01@).

a)

b)

c)

n² (r)

n² (r)

n² (r)

r -a

0

+a

r -a

0

+a

r -a

0

Fig. 10.2: Refractive index profiles of different fibers in a comparison: a) step index, b) graded index, c) multi step index with 5 layers

+a

766


10.1.2 Modeling Approaches Light guiding in polymer fibers is based in all conventional optical fibers on the principle of total internal reflection. The fiber core has a greater refractive index than that of the surrounding cladding. Basically, the same approaches and methods can be used for describing wave guidance as with glass fibers. However, polymer fibers have characteristics which make some approaches difficult or even impossible. For example, many approaches proceed from very weakly attenuating fibers which can no longer quite be guaranteed with a basic attenuation of about 120 dB/km at a wavelength of 650 nm. The greatest limitation when describing the propagation characteristics is surely the extremely large number of modes which are capable of propagating. In principle, all modes of a polymer fiber can be calculated. This, however, requires a lot of memory, computation time and very good resolution, too. For these reasons many simplified descriptions have been established in which either the mode groups are calculated or the light propagation is dealt with the ray theory and additional wave phenomena outside the ray theory are described. Such hybrid approaches in particular are often used. The most current procedures for describing light propagation in polymer fibers will subsequently be described. 10.1.2.1 Approaches with Wave Theory The propagation characteristics of optical fibers are generally described by the wave equation which results directly from Maxwell’s equations and characterizes the wave propagation in a fiber as a dielectric wave guide in the form of a differential equation. In order to solve the equation, you have to determine the field distributions of all modes and the attendant propagation constants ȕ which results from the use of the boundary conditions. The latter state that the respective tangential components of the electric and magnetic fields (Et and Ht which in the cylindrical coordinates are Eĳ and Ez or Hĳ and Hz respectively) ar steady at the core-cladding interface layer. This only happens for one special value of the propagation constant ȕ. The wave equation is basically a vector differential equation which can, however, under the condition of weak wave guidance be transformed into a scalar wave equation in which the polarization of the wave plays no role whatsoever and all modes exist either as x and y-polarized eigenwaves respectively, so-called linear polarized (LP) modes (>Glo71@). The prerequisite for the weak wave guiding is that the refractive indices between the core and cladding hardly differ. Then, the equations which describe the electric and magnetic fields are decoupled so that you can write a scalar wave equation. The prerequisite for weak wave guiding is fulfilled quite well in glass fibers when the difference in refractive index between the core and cladding region is below 1%. Polymer fibers have quite large numerical apertures and thus greater differences in the refractive indices which is why this approximation is only par-


767

tially valid. Nevertheless, calculations based on the scalar wave equation only show very small inaccuracies in regard to group delay.

Fig. 10.3: Examples for field distributions of the LP0,2- und LP2,2 modes in a graded index profile fiber

The models based on the solution of the wave equation in the form of a model solver differ fundamentally only in regard to the solution method and whether or not you are proceeding from a more computer-intensive vector wave equation or the more usual scalar wave equation. In the technical literature solutions for the vector wave equation with the aid of finite element method (FEM, >Bha00@ and >Liu95@), with finite differences (Finite Difference Time Domain Method - FDTD, >Xiao06@) and the beam propagation method (BPM, >Hua93@) are well-known. These are generally used for very small, mostly singlemode waveguides in which polarization characteristics play a role. Polymer fibers are quite large and receive the polarization of light for only a few centimeters. That is why analytical estimates of the scalar wave equation, the socalled WKB Method and Ray Tracing, are primarily used for the modeling of POF. 10.1.2.2 Ray Tracing Procedure The Ray Tracing procedure proceeds from the point of view that light propagation can be described within the fiber as in free space. This approximation is that much more exact the greater the dimensions of the fiber are and the greater the number of guided modes. For example, in this model no wave phenomena such as the partial penetration of the field into the cladding material are taken into account and attendant additional losses or phase rotation - the so-called Goos-Hänchen Shift - and have to subsequently be described separately (>Bun99a@ and >Bor03@).

768


10.1.3 Wave Theory Description Analytical solutions for the wave equation exist for different refractive index profiles such as step index and parable profile. Then you only have to determine the propagation constants and propagation times of the modes in order to describe the fiber. In most cases the so-called WKB Method is used with which the propagation times of very many modes can be calculated efficiently (>Ish05c@ and >Ohd05a@). 10.1.3.1 WKB Method The WKB Method is an approach for the solution of the scalar wave equation. It was developed by Wenzels, Kramer and Brillouin from whom the name derives. The solutions with the WKB Method contain simplifications which correspond to the description of the fiber with the ray theory. The WKB Method thus builds a bridge between the ray and mode description of the fiber. The WKB Method primarily makes available expressions for describing the propagation constants and the group delay. It permits quick estimates of bandwidth or the transmission capacity of a fiber. In principle, all types of fibers with any number of monotonously decreases in the refractive index profiles can be calculated whereby discontinuous places such as steps in a profile - with MSI fibers, too - can be handled without any problems because the description takes place in integral form (>Glo73@). r2

³r

1

k r dr

§ 1· ¨ p - ¸ S with : k r © 2¹

k 02 n2 (r ) - E2 -

l2 r2

Here l stands for the circumferential order of the mode or the skewness of the ray. With an ever increasing circumferential order the beam path continues to go along the edge of the fiber while the lowest circumferential order 1 = 0 describes the so-called meridional rays which intersect the fiber axis. Both integration boundaries r1 and r2 are the inner and outer reversal radii (caustic). They describe the region in which the light is concentrated within the fiber. A ray can therefore never come closer to the fiber axis than the inner reversal radius r1 and also not further away than the outer reversal radius r2.

Fig. 10.4: The inner and outer reversal radius (caustic). Left: side view on the fiber, right: view on the end face.


769

Parameter kr can be viewed as the radial component of the propagation vector. You can see that this radial component gets smaller and smaller the higher the circumferential order of the ray is. kr disappears at both reversal radii. In this way both radii can be determined. You can establish the group propagation times of the modes by differentiating the equation above to Ȧ. In this way you get the term dȕ/dȦ, which describes the group delay, from the inner derivatives. After reordering the equation you get the following expression for the propagation time. r2

W

k0 Ec

³r

1

1 n (r ) N (r ) dr kr r2 1 dr r1 k r

³

N(r) stands for the profile of the group index over radius r. 10.1.3.2 Step Index Profile Fiber The field distributions in step index profile fibers can be determined analytically and are described by Bessel functions. The conditional equation for the propagation constant results from the boundary conditions in which the transversal components of the electric and magnetic fields are always supposed to merge into each other at the core-cladding interface. u Jl' (u) Jl (u)

v K l' ( v ) Kl(v )

This equation is implicit whereby u and v correspond to the normalized propagation constants in the core and the cladding, suffice for the relation u2 + v2 = V2 (see e.g. >Sny83@) and retain the propagation constant ȕ. u

2 a k 02 ncore E2 ; u

2 a E 2 k 02 ncladding

The propagation times of the modes are independent of the circumferential order of the mode and depend exclusively on the propagation constant. W

ncore k 0 Ncore E c0

n eff

neff W gr

W gr

Here the first term describes the effective refractive index neff which experiences the guided wave. The second term describes the group delay Ĳgr which requires the light in a material with group index Ncore. This description is equivalent to beam propagation in free space with the refractive index neff and the group refractive index Ncore. We shall see in the section on ray tracing that you can describe quite well the light propagation in a fiber in which very many modes can propagate by using the beam model.

770


10.1.3.3 Graded Index Fibers with Power-Law Profile The refractive index distribution over the radius of a graded index fiber can generally be described with an power-law profile. g § ªr º · 2 °° ncore ¨ 1 - 2' « » ¸ ¨ ¬ a ¼ ¸¹ ® © ° 2 2 °¯ncladding ncore 1 - 2' ,

n2 (r )

for r d a else

Here the profile exponent is g which determines the steepness of the profile. If g becomes very large, then the profile approximates more and more that of a SI fiber. Parameter ǻ describes the profile height which stands for the difference in refractive index between the core and the cladding and also influences the numerical aperture of the fiber. There are exponential profiles as direct solutions for the group propagation times and indirectly for the bandwidths of such fibers. Fibers with power-law profiles possess the characteristic that the modes can be put in mode groups which have the same propagation constant ȕ and also similar mode delay Ĳ (at least for exponents close to g = 2). The following relationship exists for propagation constants of the mode groups and their orders: E

ncore k 0 1 - 2 '

m M

whereby M represents the highest mode group order. The propagation times of the modes are only then dependent on the propagation constant and no longer on the circumferential order. Assuming an power-law profile as indicated above, then group delay can be determined with the aid of the WKB method by differentiating the propagation constant ȕ from the angular frequency Ȧ (>Mar77@).

W

Ncore c0

1-

' >4 - 2P@ § m · ¨ ¸ g 2 ©M¹

2g g 2

1 - 2' m M 2g g 2

Here m stands for the order of the mode group and M for the highest mode group. Parameter P describes the so-called profile dispersion (see e.g. >Pre76@). P

ncore O d' Ncore ' dO

The normalized propagation times Ĳ' = (Ĳ - Ĳ0)/Ĳ0 for different exponents g and without profile dispersion (P = 0) is illustrated in Fig. 10.5. You can see that the skews become very small for parabolic profiles with g = 2. For greater exponents (g > 2) the higher modes are slower and with smaller exponents they are faster.


771

g = 2.2

norm. propagation time +0.0004

g = 2.1

+0.0002 g = 2.0

0.0000 g = 1.9

-0.0002 g = 1.8

-0.0004 norm. mode group m/M 0.0

0.2

0.4

0.6

0.8

1.0

Fig. 10.5: Normalized propagation time Ĳ‘ = (Ĳ - Ĳ0)/Ĳ0 over the mode group (m/M) for different profile exponents without profile dispersion (P = 0)

10.1.3.4 Multi Step Index Fibers For MSI fibers there are no analytical solutions which describe the propagation times or the propagation constants. The refractive index profile of this fiber is not continuous at every step which makes calculating it extremely difficult. You can find solutions for such fibers only as part of numerical mode solvers or with the WKB method. Most modeling, however, is based on ray tracing. Figure 10.6 shows the propagation times of meridional rays in MSI fibers with two and six steps acc. to >Zub04@. The abrupt transitions between the ranges of different refractive indices are also discernible in the propagation times. 1.58 1.56

transit time [a.u.]

transit time [a.u.]

1.54 1.52 1.50 1.48 1.46 1.44 1.42 1.40

E invariant 1.42

1.44

1.46

1.48

1.50 1.40

E invariant 1.42

1.44

1.46

1.48

1.50

Fig. 10.6: Transit times of meridional rays in MSI fibers with two (left) and 6 layers (right, [Zub04])

772


10.1.3.5 Determining the Mode Power Distribution The power distribution over the modes results from the launch conditions, i.e. from the radiation characteristics of the transmitter. In an ideal case you can determine this distribution as an overlapping integral of the field at the transmitter with the individual modes:

KE, l

³³A ³³A Ee

2

EeEE* , l dA

dA

2

³³A EE,

2

dA

whereby Ee represents the electric field at the transmitter and Eȕ,l the electric field of the respective mode. Șȕ,l is the coupling efficiency in the respective mode with propagation constant ȕ and circumferential order l. In addition, the field distributions Eȕ,l of all modes are needed, something which is generally not possible. Rather you try to determine the launching condition and the resulting mode distribution in the fiber from the near and far field of the transmitter. 10.1.3.6 Calculating the Transmission Function and the Output Signal When the light is coupled into the fiber, the total power is split up into the individual modes which then propagate at different speeds along the fiber so that they arrive at the detector at the end of the fiber at different times. This leads to different signal parts reaching the end of the fiber at different times resulting in a distorted signal. You can model this procedure by treating the output signal at the end of the fiber like a superposition of the input signal shifted in time by Ĳ. This weighting with the superposition of the signal parts results from the mode distribution Cm. SA (t)

¦ Km SE t - Wm m

Here SA(t) and SE(t) are the output or input signal respectively. Counting index m goes over all mode groups whereby Șm and Ĳm are the share of the total power and the propagation time of the mode group. The impulse response h(t) results analogously by accepting a Dirac (unit) impulse as the input signal. h( t )

¦ Km G t - Wm m

The transmission capacity of a fiber is often indicated as bandwidth B which states up to what frequency the individual frequency parts are attenuated by 50% at most. With such a fiber you can assume that you can transmit signals the


773

spectrum of which lies within this bandwidth. You calculate the bandwidth by obtaining the frequency response through Fourier transformation of the pulse response. H ( f ) F h ( t )

The bandwidth is then the frequency f3db with which the amplitude of the frequency response has dropped to 50% (3 dB):

H f3 dB

1 2

10.1.4 Ray-Tracing When calculating the propagation characteristics, a very large number of rays are generated at the transmitter and each ray follows its own path along the fiber. In doing so, its attenuation as well as the elapsed propagation time is logged. At the end of the fiber a histogram about the rays received is drawn up which includes propagation time, attenuation, location or propagation direction (>Zub02a@). This procedure is based on tracing very many rays to obtain a statistic and it is here that you can see the main disadvantage of this procedure: it requires great computing power and the result approximates the exact solution in step with every further ray calculated according to the Law of Large Numbers.

Fig. 10.7: Emission characteristics (left) of a light source in the Ray-Tracing model and simulated output signal in comparison with a measurement result ([Zub04])

The emission characteristics of the light source are modeled by a probability distribution according to which the rays to be traced are selected. A high power density corresponds to a high probability that just such a ray will be selected. The power density is calculated at the receiver on the basis of a histogram in which the rays have been counted which exit the fiber within a certain time interval (pulse response), their propagation direction lies within a certain solid angle (far field) or they exit from the fiber within a certain area (near field).

774


10.1.4.1 Step Index Fibers The path through the fiber is calculated just like a ray in classic optics. With step index fibers the ray is traced until it hits the core-cladding interface where the reflection of the ray is taken into account by re-calculating the direction of propagation. If it is an ideal reflection, then the angle of incidence is the same as the angle of reflection. Mode coupling or scattering at the interface can be explained by random deviations from the ideal direction of propagation. Skews between the individual eigenwaves in this model result from the additional path the greatly tilted rays have to cover in contrast to the rays running parallel to the fiber axis which only have to cover the fiber length itself. A path results which is proportional to the inverse cosine of the angle in regard to the fiber axis. L(T)

L cos T

In this case L corresponds to the fiber length and ș is the angle of the ray in reference to the fiber axis. The greater the propagation angle in regard to the fiber axis, the longer the path becomes. The ray tracing procedure is then used to calculate the different propagation times of the eigenwaves dependent on the propagation angle ș in regard to the fiber axis. W(T)

r

L(T) ncore c0

L ncore cos (T) c 0

cladding

n(r) core

Fig. 10.8: Principle of beam propagation in SI fibers

10.1.4.2 Graded Index Fibers In principle, you proceed exactly as with the description of SI fibers. However, the light does not propagate in these fibers as straight, but follows a curve which is given by the spatial change of the refractive index. The trajectory can be determined according to the Eikonal equation if the refractive index profile is known (>Jost02@).


& d §¨ dR ·¸ n(r ) ds ¨© ds ¸¹

775

& n

Here s is the length of the ray path and l R describes the radius vector so that its derivative to s corresponds to the tangent at the trajectory of the ray. In SI fibers the right hand side of the equation would be absolutely zero so that the propagation direction of the ray does not change.

r

cladding

n(r)

core Fig. 10.9: Principle of beam propagation in GI fibers

10.1.4.3 Multi Step Index Fibers In MSI fibers the Eikonal equation is of course also valid, however, the description of the ray propagation in such a fiber is not trivial. The ray should split up at the interface between two steps into a reflecting and transmitted part so that the number of rays would increase exponentially with increasing length. You can also confront this problem with statistics by continuing in each case to trace only one of the two rays. Which one of the two rays is to be dropped and which one continues to be traced is the result of a probability distribution which can be calculated from the reflection and transmission coefficients. For example, the greater the reflection coefficient is, the more probable it is that the reflected ray will continue to be traced.

r

cladding

n(r)

core Fig. 10.10: Splitting of the rays in MSI fibers at the interfaces between the different layers

776


10.1.4.4 Bends Bent fibers can be dealt with quite easily with the Ray Tracing method. If the fiber to be calculated is not straight, then the ray will nevertheless still always go straight in SI fibers. However, the fiber axis and the core-cladding interface will no longer run straight. The same principle is also true for MSI fibers which do have more than one interface. It is still valid for these reflections that the angle of incidence is the same as the angle of reflection, but the direction of the fiber axis changes in the bends so that the propagation direction of the ray changes in regard to the fiber axis. At the end of the bend the ray’s direction of propagation is generally another one, i.e. mode conversion has taken place. Guided modes can also be converted into radiation modes in this process. The latter modes can then leave the core and become noticeable as additional bending attenuation (>Arr01b@ and >Durr03b@).

propagation direction before the bend

propagation direction after the bend Fig. 10.11: Description of the mode conversion in the beam propagation model; this effect will be included in the Ray-Tracing

In GI fibers mode conversion comes about more continuously. The propagation of the light ray results from the Eikonal equation. However, the ray within the bend experiences a somewhat different refractive index profile since the fiber is always moving away in one direction. The right hand side of the Eikonal equation has effectively been changed and other ray trajectories have occurred. Here, too, the ray at the end of the bend has a different beam parameter. 10.1.5 Mode-Dependent Attenuation Mode-dependent attenuation exists in multimode fibers and especially in polymer fibers with relatively high losses. >Ish96@, >Mic83@, >Yab00b@, >Lou04@, >Gol03@ and >Ish00b] have shown that the losses increase almost exponentially for the highest mode groups. The higher modes generally experience higher attenuation than the basic mode. There can be different reasons for this. In the mode descrip-


777

tion higher modes have more intensity in the vicinity of the core-cladding interface and even in the cladding itself which is why higher losses could be expected in non-ideal interfaces. In addition, some effects can also be explained as part of the Ray Tracing with some very intuitive graphic models.

mode attenuation [dB/km] 7 6 5 4 3 2 1 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 normalized mode group x Fig. 10.12: Measurement of the mode dependent attenuation dependent of the normalized mode group x = m/M ([Yab00b])

10.1.5.1 Additional Path-Dependent Attenuation of Higher Modes With Ray Tracing the skews of the individual rays are described by the paths of varying length they have to cover in the fiber. In contrast to glass fibers polymer fibers have quite high attenuations and the differences in path length can lead to varying losses. On the assumption that the attenuation of the ray occurs along the beam path - and not along the length of the fiber - then the logarithmic attenuation in dB of the modes is proportional to the path covered which is why higher modes also sustain higher attenuations. This effect can be described as follows:

S ( t, z)

§ Dz · ¨¨ - cos ( T) ¸¸ ¹ S (0 ) e ©

S ( t, 0) e - D ( T)

with D (T)

D cos (T)

Here Į is the material attenuation and S(t,z) stands for the signal at the z location.

778


Fig. 10.13: Influence of the excess loss on the far field distribution due to the longer path length (lengths 1 m, 10 m, 20 m, 50 m and 100 m, UMD launch, AN = 0.50, Įcore = 120 dB/km).

10.1.5.2 Additional Losses of Higher Modes through LossEncumbered Reflections In the beam model the higher modes propagate under a greater angle in regard to the fiber axis than the basic mode for example. Rays which represent higher modes thus experience more reflections along the link. When the core-cladding interface cannot be assumed to be ideal and losses occur with every reflection, then the result is that higher modes suffer additional losses. These kinds of additional losses can be modeled by calculating the number of reflections per length and defining a reflection factor R for every reflection. NRe fl

L tan (T) 2 a sin (\ )

Since the number of reflections also depends on whether or not we are dealing with a meridional or a helix beam, a further dependence on the angle of incidence ȥ occurs in regard to the tangential plane at the core-cladding interface. In this way the reflection-dependent additional attenuation per unit length can be described as follows: S ( t, z, \ )

S ( t, 0, \ ) RNRe fl DR

-

tan (T) ln (R) 2 a sin (\ )


779

A possible angle dependence of the reflection losses can be described by including the reflection factor R(ș).

Fig. 10.14: Influence of the excess attenuation on the far field due to the longer path length and the reflection losses (UMD launch, AN = 0.5, Įcore = 120 dB/km, R = 0.9999).

10.1.5.3 Goos-Hänchen Effect Other effects can be incorporated into the model phenomenological by absorbing them as local additional effects which do not have any mutual reciprocal effect on the other effects. Besides the additional losses at the core-cladding interface through the reflection factor, which attenuates the power of the ray with every reflection in accordance with the reflection factor, an even more exact description can be made, for example, by taking the angle-dependent attenuation caused by the Goos-Hänchen effect with every reflection into account. This occurs when the incident field is not directly reflected at the core-cladding interface but enters a little bit, depending on the angle of incidence, into the cladding material (>Bun99a@ and >For01@). DGH

dGH (T) D cladding D cos (T)

The core radius is a and Įcladding is the attenuation of the cladding material. The term dGH(ș) describes the penetration depth of the field in dependence of the angle of incidence ș and wavelength Ȝ.

780


dGH (T)

O 2 2 2 S ncore cos 2 (T) - ncladding

The propagation properties of SI fibers can be characterized quite well with these simple geometric observations and phenomenological descriptions.

Fig. 10.15: Influence of the excess loss on the far field due to the longer path length, the reflection losses and the Goos-Hänchen effect (UMD launch, AN= 0.5, Įcore = 120 dB/km, Įcladding = 5000 dB/km, R = 0.9999)

10.1.6 Mode Mixing Mode mixing is a process which is rather strong in polymer fibers and decisively influences the propagation characteristics of these fibers (>Ish96@, >Bun99a@, >Rud95@ and >Sav06@). While an entire mode is transformed in bends into a new eigenwave or a ray deterministically changes its direction, mode mixing is then rather a statistical process in which modes exchange power with each other. This effect generally occurs through irregularities in the fiber, whether they are roughness of the core-cladding interface or impurities in the core material. These irregularities are microscopic and lead to light scattering. This effect can therefore only be described with statistical means. The main effects are Rayleigh and Mie scattering which differ in the size of the scattering centers (>Cam03@). Rayleigh scattering arises through the molecular structure of matter which is why no material can have perfectly homogenous properties. Its optical density fluctuates around a mean value which represents the


781

refractive index of the material. These fluctuations are very small and have typical sizes in the range of molecules (< μm). Rayleigh scattering depends on the wavelength and decreases with greater wavelengths as of the fourth power (~Ȝ-4). Mie scattering comes from the fluctuations of the refractive index which has greater typical lengths that mostly come about because of impurities in the material such as air bubbles or specks of dust which are large compared with the wavelength of light. The ensuing scattering has more of an effect on the direction of propagation of the light and is independent of the wavelength. A typical example for Mie scattering is the white color of emulsions such as milk which comes about because of the wavelength-independent scattering of the light.

Fig. 10.16: Mode mixing by scattering on the core cladding interface (left) and inside the core material (right, [Bun06])

Investigations have shown that in standard SI fibers that Rayleigh scattering predominates in the core and Mie scattering at the core-cladding interface (>Bun06@). This shows that today’s polymer fibers are drawn from very pure material. However, some problems still do arise at the interface. The latter occur in part because the core and cladding materials have different expansion coefficients which can result in tension. On the other hand, the drawing of the fiber can cause the cladding to acquire a rough surface. When thinking about these aspects you can see that mode mixing is a complex process which plays a great role in polymer fibers and has other effects than in conventional glass fibers (>Rud95@ and >Sav06@). There are some approaches for the modeling of mode mixing which cannot be applied equally well in all propagation models (>Cal95@, >Can80@ and >Can81@). Some descriptions present themselves rather in mode models (>Har95@, >Ols75@ and >Su05@) while others are more limited to use in Ray Tracing models (>Zub02a@). 10.1.6.1 Coupled-Mode Theory Mode coupling can generally be described as over-coupling a part of the power of a mode into other modes along the fiber. This circumstance can be described for each individual mode m by the power flow equation which characterizes the power flow away from a mode and toward a mode within an infinitesimally small piece of the fiber. dPm dz

-2 D m Pm

¦ cm,n Pn - Pm m

782


Here Pm, Įm and cm,n stand for the power in mode m, its mode-dependent attenuation per unit length and the coupling coefficient between modes m and n. The first term describes the power drain through losses arising and the second term the power flows between the modes which on the one hand are proportional to the coupling coefficient and on the other proportional to the difference between the guided power of both modes. The system thus described makes sure that there is a power flow of modes which guide much power up to modes which are only stimulated a little bit (>Kahn92@, >Sav02a@ and >Sav02b@). The final state aimed for would be a uniform distribution of the power over the modes. However, since the modes incur different attenuations, the stationary state which the system aims for is more similar to a Gaussian distribution with which the higher order modes can guide less power. The coupling coefficients which describe the coupling between modes can either be described by analytical attempts which are based on observations of mode overlapping (>Djo00@, >Djo04@, >Kov05@ and >Oha81@) or are defined in a more phenomenological manner. Examples of such phenomenological descriptions could be models for Mie and Rayleigh scattering or approximations based on measurements.

Fig. 10.17: Application of the mode coupling matrix between short peaces of ideal fiber

The power flow equation shown above for a mode represents one part of an equational system which can be described concisely with the aid of a matrix multiplication (>Kru06a@). P ( z 'z) M ( 'z) P ( z)

Here the components of the power vector p(z) correspond to the power distribution over the modes at location z while M(ǻz) is the mode coupling matrix.

10.1.6.2 Diffusion Model The mode coupling model presented above is relatively complex because it describes the mode coupling among all modes. In real fibers, however, only very few


783

modes effectively interact with each other so that a simpler description of the mode coupling suffices (>Kit80@). Investigations have shown that neighboring modes, i.e. those with similar propagation constants primarily show strong mode coupling. Analytical observations have led to the conclusion that the strength of the mode coupling decreases with the fourth power of the difference. From these observations Gloge developed the diffusion model which solely differentiates between mode groups and only describes the mode coupling of neighboring mode groups (>Glo72@). If you also consider the fact that higher mode groups also contain more modes and that these are stimulated almost evenly within the mode group then you obtain the following description from the equation above: m

d Pm dz

- m Dm Pm m c m Pm 1 - Pm (m - 1) c m -1 Pm 1 - Pm

As already described above, the first term describes the power dissipation of the mode group m by attenuation. The second term stands for the coupling with the neighboring, higher mode group m + 1 and the last term for the coupling with the neighboring, lower mode group m - 1. If you now assign a propagation angle ș to each mode group, as is possible in a step index fiber, and assume that very many modes are capable of propagating so that the individual propagation angles only differ very slightly, then you can sum the last two terms to a derivative and get: dP dz

- D (T) P (T) 'T 2

1 w § wP· ¨ T d ( T) ¸ T wT © wT ¹

Here d(ș) describes the mode coupling in the form of a diffusion constant which can indeed be dependent on the angle. You can take the mode-dependent attenuation ș(Į), as described above, into account through the additional path. This results in D(T)

§ 1 · D D¨¨ 1¸¸ © cos T ¹

§ T2 · D ¸ | D¨ 1 ¨ ¸ cos T 2 © ¹

With this approximation and approach, i.e. that the coupling constant between the modes is constantly D, then the diffusion equation can be written in the following, frequently used form. dP dz

- A T2 P

D w § wP· ¨T ¸ T wT © wT ¹

This equation can be solved numerically for different launch conditions in order to determine the transition to the equilibrium mode distribution. The latter behaves as a stationary solution for dP/dz = 0 and in the case of equilibrium you get a Gaussian distribution the width of which gets wider with the increasing diffusion constant D and lower attenuation A (>Jia97@ and >Zub03@).

784


z=1m

z = 20 m rel. power

rel. power 1.0

z = 50 m rel. power

0.8 0.6 0.4 0,2 0.0 0

5

10

15

20 0 D [°]

5

10

15

20 0 D [°]

5

10

15

analytical EFDM

20 D [°]

Fig. 10.18: Examples for far field distributions, calculated using the diffusion model for four different launching conditions after lengths of 1 m, 20 m and 50 m fiber. The comparison shows a good agreement with the theoretical results (see [Djo00])

10.1.6.3 Application with the Aid of the Split-Step Algorithm The split-step algorithm can be carried out in a similar way to the imitative conversion of the coupled-mode theory and diffusion model >Agr97@. Whereas the propagation in the split-step algorithm is divided into a linear and a non-linear part and both are calculated separately, the linear propagation and mode coupling are calculated here separately (>Bre06@). If you calculate the propagation of a signal along a fiber, then a length ǻz first propagates linearly without mode mixing. After that the mode coupling is calculated in the form of a matrix multiplication which changes the power distribution over the modes. In the next step the propagation of the signal is again calculated by one step ǻz and a further mode coupling is calculated. The linear propagation and the mode coupling actually occur simultaneously which is why the step size ǻz should be kept as small as possible in order to take the former into account.


rel. power

rel. power

1.0

1.0

0.8

0.8

0.6

0.6 1m 10 m 30 m 50 m

0.4 0.2 0.0 0

5

785

10

1m 10 m 30 m 50 m

0.4 0.2 15

20 D [°]

0.0 0

2

4

6

8 t [ns]

Fig. 10.19: Calculated far field distributions (left) and impulse responses (right), using the split-step algorithm for POF with AN = 0.5, Įcore = 50 dB/km, Įcladding = 50,000 dB/km, D = 7 10-4 rad2/m, ([Bre06])

10.1.6.4 Phenomenological Approach The phenomenological approach proceeds even more pragmatically. Mode mixing is described by a random change in the propagation direction ș. Thus, local mode mixing can be described by special effects such as Mie scattering. In this case the probability distribution over the change angle represents the scattering characteristics. A wider probability distribution then corresponds to stronger mode mixing. Consequently, individual effects can be investigated independently of each another and you can acquire a better understanding of the physics in the fiber.

Fig. 10.20: Modeling of the mode mixing using the Ray-Tracing: beam propagation und random change of the propagation direction during entering (1) and exiting (2) the fiber, in the core after a characteristic length (4) and at the reflection on the core cladding interface (3), ([Zub04])

A special approach (>Zub02a@) differentiates between mode coupling in the core material and at the core-cladding interface. Whereas the scattering at the interface can be modeled by randomly changing the direction of propagation with every reflection, as described above, with material scattering the character of the distributed mode coupling along the fiber must be modeled.

786

10.2 Examples for Simulation Results

This is realized by defining a characteristic length after which a scattering process with change of direction takes place. This approach corresponds to the thermodynamic model of a particle which collides with other particles after a medium length. This model is implemented by logging the distance covered for each individual ray. When the logged length reaches the characteristic length the scattering process is started and the logging of the path length is started all over again.

10.2 Examples for Simulation Results Some further results of simulation models will be subsequently presented. Section 10.2.1 comes from the diploma thesis of C.-A. Bunge and was already mentioned in the first edition. The other results come from projects at the POF-AC Nürnberg. 10.2.1 Calculating the Bandwidth of SI Fibers The different kinds of dispersion that limit the bandwidth of optical fibers have been summarized in Chapter 1. The polymer fibers used today are generally multimode fibers so that wave-guide dispersion and polarization-mode dispersion can be neglected. This leaves mode dispersion and chromatic dispersion as the relevant processes to be considered. In [Bun99a] a comprehensive investigation of the bandwidth in SI-POF is undertaken. The work looks at different mechanisms, making the following basic assumptions: ¾Due to the large number of modes (approx. 2.4 million for 1 mm SI-POF at 650 nm) it is assumed that the angles are of continuous distribution. ¾All calculations were carried out under launching conditions that only depend on the angle. ¾The calculations were based on uniform mode distribution (UMD) launch constant far field over the range of guided rays. ¾Fixed values were entered for the attenuation in the core and cladding. ¾No inhomogeneities in the core diameter, NA or the geometry of the corecladding interface were taken into account. The following processes were investigated: ¾Geometric beam propagation in a cylindrical wave guide ¾Losses due to homogenous attenuation in the core ¾Additional losses due to differences in propagation paths ¾Additional losses due to core-cladding interface ¾Goos-Hänchen effect ¾Mode coupling ¾Effect of leaky waves


787

Figure 10.21 shows the result of a calculation for POF with an NA of 0.40 and 0.50. The values for basic attenuation were entered at 120 dB/km and 220 dB/km. A significant effect on the result came from the attenuation of the cladding material that was entered here with 50,000 dB/km and 65,000 dB/km. Above 30 m, the effect of mode related processes significantly cuts in and leads to an increase in the bandwidth of the POF. 500

bandwidth [MHz] basic attenuation 120 dB/km, AN=0.4

200

cladding attenuation 50,000 dB/km

100 basic attenuation 220 dB/km, AN=0.5

50

20

cladding attenuation 65,000 dB/km

10 5

10

20

50

100 200 length [m]

Fig. 10.21: Calculated bandwidth from [Bun99a] for SI-POF

Figure 10.22 shows a simulation that takes account of the effect of mode coupling. The mode coupling constants correspond in principle to the reciprocal of the coupling length, here that would be 30 m, 300 m and f).

bandwidth [MHz] 1000

Jf = 0.033/m Jf = 0.0033/m

100

10 no coupling length [m]

1

1

10

100

1000

Fig. 10.22: Influence of mode coupling to the bandwidth according to [Bun99a]

788


The calculations in [Bun99a] for fibers with a small diameter provided particularly interesting results, as demonstrated in Fig. 10.23. Although current data transmission relies almost exclusively on fibers that have a 1 mm core diameter, significantly smaller diameters are also of interest where parallel data links or multi-core fibers are being used (see Chapter 3). Theoretically, in a step index fiber the diameter has no effect on the bandwidth. However, in practice the diameter does play a role for the effect of the mode related process. Both mode-related attenuation as well as mode coupling are primarily determined by the core-cladding interface. Reducing the diameter increases the number of reflections and thereby the effect of these mechanisms. Also, the wave guiding of leaky beams changes.

bandwidth [MHz] 1,000

POF-NA = 0.30

500

ØPOF: 50 μm ØPOF: 100 μm ØPOF: 150 μm

200 ØPOF: 200 μm

100

ØPOF: 400 μm ØPOF: 700 μm

50

ØPOF: 1,000 μm

20 4

6

8

10

20

50

100 length [m]

Fig. 10.23: Bandwidth for POF with different core diameters ([Bun99a])

Figure 10.24 summarizes the theoretical dependence of attenuation and bandwidth on diameter (more precise parameters in [Bun99a]). As the diameter is reduced, the effect of the mode-related processes becomes evident in shorter and shorter lengths. However, this is at the expense of an increasing overall attenuation. The calculation here is based on UMD in all cases. When assuming that the launching conditions are being made with a smaller NA, as is the case in practical applications, the increases in attenuation are not nearly as conspicuous.


bandwidth [MHz]

attenuation [dB]

1000

50

500

40

200 100 50 20

ØPOF: 50 μm 100 μm 150 μm 200 μm 400 μm 700 μm 1000 μm

ØPOF: 1000 μm 700 μm 400 μm 200 μm 150 μm 100 μm 50 μm

30 20 10 0

5

10

789

20

50

5

100

10

20

50

100

length [m]

length [m] Fig. 10.24: Theoretical influence of the diameter according to [Bun99a]

Figure 10.25 shows the bandwidth and attenuation for fibers with an AN = 0.50 and AN = 0.30 for core diameters between 100 μm and 1,000 μm, both again calculated on the basis of UMD launch. In the multi-core fibers available today the single core diameters are approximately 140 μm. Based on theoretical calculations one can expect a doubling of the bandwidth while, however, the attenuation is also significantly increased.

bandwidth [MHz]

attenuation [dB/50m]

300

30

250

25

200

20 POF AN = 0.30

150

15 10

100 POF AN = 0.50

POF AN = 0.50

5

50 0

POF AN = 0.30

100

200

500 1000 core diameter [μm]

0

100

200

1000 500 core diameter [μm]

Fig. 10.25: Bandwidth and attenuation for different fiber NA and diameter

The effect of mode related processes becomes even clearer in Fig. 10.26, which shows the simulated far field distributions for a 50 μm thick SI-POF for lengths of up to 50 m.

790

1.0


rel. intensity launch point 10 m

0.8

20 m

0.6

50 m

0.4 0.2 0.0

-25

-20 -15

-10

-5

0

5

10 15 20 25 propagation angle [°]

Fig. 10.26: Far field distribution of a 50 μm thick POF ([Bun99a])

After 20 m the far field width has already dropped to ½, resulting in a bandwidth increased by a factor of 4. When light is launched from the start at a small angle, the bandwidth advantage is maintained without too great an increase in losses compared to the 1 mm POF. The qualitative descriptions of theoretical models available to date coincide very well in describing the behavior of SI-POF. There is not yet a universally applicable model available for making a quantitative estimate. Finding such a model is the aim of a work group of the European FoTON project which was formed at the end of the year 2000 (for further information see www.pofac.de). 10.2.2 A Linear POF Propagation Model

At the beginning of 2003 Dr. Christian-Alexander Bunge developed a simple model for the Audi AG for estimating the bending losses of fibers. The task was to calculate the bends and the ensuing losses from given CAD data which describe the laying of fibers. The data structure was thereby investigated and the bends along the fiber pieces were determined with the aid of a vector computation. The parameters calculated are the bending radius and angle (Fig. 10.27).

Fig. 10.27: Simulated POF cable harness


791

In a second step a model had to be created which calculates the bending losses from these two input parameters. The assumption that the losses could simply be added on, i.e. the bending losses could be modeled as linear processes, gave the model its name. On the other hand this assumption ignores the influence of previous bends on the power distribution in the fiber and therefore on the following bending losses. You get more or less an upper estimate of the losses occurring. Measurements of the bending losses on MOST fibers were conducted in dependence on the bending radius and bending angle. All measurements took place under equilibrium mode launch. The results are fitted to a simple model and applied to a simulation model. Some of the results were presented at the 2003 POF Conference in Seattle (>Bun03b@) and were subsequently compared with other simulation results on a ray tracing basis. First the attenuation at a bend with different bend angles was determined (Fig. 10.28). The bending radius was uniformly 20 mm and the bend was made once at the beginning and once at the end of the fiber. In the latter case somewhat higher losses resulted since several higher modes are come about in the fiber through mode coupling.

bending loss [dB] 0.45 0.40

end of fiber

0.35 0.30 0.25

begin of fiber

0.20

r = 20 mm

0.15 0.10 0.05 0.00 0

20

40

60

80

100

120

140 160 180 200 bending angle [°]

Fig. 10.28: Angle dependent attenuation on a bend

The measurement curves show that the bending attenuation increases almost linearly with the bending angle. However, the curves do not intersect the y-axis at exactly zero. The assumption in this model is that in a bend losses occur at the transition between the straight fiber in front of and behind the bend and the bent piece of fiber.

792


Figure 10.29 shows the dependence of the bending losses on the bending radius, for measurements at different positions in the fiber. The coupling-in NA was adjusted here at 0.34, a value close to the equilibrium mode distribution. 2.5

bending loss [dB]

after 5 m after 10 m after 15 m after 20 m

2.0 1.5

AN = 0.34

1.0 0.5 0.0 0

5

10

15


Fig. 10.29: Bending loss in dependence of the bending radius

The losses for smaller bending radii increase approximately inversely proportional with r (See Chap. 2 for other examples.). How big the influence of the numerical aperture at the coupled-in location actually is on the bending losses is illustrated in Figs. 10.30 and 10.31. For radii of 5 mm to 25 mm the bending losses were determined after different fiber lengths for the launchin NA up to 0.65 (overfilled). 5.5 bending loss [dB] 5.0 4.5 4.0 after 5 m 3.5 after 10 m 3.0 after 15 m after 20 m 2.5 2.0 1.5 1.0 0.5 0.0 0.0 0.1 0.2

R = 5 mm

launch-NA 0.3

0.4

0.5

0.6

0.7

Fig. 10.30: NA dependent bending loss at 5 mm bending radius (360° bend)

10.3 Measurement and Simulation of Bandwidth of PF-GI-POF

793

bending loss [dB] 0.7 0.6 R = 25 mm 0.5 after 5 m after 10 m after 15 m after 20 m

0.4 0.3 0.2 0.1 0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6 0.7 launch NA

Fig. 10.31: NA dependent bending loss at 25 mm bend radius (360° bend)

With smaller bending radii NAs up to 0.30, i.e. approximately up to the value of the equilibrium mode distribution, are relatively problem-free. In combination with tight bends coupling in with large angles leads to high losses. The measurements show that the bending losses primarily depend on the mode distribution. Very exact calculations of the bending losses on a given link without exactly knowing the mode distribution of the transmitter are in principle not possible. To compound matters the fibers from different manufacturers have mode-dependent losses which also find expression in the bending behavior. The greatest error, however, lies in the fact that every tight bend also changes the mode distribution itself. Subsequent bends thus actually acquire another launch NA. A possible expansion of this procedure is therefore the so-called nonlinear model in which the bends in their combinations are taken into account. The model parameters here too were calculated from experimentally established data. The exact knowledge of the bending losses allows a guarantee of the power rating in construction, even with tight bends, i.e. optimal utilization of the components at hand.

10.3 Measurement and Simulation of Bandwidth of PF-GI-POF Most recently the Georgia Institute of Technology conducted a series of measurements and simulations for influencing the mode mixing in perfluorinated GI-POF (>Ral06@, >Ral07@ and >Poll07@). Section 6.3.6.3 reported already on transmission of up to 40 Gbit/s over a 50 μm diameter PF-GI-POF.

794


Graded index profile glass fibers can completely eliminate multimode dispersion only when the index coefficient amounts to exactly 2 and the chromatic dispersion can be ignored. In reality both requirements cannot be met so that different propagation times still always appear for the modes. These differences in propagation time are measured as differential mode delay (DMD). With GI fibers a small spot of light is coupled in at different locations over the fiber cross-section and the propagation time of a short impulse is measured. In SiO2 GI fibers practically no mode coupling arises (the mode coupling coefficient is 1.5 10-4 m-1). The consequence of this is that the individual modes clearly run apart after long fiber lengths and are visible as peaks in the impulse response. An example of a measured impulse response (1.1 km of a 50 μm GI-GOF) is shown in Fig. 10.32.

signal [a.u.] 1.0

0.8 50 μm MM-GOF length 1,100 m 0.6

0.4

0.2

0.0 0

1

2

3

4

5 time [ns]

Fig. 10.32: Exemplary pulse response of a GI-GOF

After a link of a good kilometer the different mode groups have recognizably run apart and form individual maxima in the pulse response. The launch of only one mode group can, however, significantly increase the possible capacity. A very much larger mode coupling arises in PF-POF which is stated in >Ral06@ as being > 1.5 m-1, i.e. at least four times the order of magnitude above the value for silica glass fibers. A value of 10 m-1 has been established in >Ral07@. Figure 10.33 shows the influence of such a large mode coupling on the pulse response of a GI fiber in a simulation.


795

Fig. 10.33: Simulated pulse responses for different mode coupling values ([Ral07])

If the mode coupling lies in those ranges typical for glass fibers, then the peaks belonging to the different mode groups will be somewhat rounded, but the width of the pulse response will remain more or les constant. With very strong coupling a significantly narrower impulse, almost in Gaussian form, will arise. The reason for the lower pulse broadening is that individual modes can no longer propagate constantly over the entire fiber length at maximum or minimum speed, but are interchanged again and again on their way. The influence of growing mode coupling on the maximum impulse broadening (DMD) with different profile coefficients is shown in Fig. 10.34. DMD [ps] 400 mode coupling length for PF-GI-POF: 10 m to 100 m

320

240 D = 1.9/2.1 D = 1.9 D = 2.0 D = 2.1

160

80

0 0.01

mode coupling coefficient [m-1] 0.1

1

10

100

Fig. 10.34: Simulated DMD at different mode coupling ([Ral07])

1000

796


In addition to fibers with profile coefficients of 1.9, 2.0 and 2.1 a fiber with a refractive index shape with different profile coefficients inside and outside was viewed. With a small mode coupling you see a strong influence of the deviation of the profile coefficients from an optimal 2.0. With greater mode coupling the influence is diminished considerably and the skews are generally reduced. This result does not only explain the comparably large bandwidth of the GI-POF compared with glass fibers with identical index profiles, but also explains the independence of the bandwidth and the DMD of the launching conditions. Measurements of the pulse response for different launch positions on a 200 m long PF-GI-POF (Øcore: 50 μm; Chromis Fiberoptics) is shown in Fig. 10.35.

offset [μm] 35 30 25 20 15 10 5 0 -5 -10 -15 -20 -25 -30 0

50

100

150

200

250

300

delay [ps] Fig. 10.35: DMD measurement on a 200 m long PF-GI-POF at 800 nm ([Ral06])

The pulse width regardless of the launching position amounts to about 56 ps limited by the receiver. The peak positions fluctuate by only a few picoseconds. The different levels are determined by the mode-dependent attenuation. The following picture results from the simulations for data transmission:

10.4 Simulation of Optical Receivers and Large Area Photodiodes

797

¾ PF-GI-POF work in a range primarily determined by the mode coupling. ¾ Bandwidth and pulse broadening are for the most part independent of the launching conditions as of a certain length. ¾ Typical mode coupling lengths lie between 10 m and 100 m. ¾ The bandwidth drops a bit with the root of the length. ¾ For a transmission of 40 Gbit/s over 100 m of PF-GI-POF a penalty of at most 5 dB occurs when the profile exponent lies between 2.0 and 2.1. A good corresponding value of 4 dB at 100 m and 40 Gbit/s results from measurements with the aid of pulse broadening. One disadvantage of large mode coupling lies in the additional attenuation since a part of the light is always also coupled into non-guided modes. The fiber used in the articles cited has losses of about 50 dB/km at 800 nm and about 150 dB/km at 1,550 nm. Asahi Glass has already manufactured PF-GI-POF with less than 10 dB/km attenuation. These fibers should consequently have less mode coupling. They are used in connections up to 1,000 m where smaller coupling attenuation already causes the same effects. According to >Ral06@ mode coupling in SI-POF even lies at 670 m-1, whereby you have to take into consideration that the mode dispersion here is much greater. Different measurements have shown that after about 100 m the influence of the mode coupling has for the most part been eliminated even with SI-POF.

10.4 Simulation of Optical Receivers and Large Area Photodiodes We have already pointed out in Chapter 4.4 (receivers) and Chapter 6 (systems) that the capacity of large area photodiodes are generally viewed as the limiting factor for the use of large core fibers with high data rates. Many experts were therefore surprised when several years ago the transmission of several gigabits with thick fibers and the corresponding large area diodes was stated in a series of published reports (see Chapter 6.3). Had all the experts been wrong? The reason for this misunderstanding lies in some fundamental differences between photoreceivers in optical telecommunication engineering so far and in POFspecific solutions. The influence of substrate resistance on the function of the receivers has for the most part been overlooked so far. In order to achieve high bandwidths despite the high capacity of photodiodes, receiver designs with low input resistance have to be chosen which means increased importance for the substrate resistance of the photodiodes. In a joint project among Schott Mainz, the Fraunhofer Institute for Integrated Circuits Erlangen, DieMount Wernigerode and the POF-AC different large photodiodes (diameter between 250 μm and 1,600 μm, produced by CIS Erfurt) are currently being investigated in combination with different receiver circuits. By modeling the receivers, the parasitic parameters can then be determined as exactly as possible (>Skl07@).

798


The different diodes have substrate layers about 300 μm thick. With a diameter of 1,300 μm, for example, and a specific resistance of the silicon of 50 :cm a bulk resistance of the substrates of about 113 : results. This is greater than the typical input resistance of a HF amplifier. If you reduce the size of the photodiode significantly, the capacitance will indeed drop, but at the same time the substrate resistance will increase so that little changes at the RC time constant. A simple electrical equivalent circuit diagram was used for the simulation of the measured impulse responses (shown in Fig. 10.36). The photodiode has been reproduced as the ideal current source with parallel capacitance, a relatively high insulation resistance and direct-axis inductance. In addition, there is the substrate resistance as well as the serial resistance and the input capacitance of the following amplifier stage.

RB

Iph

LS

CP

CPD Rp 50 M:

photodiode

Rload 25 :

input of the electrical amplifier circuit

Fig. 10.36: Electrical equivalent circuit for the photodiode

Figure 10.37 shows a comparison between the measured pulse response - at a wavelength of 650 nm and an optical pulse width of < 200 ps - and the simulated pulse response. Here great value is placed on agreement of the pulse widths. In Fig. 10.37 an example of the behavior of a photodiode with a diameter of 1,300 Pm at 12 V reverse voltage is shown. There is good agreement between the simulation and the measurement.

799


amplitude [V] 0.25 simulation 0.20

0.15

0.10

0.05

0.00 measurement -0.05 0

1

2

t [ns]

3

4

5

Fig. 10.37: Simulated and measured impulse response for a 1,300 μm large photodiode

Table 10.1 illustrates the parameters with which three of the different photodiodes used so far were simulated. Table 10.1: Fitted parameters of the different photodiodes ([Skl07]) Photo Diode

Rb []

CPD [pF]

Ls [nH]

CP [pF]

:

850 μm

176.4

1.8

8

8.5

: 1,300 μm

75.4

6.2

8

8.5

: 1,600 μm

49.8

8.5

8

8.5

In this model the photodiode capacitance changes proportionally to the area, the parasitic influences, however, are in a similar order of magnitude. Advantageous with larger diodes is the substrate resistance which clearly drops. These results also explain quite well the relatively slight dependence of the measured bandwidth of the receiver from the reverse voltage and the diameter of the photodiode used as shown in Fig. 10.38.

800


1000

BW3 dB [MHz]

900 PD:

800

850 μm 1,300 μm 1,600 μm

700 600 500 400 300

UPD [V] 0

4

8

12

16

20

Fig. 10.38: Dependence of the measured receiver bandwidth from the photo diode size and reverse voltage

The following important rules ensue when using thick fibers at high data rates: ¾In addition to the low capacitance you have to above all pay attention to the low substrate resistance in the production of the photodiodes. ¾High bandwidths can be attained mainly with receiver concepts which make small input resistance possible. ¾The size of the photodiode only plays a minor role. As a rule, a larger photodiode allows a more efficient coupling of the fiber so that there is all told an improvement over a small photodiode. ¾The dependence of the receiver bandwidth on the bias voltage is relatively small. Should the available bandwidth be completely used, then you can also work with low voltages.

Fig. 10.39: Data transmission with a 1.6 mm photo diode, 12 V reverse voltage, 1 Gbit/s over 50 m OM-Giga


801

Proceeding from this knowledge, the transmission of a 1 Gbit/s data rate was realized using a receiver with a 1,600 μm large photodiode. The eye diagram after a 50 m fiber link (OM-Giga) is shown in Fig. 10.39. The eye is completely open although no equalizer was used. The results are hardly worse than the results achieved so far with 800 μm photodiodes. As Figure 10.40 shows, error free transmission is even possible with a photodiode voltage of only 3.3 V.

Fig. 10.40: Data transmission with a 1.6 mm photo diode, 3.3 V reverse voltage, 1 Gbit/s over 30 m OM-Giga, world wide first demonstration!

A reduction in the eye opening can clearly be seen. However, using a passive equalizer could compensate for this without any problem. This equalizer should be considerably less technically complex than generating a high photodiode voltage with a charge pump. If a receiver with a higher input resistance is used then the diode capacitance will have a stronger effect which will be investigated in the next phase of this project.

11. POF Clubs

This chapter is intended to provide a look at the international scientific and technical activities in the field of polymer fibers. Many established groups have in the meantime become aware of this technology and have been dealing with it. A detailed treatment of all these aspects is beyond the scope of this book. We shall therefore only present the most important groups and events which have to do with POF.

11.1 The Japanese POF Consortium Of the interest groups existing today in the field of polymer optical fibers the Japanese POF Consortium can look back on the greatest amount of activity. It was founded in 1994 and has been led since by Professor Yasuhiro Koike who has achieved international recognition for his numerous publications, especially on graded-index profile polymer optical fibers. A total of approximately 70 institutes and manufacturers are represented in the Japanese POF Consortium. Table 11.1 shows its status in 1999 ([Pol99], [Koi96d]). Table 11.1: Members of the Japanese POF Consortium Alps Electric Co., Ltd. Asahi Chemical Industry Co, Ltd. Bridgestone Corporation Fujikura Ltd. Fujitsu Ltd. Hayashi Telempu Co. Ltd. Hirose Electric Co. Ltd. Hitachi, Ltd. Japan Synthetic Rubber Co., Ltd. Kurabe Industrial Co., Ltd. Kyushu Matsushita Electric Co. Ltd. Mitsubishi Cable Industries Ltd. Mitsubishi Gas Chem. Co. Inc. Mitsubishi Rayon Co. Ltd. MRC Techno Research Inc. Nippon Shokubai Co. Inc.

AMP Japan Ltd. Asahi Glass Co. Ltd. Enplas Laboratories Inc. Fujitsu Kasei Ltd. Hamamatsu Photonetics K.K. Hewlett Packard Japan, Ltd. Hitachi Cable Ltd. Hoechst Industry Ltd. Keio University Kyocera Corp. Matsushita Electric Ind. Co., Ltd. Mitsubishi Electr. Corp. Mitsubishi Material Corp. Molex Japan Co. Ltd. NEC Corporation Nissei Electric Co. Inc.

804

11 POF Clubs

Table 11.1: Members of the Japanese POF Consortium, continued NTT Advanced Technology Corp. OMRON Corporation SC Machinex Corp. Sharp Corporation Siemens K.K. Sumitomo Chemical Co. Ltd. Sumiomo Elect. Ind., Ltd. TDK Corporation The Furukawa Electric Co. Ltd. Tokyo Telecommun. Network Corp. Toshiba Corporation University of Tokyo Yazaki Electric Wire Co. Ltd.

NTT Corporation Optronics Co. Ltd. Seiko Epson Corporation Showa Electr. Wire&Cable Co. Ltd. Sony Corporation Sumitomo Corporation Sumitomo Wiring Syst. Ltd. Teijin Limited Tokyo Institute of Technology Toray Industries Inc. Toyokuni Electric Cable Co. Ltd. Yamanashi University Yokohama National University

11.2 HSPN and PAVNET In the U.S.A., the IGI Company in Boston can be viewed as the most important representative of POF-interested parties. IGI regularly publishes POF News and sells different studies on developments within the telecommunications field, including polymer optical fibers. IGI organizes the annual POF World events which are intended primarily for commercial users. The current managing director is Paul Polishuk who is also the head of the POF Interest Group with members worldwide. Of international importance were two consortiums in the U.S.A. which have been working in succession for several years on the development of polymer optical fiber systems, primarily for use in avionics. The HSPN Consortium (High Speed Plastic Network) was founded in 1994, the aim of which, among others, was the development of 650 nm VCSEL by Honeywell. PMMA-based GI-POF were to be developed concurrently. Both products were able to be demonstrated under laboratory conditions, but have not yet been developed for series production. Figure 11.1 shows the structure of the HSPN project. At the end of the project in 1997 the successor organization PAVNET (Plastic Fiber and VCSEL Network) was founded. The latest member was Lucent Technologies (see Fig. 11.2). The goals are: ¾PF-GI POF with < 60 dB/km at 500 to 2,000 nm ¾Expansion of the temperature range to +125 °C ¾use of existing VCSEL technology at 850 and 1,300 nm ¾622 Mbit/s over 30 m, later 2,500 Mbit/s over 100 m

11 POF Clubs

805

Packard-Hughes I n t e rc o n n e c t Program Management Boei ng Aircraft Aircraft Applications

Boston Optical Fiber Graded Index POF Aircraft Applications

Honeywell OptoElectronic Modules

Packard-Hughes Interconnect Fiber Termination & Auto/LAN App.

Fig. 11.1: Structure of HSPN according to [Cir96]

Packard-Hughes Interconnect Program Management & Administration Project Integration

The Boeing Company

Boston Optical Fiber

Aircraft Interconnects

POF Design Material & Process

Honeywell Technology Center Electro-optical Design

Lucent Technology Telephone System Application Central Office Switching

PackardHughes Interconnect Interconnection Solutions

Fig. 11.2: PAVNET consortium

This group has not been internationally active over the past years. The reasons for this may be problems with the technologies for red VCSEL and the GI-POF. In contrast to the Japanese approach, Boston Optical Fiber used a Teflon-based material, but so far, the losses have still been in the vicinity of some 1,000 dB/km (cf. [Ily00]). During the some years in the late 90ies Mitel in particular has published in the field of VCSEL. The greatest amount of work on the North American continent was published by Lucent Technologies at the beginning of this decade. In the summer of 2000 they announced their own GI-POF production on a CYTOP®-basis [Luc00]. A few years ago the production of PF-GI-POF was detached to a subsidiary and has since been carried on by OFS. A continuous production procedure already described in Chapter 2 has in the meantime been developed by Chromis Fiberoptics. In the meantime the POF Trade Organization (POFTO) has developed out of the former POF Interest Group into an international trade and information platform. Current activities were presented in [Pol06a]. Members of POFTO are indicated in Fig. 11.3.

806

11 POF Clubs

Fig. 11.3: Members of the POF Trade Organization ([Pol06a])

The goals of POFTO are to: ¾make POF applications known to final customers and their representations ¾improve awareness of POF ¾offer training on economy and design possibilities of POF ¾setting up POFTO groups in all countries ¾be active for open competition between POF, copper and glass ¾push for the admission of POF in all standards ¾develop a certification program for all installers The POFTO is managed by several directors each of whom represents important industrial partners or industrial consortiums. ¾Richard Beach (Beach Communications) ¾Paul Mulligan (FiberFin) ¾Randy Dahl (Industrial Fiber Optics) ¾Paul Polishuk (Information Gatekeepers, Inc.) ¾Ken Eben (Mitsubishi International) ¾Arlan Stehney (IDB Forum) Different subcommittees are organized within the group. ¾members ¾standards ¾training ¾marketing ¾home networks ¾long term planning ¾optical interconnections ¾automotive ¾industrial control systems ¾entertainment electronics

11 POF Clubs

807

The most recent American activity is the POF@10G-Consortium. Current members of this group are: ¾Archcom Technologies ¾Asahi Glass Company ¾PhyWorks ¾Chromis Fiberoptics ¾Picolight ¾Nexans Their goal is the commercial introduction of POF-based systems for 10 Gbit/s over 100 m as a reasonably priced and simple alternative to copper and glass fiber solutions. By using suitable VCSEL the power requirements in particular of these systems can be reduced. Today’s copper systems require about 15 W per transceiver; only about 1.5 W would be necessary with the POF version. Furthermore, the cable cross-section of an approximately 8 mm diameter for a shielded copper cable would drop to 2.2 u 4.5 mm2 for a duplex GI-POF.

11.3 The French POF Club The French POF Club was founded as early as 1987. The director of the group for many years was Michel Bourdinaud. The first international POF conference took place in Paris in 1992 and was organized by IGI Europe. By 1994, approximately 200 members were registered in the French Plastic Optical Fibre Club (FOP Club, [Bou94]). It is part of the French Optical Society (SFO) and is supported by the French Atomic Energy Commission. The background to this involvement is the idea of using scintillating polymer optical fibers for detection of elementary particles (e.g. [Far94], [Des94], [Bar96]). Membership in the FPO is free of charge. Financing is taken care of by the SFO (French Optical Society), the CEA (Commissariat à l’Energie Atomique) and small contributions at conferences. Participants include representatives from universities, research institutions, industry and government or military institutions respectively. There are 50 to 80 participants at the bi-annual meetings. In 1994, the FOP published the first comprehensive book on polymer optical fibers [FOP94] which has been available in an English translation [FOP97] since 1997.

11.4 The Information Technology Society (ITG) sub committee (FG) 5.4.1 “Polymer Optical Fibers” In Germany, there has been considerable interest in POF for some time now, in particular through the activities of the chemical industry (Hoechst, Bayer). Until 1996 there was no national interest group in the field. The creation of just such a group goes back to a meeting of various German participants at the POF Confe-

808

11 POF Clubs

rence in Paris (October 1996). After some preliminary preparations it was decided by the sub department 5.4 “Communication Cable Networks” of the Information Technology Society (ITG) within the Association for Electrical, Electronic & Information Technologies (VDE) to found the sub committee (FG) 5.4.1 “Polymer Optical Fibers” on December 3rd, 1996. The head of the sub committee and chairman of the ITG-FA 5.4 since 1999 has been Olaf Ziemann of the POF-AC Nürnberg (all information on the sub committee and the sub department can be found under www.pofac.de). The 24 meetings held so far are as follows (see Fig. 11.4): ¾16.01.1997 Berlin (Federal Institute for Materials Research and Testing, BAM) ¾12.05.1997 Nuremberg (Nuremberg University of Applied Sciences) ¾12.04.1997 Cologne (in cooperation with the cable conference) ¾28.04.1998 Darmstadt (Technology Center of the Deutsche Telekom) ¾05.-08.10.1998 International POF Conference in Berlin (BAM) ¾10.12.1998 Ulm (University of Ulm in cooperation with Daimler/Chrysler) ¾20.04.1999 Jena (Fraunhofer Institute for Optics and Fine Mechanics) ¾16.09.1999 Stuttgart (Lapp Kabel GmbH) ¾09.03.2000 Mönchengladbach (Alcatel Cable) ¾19.10.2000 Potsdam (University of Potsdam) ¾27.03.2001 Gelsenkirchen (University of Applied Sciences) ¾24.10.2001 Giessen-Friedberg (University of Applied Sciences) ¾24.04.2002 Leipzig (Telekom Univ. of Applied Sciences) ¾10.07.2002 Munich (BMW) ¾10.12.2002 Colonge (in cooperation with the cable conference) ¾26.03.2003 Offenburg (University of Applied Sciences) ¾25.06.2003 Munich (during the Laser 2003 exhibition) ¾05.11.2003 Mainz (IMM) ¾09.03.2005 Erfurt (DieMount, CIS and IMMS) ¾27.-30.09.2004 Nuremberg (International POF conference) ¾08.03.2005 Wetzikon, Switzerland (Reichle & De Massari) ¾21.11.2005 POF-AC Nürnberg ¾12.05.2006 Oldenburg (BFE) ¾25.10.2006 Munich (on the Systems 2006 exhibition) ¾17.07.2007 Erlangen (Fraunhofer Institute for Integrated Circuits) ¾17.09.2007 Berlin (POF-Day on the ECOC 2007 exhibition) Between 30 and 130 visitors attend the respective meetings. The large number of people attending past meetings attests to the increased interest in polymer optical fibers in Germany. In Europe, Germany is at present the country with the most POF research and application work which was also mirrored in the number of German papers given at international conferences “Plastic Optical Fibers & Applications” since 1992 (Fig. 11.5). The highlight in the work so far of the ITG Sub Committee and the POF-AC Nürnberg was the organization of the 13th International POF Conference in 2004 in the Nuremberg Conference Center. In addition to the scientific session program, a trade exhibition with over 30 exhibitors was held for the first time.

11 POF Clubs

21. SCM

809

POF1998: BAM

10. SCM

1. SCM

3. SCM

24. SCM

14. SCM

9. SCM

8. SCM

18. SCM 12. SCM

11. SCM 6. SCM 17. SCM 23. SCM 4. SCM POF2004: CCN

7. SCM

2. SCM

15. SCM

20. SCM

5. SCM 19. SCM

22. SCM

16. SCM

13. SCM

Fig. 11.4: Recent meetings of the ITG sub committee 5.4.1 (as 2007) 110 100 Papers from Germany 90 Papers other countries 80 70 60 50 40 30 20 10 0 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 Fr. NL Jap. USA Fr. USA Ger. Jap. USA NL Jap. USA Ger. HK Kor.

Fig. 11.5: German participation on international POF conferences

The basic goals of the sub committee are: ¾exchange and evaluation of experience and information in the field of production and application of polymer optical fibers and waveguides, ¾arrangement of round-table discussions, workshops, seminars and congresses on the subject of polymer optical fibers, e.g. POF’98 and ’04 in Germany,

810

11 POF Clubs

¾working out guidelines and recommendations in regard to application of polymer optical fibers including collaboration (through DKE) in national and international standardization organizations, ¾cooperation with domestic and foreign associations, e.g. French POF Club, Japanese POF Consortium, etc. ¾initiation and cooperation with national and international research and development projects in the field of production and application of POF, ¾publishing technical-scientific books and articles, exchanging information, coordinating the purchasing and use of special and expensive measuring and testing devices. The main emphasis of the sub committee work is in the following areas: ¾international contacts ¾marketing analysis, applications, comparison with other media ¾measuring techniques ¾production of fiber / cable (GI-POF in Europe?) ¾standards including eye safety ¾active and passive components (plugs, diodes, etc.) ¾sensor technology ¾lighting technology / display systems ¾use in automobiles The sub committee meets mostly twice a year in addition to participating in the international POF conferences and on occasion in meetings with other international groups. The sub committee meetings are generally conducted in German. International guests can, of course, give talk in English. In almost all meetings so far small exhibitions with posters and product presentations have been organized. Especially after the respective POF conferences German papers can be presented to a wide national public.

11.5 The Polymer Optical Fiber Application Center (POF-AC) at the University of Applied Sciences Nürnberg In October 2000, the POF-AC (The Polymer Optical Fiber Application Center) was founded in Nuremberg as an institute of the Nuremberg University of Applied Sciences. The project receives financial support amounting to € 2.3 Mio from the High-tech Offensive of the State of Bavaria. The institute’s goals are: ¾providing support when introducing the new technology ¾offering measuring equipment for characterizing POF ¾carrying out contractual investigations and developments ¾setting up of demonstration and pilot systems ¾database for all POF relevant information ¾simulating components and systems ¾maintaining close contacts to universities and other research institutes

11 POF Clubs

811

¾initiating and coordinating promotional projects ¾training courses ¾conducting national and international meetings and workshops The institute was officially opened after the completion of the introduction and training phase on September 25th, 2001 within the framework of the 10th International POF Conference. The institute’s fields of work are shown in Fig. 11.6 (taken from the report at the end of the grant phase).

trainings

characterising

pilot projects optoelectronics tools measurement devices

POF applications

connector techniques illumination systems coupler splitter

interface cards

others

Fig. 11.6: Working areas of the POF-AC

Since 2006, the institute has been financed exclusively through industrial projects and sponsored research projects. Between 2001 and 2005 approximately 200 individual projects were carried out. The projects have been divided among different working areas and are shown in Fig. 11.7.

passive components training/consulting active components sensors simulation data communication devices general optics fiber measurements Fig. 11.7: Splitting of the working areas at the POF-AC

All work dealing with lighting technology and non-fiber measuring techniques fall into the category of general optics. With fiber measurements, investigations of the optical characteristics as well as other factors, e.g. long-time and climate measurements, are taken into account.

812

11 POF Clubs

We do not wish to discuss the possible individual measurements here. Many examples of measurement results and transmission experiments with the most diverse polymer and glass fibers have already been presented in Chapters 2 and 6. The following list of the POF-AC‘s outstanding activities has been excerpted from the five-year final report: ¾ Characterization o An attenuation measuring station for standard POF was developed at the POF-AC and has since become a constituent element in the standards. o Using the bandwidth measuring station at the POF AC, measurements with the most diverse fibers have been carried out for different projects. o Measurements of the bit rate and bit error probability have been conducted on POF, glass fibers and PCS. ¾ Connection Techniques o The attenuation of POF end faces with the most diverse processing procedures have been investigated at the POF-AC. o Investigations were carried out as part of a combined project as to how POF and MC-GOF react with couplings under the most diverse conditions. ¾ Lighting Systems o In an award-winning diploma thesis advertising pillar lighting was developed which not only produced a much more uniform distribution of light, but also saved ¾ of the power. o By LED/POF combination, a special illumination for indoor plants was developed. ¾ Couplers and Splitters o The optimal configuration for POF-Y-splitter were simulated for a partner. o An automated coupler measuring set up was built as part of a diploma thesis. ¾ Interface Cards o Different PC at the POF-AC are connected via POF interfaces to the university network. The work involved includes the testing and adapting of different transceivers. ¾ Tools and Measuring Devices o Several devices of the fiber multiplexer for POF and PCS developed at the POF-AC for long-term investigations on up to 40 fibers have been delivered. o Devices which have been sold in numerous quantities include stabilized laser sources, laser transmitters and broadband measuring receivers. o One invention at the POF-AC forms the basis for tools used for POF preparation and sold commercially. ¾ Optoelectronics o Diverse photodiodes were tested in different projects in regard to their performance in fast data connections. o Laser transmitters with a data rate of up to 2.7 Gbit/s were set up and tested at 650 nm, 780 nm and 850 nm. ¾ Pilot Projects o Nine companies exhibited POF products as part of the “Nuremberg Demo Apartment”.

11 POF Clubs

813

o As of 2008, a broadband POF network is to be installed in the model house “Novascape” by Esser Design Network. o The POF-AC will set up a Gbit POF demonstrator in the European POF-ALL project. ¾ Training o Especially in the first two years about 15 training courses for different companies were carried out. o In the past two years a series of studies for international clients, e.g. Infineon, Deutsche Telekom, Agilent, and Omura Consulting, has been drawn up. o Representatives from POF-AC have been invited to international conferences to conduct workshops and tutorials, e.g. at the Carrier Ethernet Forum 2005. An example of the activities going beyond polymer fibers is the Mikrodreh project involving the Schleifring and Spinner companies, the Bavarian Laser Center and the POF-AC Nürnberg. The POF-AC developed a “squint angle measuring station” which is necessary for the precise adjustment (

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