Optoelectronics and Fiber Optic Technology

Optoelectronic and Fiber Optic

Technology

About the Author Ray Tricker (MSc, IEng, FIIE (elec), FinstM, FIQA, MIRSE) as well as being the Principal Consultant and Managing Director of Herne European Consultancy Ltd - a company specializing in ISO 9000 and ISO 14000 Management Systems - is also an established Butterworth-Heinemann author. He served with the Royal Corps of Signals (for a total of 37 years) during which time he held various managerial posts culminating in being appointed as the Chief Engineer of NATO ACE COMSEC. Most of Ray's work since joining Herne has centred on the European railways. He has held a number of posts with the Union International des Chemins de fer (UIC) (e.g. Quality Manager of the European Train Control System (ETCS), European Union (EU) T500 Review Team Leader, European Rail Traffic Management System (ERTMS) Users Group Project Co-ordinator, HEROE Project Co-ordinator) and currently (as well as writing books for Butterworth-Heinemann) he has prepared a complete Quality Management System for the European Rail Research Institute (ERRI) in Holland, and in assisting them in gaining ISO 9000 accreditation. He is also consultant to the Association of American Railroads (AAR) and Transportation Technology Centre Inc. (TTCI) advising them on ISO 9001:2000 compliance and has recently been appointed as a member of a Notified Body (for the harmonization of the trans-European, high speed, railway network).

To Claire

Optoelectronic and Fiber Optic Technology Ray Tricker

Newnes

OXFORD AMSTERDAM BOSTON LONDON NEW YORK PARIS SAN DIEGO SAN FF~NCISCO SINGAPORE SYDNEY TOKYO

Newnes An i m p r i n t of Elsevier Science Linacre House, Jordan Hill, Oxford OX2 8DP 225 W i l d w o o d Avenue, Woburn, MA 01801-2041 First p u b l i s h e d 2002 C o p y r i g h t 9 2002, Ray Tricker. All rights reserved. The right of Ray Tricker to be identified as the a u t h o r of this w o r k has been asserted in accordance with the Copyright, Designs and Patents Act 1988 All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1T 4LP. Applications for the copyright holder's written permission to reproduce any part of this publication should be addressed to the publishers

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Contents Foreword

xiii

Preface

xv

Acknowledgements 1

xvii

The history of fiber optics 1.1 Background 1.2 Basic principles of optical line transmission 1.3 Advantages of optical fibers and optoelectronic signalling 1.3.1 Immunity to electrical and magnetic fields 1.3.2 Low attenuation 1.3.3 Wide transmission bandwidth 1.3.4 Small physical size and weight 1.3.5 Increased flexibility 1.3.6 Electrical insulation 1.3.7 Immunity to electromagnetic interference and interception 1.3.8 Electrical protection 1.3.9 Analog and digital transmission 1.3.10 Receiver sensitivity 1.4 Disadvantages of optical fibers and optoelectronic signalling 1.4.1 Cost 1.4.2 Jointing and test procedures 1.4.3 Tensile stress 1.4.4 Short links 1.4.5 Fiber losses 1.4.6 Other losses 1.5 Practical applications of optoelectronics 1.5.1 Telephone networks 1.5.2 Urban broadband service networks 1.6 The future of optoelectronics 1.6.1 All-optical network 1.6.2 Computers 1.6.3 Transmission speed 1.6.4 Integration

1 19 23 24 25 25 25 26 27 27 28 28 28 29 29 29 30 30 30 31 32 32 32 33 33 34 34 34

vi

Contents

Theory 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10

2.11

2.12 2.13 2.14

2.15 2.16

2.17

2.18

Reflection and refraction Refractive index Total internal reflection 2.3.1 Shell's law Angle of incidence Critical angle Acceptance cone Acceptance angle Numerical aperture Structure of a fiber Modes 2.10.1 Multimode step index 2.10.2 Multimode graded index 2.10.3 Single-mode step index Losses 2.11.1 Attenuation 2.11.2 Absorption 2.11.3 Scattering 2.11.4 Bending losses Optical power and power density Optical fiber input power Dispersion losses 2.14.1 Material dispersion 2.14.2 Waveguide dispersion 2.14.3 Chromatic dispersion Optical transitions in semiconductors 2.15.1 Semiconductor optical transitional mechanisms Usable b a n d w i d t h of optoelectronic transmission systems 2.16.1 Cut-off frequency of an optical fiber 2.16.2 Fiber bandwidth Modulation and demodulation of the subcarrier 2.17.1 Modulation techniques 2.17.2 Frequency modulation 2.17.3 Intensity (power) modulation Signal-to-noise ratio 2.18.1 Optical receiver thermal noise 2.18.2 Quantum noise (of light) 2.18.3 Dark current noise 2.18.4 Laser noise 2.18.5 Mode partition noise 2.18.6 Modal noise (in multimode fibers)

36 38 39 39 41 45 46 47 48 49 49 51 51 52 52 53 53 53 54 55 57 57 58 58 58 59 61 61 62 63 63 65 65 67 67 68 69 69 69 69 69 69

Contents

2.19 Types 2.19.1 2.19.2 2.19.3 2.20 Types 2.20.1 2.20.2 2.20.3 2.20.4 2.20.5 3

of mixers End fire mixers Transmissive mixer Biconical tapered transmissive mixer of multiplexing Fiber multiplexing Electrical multiplexing Wavelength division multiplexing Dense wavelength division multiplexing Time division multiplexing

Fibers and cables 3.1 3.2

3.3 3.4

3.5 3.6

3.7

3.8 3.9 3.10

3.11

Single-mode fiber Multimode fiber 3.2.1 Multimode step index fiber 3.2.2 Multimode graded index fiber Manufacturing processes and designs of optical waveguides Manufacturing techniques 3.4.1 Preparation of the glass preform 3.4.2 Forming the glass rod 3.4.3 Rare earth-doped fibers 3.4.4 Cap sealing Plastic fiber optic cables Cable characteristics and specifications 3.6.1 Tensile strength of the cable 3.6.2 Buffered fiber Types of cable 3.7.1 Tight buffered cable 3.7.2 Loose tube 3.7.3 Slotted core cable Fiber optic cable ducts 3.8.1 Tube identification Cable construction Types of ducted cable 3.10.1 External duct cable 3.10.2 Universal duct cable 3.10.3 Universal ruggedized cable 3.10.4 Water protection 3.10.5 Rodent protection 3.10.6 Metallic armouring 3.10.7 Hazardous environmental cable Installation possibilities 3.11.1 Training

vii

70 70 71 71 71 72 72 72 74 74 76 77 79 79 81 82 85 85 85 90 90 91 92 93 93 95 95 95 96 97 97 98 99 99 100 100 101 101 102 103 104 105

viii

4

5

Contents

T r a n s m i t t e r s - light emitting diodes and lasers

106

4.1 Light source 4.2 Conversion of electrical energy into light waves 4.3 Light emitting diodes 4.3.1 LED preparation 4.4 Types of LED 4.4.1 Edge emitting diodes 4.4.2 High radiance light emitting diodes 4.4.3 Diffused gallium arsenide LEDs emitting at 900 nm 4.4.4 Aluminium gallium arsenide/gallium arsenide (A1GaAs/GaAs) high radiance LEDs emitting a t 830 nm 4.4.5 InGaAsP/InP high radiance LEDs emitting at 1300nm 4.4.6 Indium phosphide 4.5 Laser diodes 4.5.1 Advantages 4.5.2 Disadvantages 4.5.3 Design of a laser diode 4.6 Basic differences between gain-guided (current-induced) and index-guided (built-in waveguide) laser diodes 4.6.1 Types of laser diodes 4.6.2 Microdisks 4.6.3 Multiple signalling

106 106 107 110 111 111 112 112

124 126 128 129

R e c e i v e r s - photodiodes

132

5.1 Conversion of light waves into electrical energy 5.2 Design characteristics of an optical receiver 5.2.1 The PIN photodiode 5.2.2 Avalanche photodiodes 5.2.3 Silicon photodiodes 5.2.4 Germanium photodiodes 5.2.5 Indium gallium arsenide photodiodes 5.3 Types of receiver 5.3.1 Analog fiber optic receiver 5.3.2 Digital fiber optic receiver 5.4 Fiber amplifiers 5.4.1 Laser amplifier repeater 5.4.2 Erbium-doped fiber amplifiers 5.4.3 Erbium-doped fluoride fiber amplifiers 5.4.4 Erbium-doped tellurite fiber amplifiers 5.4.5 Praseodymium-doped fluoride fiber amplifiers 5.4.6 Semiconductor optical amplifier

132 134 134 135 135 136 137 137 137 137 138 139 139 139 140 141 141

113 114 115 116 119 120 122

Contents

6

ix

Connectors and couplers

142

6.1 Connectors 6.1.1 Connector requirements 6.1.2 Quality of the connector 6.1.3 Connector attenuation 6.1.4 Types of connector 6.2 Couplers 6.2.1 End fire coupling 6.2.2 Lens coupling 6.2.3 Multiport couplers 6.2.4 Coupling losses 6.2.5 Attaching a fiber to a connector 6.2.6 Connecting an optical fiber cable to an LED 6.2.7 Connecting a fiber cable to an integrated circuit 6.3 Fiber jointing techniques 6.3.1 Epoxy splices 6.3.2 Heat-cured epoxy 6.3.3 Room temperature-cured epoxy 6.3.4 Pre-injected epoxy 6.3.5 UV adhesive 6.3.6 Cyanoacrylate adhesive 6.3.7 Anaerobic adhesive 6.3.8 Acrylic adhesive 6.3.9 Crimping 6.3.10 No epoxy and no crimping 6.3.11 Advantages and disadvantages of various types of termination methods 6.4 Splicing 6.4.1 Splice losses 6.4.2 Alignment errors during splicing 6.4.3 Mechanical splicing 6.4.4 Fusion splicing 6.4.5 Closures for fiber optic cables 6.4.6 Repeaters and regenerators

142 143 143 144 145 152 153 154 154 158 160 161 162 165 166 166 167 167 167 167 167 168 168 169

Communication systems

183

7.1 7.2 7.3 7.4

184 187 187 188 189 189 191

Local networks Long-distance networks Telephone networks Data networks 7.4.1 Local area network 7.4.2 Metropolitan area network 7.4.3 Wide area network

169 169 169 171 172 174 179 180

x

8

9

Contents

7.5 Designing an optical fiber cable system 7.5.1 Fiber versus cable 7.5.2 Digital or analog 7.5.3 ATM versus fast Ethernet 7.5.4 Multiplexing 7.5.5 Channel and cable capacity 7.5.6 Fiber loss 7.5.7 Cable routing 7.5.8 Distribution frames 7.6 Optical path loss budget 7.7 Installation techniques 7.7.1 Storage and handling 7.7.2 Cable laying 7.7.3 Installing fiber optic cables in buildings 7.7.4 Blown optical fiber

191 192 193 194 194 196 196 197 201 201 204 205 206 209 211

Optoelectronic test techniques

216

8.1 Testing fiber optics 8.1.1 Production testing 8.1.2 Installation testing 8.1.3 In-service field testing 8.2 Examples of test equipment 8.2.1 Production test kits 8.2.2 Laser light sources 8.2.3 Power meters 8.2.4 Fiber optic tracers 8.2.5 Power and attenuation measuring test sets 8.2.6 Other field testers 8.2.7 Optical time domain reflectometer 8.2.8 Optical fault locator 8.2.9 OFL versus OTDR 8.2.10 Bandwidth measuring test sets

216 217 218 219 219 22O 220 221 221 223 224 225 23O 231 231

Future developments

233

9.1 9.2 9.3

The global market UK market Development trends 9.3.1 Porous silicon 9.3.2 Ballistic transport 9.3.3 Fluoride glasses

233 235 236 237 237 237

Contents

9.4

9.5

9.6 9.7

9.8

9.9

9.10

9.11

9.3.4 Polarization 9.3.5 Synchronous optical network 9.3.6 Solitron pulses Lasers and amplifiers 9.4.1 Distributed feedback laser 9.4.2 Tuneable lasers 9.4.3 Rare earth-doped fiber lasers and amplifiers 9.4.4 Modular laser source 9.4.5 Erbium-doped fiber amplifier Fiber cables 9.5.1 Re-emergence of multimode fibers 9.5.2 Moulded-clad single-mode fibers 9.5.3 Multi-core fibers 9.5.4 Encapsulated fibers Moveable fibers 9.6.1 Optical bus Transmission systems 9.7.1 Development trends in higher-order optical fiber transmission systems 9.7.2 Transmitters and receivers 9.7.3 Fiberless transmission 9.7.4 Optical Ether 9.7.5 Optical radio Industry 9.8.1 Leak detection 9.8.2 Optical fiber drill bit temperature monitor 9.8.3 Optical fiber soldering iron 9.8.4 Optical computing 9.8.5 Optical multiplexers 9.8.60pto-isolators 9.8.70ptoelectronic switches 9.8.8 High speed optical switches Military 9.9.1 Laser dazzler 9.9.2 Plastic opto-chips 9.9.3 Fiber optic gyroscope Local government 9.10.1 Optical sensors 9.10.2 Office lighting 9.10.3 Decorations 9.10.4 Fiber optic road signs Medicine 9.11.1 Lasers - medical uses 9.11.2 Flexible fiber optic curvature sensors

xi

238 238 238 239 240 240 240 241 243 243 243 243 243 244 246 246 246 246 247 248 249 250 250 250 251 251 251 252 252 252 252 253 253 253 255 256 256 256 257 257 258 258 259

xii

Contents

9.12 Other possibilities 9.12.1 9.12.2 9.12.3 9.12.4

Internet Cars and planes Optical processing and computing Mining

9.13 Conclusion

259 259 260 260 261 261

Appendix A

Optoelectronic and fiber optic standards

263

Appendix B

Fiber optic chronology

276

Glossary

278

Acronyms and abbreviations

290

Books by the same author

300

Useful links

301

Index

305

Foreword During the last couple of decades, fiber optics has become increasingly popular in both commercial and military environments and although optoelectronics is based on a comparatively simple technology, it is nevertheless essential for today's engineers to be aware of the basic fundamentals and capabilities of this modern technique. In order to understand new electronic engineering principles, however, it is usually necessary for the reader to have a thorough technical foundation and for the author to derive and explain mathematical theories and progressively develop formulae, etc. to enable the reader to continue his studies with a complete understanding of the fundamentals surrounding this new technology. Although an understanding of the mathematical principles applicable to optoelectronics is of course necessary, it is the aim of this book to provide only a basic introduction to the basics of optoelectronics. A book that introduces the topic, and then leaves it up to others to provide a more in-depth explanation. A book that is easy to read, hard to put down(!) and (bearing in mind the saying that a picture is worth a thousand words) has lots of diagrams, pictures and graphics to aid understanding. As well as being a background reference manual for practising electronic and telecommunications engineers, technicians and students, it will also serve as a user-friendly overview of this increasingly popular subject, especially to those who would like to get to grips with this new area of communications technology.

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Preface For over a century, telegraph and telephone systems have been interconnected via copper cables and radio links and these have formed the basis of all suburban, urban and national telecommunication networks. But although the possibility of using a dielectric rod as a waveguide had been studied during the late 1800s and a paper had actually been written about it in the 1920s, it was not until 1966 that Charles Kao and George Hockham patented the principle of 'information transmission through a transparent dielectric medium' (i.e. glass fiber), that the use of glass fibers actually became a viable proposition. Since then, the development of optical fibers and their enormous bandwidth capability has meant that more and more communication systems are now using fibers to pass optical signals over long distances with very little l o s s - especially when compared with the losses experienced by electrical signals over copper lines. Without doubt this technology, following the development of laser diodes, has now become the preferred method for cable communication. However, in the very early days of fiber it was not evident just how dramatic optoelectronic and fiber optic technology would become, or to what extent it would impact on national and global economies. Indeed, a single glass fiber the thickness of a h u m a n hair can now carry the equivalent of 300 million simultaneous telephone c a l l s - roughly all the phone calls going on in America at any one time. But even though computer capacity is anticipated to increase by 1000-fold in the next decade (and 1 000 000-fold in the decade that follows) it is highly unlikely that an optoelectronic system would be incapable of satisfying these new requirements. In less than 20 years optical fiber has revolutionized the global telecommunications network. With the aim of providing a user-friendly background to this increasingly important technology, this book is structured as follows: Chapter 1 - a description of the history surrounding fiber optics and an introduction to some of the people who were involved in its discovery, use and development. Chapter 2 - an overview of the theory behind optoelectronics. Chapter 3 - a description of the different types of fiber that are available (e.g. single mode, multimode, step index, graded index, glass or plastic),

xvi

Preface

the manufacturing processes and techniques, cable characteristics and specifications and a look at some of the installation possibilities. Chapter 4 - a description of the optoelectronic transmitter (e.g. LED and laser) used to convert electrical signals into optical energy so that they can be propagated down an optical waveguide. Chapter 5 - a look at the other end of the fiber to see how light energy is received and converted back into an electrical signal by an optoelectronic receiver. Chapter 6 - a look at the method used to connect fibers to transceivers and how fibers can be joined a n d / o r spliced together. Chapter 7 - an overview of how fiber optics are utilized in local, long distance and global telecommunication networks. How an optical fiber cable system is designed and the installation techniques used. Chapter 8 - a look at installation and in-service testing techniques and some of the test equipment that is currently available for installers and system engineers. Chapter 9 - a look into the near and future possibilities associated with optoelectronic and fiber optic technology. New and perceived developments are described and an indication of how government, military and industry are increasingly making use of fiber optics is discussed. These chapters are supported by appendices that list some of the European and International standards currently available and concludes with a list of all the main acronyms and abbreviations associated with the optoelectronic and fiber optic technology, together with a detailed glossary of terms and a full index.

Acknowledgements Whilst every effort has been made to contact the copyright holders of the pictures, diagrams and drawings used to illustrate this book, it has not proved possible to make contact in all cases. If we have used your images in this book without permission, then we apologize and would request that you please contact us so that we can ensure that you are given due credit in the next reprint.

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The history of fiber optics

1.1 Background For over a century, telegraph and telephone systems have been interconnected via copper cables and radio links and these have formed the basis of all suburban, urban and national telecommunication networks. Although line transmission technology has improved over the years, it was not until the last few decades when revolutionary innovations caused by the development of semiconductors, digitalization and optoelectronics dramatically altered the method, technology and cost effectiveness of communication systems. Of course, the transmission of information by visual means is not an entirely new concept, as man has been sending messages optically for centuries and I suspect that the first form of optical communication was the caveman shaking his fist (an action which left no doubt as to its meaning!). This effective communication used the sun as a light source, the hand as the modulator and the eye as the detector. Optical communication then progressed to flags and smoke signals, which enabled messages to be passed over greater distances.

2 Optoelectronics and Fiber Optic Technology

From a military perspective, all sorts of signalling techniques have been used by armies in the field. The Greeks had the torch telegraph, the Phoenicians water telegraphs, and the Roman army used coloured smoke as a means of communication. The Greeks and Phoenicians also used reflected light from mirrors to communicate by code between watchtowers. Ship captains used both mirrors and lanterns to communicate with their fleets and army generals frequently used shields to signal troop movements. In the sixth century BC, legend has it that Aeschylus (Figure 1.1) (the 'Father of Tragedy', who apparently met his death when an eagle mistook his bald head for a rock and dropped a tortoise on it!) used a long chain of bonfires from Asia Minor to Argos to pass on the news of Troy's downfall.

Figure 1.1 Aeschylus (525-456 BC) In the second century BC, Polybius (a Greek historian, born in Megalopolis) developed a two digit, five level code by which the whole of the Greek alphabet could be transmitted. It is perhaps interesting to note that it is this last method that is generally recognized as being the first example of optoelectronic signalling. The Chinese, when threatened by the Tartars, lit signal fires along the boarders of their empire, increasing brightness by artificial means to penetrate fog and rain. Genghis Khan (1115-1227) (Figure 1.2) controlled and checked his horsemen by flag telegraphy. During the next few decades, a number of experiments were carried out utilizing various other visual methods. For example, in 1633 the Marquis of Worcester developed long distance communication systems using black and white signs, and in the papers of John Norris of Hughenden Manor in Buckinghamshire there is an enigmatic comment: '3 June 1778. Did this day heliograph intelligence from Dr Franklin in

The history of fiber optics 3

Figure 1.2 Genghis Khan (1167-1227) Paris to Wycombe.' In 1781, Norris also built a 30m tower on a hill at Camberley, Surrey, from the top of which he used to signal and place bets by heliograph with Lord le Despencer at West Wycombe. Turning the pages of history once again, it was in 1795 that the Reverend Lord George Murray (the fourth son of the Duke of Athol) was commissioned to supervise the installation of a chain of shutter stations stretching from Plymouth to London (Figure 1.3).

Figure 1.3 Admiralty shutter telegraph relay station sites ( i courtesy of the Royal Corps of Signals Museum, Blandford)

4 Optoelectronics and Fiber Optic Technology

Figure 1.4 The Admiralty shutter telegraph system (artist's impression of a relay station, J. courtesy of the Royal Corps of Signals Museum, Blandford)

These stations formed the Admiralty shutter telegraph system (Figure 1.4) and consisted of a wood frame 20 feet high by 12 feet wide, containing 3 foot square shutters arranged two by two vertically, which pivoted about their horizontal axis and were operated from the building beneath by ropes and pulleys. By careful positioning of the shutters, 63 changes were possible: sufficient to cater for the whole of the alphabet, the ten numerals, as well as a number of selected phrases - for example: 9 all shutters open - station not at work; 9 all shutters shut - station about to work; next station standby to answer. The stations were normally manned by three men (one to operate the shutters, the other two to act as telescope observers or 'glassmen', see Figure 1.5) and it is recorded that an average message passed from London

The history of fiber optics 5

Figure 1.5 Model of the inside of the Blandford Camp shutter telegraph station ( i courtesy of the Royal Corps of Signals, Blandford)

to Portsmouth in 15 minutes whereas a prearranged signal could be sent between London and Plymouth and acknowledged back in three minutes. Across the channel and a few years before the Admiralty scheme (i.e. 1792), Claude Chapp6 (a French engineer and cleric who converted an old idea into a reality by inventing the 'semaphore visual telegraph') had succeeded in passing a message between Paris and Strasburg (a distance of over 400 km) using movable signal elements. Chapp6's device (the T-type t e l e g r a p h - Figure 1.6) consisted of two side wings mounted at right angles to a main wing that was positioned at the top of a mast. Changing the position of the main wing with respect to the mast and varying the attitude of the side wings to the main wing by 45 degree steps could give 196 different and easily distinguishable signals. These signals were observed from nearby stations using telescopes and then passed on to the next relay station in a similar manner (Figure 1.7). It was far more efficient than hand-carried messages, but by the m i d nineteenth century it had been replaced by the electric telegraph and all that remains today are a few 'telegraph hills' scattered about. But Chapp6's semaphore system was not the only one in existence. Similar systems were being used in Denmark, Norway, Finland and Holland (Curacao) as shown in Figure 1.8.

6 Optoelectronicsand Fiber Optic Technology

Figure 1.6 Chappe's semaphore visual telegraph (courtesy of Telemuseum.se)

Figure 1.7 Telegraph post at Conde in November 1794 (courtesy of Telemuseum.se)

The history of fiber optics 7

One of the systems used in Denmark between 1794 and 1862.

The Norwegian signalling system used on the of the mountain tops Lyderhorn and Ilsfjellet.

Dutch optical telegraph system on the island of Curacao in the early/ mid-nineteenth century.

Figure 1.8 Examples of some of the other earlier opto-signalling devices (courtesy of Telemuseum.se)

Turn the pages of history once more and it is Napoleon (1760-1821) who, having quickly recognized the importance of optical signalling, established telegraph stations in the countries in which he was campaigning and equipped his armies with optical telegraph equipment (telegraph ambutantes), similar to Col. John Macdonald's anthropological telegraph illustrated in Figure 1.9. In Prussia, General Freiherr von Muffling carried out a number of trials during the period 1819 to 1830 utilizing optical telegraph systems. The success of these studies led to the construction of the first telegraph line between Berlin and Potsdam. During this time various heliograph

8 Optoelectronicsand Fiber Optic Technology

Figure 1.9 Napoleon I (Emperor of France 1769-1821) and CoL John Macdonald's anthropological telegraph ( ,~ courtesy of the Royal Corps of Signals Museum, Blandford)

Figure 1.10 Typical heliograph

The history of fiber optics 9

(Figure 1.10) and signal lamp systems were also developed enabling signalling over distances of around 100 kilometres. The heliograph was primarily used by armies for signalling to a distant point. It comprised a wooden tripod and a mirror assembly, the back of which was covered by a brass plate with a brass stump and pivot that connected to the Morse key via a brass tube. By adjusting the knurled tube and a Morse key the signaller could signal in Morse, by sunlight, to his distant observer. By the 1850s, heliograph systems were widely used by the military (Figure 1.11).

Figure 1.11 Heliograph operators in India (..,~..courtesy of the Royal Corps of Signals, Blandford) The size of the heliograph mirror depended on its intended use. The largest (which was primarily used by headquarters and garrisons) had a mirror 12 inches in diameter, but other standard sizes included the 5-inch mirror for infantry units and the lighter 3-inch instruments for cavalry units. The range of operation depended upon the strength of the sunlight and visibility. Under normal conditions, ranges of 20-30 miles with a 3-inch mirror, 30-40 miles with a 5-inch mirror and 80-90 miles with 12-inch mirror could be expected, but these ranges were frequently exceeded in practice.

10 Optoelectronics and Fiber Optic Technology

Heliographs had one unfortunate problem, however, they could only be used on cloudless days! This problem was largely overcome by inventors such as Henry Coxwell who developed balloon signalling. Figure 1.12 represents Coxwell's method of adapting semaphore arms of various shapes and symbols to convey, according to a preconcerted arrangement, any required information to those on the ground.

Figure 1.12 Henry Coxwell's balloon signalling (courtesy of London Illustrated News, 8 October 1879)

In the 1840s, a Swiss physicist (Daniel Collodon) and a French physicist (Jacques Babinet) showed that light could be guided along jets of water for fountain displays. Then in 1850, John Tyndall (Figure 1.13), a schoolteacher, noted that light could be guided inside a clear material, and by using water that flowed from one container to another and a beam of light, he demonstrated that light used internal reflection to follow a specific path. The principle was very simple, as the water flowed from the first container, Tyndall directed a beam of light at the path of water and the light followed a zigzag path inside the curved path of water. This simple experiment marked the first research into the guided transmission of light. But it was not until the 1880s that Alexander Graham Bell (in conjunction with Sununer Tainter) patented an optical telephone system, which he called the photophone, and successfully modulated a beam of sunlight using a diaphragm mirror. This is generally recognized as the

The history of fiber optics 11

Figure 1.13 John Tyndall, British scientist (1830-1893) first mobile phone! The action of the photophone is very simple (Figure 1.14a). Rays of sunlight (or a powerful light source) are reflected by a flat mirror (h) into a system of lenses (a and b) that form a beam of light on to a silver-coated glass plate (H). A person speaking into the mouthpiece of the transmitter (see Figure 1.14b) causes the silver-coated plate to vibrate which in turn causes the reflecting surface of the plate to change shape. This affects the strength of the light rays, which are compressed by a second set of lenses (c), and then reflected to the receiver. At the distinct end (see Figure 1.14c), the light rays are received by a silver-coated copper parabolic reflecting mirror (P). By using a sensitive selenium-photoresistor (S) (comprising a cylinder of copper plates, separated by strips of mica and filled with s e l e n i u m - whose resistance varies according to the quantity of light that strikes it), connected to a battery and a telephone instrument (t), the system is able to reproduce the speaker's voice. On a good day, when the weather was sunny, the photophone had an effective range of over 200 m, but it did not work at night or when the weather was cloudy. The transmission distance was increased to 2 km when Bell incorporated the use of an electric globe following Edison's invention of the light bulb, but the user still had to carry a fairly cumbersome piece of equipment around, with two such devices being needed for two-way communication. Alexander Graham Bell's earlier invention, the telephone, proved to be a far more practical proposition, therefore, and although he dreamed of sending signals through the air, he found that the atmosphere didn't transmit light nearly as reliably as wires could carry electricity.

12 Optoelectronics and Fiber Optic Technology

Key h

Flat mirror

Very thin mirror

b

Lens

Parabolic reflector

Allun cell lens to cut off heat rays

Selenium photoresistor

c

Compression lens

Figure 1.14

t

Alexander Graham Bell's photophone

Telephone earpiece

The history of fiber optics 13 Table 1.1

Patent number

Details of the photophone's US patent registration Type Inventor

235,199

U

235,496

U

Appfied

G r a n t e d Claims P a g e s

Bell

08/28/1880

12/07/1880

18

13

Bell

09/25/1880

12/14/1880

3

3

Alexander Graham

Alexander Graham and Tainter, Sununer

Description

Apparatus for Signalling and Communicating, Called 'Photophone' PhotophoneTransmitter

All of these methods were of course affected by the visual transparency of the medium through which they were being propagated (i.e. air), and fog, rain, storms, cloud and snow were some of the problems to be contended with. Another major problem that affected these early lightwave communication systems was the inability of lightwaves to go around corners, and avoid trees, buildings and mountains. The transmission path, therefore, had to be an unobstructed line of sight between transmitter and receiver. In the same period, William Wheeler invented a system of light pipes which were lined with a highly reflective coating, and by using light from an electric arc lamp placed in the cellar, he was able to illuminate almost the whole of the house by directing the light around the home via the pipes. Later on in 1888, the medical team of Roth and Reuss of Vienna used bent glass rods to illuminate body cavities and in 1895, the French engineer Henry Saint-Rene designed a system of bent glass rods for guiding light i m a g e s - in an attempt at early television. In 1898, American David Smith applied for a patent on a bent glass rod device to be used as a surgical lamp. By the turn of the century, however, several inventors had realized that bent quartz rods could carry light, and had patented them as dental illuminators, and many doctors were using illuminated plexiglass tongue depressors. But whilst all this was happening, a new technology was slowly developing that would eventually overcome the problem of optical transmission. This technology depended on a phenomenon called 'total internal reflection', which could confine light in a material that was

14 Optoelectronicsand Fiber Optic Technology

Figure 1.15 John Logie Baird (1888-1946)

surrounded by another material with a lower refractive index, for example glass in air. During the 1920s, the famous English inventor John Logie Baird (Figure 1.15) (remembered as being an inventor of mechanical television - an earlier version of today's television- and inventions related to radar and fiber optics) and an American (called Clarence Hansell) patented the idea of using bundles of hollow pipes or transparent rods to transmit images for television a n d / o r facsimile systems. In Munich during the 1930s a German medical student (Heinrich Lamm), whilst trying to find a way of looking at inaccessible parts of the human body, successfully demonstrated, with the aid of a 'light-stick', that by shining a light bulb filament on to an image, that image could be transmitted through a short bundle of optical fibers. However, the image transmitted by the unclad fibers was very poor and his effort to file a patent was denied because of Baird's previous British patent. Later on (i.e. in 1951), Holger Hansen also applied for a Danish patent for fiber optic imaging but was turned down by the Danish patent office because of existing Baird and Hansell patents. Hansen tried to interest some companies in his invention but without success. Historically, nothing else appears to have happened concerning the use of fiber bundles until 1954, when a Dutch scientist (Abraham van Heel) from the Technical University of Delft produced a paper concerning imaging bundles in the popular British journal, Nature. He had

The history of fiber optics

Cladding layer .... Core

15

~i

1

2 Figure

1.16

Basic fiber optic

proved that instead of using 'bare' fibers with total internal reflection at the glass-air interface, if the bare fiber (glass or plastic) was covered with a transparent cladding of lower refractive index, the reflective surface would be protected from contamination, and 'crosstalk' (i.e. the leakage of signal from one fiber to another) was greatly reduced between them. Figure 1.16 above shows the difference of a ray of light entering at a small angle (1) compared with another ray that enters at a different angle (2). The result is that different rays (or modes) of light will reach the end of the fiber at different times, even though the original light source was the same. Van Heel's experiments eventually led to the development of glass-clad fibers with an attenuation of less than one decibel per metre, which, whilst it was all right for medical imaging, was much too high for the telecommunications industry. Indeed, with an exponential growth rate in telephone traffic, television and the perceived use of videotelephony even in those days, telecommunications engineers were already asking for more transmission bandwidth. The use of higher optical frequencies seemed the best possible solution and with the invention of the laser (light amplification by stimulated emission~radiation) in 1960 came the announcement in the July 22 issue of Electronics that 'Usable communications channels in the electromagnetic spectrum may be extended by development of an experimental optical-frequency amplifier.' With the emergence of the continuous wave helium-neon laser this announcement quickly became a reality. By 1965, however, there were still technical barriers to the use of lasers, and optical waveguides were also proving to be a problem. This was mainly because, although optical fibers were very similar to the plastic dielectric waveguides that were used in certain microwave applications, the core of the drawn fibers was so small that they carried light in only one waveguide mode. Another problem was that most of the telecommunications industry had already discarded the use of fibers as being too lossy! So whilst attenuation of a decibel per metre was all

16 Optoelectronicsand Fiber Optic Technology

Figure 1.17

Charles Kao

right for medical purposes (e.g. for looking inside the body), telecommunications, which had a requirement for operating over long distances, could only accept a maximum loss of 10 or 20 decibels per kilometre. Help was around the corner, however, as a team from Standard Telecommunications Laboratories (STL) headed by a young Shanghai engineer called Charles K. Kao (Figure 1.17) and another young STL engineer who specialized in antenna theory, called George Hockham, were trying to find a way around fiber attenuation. With samples collected from a whole host of fiber manufacturers, they gradually built up a database of their (i.e. fiber's) properties, which enabled them to reach the conclusion that most of the high losses in early fibers were due to impurities and not to the silica glass itself! They were convinced that with careful manufacturing, these fiber losses could be reduced below 20 decibels per kilometre. Together Kao and Hockham presented a paper at a London meeting of the Institution of Electrical Engineers containing a proposal for longdistance communications over single-mode fibers. This was reported in the 1 April, 1966 issue of Laser Focus as follows: Dr C. K. Kao observed that short-distance runs have shown that the experimental optical waveguide developed by Standard Telecommunications Laboratories (STL) has an information-carrying capacity of one gigacycle, or equivalent to about 200 TV channels or more than 200 000 telephone channels. He described STUs device as

The history of fiber optics 17

consisting of a glass core about three or four microns in diameter, clad with a coaxial layer of another glass having a refractive index about one percent smaller than that of the core. Total diameter of the waveguide is between 300 and 400 microns. Surface optical waves are propagated along the interface between the two types of glass. According to Dr Kao, the fiber is relatively strong and can be easily supported. Also, the guidance surface is protected from external influences and the waveguide has a mechanical bending radius low enough to make the fiber almost completely flexible. Despite the fact that the best readily available low-loss material has a loss of about 1000 dB/km, STL believes that materials having losses of only tens of decibels per kilometre will eventually be developed. Based on Kao and Hockham's presentation, the British Post Office set up a new research fund of s million pounds to study ways to decrease fiber loss and in a similar manner, many institutes and laboratories from around the world were also actively investigating ways of trying to reduce fiber loss. Most of this research centred on an attempt to purify the glass used for optical fibers but at the Corning Glass Works, three researchers (Robert Maurer, Donald Keck and Peter Schultz, Figure 1.18) were experimenting

Figure 1.18 The original three Coming researchers with fused silica, a material that could be made extremely pure, but which had a high melting point and a low refractive index. By carefully adding controlled levels of dopants, the Corning researchers managed to increase the refractive index of the core slightly above that of the cladding, but without raising the attenuation dramatically. The Corning researchers' experiments came to fruition in September 1970 when they announced that they had made a single-mode fiber with

18 Optoelectronics and Fiber Optic Technology

an attenuation of below 20 d B / k m in a 633-nanometer h e l i u m - n e o n line. This breakthrough is generally recognized as the start of fiber-optic communications and opened the door to the commercialization of fiber optics, first for long-distance telephone service and later for computerrelated telecommunications (such as the Internet) and even medical devices (like the modern endoscope). This fiber optic wire or 'optical waveguide fiber' was patented by Corning (patent number 3,711,262) and was capable of carrying 65000 times more information than copper wire, via a pattern of light waves which could be decoded at a destination even a thousand miles away. The team had solved the problems presented by Dr Kao and it is now an accepted fact that fiber optic communications is one of the 10 most outstanding engineering achievements of the last 25 years. It is extremely doubtful (certainly not in my lifetime!) whether a superior method for long-distance information transfer will be discovered in the near future. In the same year that Corning patented the optical waveguide fibre (i.e. 1970), Bell Labs in America and a team from the Loffe Physical Institute in St Petersburg manufactured semiconductor diode lasers capable of emitting a continuous wave at room temperature. With the emergence of better manufacturing practices and the use of longer wavelengths, fibers were gradually being produced with even lower attenuation and in 1976 telephone field trials were conducted by AT&T using these types of fibers to transmit light (at 850 nanometers) with the aid of gallium aluminium arsenide laser diodes, for several k i l o m e t r e s - without repeaters. By then losses of less than 2 d B / k m were the accepted norm, but this was rapidly reduced to about 0 . 5 d B / k m when the new lnGaAsP lasers working in the 1300 nanometer range were introduced. So, although the possibility of using a dielectric rod as a waveguide had been studied during the late 1800s and a paper had actually been written about it in the 1920s, it was not until 1966 that Charles Kao and George Hockham patented the principle of 'information transmission through a transparent dielectric m e d i u m ' (i.e. glass fiber), that the use of glass fibers actually became a viable proposition. Since the development of optical fibers, more and more communication systems are now using fibers to pass optical signals over long distances with very little loss (compared to the losses experienced by electrical signals over copper lines). Following the development of laser diodes, the use of optoelectronics has now become the preferred method for cable communication. Indeed, a single glass fiber the thickness of a h u m a n hair (see Figure 1.19) can carry the equivalent of 300 million simultaneous telephone calls - roughly all the phone calls going on in America at any one time.

The history of fiber optics

Figure 1.19

19

Fiber size (courtesy of Corning Cable Systems GmbH)

1.2 Basic principles of optical line transmission Some crystals, for example gallium arsenide (GaAs), emit light at a wavelength very close to visible light when an electric current is passed through them. Figure 1.20 shows the frequency and wavelength of rays and light making up the electromagnetic spectrum. As can be seen from Figure 1.20, visible light (with wavelengths in the 400 nm to 750 nm range) is only a small part of the light spectrum. Above this is the near infrared region (i.e. 750 nm to 1600 nm) which fiber optic transmission u s e s - primarily because the fiber propagates the light much more efficiently at these wavelengths. This light emission is produced by the release of energy (photons) from the atoms of a material and is caused when the atoms are excited by heat, chemical reaction or some other means. Today's low-loss glass fiber optic cable can offer almost unlimited bandwidth and has many advantages over previous transmission media. In a fiber optic system, the sound waves are converted to electrical signals, which are coded as a series of pulses fed as a rapid burst of laser light through the fiber optic cable. At the other end they are received, decoded and converted back to sound waves. A lightwave system (Figure 1.21), therefore, is composed of four basic parts: a transmitter, fiber optic cable, regenerator and a receiver. The transmitter is the light source, which can be a light emitting diode (LED) or a laser. This source is modulated (i.e. turned on or off in a digital system) to represent the binary digits (ls and 0s) it receives from an electrical transmission system. The transmitter then converts the electrical analog or digital signal into a corresponding optical signal.

20

Optoelectronicsand Fiber Optic Technology

1 0 22

~- Cosmic rays

1021 - 102o - i0~9

~-- Gamma rays

- -

10~8

1017

~- X-rays

- -

-

Violet

(I THz)

~- Ultraviolet light

- -

1 0 TM -

-

1012 -

109

Yellow Orange

-

Red

101o -(I GHz)

Radar

10 7

(1 MHz) 106 10s

.._..

Television and FM radio

Shortwave radio '--~-- AM radio

Radio frequencies

550 580 620 750

Infrared

- -

10 8 - -

490

Green