WDM Systems and Networks: Modeling, Simulation, Design and Engineering

WDM Systems and Networks

For further volumes: http://www.springer.com/series/6976

Optical Networks Series Editor

Biswanath Mukherjee University of California Davis, CA

Neophytos (Neo) Antoniades Georgios Ellinas Ioannis Roudas •

Editors

WDM Systems and Networks Modeling, Simulation, Design and Engineering

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Prof. Neophytos (Neo) Antoniades Department of Engineering Science and Physics College of Staten Island The City University of New York Staten Island, NY 10314 USA e-mail: [email protected]

Series Editor Biswanath Mukherjee University of California Davis, CA USA

Georgios Ellinas Department of Electrical and Computer Engineering University of Cyprus Nicosia, Cyprus e-mail: [email protected] Ioannis Roudas Department of Electrical Engineering University of Patras 26504 Rio, Greece e-mail: [email protected]

ISSN 1935-3839 ISBN 978-1-4614-1092-8 DOI 10.1007/978-1-4614-1093-5

e-ISSN 1935-3847 e-ISBN 978-1-4614-1093-5

Springer New York Dordrecht Heidelberg London Library of Congress Control Number: 2011941711 Ó Springer Science+Business Media, LLC 2012 All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)

Neo Antoniades dedicates this book to his son Andreas for his love of science Georgios Ellinas dedicates this book to his nephew Nicolas and his niece Carina Ioannis Roudas dedicates this book to Linda, Nicholas and Adrian

Foreword

High-capacity optical networks employing wavelength-division multiplexing (WDM) form the underpinnings of our telecom backbone networks. The continued developments of WDM systems and networks and related technologies are crucial for the evolution of the future Internet. Important problems include accurate modeling, simulation, design, and engineering of components, devices, subsystems, systems, and the network as a whole. The authors of this book must be complimented for capturing these important problems across this entire ecosystem. This book presents in a clear and illustrative format the technical and scientific concepts that are needed to accomplish the design of WDM systems and networks (and hence the future Internet). The book has been co-edited by Professor Neo Antoniades of The City University of New York, Professor George Ellinas of the University of Cyprus, and Professor Ioannis Roudas of the University of Patras, Greece. The editors bring nearly 40 years of combined experience on various aspects of optical WDM systems and networks in preparing this book. They provide very good balance between theory and practice, having worked in the telecom industry (Bellcore, Tellium, Corning) as well as in academe. The book is divided into three parts: (1) Tools and Methods; (2) Implementations; and (3) The Logical Layer. Tools and Methods capture the state of the art on (a) Device and network element modeling; (b) Network modeling and performance evaluation; (c) Modeling of the transport systems; and (d) Commercial software simulation tools. Particularly noteworthy is the fact that modeling and simulation methods in software tools from leading vendors (such as VPI Systems, RSoft, and Optiwave) are covered. Part II on Implementations captures the relevant problems and challenges from short range to longer range systems such as: (a) Optical interconnects; (b) In-building systems (using plastic optical fibers); (c) Radio-over-fiber (RoF) systems; (d) Passive optical networks (PON) (for Fiber-to-the-home (FTTH) applications); and (e) Optical communication systems.

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Part III on Logical Layer includes networking problems such as: (a) Physicalto-logical layer interactions and (b) Multi-domain optical networks and survivability. The editors deserve praise because chapters on the above topics have been contributed by an excellent lineup of authors, many of whom work in leading companies in the field as well as in reputable universities, and they come from many different countries from around the world. This book is highly recommended as it offers timely, accurate, and authoritative information on WDM systems and networks, particularly on their accurate modeling, simulation, design, and engineering. The reader will enjoy the book and learn a lot. USA, August 2011

Biswanath Mukherjee

Preface

The demand for more and better applications for telecommunications networks has always driven the need for more bandwidth from these networks. At the same time, as the bandwidth has steadily increased over the last few decades, more applications have emerged that utilize this bandwidth. And so the ‘‘cycle’’ continues. Fiber-optic networks have been in the forefront of the effort to provide the users with more bandwidth, enabling them to run a number of new applications that span the whole spectrum of human interaction, including day-to-day activities, entertainment, and business, among others. Optical systems and networks have seen an incredibly rapid evolution. Only a few decades separate the invention of the laser and the low-loss fiber from the wide usage of wavelength-division-multiplexed (WDM) fiber-optic communication systems in the current telecom networks. These systems are now in every part of the telecom infrastructure including undersea, backbone, metropolitan area, as well as access networks, reaching all the way into our homes and businesses. Although some parts of the network still include networking functions such as switching and regeneration in the electronic domain, the direction is more and more toward networks that are transparent to signal rate, protocol, and format, where the signal stays in the optical domain as long as possible. This book provides the latest developments in the ever-expanding field of optical communication system and network design and engineering. It presents the industry, as well as current research, in state-of-the-art architectures of WDM optical systems and networks and takes a vertically layered (across the network layers) approach to system/network modeling, design, and engineering. Modeling and simulation techniques, in conjunction with experimental verification and engineering, are presented for different systems and networks. This book is different from a number of other books on optical systems and networks that are either general textbooks on optical networks or focus exclusively on the technology and point-to-point transmission. This contributed volume looks at both networking and system design issues, and focuses on the latest research developments in a number of areas including ultra long haul (ULH), metro, and access networks, as well as enabling technologies. It can be a very good supplement to ix

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any of the general optical networks/technology books for those interested in probing and understanding this area further. The aim of the editors is to present a body of work in this book that can provide the research scientist, company engineer, as well as the university professor/researcher with a better understanding of how to more efficiently design and engineer optical communication systems and networks. The intention is to demonstrate the effectiveness of computer-aided design when it comes to network engineering and prototyping. This book is divided into three parts that can appeal to different readers, who are interested in different types of networks and different applications. The first part of the book (Chaps. 2–5) presents modeling approaches and simulation tools mainly for the physical layer (including transmission effects, devices, subsystems, and systems), whereas the second part (Chaps. 6–11), presents more engineering/ design issues for various types of optical systems (including ULH, access, and inbuilding systems). The third part of the book (Chaps. 12–13) presents mostly networking issues related to the design of provisioning and survivability algorithms for impairment-aware and multi-domain networks. Finally, Chaps. 1 and 14 provide some introductory remarks and future directions respectively.

Acknowledgments

We are indebted to our past advisors, supervisors, colleagues, students and friends, who have motivated and inspired us in the optical communications paths that we took in our careers. In particular, we express our deep gratitude to our advisors and mentors Profs. Thomas E. Stern, Gee-Kung Chang, Mohamed Ali, Alfred Levine, Syed Ahamed, Syed Rizvi, Yves Jaouen, Constantin Caroubalos, Aurel Lazar and Manolis Sangriotis, and to our former colleagues Drs. Krishna Bala, Rich Wagner, Sarry Habiby, Aly F. Elrefaie, Jean-Francois Labourdette, Eric Bouillet, Janet Jackel, Richard Vodhanel, B. Roe Hemenway, Vassilis Keramidas, K. P. Ho, and Michael Sauer. All of them offered us their invaluable advise, exceptional insight, and foresight. They provided us with valuable guidance throughout the years and helped us better understand and appreciate various aspects of optical networking. We would also be remiss if we did not extend a thank you to Professor Biswanath Mukherjee for taking the time to write the foreword for this book and for his patience and encouragement while this book was being prepared and delivered. We also wish to express our thanks to Alex Greene, Allison Michael, and the entire publishing team at Springer Science+Business Media, for their effort and patience in order to bring this project to fruition. Special thanks go to Profs. N. Madamopoulos’ and N. Antoniades’ Ph.D. student Sasanthi Peiris for her great editorial support on this project. Also, special thanks goes to all the authors who contributed chapters for this book. Finally, Georgios Ellinas is greatly indebted to his family for their understanding and patience during this undertaking. Neo Antoniades wishes to express his profound gratitude to his wife Beatriz for her support and help while working on this project, to his parents for their lifelong inspiration and love and lots of thanks to Andreas, Pablo, George and Allison for making it fun. Ioannis Roudas gratefully acknowledges his family, whose loving support, help, and understanding has made his involvement in this book possible. Neophytos (Neo) Antoniades Georgios Ellinas Ioannis Roudas

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Contents

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Modeling, Simulation, Design and Engineering of WDM Systems and Networks: An Introduction . . . . . . . . . . . . . . . . . . . Georgios Ellinas, Neophytos (Neo) Antoniades and Ioannis Roudas

Part I

Tools and Methods

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Computer Modeling of Transport Layer Effects . . . . . . . . . . . . . André Richter

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State-of-the-Art in Device and Network Element Level Modeling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ramón Gutiérrez-Castrejón

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Semianalytical Models for Network Performance Evaluation . . . . Ronald Holzlöhner, Oleg V. Sinkin and Vladimir S. Grigoryan

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Commercial Optical Communication Software Simulation Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dwight H. Richards

Part II 6

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Implementations

Optical Interconnects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nicholas Madamopoulos

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Short Range (in-building) Systems and Networks: A Chance for Plastic Optical Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . María Angeles Losada and Javier Mateo

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WDM Phase-Modulated Millimeter-Wave Fiber Systems . . . . . . . Xianbin Yu, Kamau Prince, Timothy B. Gibbon and Idelfonso T. Monroy

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Fiber to the Home Through Passive Optical Networks. . . . . . . . . Alicia López, Noemí Merayo, Juan José Martínez and Patricia Fernández

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Coherent Optical Communication Systems . . . . . . . . . . . . . . . . . Ioannis Roudas

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Design Process for Terrestrial and Undersea DWDM Network Upgrades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sergey Burtsev

Part III

Logical Layer

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Impairment-Aware Optical Networking: A Survey . . . . . . . . . . . Siamak Azodolmolky, Marianna Angelou, Ioannis Tomkos, Tania Panayiotou, Georgios Ellinas and Neophytos (Neo) Antoniades

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Provisioning and Survivability in Multi-Domain Optical Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nasir Ghani, Min Peng and Ammar Rayes

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Future Directions in WDM Systems and Networks . . . . . . . . . . . Georgios Ellinas, Neophytos (Neo) Antoniades and Ioannis Roudas

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Contributors

Marianna Angelou Athens Information Technology, Peania, Attiki, Athens, Greece, e-mail: [email protected] Neophytos (Neo) Antoniades Department of Engineering Science and Physics, College of Staten Island/The City University of New York, Staten Island, NY 10314, USA, e-mail: [email protected] Siamak Azodolmolky Athens Information Technology Peania, Attiki, Athens, Greece, e-mail: [email protected] Sergey Burtsev Xtera Communications Inc., Allen, TX, USA, e-mail: sergey. [email protected] Georgios Ellinas Department of Electrical and Computer Engineering, University of Cyprus, Nicosia, Cyprus, e-mail: [email protected] Patricia Fernández Department of Signal Theory, Communications and Telematic Engineering, University of Valladolid, Valladolid, Spain, e-mail: patfer@tel. uva.es Nasir Ghani Department of Electrical and Computer Engineering, University of New Mexico, Albuquerque, NM, USA, e-mail: [email protected] Timothy B. Gibbon Department of Photonics Engineering, Technical University of Denmark, Lygnby, Denmark, e-mail: [email protected] Vladimir S. Grigoryan Ciena Corporation, Linthicum, MD, USA, [email protected]

e-mail:

Ramón Gutiérrez-Castrejón Universidad Nacional Autónoma de México (UNAM), Mexico City, Mexico, e-mail: [email protected] Ronald Holzlöhner Laser Guide Star Department, European Southern Observatory (ESO), Garching, Germany, e-mail: [email protected]

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Alicia López Photonic Technologies Group, i3A, University of Zaragoza, Zaragoza, Spain, e-mail: [email protected] María Angeles Losada, Photonic Technologies Group, i3A, University of Zaragoza, Zaragoza, Spain, e-mail: [email protected] Nicholas Madamopoulos Department of Electrical Engineering, City College of New York/The City University of New York, New York, NY 10031, USA, e-mail: [email protected] Juan José Martínez Photonic Technologies Group, i3A, University of Zaragoza, Zaragoza, Spain, e-mail: [email protected] Javier Mateo Photonic Technologies Group, i3A, University of Zaragoza, Zaragoza, Spain, e-mail: [email protected] Noemí Merayo Department of Signal Theory, Communications and Telematic Engineering, University of Valladolid, Valladolid, Spain, e-mail: noemer@ tel.uva.es Idelfonso T. Monroy Department of Photonics Engineering, Technical University of Denmark, Lygnby, Denmark, e-mail: [email protected] Tania Panayiotou Department of Electrical and Computer Engineering, University of Cyprus, Nicosia, Cyprus, e-mail: [email protected] Min Peng Wuhan University, Wuhan, People’s Republic of China, e-mail: pengm @whu.edu.cn Kamau Prince Department of Photonics Engineering, Technical University of Denmark, Lygnby, Denmark, e-mail: [email protected] Ammar Rayes Cisco Systems, Inc., San Jose, CA, USA, e-mail: [email protected] Dwight H. Richards Department of Engineering Science, College of Staten Island/The City University of New York, Staten Island, NY 10314, USA, e-mail: [email protected] André Richter VPIphotonics Division, VPIsystems, Berlin, Germany, e-mail: [email protected] Ioannis Roudas Department of Electrical Engineering, University of Patras, 26504 Rio, Greece, e-mail: [email protected] Oleg V. Sinkin System Modeling and Signal Processing Research, Tyco Electronics Subsea Communications, Eatontown, NJ, USA, e-mail: osinkin@ subcom.com Ioannis Tomkos Athens Information Technology, Peania, Attiki, Athens, Greece, e-mail: [email protected] Xianbin Yu Department of Photonics Engineering, Technical University of Denmark, Lygnby, Denmark, e-mail: [email protected]

Chapter 1

Modeling, Simulation, Design and Engineering of WDM Systems and Networks: An Introduction Georgios Ellinas, Neophytos (Neo) Antoniades and Ioannis Roudas

Abstract Optical systems and networks have evolved enormously in the last three decades with the creation of next-generation optical components, subsystems, systems, and networks that are now utilized in all aspects of the network structure starting from the in-house/building and access networks, all the way up to the backbone and ultra long-haul infrastructures. This book addresses a variety of issues related to next-generation optical systems and networks, starting from modeling and simulation and then proceeding to design and engineering, enabling the reader to gain an understanding of the latest developments in the ever-expanding field of optical communication systems and networks and of all the open technological and research issues. It presents the state-of-the-art architectures of optical systems and networks and takes a vertical approach to system/network modeling and design, starting from the transmission effects and component level and moving all the way up to the control layer. Analysis is mixed with modeling/simulation and engineering approaches to present the current industry techniques, as well as the research state-of-the-art.

G. Ellinas (&) Department of Electrical and Computer Engineering University of Cyprus 1678 Nicosia, Cyprus e-mail: [email protected] N. (Neo) Antoniades Department of Engineering Science and Physics College of Staten Island/The City University of New York Staten Island, NY 10314, USA e-mail: [email protected] I. Roudas Department of Electrical Engineering University of Patras 26504 Rio, Greece e-mail: [email protected]

N. (Neo) Antoniades et al. (eds.), WDM Systems and Networks, Optical Networks, DOI: 10.1007/978-1-4614-1093-5_1, Ó Springer Science+Business Media, LLC 2012

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1.1 Introduction In the past few decades, innovations in optical components, subsystems, systems, and networks have made it possible to satisfy the enormously increasing network traffic demand resulting from the high-bandwidth applications required by end users. Today, 40-channel or 80-channel 10 Gbps optical communications systems are barely meeting the requirements for high-capacity transmission. Under development next-generation optical communications systems exhibit singlechannel data rates of 40 and 100 Gbps for long-haul and are targeting 10 Gbps for the access with 1 Gbps symmetric access to customers being the logical step. It is expected that data traffic will maintain an exponential increase in the foreseeable future due to the rapidly increasing use of high-speed online services and broadband applications, such as telepresence, high-definition TV (HDTV), teleconferencing, telemedicine, data storage, grid computing, metacomputing, and others. This trend keeps pushing the broadband communication infrastructure towards higher speeds, greater capacity, longer reach, and closer to the end user. Thus, the need to better design and engineer all types of optical networks, including core, metro, and access networks has been a high priority of telecommunication engineers and researchers in the last few decades. With the successful commercialization of wavelength-division multiplexing (WDM) and the advent of dense WDM (DWDM) the optical transport network (OTN) has been standardized by the ITU-T as the underlying infrastructure of the network [1]. This provides for the management of services using multiple, different wavelengths of light over the same optical fibers, with the clients of the OTN being able to occupy up to a full wavelength. Furthermore, this allows the network to transport huge amounts of data that are needed for many current and future communication services and applications. WDM was first employed in point-topoint systems where fiber was used as a direct substitute for copper. In this approach all switching and processing of the data was handled by electronics and the optical traffic-carrying signal would undergo optical-electronic-optical (OEO) conversions at every switching/routing node. The scaling limitations in signal bit rate, switch matrix port-count, and network element cost were the key motivations behind the attempt to develop large port-count transparent switches to be used in opaque network architectures [2]. The key advances within the optical networking space in the second half of the 1990s and the early 2000s were mainly due to the development of new DWDM component, subsystem, and system technologies that resulted in the second generation optical networks where the provisioning and recovery functionalities moved to the optical part of the network [3]. Clearly, the ultimate goal is the design of completely transparent optical networks, as these networks offer the most economical solution with transparency in bit rate, signal format, and protocol. However, transparent optical networks face a number of challenges, with the two main ones being the difficulty in control and management of these networks and the need for end-to-end engineering due to the accumulation of physical impairments in the optical path. A middle-of-the-path

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solution to this problem is the creation of networks with some transparency but also some opacity, with the signal undergoing OEO conversions at specified points in the network. These types of networks are called translucent network architectures that seek a graceful balance among network design, cost, and service provisioning performance by exploiting the advantages of both transparent and opaque optical networks while eliminating their drawbacks. As the traffic grows and the bit rates increase substantially, translucent (optical-bypass) core WDM networks using reconfigurable optical add/drop multiplexers (ROADMs) and tunable lasers appear to be on the road towards widespread deployment while alloptical networks using DWDM technology appear to be the sole approach for transporting huge network traffic in future core networks [4]. Over the years the functionality of the physical layer is enhanced, with OTN providing for carrier-grade operations, administration, and maintenance (OAM) for these managed wavelength services, forward error correction (FEC) for increased reach and margin, and protection switching for high availability. Focus on manageability has resulted in advances in control and management, including taking control techniques designed for the electrical transport layers of the network and adapting them to the optical transport layer. As an example, the control protocol known as generalized multiprotocol label switching (GMPLS) has been proposed for application to control functions in the physical layer. GMPLS is a generalization of multiprotocol label switching (MPLS), which was designed as an improvement over the packet-forwarding techniques used in IP networks and to ensure automated provisioning, to maintain connections, and to manage the network resources including providing quality-of-service (QoS) and traffic engineering (TE) [5]. A typical WDM system and network spans the entire networking space consisting of the in-house/building, access, metropolitan-area, long-haul, and ultra long-haul networks. The distances covered by these networks include everything from a few meters for in-house/building networks to ultra long-haul transmission links such as transoceanic cables that can range from a few hundred kilometers in distance to several thousands of kilometers. The topologies of these networks also vary from tree structures for newly proposed passive optical network (PON)-based access networks, to ring (and stacked/interconnected ring) structures in metro and long-haul networks, to arbitrary mesh topologies for backbone networks. Thus, optical networks have evolved from point-to-point WDM transmission systems, to star, bus, and ring networks to arbitrary connected mesh networks utilizing intelligent network elements such as reconfigurable optical cross-connects or ROADMs among others that enable them to support network functionalities such as automated provisioning of optical services, automated protection/ restoration capabilities to combat failure scenarios, and others. These networking equipment provide for the seamless transport of the traffic with the QoS and availability that meets the customer’s requirements. The applications that are supported in these networks range from plain old telephone service (POTS) to Internet data and business-to-business services. Clearly, future optical networks will be required to support not only point-to-point connections but multicast and

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groupcast connections as well, as the number of applications that require such connections is expected to rise significantly in the near future. New trends, new technologies, and new applications are in the forefront of optical systems and optical networks. Below we describe three simple examples that highlight new applications and new design requirements, and the need for new technologies and new techniques to address them efficiently. As an example, consider the use of single-mode fiber (SMF), multi-mode optical fiber (MMF), and plastic optical fiber (POF) for next-generation avionics platforms. Most airplanes, including commercial and military aircrafts, have silica multimode optical fiber links installed for data transfer from/to on-board surveillance equipment and for internal communications. POF with its large diameter offers a good alternative for low-bandwidth avionics applications due to cost and ease of installation and should see implementations in the near future [6]. As the bandwidth required by different on-board applications is steadily growing, there is an urgent need for technology upgrades to higher data rates. So far, MMF transmission in avionics environments has been limited to data rates on the order of several dozen to a few hundred Mbps. Recent new applications such as entertainment on board commercial airplanes require more bandwidth, so new transmission techniques need to be considered. Since MMF bandwidth is limited by modal dispersion, a state-of-the-art approach is to use advanced modulation formats, such as quadrature phase shift keying (QPSK) and orthogonal frequency division multiplexing (OFDM) [7], as described in Chap. 10, in conjunction with multiple-input multiple-output (MIMO) techniques, in order to either overcome or be able to electronically compensate modal dispersion. Multi-level modulation and orthogonal modulation have been used broadly in wireless and radio frequency (RF) applications. These advanced modulation techniques that use orthogonal carriers to transport the phase and amplitude information and parallel streams to effectively transmit and receive a high-bit-rate stream and achieve high spectrum efficiency can be applied in this application to overcome the limitations of the existing systems. SMF on the other hand is the clear favorite for military avionics platforms where the amount of information is putting high requirements on bandwidth. As this example clearly shows, a combination of technologies (types of fiber) and transmission techniques (modulation formats) is required to address this new relatively simple application. Another example of an optical networking application that requires new design and engineering approaches is the provisioning of broadband services inside buildings. For this application there can be two possible solutions: (1) a wireline approach with the usage of MMF or POF in these systems (discussed in Chap. 7), or (2) a wireless/wireline approach with the usage of radio-overfiber (RoF) technology (examined in Chap. 8). RoF has recently been proposed for the transport of wireless signals particularly inside a building and has been proposed for use over glass MMF in [8]. The idea is to provide shared bandwidth to wireless users using optical fiber and at the end on the wireless

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side, by reducing the cell size to pico-cell scale limiting the number of users and increasing their bandwidth. This is achieved since a reduced cell size maintains a high signal-to-noise ratio (SNR) at the receiver with moderate transmit power. For applications with high user density a cell may only have a radius of a few meters and as a result covering the space of a very large room will require tens of antennas. Despite the burden on the wired infrastructure that the above seems to bring, it has been demonstrated that radio-over-fiber approaches can result in a simplified overall system design for a pico-cellular network by aggregating the RF signal generation and network management at a central location [9] and using optical fiber to distribute the signal to the various antenna sites. Clearly, this is another example that shows how new systems can be designed and engineered for the delivery of services utilizing different types of fibers and millimeter waves. A third (networking) example detailing the new trends in the design of optical networks, deals with the inclusion of the physical layer impairments in the algorithmic design necessary for the development of provisioning and survivability techniques. If the network is transparent, where the signal stays in the optical domain for the entire path, efficient routing and wavelength assignment (RWA) and protection algorithms have to account for the physical layer impairments (noise, crosstalk, dispersion, etc.) that the optical signal encounters. Clearly, the impact of the physical layer impairments depends on the design and engineering of the network components and subsystem architectures (e.g., optical node architecture) and different designs will achieve different network performance. If this example is extended to translucent networks, a new interesting problem that arises is the placement of the opaque nodes (e.g., regenerators) in conjunction with the impairment-aware RWA and protection algorithms. As the aforementioned examples amply demonstrate, advances are being undertaken in all aspects of optical systems and networks. This book provides the latest developments in the ever expanding field of optical communication system and network design and engineering. It presents the state-of-the-art architectures of WDM optical systems and networks and takes a vertical approach to system/network modeling and design, starting from the transmission layer and moving all the way to the control layer. A detailed discussion of the main physical transport layer effects and models is followed by a discussion of current trends in WDM technologies and their corresponding modeling. The latest in semi-analytical system simulation techniques are applied to optical WDM systems and networks and a review of various current areas of optical communication systems and optical networks is presented. Modeling and simulation is mixed with experimental verification and engineering to present the industry as well as research state-of-the-art. The goal of this book is to provide the professional scientist, engineer, and industrial/academic researcher with the basic skills, concepts, and simulation/design techniques so as to be able to design and engineer optical communication systems and networks at various layers.

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1.2 Organization of the Book This book is a combination of a number of topics related to the design (from a modeling and simulation perspective) and engineering of next-generation optical systems and networks that include components, devices, subsystems, systems, and networks, but it is by no means an exhaustive list. This book identifies engineering and research areas and topics that are in the forefront of research in fiber-optic networks and that could be of interest to the reader. It looks at both networking and system design issues and focuses on the latest research developments in a number of areas including optical interconnection, in-building, access, and long-haul (LH) networks, as well as enabling technologies, and as such, it can be of interest to all telecommunication network engineers and researchers that want to probe and understand these areas further. The book is divided into three main parts; Readers who are interested in the modeling and simulation perspective can read the first part of the book consisting of Chaps. 2–5 that deal with the modeling methodologies of transmission effects, components, devices, subsystems, systems, and networks and the design of software simulation tools for WDM systems and networks. The second part (Chaps. 6–11), apart from the modeling methodology, also includes a more engineering flavor and examines the system engineering/design of optical interconnects, in-building systems, RoF systems, access networks, and the upgrade of terrestrial and undersea systems. This part of the book will be of particular interest to those involved in WDM systems and in optical network engineering and deployment. The final part of the book comprising Chaps. 12 and 13 moves to the control layer (logical layer) and involves algorithmic designs taking into account physical-to-logical layer interactions and the case of multi-domain networks in the design of provisioning and survivability algorithms for these networks. In summary, the reading of the book can be customized as suited to each individual’s interests in exploring the issues related to the modeling, design, and engineering of WDM systems and networks. Several other new areas of engineering and research for multiwavelength optical systems and networks are outlined in Chap. 14 as areas of future and promising work that can be of interest to the reader. The reader is encouraged to explore further the references provided in that chapter to gain a better understanding of the ongoing work and the important issues related to the research work on those topics. In more detail, the rest of the chapters in this book provide a concise treatment of the following topics: Chapter 2 gives an overview of the computer modeling for the physical (i.e., optical transport) layer effects of multiwavelength optical networks. It initially describes different physical effects causing system degradation in the WDM transport layer including fiber propagation, optical amplification, and optical signal generation effects. In terms of fiber propagation effects this chapter considers chromatic dispersion, fiber nonlinearities (including Kerr effect, self- and crossphase modulation, four wave mixing, and scattering effects), polarization mode

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dispersion (PMD) and polarization-dependent loss/gain (PDL/PDG) and then describes modeling signal propagation over SMF. Optical amplification effects include amplified spontaneous emission (ASE) noise, gain and optical signal-tonoise ratio (OSNR) ripple effects, as well as transients, while optical signal generation effects consist of different types of modulation (direct and external) and various transmitter effects including transmitter-induced distortion and chirp, as well as transmitter-induced noise. Other transmission effects discussed in Chap. 2 include crosstalk and filter concatenation-induced distortion. Chapter 3 builds on Chap. 2 and focuses on the state of the art in passive device and component-level modeling (including optical fiber, multiplexers/demultiplexers, switches, and dynamic spectrum equalizers), as well as in network element-level modeling (including optical add drop multiplexers (OADMs) and optical cross-connects (OXCs)). The chapter shows how complex structures such as wavelength selective switches and optical add drop multiplexers can be built from simple structures (e.g., switches, filters, etc), utilizing a simulation tool. Different techniques are also described on how to model more sophisticated active devices such as lasers, amplifiers, modulators, and receivers. The development of models for the network performance evaluation of multiwavelength optical networks based on the modeling described in Chaps. 2 and 3 is crucial in the deployment of these networks. Chapter 4 addresses precisely this problem by introducing semi-analytical approaches for analyzing signal statistics in optical (nonlinear) transmission systems and ascertaining the fiber network performance. The chapter describes several semi-analytical methods, including linearization methods for the accurate computation of the bit error rate (BER) through the covariance matrix analysis and momentum-based methods for computationally efficient analysis of the amplitude and timing jitter as well as signal statistics. Chapter 5 combines the ideas presented in Chaps. 2–4 by presenting and reviewing the existing commercial optical communication software simulation tools, which include modeling and simulation of optical components, devices, subsystems, systems, and networks. Different capabilities of these simulation tools are described for different layers such as the physical, systems, and network layer. As in the last few years optical interconnect technology has become less costly and with enhanced functionality, there is increased interest in developing optical interconnect technology for shorter and shorter distances, including rack-to-rack, board-to-board, inter-chip, and intra-chip applications. Chapter 6 describes the current state-of-the-art on optical interconnects including rack-to-rack, chip-tochip, and on-chip interconnects and delves into the current trends on data centers and on the 100 Gbps frontier. This chapter details the latest optical interconnect systems (including system requirements and design issues, photonic technologies utilized, and architectures developed) and outlines the significant challenges that must be overcome to provide significant performance enhancements compared to their electrical counterparts. In Chap. 7 the issue of short range (in-building) systems and networks is examined, focusing on the recent emergence of optical in-building systems including the usage of plastic optical fiber (POF) in these systems. Modeling

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approaches for POF and for effectively simulating these systems and networks are discussed and different applications and implementations are described and analyzed. Chapter 8 introduces the concepts of radio-over-fiber (RoF), focusing mainly on WDM phase modulated millimeter-wave fiber systems. RoF schemes facilitate wireless microwave signal coverage within a remote area; information is transported over an optical fiber by modulating the message-bearing radio frequency (RF) signal onto an optical lightwave carrier. RoF technology allows combining of the ultra-broadband capacity of optical networks with the flexibility and mobility of wireless access networks, which is required to meet the capacity demand of future broadband wireless applications. Different architectures for the RoF systems are considered in this chapter and compared in terms of transmitter and receiver designs, BER performance, and crosstalk in order to propose an RoF system that is highly promising for future high-frequency wireless fiber communications. There is a growing perception that copper-based access networks will soon no longer be able to meet the ever-growing consumer demand for bandwidth. This, along with a combination of regulatory and competitive forces, as well as recent rapid advances and standardization of PON technology, have prompted carriers around the world to consider PON-based fiber-to-the-curb/home systems as a possible successor to current copper-based access solutions. Among the various PON-based fiber-to-the-x (FTTx) schemes (e.g., fiber-to-the-home (FTTH), fiberto-the-curb (FTTC), or fiber-to-the-building (FTTB)), single-channel time-division multiplexed PON (TDM-PON) and multi-channel wavelength-division multiplexed PON (WDM-PON) architectures are the two most viable candidates. Chapter 9 is devoted to these types of access systems and networks, providing a concise treatment of the state of the art in PONs with an emphasis on simulation, design, and engineering. A comprehensive overview of current deployed and novel proposed protocols and subsystem architectures for passive optical networks is presented. Emphasis is given to link layer techniques that allow resource sharing without comprising bandwidth utilization or so-called dynamic bandwidth allocation techniques. Different PON architectures are presented and examined including tree and ring architectures. As the channel data rate moved up to 40 Gbps and 100 Gbps, traditional intensity modulation and direct detection (IMDD) technologies that are broadly adapted in optical communications systems are incapable of achieving the required performance levels. Although technological advances have allowed the IMDD transmitter and receiver to operate at signaling speeds as high as 40 Gbps, at these speeds the cost of binary IMDD transmitter and receiver increases dramatically since both optical and electronic components are required to operate at such high speed. In addition, several detrimental transmission effects such as chromatic dispersion (CD), polarization mode dispersion (PMD), and fiber nonlinearities become the major limiting impairments of reliable transmission as the data rate reaches 40 Gbps and beyond. The rapid evolution of long-haul optical communications systems witnessed in the last five years is mainly due to the gradual adoption of spectrally-efficient, multilevel modulation formats, in conjunction with

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polarization division multiplexing (PDM) and coherent intradyne detection assisted by electronic compensation techniques using digital signal processing (DSP). The objective of Chap. 10 is to briefly review the operating principles of long-haul coherent optical communications systems using M-ary modulation formats. This chapter initially presents the digital M-PSK transmitter and receiver optimal architectures. Subsequently it reviews the operating principles of coherent detection and the different types of coherent receivers and then focuses mainly on the modeling of coherent optical systems using QPSK modulation and the evaluation of their performance. A description of high-speed DSP algorithms utilized for coherent detection is also included in this chapter. A number of enabling technologies described in the preceding chapters (WDM, FEC, RZ modulation format, advanced fiber designs, erbium-doped fiber amplifiers (EDFAs)) have been the principal enablers of the modern undersea fiber-optic systems. Furthermore, the introduction of Raman amplification (distributed Raman amplification (DRA) and remote optically pumped amplifiers (ROPA)), improved performance fibers, and other types of modulation formats (e.g., differential phase shift keying (DPSK)) can be utilized to increase the length of the spans between repeaters and enhance the received OSNR. Building greenfield undersea and terrestrial fiber-optic DWDM systems is a very expensive proposition for the carriers. A much cheaper solution is the upgrade of existing legacy systems upgrading either only the line terminal equipment (LTE) or upgrading the LTE and adding in-line active equipment in the transmission path, while preserving the legacy fiber plant. The goal of Chap. 11 is precisely to cover the design process of such DWDM upgrades of legacy fiber-optic systems. In this chapter the current industrial state of the art in the design process for the DWDM upgrade of terrestrial and undersea long-haul optical communications systems is presented, aiming to maximize upgrade capacity and minimize cost in these systems. Specifically, this chapter examines upgrades of undersea repeatered systems, un-repeatered undersea and terrestrial systems, and terrestrial multi-span long-haul systems. The focus of this chapter is on the optical transport part of any given fiber-optic system upgrade design, including fiber plant quality (in-line fiber type, fiber loss/chromatic dispersion/PMD data), margins for repair and aging, estimation of repeater characteristics, management of multipath interference and nonlinearities, Raman amplification aspects, and capacity results based on the models developed. Chapter 12 combines the physical (transport) and logical (control) layers and examines the physical-to-logical layer interactions, dealing specifically with impairment-aware (IA) routing and protection including different impairmentaware RWA algorithms for both unicast and multicast applications and the required control plane extensions for impairment-aware optical networking. Specifically, the transmission impairments and their modeling discussed in Chaps. 2 and 3 are considered in this chapter, and a host of IA RWA and protection techniques are developed and analyzed in terms of various performance metrics. Eventually, impairment-aware control plane extensions are compiled in this chapter in order to demonstrate how an IA approach would be implemented in the network architecture.

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Chapter 13 considers only the logical layer and goes a step further by considering multi-domain network architectures, taking into account the fact that modern wireline infrastructures have morphed into a complex interconnection of distributed, decentralized domains. In this case each domain is treated as a standalone entity running its own control and management planes, as delineated by ownership (multi-carrier), location, technology type, etc [10–12]. To address this type of network architecture, issues related to provisioning and survivability in multi-domain optical networks must be investigated and analyzed. Thus, this chapter discusses multi-domain provisioning, hierarchical routing and path computation, as well as per-domain provisioning techniques. Multi-domain survivability techniques including proactive protection strategies and dynamic restoration strategies are also described. Finally, Chap. 14 concludes the work and summarizes the state of the art in the field while presenting some thoughts on the obvious question: what the future holds.

References 1. Bonenfant P, Rodriguez–Moral A (2000) Optical data networking. IEEE Commun Mag 38(3):63–70 2. Bouillet E, Ellinas G, Labourdette J–F, Ramamurthy R (2007) Path routing in mesh optical networks, 1st edn. Wiley-Interscience, New York 3. Saradhi C, Subramaniam S (2009) Physical layer impairment aware routing (PLIAR) in WDM optical networks: issues and challenges. IEEE Commun Surv Tutor 11(4):109–130 4. Rahbar A, Yang O (2008) Contention avoidance and resolution schemes in bufferless alloptical packet-switched networks: a survey. IEEE Commun Surv Tutor 10(4):94–107 5. Stern TE, Ellinas G, Bala K (2008) Multiwavelength optical networks: architectures, design, and control, 2nd edn. Cambridge University Press, Cambridge, UK 6. Gasulla I, Capmany J (2007) High-frequency radio over fiber QPSK transmission through a 5 km multimode fiber link. In: Proceedings of European conference on optical communications (ECOC), Berlin, Germany 7. Antoniades N, Losada MA, Mateo J, Richards D, Truong TK, Jiang X, Madamopoulos N (2011) Modeling and characterization of SI-POF and connectors for use in an avionics system environment. In: Proceedings of the international conference on plastic optical fibers (ICPOF), Bilbao, Spain 8. Chang G-K, Jia Z, Yu J, Wang T, Ellinas G (2008) Super broadband optical wireless access technologies. In: Proceedings of the IEEE/OSA optical fiber communication conference (OFC/NFOEC), paper OThD1, San Diego, CA 9. Lethien C, Loyez C, Vilcot J-P, Clavier L, Bocquet M, Rolland P (2009) Indoor coverage improvement of MB-OFDM UWB signals with radio over POF system. Optics Commun 282(24):4706–4715 10. Ghani N, Liu Q, Gumaste A, Benhaddou D, Rao NSV, Lehman T (2008) Control plane design in multidomain/multilayer optical networks. IEEE Commun Mag 46(6):78–87 11. Chamania M, Jukan A (2009) A survey of inter-domain peering and provisioning solutions for the next generation optical networks. IEEE Commun Surv Tutor 11(1):33–51 12. Alanqar W, Jukan A (2004) Extending end-to-end optical service provisioning and restoration in carrier networks: opportunities, issues, and challenges. IEEE Commun Mag 42(1):52–60

Part I

Tools and Methods

Chapter 2

Computer Modeling of Transport Layer Effects André Richter

Abstract This chapter introduces the reader into optical signal representations and the major physical layer effects causing system degradations in the WDM transport layer. Suitable modeling approaches are presented, and typical simulation results are demonstrated. Finally, the chapter focuses on performance degrading effects due to fiber propagation, optical amplification, and signal generation.

2.1 The Value of System-Level Simulation System-level simulations are now routinely used to theoretically evaluate the performance of transmission systems based on the parameters of existing optical equipment, and to define performance requirements for new equipment to ensure robust operation for all required application scenarios. In a sense, system simulation is used as a virtual prototyping tool that reduces cost by shortening the system construction cycle. Furthermore, simulations prove valuable for investigating possible upgrade scenarios, understanding the nature and sources for system-limiting effects, determining the full effect of component limitations, and developing new technological approaches. The most critical part in designing accurate and reliable system simulation scenarios is selection of the right parameters and accurate transport effect models.

A. Richter (&) VPIphotonics Division, VPIsystems Berlin, Germany e-mail: [email protected]

N. (Neo) Antoniades et al. (eds.), WDM Systems and Networks, Optical Networks, DOI: 10.1007/978-1-4614-1093-5_2,  Springer Science+Business Media, LLC 2012

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2.2 Simulation Domains and Signal Representations For computer simulation to be effective, optical signals need to be represented into the computer. Optical signal representations with different degrees of abstraction are important in providing flexibility for modeling various aspects of photonic networks. Since the initial publications of the fundamental concept as in Lowery et al. [1], many improvements have been introduced supporting the modeling requirements of the ever-evolving advances in optical technologies and network architectures [2]. Time-domain simulations that propagate individual time samples on an iterationby-iteration basis between modeling blocks have been used to investigate, for instance, the properties of integrated photonic circuits, high-speed transmitters, and receivers, where the analysis of bidirectional interactions in the picoseconds range are of key importance [3]. For optical systems and networking applications mixeddomain simulations (i.e. including both, the time- and frequency-domains) are more suitable where arrays of signal samples are passed between modeling blocks. Applications are mainly unidirectional propagation scenarios and bidirectional scenarios where time delays are longer than the array duration (e.g., optical switching scenarios in bidirectional ring networks), or where bidirectional interactions can be averaged in time (e.g., bidirectional Raman pumping of optical fiber). The simulation efficiency of mixed-domain simulations can be controlled by representing optical signals either by their complex, polarization-dependent sampled waveform (time-dynamic or spectral representation), or by time-averaged parameters only. Depending on the modeling scope of interest, the time-dynamic representation can be applied to a single frequency band (SFB), or to multiple (non-overlapping) frequency bands (MFBs) for which the appropriate propagation equations describe the evolution of individual frequency bands, while interacting terms are approximated using frequency decomposition (FD) techniques. Although waveform-dependent propagation effects are ignored when using a parameterized representation of optical signals, this approach provides significant simulation speed-up while keeping sufficient modeling accuracy when investigating, for instance, the impact of ‘far-away’ WDM channels or optical pumps. Parameterized Signals (PS) can also be used to track important properties along the transmission path such as power, principal states of polarization (PSP), as well as accumulated amounts of noise, chromatic dispersion (CD), self-phase modulation (SPM), differential group delay (DGD), and thus easily assess signal characteristics over topology and frequency. Additive noise that is generated by lumped or distributed optical amplifiers can be tracked separately from the optical signal, disregarding nonlinear interaction effects between noise and signal, or accounting for them at the receiver using deterministic system performance estimation algorithms. In that case, optical noise can be described by its wavelength-dependent power spectral density (PSD), which could be step-wise approximated by so-called Noise Bins (NBs) of constant PSD. Further on, the parameterized representation is useful for keeping parasitic terms resulting from optical amplification, crosstalk,

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and scattering processes (collectively called Distortions) separate from the optical signal, and thus, allowing the investigation of their impact on the signal quality.

2.3 Device Modeling Different levels of abstraction in device modeling help to address the tradeoffs between the model complexity, simulation accuracy and computation speed [4] thus allowing the researcher to choose the required depth of detail. Typically, detailed models require in-depth parameters, whereas behavioral models operate on measured characteristics, or information from data sheets. At one extreme, detailed physical models represent components based on material and structural parameters. These parameters may be difficult to obtain, being proprietary, or difficult to derive from external measurements of a packaged device. Detailed physical models may require intensive computation, but can be used to design new devices and predict their performance. Black-box models are based on the physics of the device, but can be described in terms of behavioral parameters (instead of in-depth parameters), which can be directly derived from external measurements. Linear devices (such as filters) or well-specified devices (such as transmitters with a digital input) can be represented by their measured performance alone. Although novel devices cannot be designed directly using these models, the system performance of a module is easily and accurately assessed using these methods. Data sheets often provide characteristics as a series of parameters fitted to measurements (such as rise-time, spectral width). Although such data are often gathered using long-term measurements, their wide availability makes it useful for systemslevel simulation. Attention should be given so that long-term averages do not misrepresent the worst case.

2.4 Fiber Propagation Transmission effects influencing signal propagation over single-mode optical fiber can be grouped into several general categories arising from the fiber properties itself. There are first optical power loss effects, which are either regarded as being equally distributed along the fiber distance (e.g., power attenuation) or occur at isolated points along the fiber due to discrete events such as splices and fiber ends. Further on, the velocity of light traveling through the fiber depends on the wavelength, which results into dispersive effects of information-carrying optical signals. The so-called group velocity dispersion (or CD) is a linear propagation effect, which could theoretically be reversed after fiber propagation by means that are independent of the transmitted signal, for example, using components with opposite dispersion such as using dispersion compensating fiber (DCF) or fiber

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Bragg gratings (FBG). Additionally, power loss and dispersive propagation effects might be dependent on the local polarization state in time and frequency due to manufacturing imperfections, stress or other effects changing the properties of the perfectly circular optical single-mode fiber. Important effects are polarization mode dispersion (PMD) and polarization-dependent loss/gain (PDL/PDG). In addition to the above-mentioned linear effects linear scattering processes such as Rayleigh scattering, spontaneous Raman- and Brillouin- scattering are also important to consider. Rayleigh scattering can be understood as distributed reflection of the light wave on microscopic in-homogeneities of the fiber medium. This effect significantly contributes to the fiber loss. Furthermore, double Rayleigh scattering leads to a reflected lightwave propagating in the same direction as the originating signal and therefore interferes with it. While the reflection strength is typically small, the effect may become significant in high amplifying media (for instance Raman amplifiers with high pump power). Spontaneous Brillouin and Raman scattering effects are caused by the scattering of the lightwave on thermally induced acoustic waves or molecular oscillations of the fiber medium (also referred to as acoustic and optical phonons, respectively). The importance of these effects results from the fact that the spontaneously scattered signal becomes a seed for further amplification by the much stronger nonlinear (stimulated) scattering effects, which are considered below. Besides polarization-independent or dependent linear effects, signal transmission over optical fiber may suffer from effects, which result from the nonlinear interactions of the fiber material and the light traveling through it. Nonlinear propagation effects can generally be categorized as effects involving instantaneous or almost instantaneous electronic contributions—the optical Kerr effect and its various manifestations as SPM, cross-phase modulation (XPM), and four-wave mixing (FWM) as well as nonlinear effects with a noticeable time response: stimulated Brillouin and Raman scattering (SBS and SRS) respectively. Depending on the response time the spectral bandwidth of these nonlinear effects varies from almost unlimited (in telecommunications terms) for the Kerr effect, intermediate for the Raman effect (in the vicinity of 13 THz) and rather small for the Brillouin effect (linewidth of about 100 MHz and a Stokes shift of about 10 GHz). Finally, it is important to note that the nonlinear propagation effects occur in coexistence to CD, polarization effects, and attenuation, and may produce complex degradations or beneficial interactions (depending on the system parameters under investigation). For example, the FWM efficiency depends strongly on the CD of the fiber; SPM can support very stable pulse propagation in the presence of the right amount of CD; the impact of PMD can be reduced by nonlinear fiber interactions; the polarization dependence of SRS may introduce PDG. Thus, accurate fiber modeling needs to include nonlinear and linear phenomena, so that detrimental and beneficial interactions between these characteristics are predicted correctly.

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17

2.4.1 Linear Propagation Effects 2.4.1.1 Attenuation Fiber attenuation is mainly caused by absorption and scattering processes. It determines the resulting exponential decay of optical input power propagating through the fiber. Absorption arises from impurities and atomic effects in the fiber glass. Scattering is mainly due to intrinsic refractive index variations with distance (Rayleigh scattering) and imperfections of the cylindrical symmetry of the fiber. The usable bandwidth ranges from approximately 800 nm (increased Rayleigh scattering scales with k-4 [5, 6]) to approximately 1,620 nm (infrared absorption due to vibrational transitions). Older types of single-mode fiber show additionally an attenuation peak at approximately 1,400 nm due to the absorption of water molecules. Typically being specified in logarithmic units as dB/km, sometimes it is useful to work with attenuation values in linear units so:   loge ð10Þ ð2:1Þ a¼ adB=km  0:23026adB=km 10 For single channel applications a can be assumed to be wavelength-independent. However, for applications covering a large spectral range, such as multi-band WDM or Raman amplification, the spectral dependency of fiber attenuation should be accounted for accurately in simulations. The effective length Leff defines the equivalent fiber interaction length with respect to constant power [7] so that: Leff ¼

Zz

eaz1 dz1 ¼

1  eaz a

ð2:2Þ

0

In the above, Leff is an important quality measure when rescaling the signal evolution to account for attenuation and periodic amplification, and helpful when performing quick system performance estimations. 2.4.1.2 Chromatic Dispersion The CD or group velocity dispersion (GVD) represents a fundamental linear propagation limitation in optical fiber. It describes the effect that different spectral components propagate with different group velocities. CD alone causes spreading of chirp-less pulses along the fiber, although this statement is not true in general. For instance, in combination with nonlinear propagation effects other pulse propagation characteristics are detectable. Note that CD is a collective effect of material and waveguide dispersion [6, 8]. Depending on the manufacturing process and the radial structure of the fiber, different fiber types with various CD profiles can be designed. In standard single-mode fibers (SSMF) the GVD is positive (i.e. shorter wavelengths propagate faster) for wavelengths longer than

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approximately 1.3 lm and negative for shorter wavelengths. Thus, 1.3 lm is the point of zero GVD for SMFs. The fiber design allows shifting the point of zero dispersion and production of zero-dispersion fibers at 1.5 lm called dispersionshifted fibers (DSF) or fibers with low positive or negative dispersion at that wavelength called non-zero dispersion-shifter fibers (NZ-DSF), as well as fibers with high negative dispersion values for dispersion compensation called DCF. The impact of GVD on pulse propagation becomes clearer when expanding the modal propagation constant b(x) in a Taylor series around an arbitrary frequency x0 as shown in Agrawal [9]: x 1 1 bðxÞ ¼ nðxÞ ¼ b0 þ b1 ðx  x0 Þ þ b2 ðx  x0 Þ2 þ b3 ðx  x0 Þ3 ð2:3Þ c 2 6 where n(x) is the effective refractive index of the fiber, c is the speed of light in a vacuum, and bk ¼ ok =o xk bðxÞ at x ¼ x0 for k ¼ 0; 1; 2; 3 ði:e: partial derivative of bðxÞ with respect to x evaluated at x0 Þ In the above: • b0 accounts for a frequency-independent phase offset during propagation corresponding to the propagation constant at a certain reference frequency. • b1 defines the inverse of the group velocity vg determining the speed of energy propagated through the fiber. • b2 defines the GVD and describes the frequency dependence of the inverse of vg. It is responsible for the broadening of an initially chirp-less pulse due to the fact that its Fourier components propagate with different group velocities. This also leads to pulse chirp when the leading and trailing edges of the pulse contain light with different frequencies. Equivalently, it defines the different propagation speeds of pulses in frequency-separated channels (such as in WDM systems). • b3 is known as the slope of the GVD or second order GVD. This term accounts for the frequency dependence of the GVD and therefore for the different broadening properties of signals or signal portions propagating at different frequencies. This term is critical for wideband transmission systems, for systems operating in frequency regions where b2 is close to zero as well as systems utilizing DCFs to compensate dispersion for multiple WDM channels simultaneously. In general, it is of more interest to determine the dependence of the inverse of the group velocity vg on wavelength rather than on frequency. This dependence is described by the dispersion parameter D and its slope with respect to wavelength, S. The following relationships inter-relate the above parameters: D¼

d 1 2pc ¼  2 b2 dk vg k

  dD ð2pcÞ2 1 1 S¼ ¼ b þ b dk k 3 pc 2 k3

ð2:4Þ

ð2:5Þ

2 Computer Modeling of Transport Layer Effects

d 1 k2 ¼ D dx vg 2pc

ð2:6Þ

db2 k3 ¼ ðkS þ 2DÞ dx ð2pcÞ2

ð2:7Þ

b2 ¼ b3 ¼

19

D is typically measured in ps/nm-km and can be interpreted as describing the broadening DT of a pulse with bandwidth Dk after propagation over a distance z, or equivalently, the time offset DT of two pulses after distance z that are separated in the spectral domain by Dk. DT is given below as: DT  Dk

dT d 1 ¼ DkzD ¼ Dkz dk dk vg

ð2:8Þ

From the above, the walk-off length Lw can be defined as the distance it takes for one pulse with duration T0 traveling at frequency x1 to overtake another pulse traveling at frequency x2 and is thus given by: LW ¼

T0 T0  jb1 ðx2 Þ  b1 ðx1 Þj jDkDj

ð2:9Þ

The dispersion length LD characterizes the distance over which dispersive effects become important in the absence of other effects. Specifically, it defines the pffiffiffi distance over which a chirp-free Gaussian pulse broadens by a factor of 2 due to GVD and is given by: LD ¼

T02 2pc T 2 ¼ 2 0 j b2 j k jDj

ð2:10Þ

However, LD can also be applied as an approximation to determine the relevance of CD effects on other intensity modulated pulses. Since LD is inversely proportional to the square of the signal bandwidth, as an example, the CD requirement increases by a factor of 16 when increasing the signal bitrate from 10 to 40 Gbit/s using the same modulation format and filter bandwidths that are proportional to the signal rate. In consequence, CD limits uncompensated 40 Gbit/s NRZ transmission over SSMF to approximately 4 km and over NZ-DSF to approximately 20 km. Additional dispersion variations due to changes in temperature along the link accumulate fluctuations as shown in Kato et al. [10] that have to be compensated adaptively, especially in long-haul applications. So it would be natural to think that low- or zero-dispersion fibers are the optimum choice for WDM systems design. However, instead of using DSFs with very small local GVD values, it is more appropriate to use transmission fibers with larger local dispersion values (e.g., SSMF or NZ-DSF) and place DCFs at regular distances along the link compensating for all or part of the accumulated CD the optical signal experienced up to that point. If optical fiber were a purely linear

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Fig. 2.1 Dispersion management for a long-haul transmission experiment where dispersion compensation is placed at regular intervals along the fiber as described in Liu et al. [11]

transmission medium it would not matter at which point along the link CD is compensated. However, as will be described in the following section, fiber propagation is slightly dependent on the intensity of the light traveling through it, thus resulting in nonlinear propagation effects. In modern WDM transmission systems with channel symbol rates of 10 Gbit/s and beyond, dispersion management taking care of the careful adjustment of local CD values (not too small) and maximum accumulated CD before dispersion compensation (not too large) helps to control these nonlinear fiber impairments (Fig. 2.1).

2.4.2 Nonlinear Propagation Effects 2.4.2.1 Kerr Effect The Kerr effect denotes the phenomenon in which the refractive index of an optical fiber n(x, t) is slightly dependent on the intensity of the optical signal IðtÞ ¼ jEðtÞj2 , with E(t) being the electric field, passing through the fiber. As a result: nðx; tÞ ¼ n0 ðxÞ þ n2 I ðtÞ

ð2:11Þ

where n0 is the linear refractive index and n2 is the nonlinear refractive index of the fiber. When the electric field intensity of the transmitted signal bit stream varies in time it induces an intensity-dependent modulation of the refractive index, and hence, modulation of the phase of the transmitted signal. Compared to other nonlinear media, n2 is very small (typically on the order of 10-20 [m2/W]). However, even this weak fiber nonlinearity becomes fairly relevant for signal propagation as field intensities of several mWs are focused in a small fiber core (in the order of several tens of lm2) over interaction lengths of tens to hundreds of kilometers. As a result, effects such as nonlinear interaction between signal pulses might accumulate during transmission and become of system limiting importance. In conclusion, the Kerr nonlinearity results in several intensity-dependent propagation effects, with the most important ones being SPM, XPM, and FWM.

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Self- and Cross-Phase Modulation SPM and XPM occur when a temporal variation of the optical signal intensity induces a temporal phase shift on the originating signal (SPM) or on other co-propagating signals at different wavelengths (XPM). For example, in the case where several optical signals at different wavelengths each with intensity I(t) and initial phase u0 are launched into a fiber, the phase modulation of the signal corresponding to channel m depends on the local power distribution of all other channels as follows: um ðt; zÞ  u0;m ¼ ulin;m þ uSPM;m þ uXPM;m X 2p 2p 4p n0;m z þ n2 zIm ðtÞ þ n2 z ¼ Ik ðtÞ k k k k6¼m

ð2:12Þ

where um(t, z) is the phase modulation of channel m, u0, m is the initial phase of channel m, n0, m is the linear refractive index of channel m, n2 is the nonlinear refractive index, and k is an index denoting the neighboring WDM channels of channel m. Furthermore: • ulin, m corresponds to the accumulated linear phase shift due to transmission. • uSPM, m corresponds to the accumulated nonlinear phase shift due to SPM in channel m. The SPM-induced phase shift is proportional to the local signal intensity. It induces chirp (time-varying frequency shift) and spectral broadening, so as an example, pulses behave differently in the presence of GVD and optical filtering. • uXPM, m corresponds to the accumulated nonlinear phase shift due to XPM in channel m describing the phase modulation that is induced by intensity fluctuations in neighboring WDM channels. XPM introduces additional nonlinear phase shifts that interact with the local dispersion as well. It must be noted that XPM occurs only over distances where optical intensities at different frequency components co-propagate, e.g. pulses propagating in different WDM channels are overlapping. In general, the XPM effect reduces with increased CD as pulses at different frequencies propagate faster through each other, e.g. Lw (as defined in Sect. 2.4.1.2) becomes smaller. For the same reason, XPM scales inversely with the channel spacing. Lw is also called the collision length Lc as it accounts for the distance where two pulses at different frequencies collide (and thus, interact nonlinearly due to XPM) during propagation. Four-Wave Mixing Parametric interactions between optical field intensities at different frequencies might induce the generation of inter-modulation products at new frequencies when propagating optical signals over wide spectral ranges through the fiber.

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This nonlinear effect is called FWM. FWM can occur between channels in WDM systems, between optical noise and channels, and between the tones within one channel. More generally noted, FWM occurs for instance when two photons at frequencies x1 and x2 are absorbed to produce two other photons at frequencies x3 and x4 satisfying the relation below: x1 þ x 2 ¼ x 3 þ x 4

ð2:13Þ

It could also be understood as mixing of three waves producing a fourth one according to the electrical field equations below: Eklm ¼Ek El Em ¼jEk jjEl jjEm jejðxk þxl xm Þt ej½bðxk Þþbðx1 Þbðxm Þz

ð2:14Þ

where Ei is the electric field of the wave propagating at frequency xi, b(xi) is the modal propagation constant at xi, and xklm = xk ? xl-xm is the carrier frequency of Eklm. The energy of Eklm is given as the superposition of mixing products of any three waves for which xklm = xk ? xl-xm holds (with the degenerate case for xk = xl). Note that the propagation constant is frequencydependent, so efficient interactions only occur if contributions to Eklm given at different times add up over distance, e.g., the phase mismatch Db between the inter-modulating fields tends to go to zero (phase matching condition): Db ! 0;

DbðxÞ ¼ bðxk Þ þ bðxl Þ  bðxm Þ  bðxklm Þ

ð2:15Þ

The power of the newly created wave is proportional to the power of the three interacting waves assuming no pump depletion due to FWM occurs [12] resulting in: Iklm ðzÞ ¼ jEklm ðzÞj2 ¼ gc2klm jEk ðzÞj2 jEl ðzÞj2 jEm ðzÞj2 eaz L2eff

ð2:16Þ

where cklm is the nonlinear index of the fiber at xklm, a is the fiber attenuation, z is the propagation distance, Leff is the effective length, and g is the so-called FWM efficiency given by: g¼

a2 1 þ 4eaz sin2 ðDbz=2Þ a2 þ Db ð1  eaz Þ2

ð2:17Þ

FWM between the WDM channels scales inversely with channel spacing in case of non-zero GVD. Increasing the local dispersion results in an increased walkoff of the Fourier components and thus, it results in phase mismatch after shorter propagation distances, which leads to an even steeper decrease of the FWM efficiency with channel separation. Fig. 2.2 presents the results of a case study where four 10 Gbit/s NRZ signals are propagated over 50 km of DSF and NZ-DSF fiber segments, respectively. Figure 2.2a shows the spectrum after transmission over 10 km of DSF, while Fig. 2.2b shows the corresponding spectrum after transmission over 10 km of

2 Computer Modeling of Transport Layer Effects

23

Fig. 2.2 Spectrum of four 10 Gbit/s NRZ channels after transmission over 10 km of: a DSF; b NZ-DSF segments. Power profiles of channel 3 with FWM products for propagation over: c DSF; d NZ-DSF segments

NZ-DSF. The larger local dispersion of the NZ-DSF with respect to the DSF results in an increased walk-off between the interacting tones (e.g., WDM channels and FWM distortions outside the WDM spectrum that are created during propagation). Hence, the FWM efficiency is reduced resulting in mixing products of lower power and faster power oscillations during propagation. This can be observed when comparing Fig. 2.2c showing the power levels of channel 3 and the FWM products versus DSF distance and Fig. 2.2d showing the power profiles for the corresponding NZ-DSF case.

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The number of generated FWM components that are created when signals at N wavelengths mix together grows with N2(N-1)/2. Using a sampled signal representation, all important components that fall into the simulation bandwidth are automatically included when solving the fiber propagation equations (see Sect. 2.4.4.1). Note that the simulation bandwidth should be chosen wide enough in order to avoid misleading results due to aliasing effects of the generated FWM components [13]. When accounting for FWM distortions using parametric modeling, the number of calculated FWM components shall be limited to avoid excessive numerical effort by considering only FWM distortions that would reach a certain power threshold, and thus, are important to be accounted for in system performance analysis.

Kerr Nonlinearities and CD Similar to the dispersion length LD, a scale length for Kerr nonlinearities in optical fiber could be defined: the nonlinear length LNL is given as the distance over which the phase of a pulse with intensity I changes by one rad due to the Kerr nonlinearity in a fiber with a nonlinear coefficient c LNL ¼

1 cI 2

ð2:18Þ

LNL denotes the distance where nonlinear effects become important (in the absence of other effects), also called nonlinear scale length. The ratio between LD and LNL describes the dominating behavior for signal evolution in optical fiber. For LD \ LNL linear effects dominate the signal propagation. In these quasi-linear systems pulse powers change rapidly in time due to dispersion-induced pulse spreading and compression over LNL, and thus, the impact of nonlinear propagation effects is averaged [14]. Those nonlinear pulse-to-pulse interactions can be best described by time-equivalent terms of FWM and XPM, namely intra-channel four-wave mixing (I-FWM) and intra-channel cross-phase modulation (I-XPM) as described in Essiambre et al. [15], Mamyshev and Mamyshev [16]. Typically, I-FWM induces pulse echoes while I-XPM induces timing jitter. Pulse-to-pulse interactions in quasi-linear systems become important for bitrates of 40 Gbit/s and higher. With a careful selection of launch power, pre-chirp as well as the dispersion map, degradations due to I-XPM and I-FWM can be drastically reduced (see for instance [17–19]). Of special interest is also the case when LD * LNL. In these nonlinearitysupporting systems cancelation of dispersive and nonlinear effects can be observed for carefully chosen system settings such as in soliton or dispersion-managed soliton (DMS) systems. Choosing system parameters such as pulse width, separation, power, as well as dispersion map and amplifier position and separation carefully, pulse chirp introduced by CD and SPM may combine and be used

2 Computer Modeling of Transport Layer Effects

25

Fig. 2.3 Eye diagrams of a 10 Gbit/s NRZ signal after propagation over four spans of 80 km SSMF for three different dispersion compensation schemes: a pre-compensation; b symmetric compensation; c post-compensation. For each case, eye diagrams without consideration of Kerr nonlinearities are displayed on the left and including Kerr nonlinearities on the right

advantageously for stabilizing pulse propagation over very long distances. However, XPM introduces timing jitter, which in addition to timing jitter induced by added optical noise along the link is one of the ultimate limitations for such systems (see for instance [20]). Fig. 2.3 shows results of a single channel 10 Gbit/s NRZ transmission over four spans of 80 km SSMF using different methods of dispersion compensation. Figure 2.3a shows eye diagrams (with and without considering Kerr nonlinearities) for the case that 100% of the accumulated dispersion of each SSMF span is compensated before entering the fiber (pre-compensation). Figure 2.3b shows the corresponding results when applying symmetrical compensation, e.g., equal parts of dispersion compensation are performed before and after the SSMF. Figure 2.3c shows the eye diagrams when the accumulated dispersion of each SSMF span is compensated after the fiber (post-compensation). Appropriate DCMs are placed at mid-stage access EDFAs before and/or after each span, which also set the signal

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A. Richter

power into the transmission fiber at 10 dBm. Admittedly this value is rather large, though, it is chosen here in order to demonstrate the importance of considering Kerr nonlinearities when designing for an optimum dispersion map and amplifier placement.

2.4.2.2 Scattering Effects Stimulated Raman Scattering Stimulated Raman scattering (SRS) is one of the most important scattering effects in optical fiber. It denotes the inelastic process in which light is scattered by material scattering centers, which causes vibrational excitation of molecules. As a result, energy is lost to the material with the effect that lower energy photons are created from higher energy photons. Effectively, this means that signals at shorter wavelengths are attenuated and signals at longer wavelength are amplified. SRS has an important application for WDM systems, as relatively high distributed amplifications may be achieved over the length of the transmission fiber. However, it may also induce the Raman-induced tilt of WDM channel powers when the shorter wavelength channels become attenuated at the expense of longer wavelength channels. Raman scattering is present at all optical frequencies, e.g., Raman amplification can be achieved at any wavelength (if an appropriate pump is available). The amplification range using a single pump wavelength is broadband (about 6 THz at 1.5 lm) but also strongly shaped. Raman amplifiers providing flat gain over large bandwidths require multiple pumps at different wavelengths [21] and a careful adjustment of the individual pump powers [22, 23]. Besides stimulated Raman scattering, spontaneous Raman scattering occurs in the fiber as well producing broadband optical noise that is strongly wavelengthand temperature-dependent. Additionally, double Rayleigh scattering may produce significant amounts of additional noise. The pumping conditions (number and power of co- and counter-propagating pumps) play an important role on the noise characteristics, as shown clearly in Figure 2.4. Compared with EDFAs compensating fiber loss at the ends of links, Raman amplification is low noise due to its distributed nature. Hence, Raman amplification allows lower launched powers while maintaining the OSNR, which reduces other nonlinear effects in the fiber.

Stimulated Brillouin Scattering Stimulated Brillouin scattering (SBS) occurs when forward- and backwardpropagating lightwaves induce an acoustic wave due to electrostriction with a frequency that equals the frequency difference of the optical waves, and a propagation constant that equals the sum of the propagation constants of the optical waves (phase matching). In other words, the acoustic wave forms a

2 Computer Modeling of Transport Layer Effects

27

Fig. 2.4 Effective noise figures for different pumping conditions after optimization of 12 Raman pumps to generate flat gain over 80 nm signal bandwidth after transmission over 100 km NZ-DSF (From Breuer et al. [21], Fig. 9. Reproduced by permission of  2004 The Institute of Electrical and Electronics Engineers)

traveling index grating, which reflects the incident light wave with a downward Doppler shift. The strength of SBS depends significantly on the type and modulation bandwidth of the transmitted signal. SBS is strongest for narrowband signals, as a strong coherent grating is produced. It may be detrimental for optical communication systems, as it strongly reflects un-modulated signals and narrow-band pumps. However, the impact of SBS can be effectively lowered by employing techniques that spread the signal power more evenly in frequency (i.e., phase modulation).

2.4.3 Polarization-Dependent Propagation Effects 2.4.3.1 Polarization Mode Dispersion (PMD) Due to manufacturing imperfections, fibers are not ideally circular and are additionally subject to temperature changes, vibrations, bending, and twisting, which make them birefringent. All of these random mechanisms cause local stress and redefine the local birefringence axes along the fiber length. Hence, light that is propagating along the single-mode fiber is split into two local polarization modes traveling at different velocities. PMD describes this dependence of the propagation velocity of optical signals on the local state of polarization. At any point along the fiber, light travels as a combination of two polarization eigenstates, which randomly vary with fiber length. For instance, signal power originally sent into one state may partially transfer to the other along the fiber. As a final result, this means that different parts of a signal pulse travel with different velocities causing pulse spreading, which limits the bandwidth of the

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transmission system. With this model, PMD at a fixed frequency can be described by two PSPs representing the pair of orthogonal polarization states of slowest and fastest propagation resulting from the integrated polarization characteristics over the whole fiber at that frequency. The maximum difference between group delays of any polarization state (e.g., the delays of the two PSPs) is defined as the DGD Ds. The intrinsic PMD coefficient DPMD,i determines the amount of Ds per unit length z for very short fiber lengths where birefringence can be assumed to be uniform [24]. Using Taylor series expansion with respect to frequency and considering the first two terms only, DPMD,i is given by DPMD;i ¼

 ns  nf x d   Ds d  þ ¼ b  b1;f ¼ ns  nf c z dx 1;s c dx

ð2:19Þ

where b1,s is the inverse of the group velocity in the slow PSP, and b1,f is the inverse of the group velocity in the fast PSP, where ns and nf are the effective refractive index for the slow and fast PSP, respectively. For transmission fiber lengths of interest, however, it cannot be assumed that the orientation and value of birefringence remain constant. The correlation length Lcorr provides a measure for the strength of the accumulated random change in polarization states. It is defined as the distance where the average power in the orthogonal polarization mode, Porth, is within 1/e2 of the power in the starting mode, Pstart, when omitting the effect of attenuation [25] jhPstart ðLcorr Þi  hPorth ðLcorr Þij ¼

Ptotal e2

ð2:20Þ

For typical transmission fibers, Lcorr is only several meters. Here, the randomly and continuously varying DGD over z represents the characteristics for a random walk problem [26], and thus, scales with the square root of z (each incremental section may add or subtract to the effects of previous sections). In consequence, the PMD coefficient of transmission fibers DPMD determines the amount of average pffiffi DGD per z [25]. The stochastic process of varying birefringence can be evaluated analytically with the result that Ds follows a Maxwellian probability density function (PDF) [27]. The Maxwellian PDF originates from the quadratic addition of three Gaussian processes following an analogy with the statistical model of Brownian motion [28]. It can be used to describe the random distribution of the DGD for fiber distances that are much larger than Lcorr as follows: PðDsÞ ¼

p Ds2 p4Ds2 =hDsi2 e 16 hDsi3

ð2:21Þ

Note that DPMD,i is not only time-dependent due to the drift of the environmental parameters but shows also a frequency dependence, which is indicated by the second term in Eq. 2.19. Using the Poincare Sphere as representation of light polarization [29] the PMD vector is, by definition, a Stokes vector pointing in the direction of the slow PSP with a length equal to the DGD [26].

2 Computer Modeling of Transport Layer Effects 20

Total Total Measurements Measurements : 97709770 PMD - Coefficient - Average ps / √ps/ km PMD-Coefficient Average : 0.170.17 PMD - Coefficient - Maximum: PMD-Coefficient Maximum: 4.794.79 ps / √ps/ km km

18 16 14 12

class width 0.5 ps/√km km

class width 0.02 ps/ km

10 8 6 4 2

> 3.5

2.5 to 3.0

1.5 to 2.0

0.5 to 1.0

0.42 to 0.44

0.46 to 0.48

0.34 to 0.36

0.30 to 0.32

0.26 to 0.28

0.30 to 0.32

0.22 to 0.24

0.18 to 0.20

0.10 to 0.12

0.14 to 0.16

0.06 to 0.08

0