40 INTRODUCTION TO OPTICAL NETWORKS networks are now widely deployed. Today it is common to have high-speed optical interfaces on a variety of other devices such as IP routers and ATM switches. As these first-generation networks were being deployed in the late 1980s and early 1990s, people started thinking about innovative network architectures that would use fiber for more than just transmission. Most of the early experimental efforts were focused on optical networks for local-area network applications, but the high cost of the technology for these applications has hindered commercial viability of such networks. Research activity on optical packet-switched networks and local-area optical networks continues today. Meanwhile, wavelength-routing networks became a major focus area for several researchers in the early 1990s as people realized the benefits of having an optical layer. Optical add/drop multiplexers and crossconnects are now available as commercial products and are beginning to be introduced into telecommunications networks, stimulated by the fact that switching and routing high-capacity connections is much more economical at the optical layer than in the electrical layer. At the same time, the optical layer is evolving to provide additional functionality, including the ability to set up and take down lightpaths across the network in a dynamic fashion, and the ability to reroute lightpaths rapidly in case of a failure in the network. A combination of these factors is resulting in the introduction of intelligent optical ring and mesh networks, which provide lightpaths on demand and incorporate built-in restoration capabilities to deal with network failures. There was also a major effort to promote the concept of fiber to the home (FTTH) and its many variants, such as fiber to the curb (FTTC), in the late 1980s and early 1990s. The problems with this concept were the high infrastructure cost and the questionable return on investment resulting from customers' reluctance to pay for a bevy of new services such as video to the home. However, telecommunications deregulation, coupled with the increasing demand for broadband services such as Internet access and video on demand, is accelerating the deployment of such net- works by the major operators today. Both telecommunications carriers and cable operators are deploying fiber deeper into the access network and closer to the end user. Large businesses requiring very high capacities are being served by fiber-based SONET/SDH or Ethernet networks, while passive optical networks are emerging as possible candidates to provide high-speed services to homes and small businesses. This is the subject of Chapter 11. Summary We started this chapter by describing the changing face of the telecom industry the large increase in traffic demands, the increase in data traffic relative to voice traffic, the deregulation of the telecom industry, the resulting emergence of a new set of Further Reading 41 carriers as well as equipment suppliers to these carriers, the need for new and flexible types of services, and an infrastructure to support all of these. We described two generations of optical networks in this chapter: first-generation networks and second-generation networks. First-generation networks use optical fiber as a replacement for copper cable to get higher capacities. Second-generation networks provide circuit-switched lightpaths by routing and switching wavelengths inside the network. The key elements that enable this are optical line terminals (OLTs), optical add/drop multiplexers (OADMs), and optical crossconnects (OXCs). Optical packet switching may develop over time but faces several technological hurdles. We saw that there were two complementary approaches to increasing transmis- sion capacity: using more wavelengths on the fiber (WDM) and increasing the bit rate (TDM). We also traced the historical evolution of optical fiber transmission and networking. What is significant is that we are still far away from hitting the fundamental limits of capacity in optical fiber. While there are several roadblocks along the way, we will no doubt see the invention of new techniques that enable progressively higher and higher capacities, and the deployment of optical networks with increasing functionality. Further Reading The communications revolution is a topic that is receiving a lot of coverage across the board these days from the business press. A number of journal and magazine special issues have been focused on optical networks [GLM+00, CSH00, DYJ00, DL00, Alf99, HSS98, CHK+96, FGO+96, HD97, Bar96, NO94, KLHN93, CNW90, Pru89, Bra89]. Several conferences cover optical networks. The main ones are the Optical Fiber Communication Conference (OFC), Supercomm, and the National Fiber-Optic En- gineers' Conference. Other conferences such as Next-Generation Networks (NGN), Networld-Interop, European Conference on Optical Communication (ECOC), IEEE Infocom, and the IEEE's International Conference on Communication (ICC) also cover optical networks. Archival journals such as the IEEE's Journal of Lightwave Technology, Journal of Selected Areas in Communication, Journal of Quantum Electronics, Journal of Selected Topics in Quantum Electronics, Transactions on Networking, and Photonics Technology Letters, and magazines such as the IEEE Communications Magazine and Optical Networks Magazine provide good coverage of this subject. There are several excellent books devoted to fiber optic transmission and compo- nents, ranging from fairly basic [Hec98, ST91] to more advanced [KK97a, KK97b, 42 INTRODUCTION TO OPTICAL NETWORKS Agr97, Agr95, MK88, Lin89]. The 1993 book by Green [Gre93] provides specific coverage of WDM components, transmission, and networking aspects. The historical evolution of transmission systems described here is also covered in a few other places in more detail. [Hec99] is an easily readable book devoted to the early history of fiber optics. [Wil00] is a special issue consisting of papers by many of the optical pioneers providing overviews and historical perspectives of various aspects of lasers, fiber optics, and other component and transmission technologies. [AKW00, Gla00, BKLW00] provide excellent, although Bell Labs-centric, overviews of the historical evolution of optical fiber technology and systems leading up to the current generation of WDM technology and systems. See also [MK88, Lin89]. Kao and Hockham [KH66] were the first to propose using low-loss glass fiber for optical communication. The processes used to fabricate low-loss fiber today were first reported in [KKM70] and refined in [Mac74]. [Sta83, CS83, MT83, Ish83] describe some of the early terrestrial optical fiber transmission systems. [RT84] describes one of the early undersea optical fiber transmission systems. See also [KM98] for a more recent overview. Experiments reporting more than 1 Tb/s transmission over a single fiber were first reported at the Optical Fiber Communication Conference in 1996, and the num- bers are being improved upon constantly. See, for example, [CT98, Ona96, Gna96, Mor96, Yan96]. Recent work on these frontiers has focused on (1) transmitting terabits-per-second aggregate traffic across transoceanic distances with individual channel data rates at 10 or 20 Gb/s [Cai01, Bak01, VPM01], or 40 Gb/s channel rates over shorter distances [Zhu01], or (2) obtaining over 10 Tb/s transmission capacity using 40 Gb/s channel rates over a few hundred kilometers [Fuk01, Big01]. Finally, we didn't cover standards in this chaptermbut we will do so in Chapters 6, 9, and 10. The various standards bodies working on optical networking include the International Telecommunications Union (ITU), the American National Standards Institute (ANSI), the Optical Internetworking Forum (OIF), Internet Engineering Task Force (IETF), and Telcordia Technologies. Appendix C provides a list of relevant standards documents. References [Agr95] G.P. Agrawal. Nonlinear Fiber Optics, 2nd edition. Academic Press, San Diego, CA, 1995. [Agr97] G.P. Agrawal. Fiber-Optic Communication Systems. John Wiley, New York, 1997. [AKW00] R.C. Alferness, H. Kogelnik, and T. H. Wood. The evolution of optical systems: Optics everywhere. Bell Labs Technical Journal, 5(1):188-202, Jan March 2000. References 43 [Alf99] R. Alferness, editor. Bell Labs Technical Journal: Optical Networking, volume 4, Jan Mar. 1999. [Bak01] B. Bakhshi et al. 1 Tb/s (101 • 10 Gb/s) transmission over transpacific distance using 28 nm C-band EDFAs. In OFC 2001 Technical Digest, pages PD21/1-3, 2001. [Bar96] R.A. Barry, editor. IEEE Network: Special Issue on Optical Networks, volume 10, Nov. 1996. [Big01] S. Bigo et al. 10.2 Tb/s (256 x 42.7 Gbit/s PDM/WDM) transmission over 100 km TeraLight fiber with 1.28bit/s/Hz spectral efficiency. In OFC 2001 Technical Digest, pages PD25/1-3, 2001. [BKLW00] W. E Brinkman, T. L. Koch, D. V. Lang, and D. W. Wilt. The lasers behind the communications revolution. Bell Labs Technical Journal, 5(1 ):150-167, Jan March 2000. [Bra89] C.A. Brackett, editor. IEEE Communications Magazine: Special Issue on Lightwave Systems and Components, volume 27, Oct. 1989. [Cai01] J X. Cai et al. 2.4 Tb/s (120 x 20 Gb/s) transmission over transoceanic distance with optimum FEC overhead and 48% spectral efficiency. In OFC 2001 Technical Digest, pages PD20/1-3, 2001. [CHK+96] R.L. Cruz, G. R. Hill, A. L. Kellner, R. Ramaswami, and G. H. Sasaki, editors. IEEE JSAC/JLT Special Issue on Optical Networks, volume 14, June 1996. [CNW90] N.K. Cheung, G. Nosu, and G. Winzer, editors. IEEE JSAC: Special Issue on Dense WDM Networks, volume 8, Aug. 1990. [CS83] J.S. Cook and O. I. Szentisi. North American field trials and early applications in telephony. IEEE JSAC, 1:393-397, 1983. [CSH00] G.K. Chang, K. I. Sato, and D. K. Hunter, editors. 1EEEIOSA Journal of Lightwave Technology: Special Issue on Optical Networks, volume 18, 2000. [CT98] A.R. Chraplyvy and R. W. Tkach. Terabit/second transmission experiments. IEEE Journal of Quantum Electronics, 34(11):2103-2108, 1998. [DL00] S.S. Dixit and R J. Lin, editors. IEEE Communications Magazine: Optical Networks Come of Age, volume 38, Feb. 2000. [DYJ00] S.S. Dixit and A. Yla-Jaaski, editors. IEEE Communications Magazine: WDM Optical Networks: A Reality Check, volume 38, Mar. 2000. [FGO+96] M. Fujiwara, M. S. Goodman, M. J. O'Mahony, O. K. Tonguez, and A. E. Willner, editors. IEEE/OSA JLTIJSA C Special Issue on Multiwavelength Optical Technology and Networks, volume 14, June 1996. 44 INTRODUCTION TO OPTICAL NETWORKS [Fra93] A.G. Fraser. Banquet speech. In Proceedings of Workshop on High-Performance Communication Subsystems, Williamsburg, VA, Sept. 1993. [Fuk01] K. Fukuchi et al. 10.92 Tb/s (273 x 40 Gb/s) triple-band/ultra-dense WDM optical-repeatered transmission experiment. In OFC 2001 Technical Digest, pages PD24/1-3, 2001. [GJR96] P.E. Green, E J. Janniello, and R. Ramaswami. Muitichannel protocol-transparent WDM distance extension using remodulation. IEEE JSA C/JLT Special Issue on Optical Networks, 14(6):962-967, June 1996. [Gla00] A.M. Glass et al. Advances in fiber optics. Bell Labs Technical Journal, 5(1):168-187, Jan March 2000. [GLM+00] O. Gerstel, B. Li, A. McGuire, G. Rouskas, K. Sivalingam, and Z. Zhang, editors. IEEE JSA C" Special Issue on Protocols and Architectures for Next-Generation Optical Networks, Oct. 2000. [Gna96] A.H. Gnauck et al. One terabit/s transmission experiment. In 0FC'96 Technical Digest, 1996. Postdeadline paper PD20. [Gre93] P.E. Green. Fiber-Optic Networks. Prentice Hall, Englewood Cliffs, NJ, 1993. [HD97] G.R. Hill and P. Demeester, editors. IEEE Communications Magazine: Special Issue on Photonic Networks in Europe, volume 35, April 1997. [Hec98] J. Hecht. Understanding Fiber Optics. Prentice Hall, Englewood Cliffs, NJ, 1998. [Hec99] J. Hecht. City of Light: The Story of Fiber Optics. Oxford University Press, New York, 1999. [HSS98] A.M. Hill, A. A. M. Saleh, and K. Sato, editors. IEEE JSAC" Special Issue on High-Capacity Optical Transport Networks, volume 16, Sept. 1998. [Ish83] H. Ishio. Japanese field trials and applications in telephony. IEEE JSAC, 1:404-412, 1983. [KH66] K.C. Kao and G. A. Hockham. Dielectric-fiber surface waveguides for optical frequencies. Proceedings of IEE, 133(3):1151-1158, July 1966. [KK97a] I.P. Kaminow and T. L. Koch, editors. Optical Fiber Telecommunications IIIA. Academic Press, San Diego, CA, 1997. [KK97b] I.P. Kaminow and T. L. Koch, editors. Optical Fiber Telecommunications IIIB. Academic Press, San Diego, CA, 1997. [KKM70] E P. Kapron, D. B. Keck, and R. D. Maurer. Radiation losses in glass optical waveguides. Applied Physics Letters, 17(10):423-425, Nov. 1970. [KLHN93] M.J. Karol, C. Lin, G. Hill, and K. Nosu, editors. IEEE/OSA Journal of Lightwave Technology: Special Issue on Broadband Optical Networks, May/June 1993. References 45 [KM98] E W. Kerfoot and W. C. Marra. Undersea fiber optic networks: Past, present and future. IEEE JSA C" Special Issue on High-Capacity Optical Transport Networks, 16(7):1220-1225, Sept. 1998. [Kra99] J.M. Kraushaar. Fiber Deployment Update: End of Year 1998. Federal Communications Commission, Sept. 1999. Available from http://www.fcc.gov. [Lin89] C. Lin, editor. Optoelectronic Technology and Lightwave Communications Systems. Van Nostrand Reinhold, New York, 1989. [Mac74] J.B. MacChesney et al. Preparation of low-loss optical fibers using simultaneous vapor deposition and fusion. In Proceedings of l Oth International Congress on Glass, volume 6, pages 40-44, Kyoto, Japan, 1974. [MK88] S.D. Miller and I. P. Kaminow, editors. Optical Fiber Telecommunications II. Academic Press, San Diego, CA, 1988. [Mor96] T. Morioka et al. 100 Gb/s x 10 channel OTDM/WDM transmission using a single supercontinuum WDM source. In 0FC'96 Technical Digest, 1996. Postdeadline paper PD21. [MT83] A. Moncalvo and E Tosco. European field trials and early applications in telephony. IEEE JSAC, 1:398-403, 1983. [NO94] K. Nosu and M. J. O'Mahony, editors. IEEE Communications Magazine: Special Issue on Optically Multiplexed Networks, volume 32, Dec. 1994. [Ona96] H. Onaka et al. 1.1 Tb/s WDM transmission over a 150 km 1.3/~m zero-dispersion single-mode fiber. In 0FC'96 Technical Digest, 1996. Postdeadline paper PD19. [Pru89] P.R. Prucnal, editor. IEEE Network: Special Issue on Optical Multiaccess Networks, volume 3, March 1989. [RT84] P.K. Runge and P. R. Trischitta. The SL undersea lightwave system. IEEE/OSA Journal on Lightwave Technology, 2:744-753, 1984. [ST91] B.E.A. Saleh and M. C. Teich. Fundamentals of Photonics. Wiley, New York, 1991. [Sta83] J.R. Stauffer. FT3Cma lightwave system for metropolitan and intercity applications. IEEE JSAC, 1:413-419, 1983. [VPM01] G. Vareille, E Pitel, and J. E Marcerou. 3 Tb/s (300 • 11.6 Gbit/s) transmission over 7380 km using 28 nm C§ with 25 GHz channel spacing and NRZ format. In OFC 2001 Technical Digest, pages PD22/1-3, 2001. [Wil00] A.E. Willner, editor. IEEE Journal of Selected Topics in Quantum Electronics: Millennium Issue, volume 6, Nov./Dec. 2000. 46 INTRODUCTION TO OPTICAL NETWORKS [Yan96] Y. Yano et al. 2.6 Tb/s WDM transmission experiment using optical duobinary coding. In Proceedings of European Conference on Optical Communication, 1996. Postdeadline paper Th.B.3.1. [Zhu01] B. Zhu et al. 3.08 Tb/s (77 x 42.7 Gb/s) transmission over 1200 km of non-zero dispersion-shifted fiber with 100-km spans using C- L-band distributed Raman amplification. In OFC 2001 Technical Digest, pages PD23/1-3, 2001. - Technology This Page Intentionally Left Blank Propagation of Signals in Optical Fiber O PTICAL FIBER IS A REMARKABLE communication medium compared to other media such as copper or free space. An optical fiber provides low-loss trans- mission over an enormous frequency range of at least 25 THz~even higher with special fibers~which is orders of magnitude more than the bandwidth available in copper cables or any other transmission medium. For example, this bandwidth is sufficient to transmit hundreds of millions of phone calls simultaneously, or tens of millions of Web pages per second. The low-loss property allows signals to be transmitted over long distances at high speeds before they need to be amplified or regenerated. It is due to these two properties of low loss and high bandwidth that optical fiber communication systems are so widely used today. As transmission systems evolved to longer distances and higher bit rates, dis- persion became an important limiting factor. Dispersion refers to the phenomenon where different components of the signal travel at different velocities in the fiber. In particular, chromatic dispersion refers to the phenomenon where different frequency (or wavelength) components of the signal travel with different velocities in the fiber. In most situations, dispersion leads to broadening of pulses, and hence pulses cor- responding to adjacent bits interfere with each other. In a communication system, this leads to the overlap of pulses representing adjacent bits. This phenomenon is called Inter-Symbol Interference (ISI). As systems evolved to larger numbers of wave- lengths, and even higher bit rates and distances, nonlinear effects in the fiber began to present serious limitations. As we will see, there is a complex interplay of nonlinear effects with chromatic dispersion. We start this chapter by discussing the basics of light propagation in optical fiber, starting from a simple geometrical optics model to the more general wave 49 . Research activity on optical packet-switched networks and local-area optical networks continues today. Meanwhile, wavelength-routing networks became a major focus area for several researchers. researchers in the early 1990s as people realized the benefits of having an optical layer. Optical add/drop multiplexers and crossconnects are now available as commercial products and are beginning. High-Capacity Optical Transport Networks, volume 16, Sept. 19 98. [Ish83] H. Ishio. Japanese field trials and applications in telephony. IEEE JSAC, 1:40 4-4 12, 1 983 . [KH66] K.C. Kao and G. A.