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
.
[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.
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Technology
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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