Page 271 - Optical Communications Essentials

Performance Impairments

Performance Impairments 261

chromatic and polarization mode dispersions cause optical signal pulses to
broaden as they travel along a fiber. Nonlinear effects occur when there are
high power densities (optical power per cross-sectional area) in a fiber. Their
impact on signal fidelity includes shifting of power between wavelength chan-
nels, appearances of spurious signals at other wavelengths, and decreases in sig-
nal strength. These nonlinear effects can be especially troublesome in high-rate
WDM links.
When any of these dispersion or nonlinear effects contribute to signal impair-
ment, there is a reduction in the signal-to-noise ratio (SNR) of the system from
the ideal case. This reduction in SNR is known as the power penalty for that
effect, which generally is expressed in decibels.
Chromatic dispersion originates from the fact that each wavelength travels at
a slightly different velocity in a fiber. Whether one implements high-speed single-
wavelength or WDM networks, this effect can be mitigated by the use of dispersion-
compensating fiber or a chirped Bragg grating. Polarization mode dispersion
(PMD) arises in single-mode fibers because the two fundamental orthogonal
polarization modes in a fiber travel at slightly different speeds owing to fiber
birefringence. This effect cannot be mitigated easily and can be a very serious
impediment for links operating at 10Gbps and higher.
Two categories of nonlinear effects can place limitations on system perform-
ance. The first category encompasses the nonlinear inelastic scattering
processes. These are stimulated Raman scattering (SRS) and stimulated
Brillouin scattering (SBS). The second category of nonlinear effects arises from
intensity-dependent variations in the refractive index in a silica fiber. This pro-
duces effects such as self-phase modulation (SPM), cross-phase modulation
(XPM), and four-wave mixing (FWM). Table 15.1 gives a summary of these
effects.

Further Reading

1. Telcordia, SONET Transport Systems, Common Generic Criteria, GR-253-CORE, Piscataway,
N.J., September 2000.
2. L. Grüner-Nielsen, S. N. Knudsen, B. Edvold, T. Veng, D. Magnussen, C. C. Larsen, and
H. Damsgaard, “Dispersion compensating fibers,” Optical Fiber Technology, vol. 6, pp. 164–180,
2000.
3. M. Karlsson, J. Brentel, and P. A. Andrekson, “Long-term measurement of PMD and polarization
drift in installed fibers,” J. Lightwave Technology, vol. 18, pp. 941–951, July 2000.
4. H. Sunnerud, C. Xie, M. Karlsson, R. Samuelsson, and P. A. Andrekson, “A comparison between
different PMD compensation techniques,” J. Lightwave Technology, vol. 20, pp. 368–378, March
2002.
5. F. Forghieri, R. W. Tkach, and A. R. Chraplyvy, “Fiber nonlinearities and their impact on trans-
mission systems,” Chap. 8, pp. 196–264, in I. P. Kaminow and T. L. Koch, eds., Optical Fiber
Telecommunications—III, vol. A, Academic, New York, 1997.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.