Page 258 - Optical Communications Essentials

Performance Impairments

248 Chapter Fifteen

result is that a pulse spreads out progressively as it travels along a link. This
spreading leads to interference between adjacent pulses (called intersymbol
interference) and may limit the distance a pulse can travel. The three main
types of dispersion are modal dispersion, chromatic dispersion, and polariza-
tion mode dispersion.
Modal dispersion occurs only in multimode fibers in which each mode travels
at a different velocity. Multimode fibers are used mainly for short-distance
communications (such as local-area networks) since modal dispersion
limits the link length. See Chap. 4 for more details on this effect.
Chromatic dispersion originates from the fact that each wavelength travels at
a slightly different velocity in a fiber. Two factors contribute to chromatic
dispersion, as Sec. 15.2 describes. Whether one implements high-speed
single-wavelength or WDM networks, this effect can be mitigated by the
use of various dispersion compensation schemes.
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.
■ Nonlinear effects, which arise at high power levels because both the attenu-
ation and the refractive index depend on the optical power in a fiber. The
nonlinear processes can be classified into the following two categories:
Nonlinear inelastic scattering processes, which are interactions between
optical signals and molecular or acoustic vibrations in a fiber
Nonlinear variations of the refractive index in a silica fiber that occur because
the refractive index is dependent on intensity changes in the signal
■ Nonuniform gain across the desired wavelength range of optical amplifiers
in WDM links. This characteristic can be equalized over the desired wave-
length range by techniques such as the use of grating filters or variable opti-
cal attenuators (VOAs), as described in Chap. 10.
■ Reflections from splices and connectors that can cause instabilities in laser
sources. These can be eliminated by the use of optical isolators.

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.

15.2. Chromatic Dispersion
The index of refraction of silica varies with wavelength; for example, it ranges
from 1.453 at 850nm to 1.445 at 1550nm. Furthermore, as described in Chap. 6,
a light pulse from an optical source contains a certain slice of wavelength
spectrum. For example, a modulated laser diode source may emit pulses that

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