Page 257 - Optical Communications Essentials

Source: Optical Communications Essentials

Chapter
15

Performance Impairments

The information-carrying capacity of a fiber is limited by various internal distor-
tion mechanisms, such as signal dispersion factors and nonlinear effects. The two
main distortion categories for high-performance single-mode fibers are chromatic
and polarization mode dispersions, which cause optical signal pulses to broaden as
they travel along a fiber. Nonlinear effects occur when there are high power den-
sities (optical power per cross-sectional area) in a fiber. Their impact on signal
fidelity includes shifting of power between wavelength channels, appearances of
spurious signals at other wavelengths, and decreases in signal strength. These
nonlinear effects can be especially troublesome in high-rate WDM links.
Important challenges in designing such multiple-channel optical networks include

■ Transmission of the different wavelength channels at the highest possible bit
rate
■ Transmission over the longest possible distance with the smallest number of
optical amplifiers
■ Network architectures that allow simple and efficient network operation, con-
trol, and management

15.1. Impairment Effects
The design of a WDM network requires the use of proven design practices when
one is selecting optical fibers, specifying component types and the values of
their operating parameters, and laying out networks to mitigate performance
degradation processes. Signal impairment effects that are inherent in optical
fiber transmission links and which can seriously degrade network performance
include these:
■ Dispersion, which refers to any effect that causes different components of a
transmitted signal to travel at different velocities in an optical fiber. The
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