Page 238 - Optical Communications Essentials

Constructing the WDM Network Puzzle

228 Chapter Thirteen

wavelength travels at a slightly different velocity through the fiber, the pulse
progressively spreads out and eventually causes neighboring pulses to overlap
and interfere. This dispersion issue becomes greater with increasing data rate
and longer distances. Various static and dynamic dispersion compensation
methods are in use to mitigate these interference effects including dispersion-
compensating fiber and tunable fiber Bragg gratings. Chapter 15 presents more
details on chromatic dispersion compensation in WDM systems.

13.2.9. Polarization mode dispersion compensation
Polarization mode dispersion (PMD) results from the fact that light signal
energy at a given wavelength in a single-mode fiber actually occupies two
orthogonal polarization states or modes (see Fig. 4.10). At the start of the fiber
the two polarization states are aligned. However, fiber material is not perfectly
uniform throughout its length. In particular, the refractive index varies slightly
across any given cross-sectional area, which is known as the birefringence of the
material. Consequently, each polarization mode will encounter a slightly differ-
ent refractive index, so that each will travel at a slightly different velocity. The
resulting difference in propagation times between the two orthogonal polariza-
tion modes will cause pulse spreading. This is the basis of polarization mode dis-
persion. PMD is not a fixed quantity but fluctuates with time due to factors
such as temperature variations and stress changes on the fiber. It varies as the
square root of distance and thus is specified as a maximum value in units of
ps/ k m . A typical value is D PMD 0.05ps/ k m . Since PMD is a statistically
varying parameter, it is more difficult to control than chromatic dispersion,
which has a fixed value. Chapter 15 presents more details on PMD compensation.

13.3. WDM Network Applications
WDM systems are the traditional commercial choice to alleviate traffic conges-
tion by increasing the bandwidth of existing fiber optic backbones. An import-
ant point is that WDM networks are bit-rate- and protocol-independent, which
means they can carry various types of traffic at different speeds concurrently.
This section gives some examples of DWDM and CWDM networks.

13.3.1. DWDM networks
Dense WDM enables large channel counts within a limited spectral band, such
as the C-band, but can be expensive to implement. However, DWDM is cost-
effective in long-haul transport networks and large metro rings. In these cases
the cost is justified since it is distributed over many high-capacity long-distance
channels.
Figure 13.11 shows a generic long-haul DWDM network. Such networks typ-
ically are configured as large rings in order to offer reliability and survivability
features. For example, if there is cable cut somewhere, the traffic that was sup-
posed to pass through that fault can be routed in the opposite direction on the

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