318 CHAPTER 7. DISPERSION MANAGEMENT
(a) (b)Figure 7.20: Schematic illustration of (a) optical and (b) electrical PMD compensators. (After
Ref. [210];©c2000 Elsevier; reprinted with permission.)
Consider a few relevant examples. In the case of “old” fiber links installed us-
ing standard fibers, the condition (7.9.7) becomesB^2 L< 104 (Gb/s)^2 -km if we use
Dp=1 ps/
√
km as a representative value. Such fibers require PMD compensation at
B=10 Gb/s if the link length exceeds even 100 km. In contrast, modern fibers have
typicallyDp< 0 .1 ps/
√
km. For systems designed using such fibers,B^2 Lcan exceed
106 (Gb/s)^2 -km. As a result, PMD compensation is not necessary at 10 Gb/s but may
be required at 40 Gb/s if the link length exceeds 600 km. It should be stressed that these
numbers represent only an order-of-magnitude estimate. A more accurate estimate can
be obtained following the PMD theory developed in recent years [192]–[195].
The preceding discussion shows that PMD limits the system performance when the
single-channel bit rate is extended to beyond 10 Gb/s. Several techniques have been
developed for PMD compensation in dispersion-managed lightwave systems [197]–
[214]; they can be classified as being optical or electrical. Figure 7.20 shows the basic
idea behind the electrical and optical PMD compensation schemes. An electrical PMD
equalizer corrects for the PMD effects within the receiver using a transversal filter. The
filter splits the electrical signalx(t)into a number of branches using multiple tapped
delay lines and then combines the output as
y(t)=N− 1∑
m= 0cmx(t−mτ), (7.9.8)whereNis the total number of taps,τis the delay time, andcmis the tap weight for
themth tap. Tap weights are adjusted in a dynamic fashion using a control algorithm
in such a way that the system performance is improved [210]. The error signal for the
control electronics is often based on the closing of the “eye” at the receiver. Such an
electrical technique cannot eliminate the PMD effects completely as it does not con-
sider the PMD-induced delay between the two PSPs. On the positive side, it corrects
for all sources of degradation that lead to eye closing.
An optical PMD compensator also makes use of a delay line. It can be inserted pe-
riodically all along the fiber link (at the amplifier locations, for example) or just before
the receiver. Typically, the PMD-distorted signal is separated into two components
along the PSPs using a polarization controller (PC) followed by a polarization beam
splitter; the two components are combined after introducing an adjustable delay in one
branch through a variable delay line (see Fig. 7.20). A feedback loop is still needed
to obtain an error signal that is used to adjust the polarization controller in response to
the environmental changes in the fiber PSPs. The success of this technique depends on