504 CHAPTER 10. COHERENT LIGHTWAVE SYSTEMS
Figure 10.13: Four-port coherent DPSK receiver employing both phase and polarization diver-
sity. (After Ref. [80];©c1987 IEE; reprinted with permission.)
The most commonly used approach solves the polarization problem by using a
two-port receiver, similar to that shown in Fig. 10.11, with the difference that the
two branches process orthogonal polarization components. Such receivers are called
polarization-diversity receivers[78]–[82] as their operation is independent of the polar-
ization state of the signal received. The polarization-control problem has been studied
extensively because of its importance for coherent lightwave systems [83]–[90].
Figure 10.12 shows the block diagram of a polarization-diversity receiver. A polar-
ization beam splitter is used to separate the orthogonally polarized components which
are processed by separate branches of the two-port receiver. When the photocurrents
generated in the two branches are squared and added, the electrical signal becomes
polarization independent. The power penalty incurred in following this technique de-
pends on the modulation and demodulation techniques used by the receiver. In the
case of synchronous demodulation, the power penalty can be as large as 3 dB [85].
However, the penalty is only 0.4–0.6 dB for optimized asynchronous receivers [78].
The technique of polarization diversity can be combined with phase diversity to
realize a receiver that is independent of both phase and polarization fluctuations of
the signal received [91]. Figure 10.13 shows such a four-port receiver having four
branches, each with its own photodetector. The performance of such receivers would
be limited by the intensity noise of the local oscillator, as discussed in Section 10.5.2.
The next step consists of designing a balanced phase- and polarization-diversity re-
ceiver by using eight branches with their own photodetectors. Such a receiver has been
demonstrated using a compact bulk optical hybrid [92]. In practical coherent systems, a
balanced, polarization-diversity receiver is used in combination with narrow-linewidth
lasers to simplify the receiver design, yet avoid the limitations imposed by intensity
noise and polarization fluctuations.
10.5.4 Fiber Dispersion
Section 5.4 discussed how fiber dispersion limits the bit-rate–distance product (BL)of
direct-detection (IM/DD) systems. Fiber dispersion also affects the performance of