10.5. SENSITIVITY DEGRADATION 503
Figure 10.12: Schematic of a polarization-diversity coherent receiver.and affects the receiver performance. In particular, if the polarization states ofEsand
ELOare orthogonal to each other (θ= 90 ◦), the signal disappears (complete fading).
Any change inθaffects the BER through changes in the receiver current and SNR.
The polarization state ˆeLOof the local oscillator is determined by the laser and re-
mains fixed. This is also the case for the transmitted signal before it is launched into
the fiber. However, at the fiber output, the polarization state ˆesof the signal received
differs from that of the signal transmitted because of fiber birefringence, as discussed in
Section 2.2.3 in the context of single-mode fibers. Such a change would not be a prob-
lem if ˆesremained constant with time because one could match it with ˆeLOby simple
optical techniques. The source of the problem lies in the polarization-mode dispersion
(PMD) or the fact that ˆeschanges randomly in most fibers because of birefringence
fluctuations related to environmental changes (nonuniform stress, temperature varia-
tions, etc.). Such changes occur on a time scale ranging from seconds to microseconds.
They lead to random changes in the BER and render coherent receivers unusable unless
some scheme is devised to make the BER independent of polarization fluctuations. Al-
though polarization fluctuations do not occur in polarization-maintaining fibers, such
fibers are not used in practice because they are difficult to work with and have higher
losses than those of conventional fibers. Thus, a different solution to the polarization-
mismatch problem is required.
Several schemes have been developed for solving the polarization-mismatch prob-
lem [72]–[77]. In one scheme [72], the polarization state of the optical signal received
is tracked electronically and a feedback-control technique is used to match ˆeLOwith
eˆs. In another, polarization scrambling or spreading is used to force ˆesto change ran-
domly during a bit period [73]–[76]. Rapid changes of ˆesare less of a problem than
slow changes because, on average, the same power is received during each bit. A third
scheme makes use of optical phase conjugation to solve the polarization problem [77].
The phase-conjugated signal can be generated inside a dispersion-shifted fiber through
four-wave mixing (see Section 7.7). The pump laser used for four-wave mixing can
also play the role of the local oscillator. The resulting photocurrent has a frequency
component at twice the pump-signal detuning that can be used for recovering the bit
stream.