Polarization-Multiplexed DPSK
A commonly used means to double the spectral efficiency is via PMUX, with a
trade-off of increased complexity at both the transmitter and the receiver. At the
transmitter, two sets of modulators are usually needed to individually modulate the
two polarization states of a polarization-multiplexed DPSK signal. Since the polar-
ization of a signal after fiber transmission is uncertain and time-varying, adaptive
polarization demultiplexing is needed, either through optical means or electronic
means. Furthermore, in WDM transmission, interchannel cross-phase modulation
(XPM) may lead to nonlinear polarization scattering, which causes signal polariza-
tion to vary on a bit-by-bit basis and degrade the performance of adaptive ...