Abstract
Spectrally-efficient optical communications systems employ polarization division
multiplexing (PDM) as a practical solution, in order to double the capacity of a fiber
link. Polarization demultiplexing can be performed electronically, using
polarization-diversity coherent optical receivers. The primary goal of this paper is the
optimal design, using the maximum-likelihood criterion, of polarization-diversity
coherent optical receivers for polarization-multiplexed optical signals, in the absence
of polarization mode dispersion (PMD). It is shown that simultaneous joint estimation of
the symbols, over the two received states of polarization, yields optimal performance,
in the absence of phase noise and intermediate frequency offset. In contrast, the
commonly used zero-forcing polarization demultiplexer, followed by individual
demodulation of the polarization-multiplexed tributaries, exhibits inferior performance,
and becomes optimal only if the channel transfer matrix is unitary, e.g., in the absence
of polarization dependent loss (PDL), and if the noise components at the polarization
diversity branches have equal variances. In this special case, the zero-forcing
polarization demultiplexer can be implemented by a 2$\,\times\,$2 lattice adaptive
filter, which is controlled by only two independent real parameters. These parameters
can be computed recursively using the constant modulus algorithm (CMA). We evaluate, by
simulation, the performance of the aforementioned zero-forcing polarization
demultiplexer in coherent optical communication systems using PDM quadrature phase shift
keying (QPSK) signals. We show that it is, by far, superior, in terms of convergence
accuracy and speed, compared to conventional CMA-based polarization demultiplexers.
Finally, we experimentally test the robustness of the proposed constrained CMA
polarization demultiplexer to realistic imperfections of polarization-diversity coherent
optical receivers. The PMD and PDL tolerance of the proposed demultiplexer can be used
as a benchmark in order to compare the performance of more sophisticated adaptive
electronic PMD/PDL equalizers.
© 2010 IEEE
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