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In this paper, the authors study the impact of Quadrature Imbalance (QI) on the performance of optical communications systems using Polarization-Division-Multiplexed (PDM) Quadrature Phase-Shift Keying (QPSK), with coherent detection and digital signal processing. They compare, via simulation, the performance of three QI compensation algorithms, suitable for PDM QPSK coherent optical receivers, including a novel, blind, adaptive, constrained equalizer, based on the constant modulus algorithm. They show that dedicated QI compensation is mandatory and cannot be substituted by conventional adaptive electronic equalizers designed for inter-symbol interference mitigation.
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