Quadrature Imbalance Compensation for PDM QPSK Coherent Optical Systems

In this letter, we 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. We 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. We show that dedicated QI compensation is mandatory and cannot be substituted by conventional adaptive electronic equalizers designed for intersymbol interference mitigation.