Compensation for In-Phase/Quadrature Imbalance in Coherent-Receiver Front End for Optical Quadrature Amplitude Modulation

We propose a novel method of compensation for imbalance between in-phase (I) and quadrature (Q) channels in the front-end circuit of digital coherent optical receivers. Adaptive finite-impulse-response (FIR) filters in the butterfly configuration, which are commonly used for signal equalization and polarization demultiplexing, are modified so as to allow for adjustment of any imbalance between the IQ channels. IQ imbalances under consideration include the gain mismatch, the phase mismatch, and the timing-delay skew. Computer simulations for the dual-polarization quadrature-amplitude-modulation (QAM) format up to an order of 256 show that such IQ imbalances can severely degrade the system performance, especially for higher order QAM; however, using the proposed scheme, we can compensate for them without any significant penalty over a wide range of imbalances.