Quadrature imbalance compensation techniques for DP-QPSK coherent systems

In this paper, four quadrature imbalance (QI) compensation schemes for coherent DP-QPSK optical systems are compared: the Gram-Schmidt Orthogonalization Procedure (GSOP) and Ellipse Fitting methods, applied both before and after electronic polarization demultiplexing. Using extensive Monte-Carlo simulations, we assessed the performance of the four schemes in the presence of optical front-end imperfections, and at a 45° angle between the orientations of the received polarization multiplexed signals and of the local oscillator (maximum mixture). It is shown that, in a small scale of front-end imperfections, the four methods efficiently compensate for QIs. However, when this scale increases, using the GSOP before polarization demultiplexing exhibits the lowest penalty for the system.