I. Introduction
With the widely deployment of the dense wavelength division multiplexing (DWDM) system, the wavelength selection switch (WSS), which caused large polarization dependent loss (PDL), is widely introduced in the polarization division multiplexing (PDM) systems. PDL leads to communication system optical signal-to-noise ratio (OSNR) penalty and there is no feasible solution to compensate for or mitigate the PDL effect, resulting in PDL becoming one of the major factors affecting the performance of PDM systems.