Using the Stokes space for equalization of polarization impairments in digital coherent optical receivers

Random polarization rotations caused by residual fiber birefringence is an important physical effect in optical coherent systems employing polarization-division multiplexing. The specificity of the large penalties induced by polarization-related impairments make polarization-dedicated equalization subsystems mandatory for the design of flexible coherent receivers. It has recently been shown that 3-dimensional Stokes space based approaches can improve polarization demultiplexing and polarization dependent losses compensation task in coherent receivers, particularly in terms of convergence speed and transparency to higher-level M-ary signals. This paper reports recent advances on Stokes space based digital signal processing for coherent optical communications. The performance versus computation efforts of different methods used to compute a symmetry plane in the Stokes space, e.g. the geometric approach and Kalman filtering, is assessed for both polarization-dependent losses compensation and PolDemux subsystems. The applicability and robustness of the adaptive Stokes algorithm are tested in a scenario based on an ultra-dense WDM access network. Experimental results demonstrate the suitableness of Stokes PolDemux solution for metro and access networks with flexible transceivers based on higher-level M-ary signals.