Experimental Demonstration of Polarization-Dependent Loss Monitoring and Compensation in Stokes Space for Coherent Optical PDM-OFDM

We have recently demonstrated polarization demultiplexing in Stokes space (SS-PDM), which is susceptible to polarization-dependent loss (PDL). In this paper, we apply the method of PDL monitoring and compensation in Stokes space to enhance SS-PDM. It is applied for each subcarrier in the frequency domain after SS-PDM. Because each subcarrier has a much lower speed and narrower bandwidth, the polarization effects that it experiences can be treated as flat. This method can work for different modulation formats on orthogonal frequency-division multiplexing (OFDM) subcarriers and only needs a small number of samples for convergence. In simulation, when the PDL is less than 3.5 dB, it significantly improves the performance of SS-PDM and is comparable with the conventional channel estimation based on training symbols (TSs). We also conduct the experiments of 66.6-Gb/s coherent optical PDM-OFDM with 4QAM subcarrier modulation over 5440-km SSMF and 133.3-Gb/s coherent optical PDM-OFDM with 16QAM subcarrier modulation over 960-km SSMF, respectively. The experimental results show that this method improves the performance of SS-PDM significantly and can match the performance of channel estimation based on TSs. To the best of our knowledge, this is the first experimental validation of PDL monitoring and compensation in Stokes space.