Robust PDL Compensation and Monitoring Scheme Using Frequency Domain Pilot Tones for Coherent Digital Subcarrier Multiplexing System

A robust digital polarization-dependent loss (PDL) compensation and monitoring scheme for the coherent digital subcarrier multiplexing (DSCM) system using frequency domain pilot tone (FPT) is proposed and demonstrated. In the transmitted polarization multiplexed DSCM signal, two FPTs with different and asymmetric frequencies are inserted into the X and Y polarizations, respectively. The polarization-dependent matrix, including polarization variation and PDL, can be obtained from the inserted FPTs in the digital signal processing (DSP) at the receiver side. The proposed PDL compensation technique avoids the singularity problem in conventional adaptive equalization schemes and achieves performance close to that of ideal polarization compensation, where the inverse of the channel matrix is used directly to compensate for the received signal. In addition, the proposed scheme can still track polarization variation up to 10 Mrad/s in the presence of 3-dB PDL. For the PDL monitoring, the proposed scheme is insensitive to chromatic dispersion (CD) and shows strong robustness to amplification of spontaneous emission noise (ASE), polarization variation and transmitter (Tx) in-phase/quadrature (IQ) impairments. Both simulation and experimental results indicate that the mean estimation error is less than 0.26 dB for PDL larger than 2 dB in the presence of severe ASE (the resulting BER is about 1e-1). When the CD varies from 0 to 200 km fiber transmission, or the polarization variation rate varies from 0 to 10 Mrad/s, there is no significant deterioration in the mean estimation error and variance of the PDL estimation. Even with Tx IQ skew of 20 ps, IQ phase imbalance of 40 $^{\circ }$ and IQ amplitude imbalance of 4 dB in both polarizations, an estimation error less than 0.5 dB can be achieved for PDL ranging from 1 to 10 dB. Therefore, the proposed scheme provides a robust PDL compensation and monitoring solution for coherent DSCM system.