Training Symbol Assisted in-Band OSNR Monitoring Technique for PDM-CO-OFDM System

We proposed and demonstrated an in-band optical signal-to-noise ratio (OSNR) monitoring method for a polarization-division-multiplexed coherent optical orthogonal frequency-division-multiplexing (PDM-CO-OFDM) system. OSNR of the PDM signals on both polarization tributaries can be simultaneously monitored by calculating the electrical SNR with low complexity by taking advantage of the uncorrelation property between the noise and training symbol, which is used for conventional frame synchronization. This OSNR monitoring technique is verified in 103.57-Gb/s 16 quadrature amplitude modulation-based PDM CO-OFDM transmission system with 240-1440-km standard single-mode fiber (SSMF) links. Experimental results show that after calibration, the OSNR estimation error is less than 0.6 dB in a dynamic range of 12-27 dB, the estimation remains accurate even after 1440-km SSMF transmission, regardless of the chromatic dispersion and polarization mode dispersion in the fiber. To further investigate the robustness of the proposed OSNR monitoring method, additional simulation is implemented on VPI and MATLAB platform. It turned out to be insensitive to nonlinear effect within reasonable range as our monitoring technique shows good performance with launching power below -2 dBm after 500-km SSMF transmission. The proposed technique could still accurately monitor the individual OSNR of each polarization tributary of the PDM signal even in the presence of 5-dB polarization-dependent loss.