Polarization-Fading-Free Phase Recovery and Robust RSOP Tracking Using Frequency-Domain Pilot Tones in Optical DSCM Systems

In this letter, we propose a novel cascaded architecture based on frequency-domain pilot tones (FPTs) to achieve joint frequency offset estimation (FOE), carrier phase estimation (CPE) and rotation of state of polarization (RSOP) tracking in optical coherent digital subcarrier multiplexing (DSCM) systems. At the transmitter side, two FPTs are inserted at different frequencies in two polarizations respectively. At the receiver side, the power spectra of the four FPTs polarization crosstalk-induced are combined for polarization-fading-free FOE. Then, the four FPTs are employed together to cancel the effect of polarization on the FPTs’ power, achieving accurate and polarization-fading-free CPE. This avoids the estimated phase being severely impaired by noise when using a single low-power FPT for CPE. Subsequently, the FPTs are employed to estimate the Jones matrix for polarization demultiplexing. Simulation results demonstrate that the proposed scheme significantly reduces the root-mean-square error (RMSE) of phase estimation at an optical signal-to-noise ratio (OSNR) of 25 dB thanks to canceling the impact of polarization fading on CPE. Due to the accurate CPE, the proposed scheme achieves a Q-factor improvement of 0.9 dB compared to existing schemes. Experimental results further confirm that the proposed scheme reduces a 0.7 dB penalty in received optical power (ROP). The proposed cascaded carrier phase and polarization tracking scheme provides an efficient technical solution for robust carrier phase estimation and polarization tracking in future high-speed, high-order modulation optical communication systems.