Polarization Controller Based on Variable-Step Greedy Linear Descent for Self-Homodyne Coherent Transmission Systems

A self-homodyne coherent (SHC) transmission system that has a good prospect in terms of short-reach interconnections can simplify digital signal processing (DSP) and reduce the power consumption of laser diodes. However, the polarization control of the carrier becomes a pivotal part of these systems, and different from the traditional polarization control on a certain state of polarization (SOP), it only needs to lock the two polarization lights after the polarization beam splitter (PBS) in a state of equal power. Half-wave plates or Mach–Zehnder interferometers can accomplish the above goals. In order to evaluate the performance of these polarization control structures in the SHC system, we modeled them on the basis of theoretical analysis. Furthermore, a variable-step greedy linear descent (GLD) algorithm is proposed to solve the power fluctuation problem caused by the accelerated change of SOP near the pole of the Poincaré sphere. The simulation results indicate that the variable-step GLD algorithm can effectively improve the tracking ability of the polarization control loop up to approximately 1.5 times of the GLD algorithm and the gradient descent (GD) algorithm.