Electronic Polarization Control Algorithms for Coherent Optical Transmission

In this paper, we review the nondata-aided constant-modulus algorithm (CMA) and a data-aided decision-directed algorithm (DDA) for polarization control and propose different extensions to both algorithms to improve their performance. The first extension to the CMA enables a common carrier recovery (CCR) through differential phase compensation (DPC-CMA). The second extension adapts the CMA for quadrature amplitude modulation signals (CMA-QAM). Both extensions can be combined to form a DPC-CMA for QAM signals (DPC-CMA-QAM). A new, modified DDA (MDDA) considerably increases polarization tracking speeds compared to the original DDA (ODDA). It is also usable for QAM signals. The algorithms are compared in simulations of QPSK and 16-QAM transmission systems. The results show that the DPC extension for the CMA in combination with CCR doubles laser linewidth tolerance and also the CMA-QAM triples polarization control speed compared to the standard CMA for QAM signals. The MDDA is similar to 1.6-4 times faster than the CMA variants and is, at least when QAM signals are transmitted, more hardware-efficient.