Development and evaluation of a digital signal processing for single polarization QPSK modulation format

Single polarization high-speed optical transmission is important for bidirectional free-space optical communication system in order to have enough isolation up-link and down-link by signal discrimination using orthogonal polarization states. At recent advance in digital coherent technology, polarization re-combining in combination with polarization diversity receiver is widely used to suppress performance degradation when polarization states of signal and local oscillator are misaligned by system vibration or shocks. However, in order to implement the re-combining function in digital signal processing, appropriate algorithm is required for realizing the system stability. In this paper, we demonstrate a new algorithm implementation for single polarization receiver with maximal-ratio-combining (MRC) technique. First we exhibited the problem for state-of-the-art polarization re-combining in instability due to singularity condition at 45-degree-azimuth elliptic polarization. In order to overcome this problem, we proposed a newly MRC algorithm added splitting ratio dependent phase correction coefficients and achieved stable re-combining at 45-degree-azimuth elliptic polarization signal. And we successfully demonstrated the stable receiving for 50-Gb/s single polarization QPSK signal with all polarization states by our digital coherent receiver platforms added the newly MRC algorithm, compared with previous-proposed MRC algorithm.