Applications Of Polarization Demultiplexing In Stokes Space In Coherent Optical Communications

Coherent optical communication can tolerate fiber dispersion well and expand the capacity of the C-band in fiber while its dynamic range and sensitivity are very high. Therefore, it is very possible for coherent optical communication to be the next generation of optical communication system. Because polarization multiplexing can double the transmission rate and be equalized flexibility in the electric field, it will be used more extensive in the future. However, the two separate and orthogonal polarization states of the polarization multiplexing system can be affected in fiber and generate cross-talk from each other, the signals in the receiver are the superposition of the source in different levels. Therefore, polarization demultiplexing must be applied in the receiver for polarization multiplexing system. Polarization demultiplexing in Stokes space(SS-PDM) could tolerate chromatic dispersion(CD) and polarization dependent loss(PDL). The principle of this method is trying to find an inverse matrix according to the distribution of the signals in Stokes space and applying the inverse matrix to recover the original polarization states. In this paper, we apply SS-PDM to coherent optical systems according to the advantages and characteristics of SS-PDM.1. We first use SS-PDM in coherent optical PDM-OFDM system, which could realize polarization multiplexing without inserting train symbols in the transmitter. The proposed approach performs well for different modulation formats of OFDM subcarrier and reduces overhead. We verify this method in experiment by transmitting 66.6-Gb/s PDM-OFDM signal with 4QAM subcarrier modulation over 5440 km SSMF and 133.3-Gb/s PDM-OFDM signal with 16 QAM subcarrier modulation over 960 km SSMF respectively. We also analyze of the convergence speed of this method which only need tens of symbols.2. We propose a modified method of constant modulus algorithm(CMA) based on polarization demultiplexing in Stokes Space for polarization demultiplexing(SS-PDM) in optical coherent receivers. The singularity problem of CMA is avoided effectively in a much wider range of PDL and its convergence speed is also improved. We demonstrate this modified CMA in a simulation of 100-Gbit/s PDM-QPSK with PDL of different values. We also experimentally demonstrate this method.3. We propose an approach of PDL monitoring and compensation in Stokes space and apply it to CO-OFDM system. PDL monitoring is carried out by computing the mean point of the signal samplesrepresented in the three-dimensional Stokes space. The presented compensation technique is based on rotations and translations of the signal samples in the Stokes space. This method is verified by simulations. For CO-OFDM, We also conduct experiments of 66.6-Gb/s coherent optical PDM-OFDM with 4QAM subcarrier modulation over 5440 km SSMF and 133.3-Gb/s coherent optical PDM-OFDM with 16 QAM subcarrier modulation over 960 km SSMF to demonstrate the PDL compensation.