Research On Modulation Schemes And Automatic Polarization Control Technology In Coherent Optical Communication System

Compared with the system using intensity modulation with direct detection, coherent optical system presents many remarkable features such as higher sensitivity, better frequency-selectivity, and multi-level modulation formats applications. The multi-level modulation formats and polarization control, which are key techniques in the coherent optical communication system, relate firmly to the system performance. The phase diversity reception of the amplitude modulation signal, the application of quadrature amplitude modulation (QAM) signal in the coherent optical system, and the automatic polarization control in the coherent optical system are studied in this thesis. The works in the dissertation include1. The scheme of homodyne phase diversity reception for amplitude modulation signal is proposed in the dissertation. In the theoretical analysis, it is proved that this technology enables the elimination of phase noise and its sensitivity is better than that of heterodyne detection. Based on the scheme, a 2.5Gbit/s pseudo random binary sequence (PRBS) received successfully. The sensitivity is -24.9dBm when the bit-error-rate (BER) is 10^-9.2. A novel coherent heterodyne system for square QAM is proposed. In the experiment, a 2.5Gbit/s PRBS with the intermediate frequency (IF) of 10GHz is transmitted. The sensitivity is -21.4dBm when the bit-error-rate (BER) is 10^-8 . Based on this scheme, a coherent QAM subcarrier multiplexing system is presented. The theoretical demonstration and simulation are given for the analysis of the spectral efficiency and tolerance of linewidth. The simulation result shows its spectral efficiency achieve more than 0.8bit/s/Hz.3. An automatic polarization controller technology used in the coherent optical communication system is presented in the dissertation. The random polarization of received optical signal can be transformed into a steady fixed linear polarization in 10 seconds. After the optimization of the algorithm, the time can be limited in 4 seconds. The circuit and algorithm designs are detailed introduced in the dissertation. The tiny fluctuation of polarization can be eliminated in several milliseconds.