Optical 8DPSK Modulation Principle And Simulation Of PMD Compensation

Recently, the demand of transmission rate is rising rapidly. The solution of increasing data bandwidth is DWDM (Optical Dense Wavelength Division Multiplexing), but people need higher bandwidth for single channel transmission. So far, it is to use narrower laser pulse to gain higher bandwidth, but this solution is limited. When laser pulse is very narrow, it will be extremely serious SPM (Self Phase Modulation) side effect. And when optical power amplification, the chirp effect will decrease Bit-Error-Rate badly. As a result, many kinds of novel multi-phase modulation format are put forward, such as the hottest format DPSK (Differential Phase-Shift Keying), DQPSK (Differential Quartered Phase Shift Keying), 8DPSK (Differential 8-Phase Shift Keying ), 16DPSK, and Star QAM. From equation R_i = R_slog₂M (R_i is bit rate, R_s is symbol rate, M is modulation phase number), DQPSK can support 2 times the data bandwidth than DPSK as the same optical pulse rate, and 8DPSK can support 3 times data bandwidth. The entire above format must be considered of high optical signal-noise-ratio, tolerance of dispersion, and robustness of SPM. This thesis describes the principle of phase-shift modulation and amplification-shift modulation, modulation and demodulation structures of DQPSK and 8DPSK in detail. With an example, we verified the modulation and demodulation principle of 8DPSK.In optical transmission, the reasons of shortening transmitting length are power attenuation which caused by Rayleigh scattering, and optical dispersion. Chromatic dispersion is perfectly solved, but when single channel rate reaches 10Gbit/s, especially 40Gbit/s, the impairment of Polarization Mode Dispersion (PMD) will emerge obviously. For example, in old optical fiber the coefficient of PMD is 0.5ps/(?), optical signal can transmit only 25km. We simulated 2-stage PMD adaptive compensation based on using Degree of Polarization (DOP) as feedback signal. The simulation system uses Particle Swarm Optimization (PSO) search algorithm to narrow and stalk the PMD signal in transmission, and controls Polarization Controller (PC). It realizes the PMD adaptive compensation. For illustrating the impairment of PMD and the result of PMD compensation, we choose 15 symbol, plot 8DPSK optical power diagram, phase diagram, and electrical power diagram after demodulation which are before and after PMD channel. And we plot DGD distribute diagram before and after PMD channel, and analysis the tolerance of all kinds of NRZ-8DPSK and RZ-8DPSK format in PMD channel and the result of PMD compensation.