Analysis And Compensation Methods Of Phase Noise For Optical Modulation System Based On DPSK

Free-space optical communication is the research focus and frontier in communication field at present.Among numerous modulation formats which are suitable for free space optical communication system,DPSK optical modulation format is of the most attention.However,the DPSK optical modulation pattern uses phase to carry the information which is very sensitive to phase noise,signal phase fluctuation made by phase noise will make information distortion,limit the transmission distance.This paper analyzes the main factors that produce phase noise in DPSK optical modulation system and make the compensation,so that the phase noise in the system is reduced and the emission performance is improved.According to the modulation principle of DPSK optical signal,the following work is completed:(1)Design the free-space optical communication DPSK modulation system,and verify the feasibility of the system under the ideal condition by Optisystem which is the optical simulation software;(2)Analyze the main factors of the phase noise in the system,they are as follows: semiconductor laser line width broadening;semiconductor laser frequency instability;phase modulator half-wave voltage varies with temperature;(3)Study the phase noise compensation methods of the system.Using blazed grating external cavity feedback line width compensation technology,extending the effective cavity length of the semiconductor laser and suppressing the spontaneous emission,reduce the linewidth of the laser,compensating the phase noise caused by the line width broadening of the semiconductor laser;using acousto-optic frequency-shifting modulation-free frequency locking technology,stabilize the laser frequency by controlling the effective cavity length of the laser,compensating the phase noise produced by the frequency drift of the semiconductor laser;using digital PID half-wave voltage compensation technology,controlling the driving voltage of the modulator,so that the peak-to-peak value is always equal to the modulator half-wave voltage,compensating the phase noise caused by the half-wave voltage drift with temperature of the phase modulator;(4)Verify the feasibility of the system phase noise compensation technology through the simulation and experiment,analyze the overall performance of the system by comparing the output signal waveform and eye diagram.Experimental results show,after compensating for the main source of phase noise in the system,the stability of the output signal in system has been improved,the bit error rate got better,the launch performance of system has been greatly improved,these show the correctness of this design.