Research On Gain And Cross Point Control In BPSK Optical Modulation System

Due to the high speed and large capacity,optical communication has attracted an increasing amount of attention.At the transmitter of the optical communication,the phase modulation methods of optical signal mainly contain BPSK,DPSK and QPSK,etc.And the phase modulator has been widely used due to its advantages.The RF driving signal voltage of the phase modulator must be controlled so as to keep the output signal voltage of the RF driver always stay at the half-wave voltage of the phase modulator,which ensures the phase modulator to achieve the ideal working condition.As temperature and high frequency continuous wave signal can change the modulation property of electro-optic modulator,the modulated signal duty cycle can be changed.In the eye diagram it shows that the cross eye point is floating up and down,resulting in the system bit error rate increasing.In order to solve the above two problems,the gain of optical phase modulator drive signal and cross point control in BPSK optical modulation system have been studied in this paper.Specific work is as follows: 1)The BPSK optical modulation system is designed and the system components are studied.2)Driving signal of the optical phase modulator making influence on the optical signal is analyzed.The closed-loop adaptive control algorithm is used to control the output voltage of the RF amplifier,making it stabilizing at the optical phase modulator half-wave voltage.And the gain control circuit of the optical phase modulator driving signal is designed.3)The influence of the cross point on the signal is analyzed.The digital PID control algorithm is adopted to control the signal zero-crossing level,which can improve the floating of cross eye point,and the automatic cross point control circuit is also designed in this paper.Finally,the experimental results show that the output voltage of the RF amplifier can be kept at about 5V and the error of the output signal is ±0.05 V.The maximum gain that the system can provide is 28 dB.The automatic cross point control circuit can make the percentage of cross eye stable at around 50%.The feasibility of the gain control and the cross point control schemes have been proved by the experiment.