Research On Adaptive Bias Voltage Control Technique For Mach-Zehnder Modulator

With the continued growth of the information service capacity, the single channel transmission rate of the optical fiber communication system is also increasing. It is one of the mainstream technology solutions to adopt advanced modulation formats and coherent receiving technology to improve the transmission capacity of the optical fiber communication system. For now, Mach-Zehnder modulator(MZM) is a key device of high-speed optical fiber communication system, and the accuracy of its bias voltage will affect the quality of the modulated optical signal. This dissertation mainly focuses on the research of the principle, system design, performance simulation and experimental verification of the adaptive bias voltage control technique for MZM.(1) From the structure and principle of MZM, the sources of bias drift have been analyzed, and the correction formula of MZM output average optical power has been given, which considers two sources of drift including the random phase and half-wave voltage.(2) An adaptive control scheme based on the derivative of MZM output average optical power with respect to the bias voltage has been proposed. By adjusting the bias voltage in the step voltage size, such that the derivative converges to the theoretical value, enabling precise control of the bias voltage. A MZM adaptive bias voltage control circuit has been designed based on the scheme. The noise source and type of the circuit has been analyzed. A method to reduce the influence of circuit noise on the control accuracy of bias voltage has been proposed. Specifically, we can increase the average number of times of MZM output average optical power and the calculated derivative. The influence of the step voltage and average number of times on the control accuracy of bias voltage has been analyzed by simulation. The simulation results indicate that the control accuracy of bias voltage will be improved and the feedback control time will be reduced with the appropriate step voltage and average number of times.(3) A signal generation device has been built based on the adaptive bias voltage control technique for MZM, which includes the generation of on-off keying(OOK) and binary phase shift keying(BPSK). The influence of the step voltage and average number of times on the control accuracy of bias voltage has been studied by experiment. The experimental results reveal that the quality of the optical signal is good when the step voltage is 0.2V, the average numbers of times of MZM output average optical power and the calculated derivative are 200 and 5. In this case, the control accuracy of bias voltage is high and the control feedback time is short.