Research On Auto-Bias-Control Technique For Modulators In CO-OFDM System

With the global information age coming, there has been a significant increase in thedata services of the Internet and telecommunication networks in recent years, and urgentneeds for researching the long-distance, high-speed and high-capacity optical fibertransmission systems. The coherent light OFDM (CO-OFDM) technology, combined withthe advantages of coherent optical detection and optical OFDM technology, is moresuitable for long-distance and high-capacity optical fiber transmission systems.The commonly used IQ modulator, which is used to implement the RTOupconverting, is made from LiNbO3material.Because the LiNbO3crystal is very sensitiveto temperature and other environmental variables, the bias point of the modulatorconstantly drifted, which has a negative impact on the the quality of signal transmissionand the long-term stability of the CO-OFDM systems.In this paper, the working principle of IQ modulator was studied, and a novel schemefor the auto-bias-control of IQ modulator was proposed. The perfect bias point could beachieved by monitoring the1st and2nd harmonic component of the dithering signals,which were added to the bias voltages, and adjusting the biases.Simulations and experiments were completed to verify the effect of the proposedauto-bias-control scheme. The results showed that this scheme was correct and effective,and especially accurate near the optimal bias point; The cost of signal-to-noise ratiointroduced by the dither signals could be ignored.An auto-bias-control system for Optical IQ modulators was realized based on theproposed scheme by using FPGA and micro processor devices.The test result of thissystem showed that it could effectively trace the optimal bias point and keep the biasesstaying at the optimal point for a long term.