An Automatic Bias Voltage Control Technique for High-order Modulation Formats

In order to maintain the stability of the transmitter to ensure the quality of the output signal, we propose a novel technique of automatic bias control based on the pilot signal that can be used in high-order modulation formats. The proposed scheme requires loading a pair of orthogonal pilot signals to the modulator and tapping the frequency-shifted local oscillation light with the signal light output from the modulator to achieve coherent detection. The control of the bias voltage is achieved by means of correlation operation and monitoring the value of the correlation coefficient. This scheme combines coherent detection and correlation detection techniques, which can effectively improve the sensitivity and control accuracy of the system. The feasibility of this scheme in high-order modulation systems and the improvement of sensitivity are also fully verified by simulation.