Mach-Zehnder modulator bias controller based on power and harmonics synchronous detection

In order to overcome the drift of Mach-Zehnder(MZ) modulator operating point by the temperature, pressure and other factors, it is necessary to compensate the voltage shift of DC bias. The previous harmonic detection method based on small dither signal has the advantage of high bias point positioning accuracy. However, a certain noise will be introduced due to the additional small signal in the loop, which will affect the purity of the radio frequency signal. The power detection working point bias method is easily influenced by the power vibration of light source, which reduces the stability and positioning accuracy of the bias point. In this paper, a MZ modulator bias point control technique was proposed, combining the average power and the harmonic detection. The optimal operating point was determined by the harmonic detection, and the ratio of the optical power before and after the electro-optic modulator was used as the reference value in feedback control. It improves the positioning accuracy and the noise performance of the bias control. Meanwhile, it overcomes the influence of the disturbance of laser power. In this method, a scheme realizing the arbitrary adjustment of bias points by controlling the ratio of optical power was also proposed and large range flexible bias point control was realized. Experimental results show that the proposed scheme has the advantages of high positioning accuracy, good stability and wide bias point, which meets well with the requirements of the MZ modulator bias point control.