Design of PDH frequency tracking system based on system identification and a novel PID control scheme

This article proposes a new high-precision PDH frequency tracking scheme based on system identification theory and combined with a new PID servo system. In traditional PDH scheme, the resonant frequency of the resonant cavity is often used as the reference, and a PID servo circuit is used to drive the PZT actuator of the laser to track the resonant cavity, and the frequency discrimination module is the controlled object. However, this scheme has many problems. Firstly, the resonant frequency is easily affected by environmental factors such as temperature and vibration. Secondly, the PZT and its associated circuits, which serve as the actuator, have significant delays. Lastly, traditional scheme often considers the frequency discrimination mechanism as a linear mechanism, ignoring the influence of noise. To solve this problem, we set the PZT control voltage signal that is not easily affected by the environment and can make the frequency discrimination error zero as the reference instead of the traditional susceptible resonant frequency. Secondly, we use the PZT and frequency discrimination module as the controlled objects. Finally, we use system identification theory to achieve high-precision identification of the transfer function by constructing a reasonable order and delay time. Based on the above analysis, a PDH loop is constructed to verify the above analysis. Experimental results have shown that the new scheme has significant advantages compared to traditional scheme, including reducing the minimum error from approximately 10kHz to around 2kHz, and ensuring long-term stable tracking.