Optimization of the PDH frequency locking system based on the crystal birefringence theory and orthogonal demodulation scheme

We have developed an orthogonal digital demodulation scheme for Pound-Drever-Hall (PDH) frequency locking systems using the field-programmable gate array (FPGA). This scheme enhances the error model by incorporating crystal birefringence theory. Our approach mitigates the impact of laser polarization deviation and environmental interference on the phase instability of the photoelectric detector (PD) output signal by separating the effective amplitude signal from interfering phase information using narrowband signal decomposition and orthogonal baseband transformation theory. Additionally, we introduced a frequency lock-point localization algorithm for precise frequency stabilization in complex environments. Our research results show a significant reduction in maximum error from approximately 200 Hz in traditional schemes to around 100 Hz, while also stabilizing the signal's linear range.