Dual-Control-Loop Design for Frequency Tracking Systems Based on Pound-Drever-Hall Technology

Optical resonators are crucial in modern optics, impacting laser physics, quantum optics, spectroscopy, and precision measurement. Precisely matching and locking the laser output frequency to the resonator’s resonant frequency are vital. The Pound-Drever-Hall (PDH) technique, a widely-used active frequency stabilization method, typically adjusts the laser’s resonant frequency for tracking. However, this approach is highly dependent on the laser’s performance and may fail with poor-quality lasers. To address this, a dual-loop frequency auto-tracking method based on PDH principles and FPGA technology is proposed. It creates a dual-control-loop system with high and low-frequency loops. A single-sideband modulator, built with dual Mach-Zehnder electro-optic modulators, offers a stable frequency reference for the laser. The FPGA-based dual loops include a low-frequency loop to adjust the laser’s piezoelectric ceramic transducer (PZT) and a high-frequency module to fine-tune the single-sideband modulator. Experiments show that compared to traditional methods, this dual-loop approach reduces the error from around 200 Hz to roughly 100 Hz. It enhances frequency-tracking accuracy by adding a high-frequency control loop to adjust the single-sideband modulator. This method effectively solves the issue of low lock precision caused by solely relying on PZT adjustments in traditional PDH schemes.