Laser feedback control system for three-wave polarimeter-interferometer on J-TEXT tokamak

The three-wave far-infrared polarimeter-interferometer on J-TEXT tokamak is a crucial diagnostic for measuring plasma electron density and current profiles. However, after more than 13 years of operation, aging lasers are experiencing power degradation and instability, rendering the original feedback system ineffective and requiring frequent manual adjustments. This study presents a novel laser feedback control system incorporating real-time data communication, frequency stabilization, and power regulation modules. The system integrates piezoelectric transducers (PZTs) of both the CO₂ pump lasers and far-infrared (FIR) lasers into a unified control loop to minimize frequency drift, while implementing adaptive adjustment based on threshold detection and directional determination through intermediate frequency (IF) changes, thereby avoiding blind adjustments. Experimental results demonstrate that the system successfully maintains laser frequency drift within 50 kHz and power fluctuations within 15%, achieving stable operation for over 8 h without manual intervention.