Optical phase locking of a narrow linewidth CW laser to an optical frequency comb with internal-external composite control

The frequency and phase locking of a slave continuous-wave laser to an optical frequency comb play a crucial role in achieving high-precision measurements under extreme conditions such as inter-satellite laser ranging and gravitational wave detection. In this letter, we demonstrate an optical phase-locked loop (OPLL) that integrates a slow control loop utilizing the laser's internal piezoelectric transducer and a fast control loop employing an external electro-optic phase modulator. The signal-to-noise ratio of the beat signal is enhanced through the application of a spatial spectral filtering technique. Furthermore, the loop filters for both the slow and fast loops are carefully designed to optimize system stability and expand the control bandwidth. Experimental results confirm that the combined internal–external composite control achieves an OPLL with a peak-to-peak residual phase error of merely 0.13 rad.