Experimental Study on Automatic Locking of Repetition Rate in Optical Frequency Comb

This paper introduces a digital PID control technology for automatically locking the repetition rate of a fiberbased femtosecond optical frequency comb. The mode-locked laser used in the experiment has a repetition rate of approximately 79 MHz. Under a temperature variation of $2^{\circ} \mathrm{C}$, the free-running repetition rate drift reaches the kilohertz level, significantly affecting the accuracy of comb-based measurements. By employing a linear phase detector as the phase discrimination module, combined with signal source modulation and digital PID control, the repetition rate of the optical frequency comb is automatically locked to a rubidium atomic clock-based timefrequency reference. After locking, the standard deviation of the repetition rate jitter is 1.5 mHz, and the frequency stability reaches 9.2E-12@ls. The automatic locking system achieves a locking time of less than 1 second and can rapidly re-lock with high precision after unlocking. The locked femtosecond optical comb meets application requirements in time-frequency transfer, combbased ranging, and other fields.