All-Optical 20-μHz-Level Repetition Rate Stabilization of Mode Locking With a Nonlinear Amplifying Loop Mirror

We demonstrated a record standard deviation <;100 μHz of stabilized repetition rate for a mode-locked laser in an environmentally stable figure-eight fiber laser cavity with a polarization maintaining active fiber located asymmetrically in the nonlinear amplifying loop. A laser diode with a maximum pumping power of 250 mW was applied to excite the polarization maintaining active fiber in the nonlinear amplifying loop and acted upon the active fiber to create refractive index change by both resonantly enhanced nonlinear effects and thermal effects. The tuning range, tuning slope, and tuning resolution of the repetition rate are measured to be 10 Hz, -55 mHz/mW, and 110 μHz, respectively, resulting in a standard deviation as low as 77 μHz for 1-s gate time and 24 μHz for 10-s gate time over 4 h.