Regeneratively mode-locked fiber laser with a repetition rate stability of 4.9 × 10^-15 using a hydrogen maser phase-locked loop

We demonstrate an ultrastable regeneratively mode-locked fiber laser that employs a phase-locked loop (PLL) circuit with a hydrogen maser. The stability for an integration time of 1 s was 6.2 × 10^-13, which is 16 times better than that of a conventional PLL laser. For an integration time of 1000 s, the stability reached as high as 4.9 × 10^-15. The repetition-rate stability was limited by the synthesizer used for the PLL operation, and there was no additional fluctuation induced by the laser operation.