Development of 19.8 MHz repetition rate optical frequency combs for dual-comb spectroscopy

We demonstrate two self-referencing optical frequency combs with a repetition rate of 19.8 MHz, which is the lowest ever reported, using nonlinear-polarization-evolution mode-locked Er:fiber lasers. The two combs are phase-locked with a common CW laser at 1.5 μm using high-speed feedback controls with servo bandwidths of 380 kHz. Over 40 absorption lines of the ν 1 + ν 3 vibration band of 13C2H2 are observed by dual-comb spectroscopy using those combs. We resolve the observed Doppler-limited lines with a 19.8 MHz frequency spacing (repetition rate), which is currently the smallest frequency spacing achieved, to the best of our knowledge. © 2020 The Japan Society of Applied Physics.