Fig. 1. (a) Schematic diagram of optical fiber frequency comb based on acetylene saturated absorption spectroscopy frequency stabilized optical reference. The black solid line represents the optical signal, the fluorescent solid line represents the free-space optical signal, and the black dashed line represents the RF signal. cw laser, continuous-wave laser; FRC, fiber-ring cavity; EDFA, erbium-doped fiber amplifier; AOM, acousto-optic modulator; ILP, in-line polarizer; EOM, electro-optic modulator; PBS, polarization beam splitter; PD, photodetector; Synth, synthesizer; BPF, bandpass filter; PID, proportional-integral-differential circuit; FBG, fiber Bragg grating; PZT, piezoelectric transducer; HC-PCF, hollow-core PCF. (b) Photo of the actual setup; (c) SEM image of the cross section of the hollow-core fiber.

Fig. 2. Optimized condition for a 2.1-m-long acetylene-filled microcell: (a) normalized sub-Doppler absorption spectrum for the P (13) line (left axis) and transmission spectrum for a fiber ring cavity with FSR of 17.1 ± 0.1 MHz (right axis); (b) same as (a) for the P (23) line; linewidth and SNR for the sub-Doppler feature of the P (13) line as a function of (c) acetylene pressure (under pump power of 29 mW) and (d) pump power (at pressure of 650 mTorr); (e) same as (c) for the P (23) line; (f) same as (d) for the P (23) line (at 600 mTorr).

Fig. 3. (a) Frequency modulation spectroscopy error signal of the P (13) transition; (b) lock point voltage and error signal of P (13) transition after being locked. The amplitude of error signal was less than 16 mV.

Fig. 4. (a) Optical spectrum of the fiber oscillator in linear scale; (b) RF spectrum of the repetition rate and the heterodyne beat note (RBW, 100 kHz; VBW, video bandwidth); (c) stabilized repetition frequency measured at 1 s gate time with SD of 0.4 mHz (data subtracted by ∼97.36 MHz); (d) stabilized beat note frequency measured at 1 s gate time. Inset shows the processed data after the periodic oscillations due to temperature fluctuations were removed.

Fig. 5. (a) Postprocessed beat note data after we added high-pass filters (HPFs); (b) fractional instability (left) and Allan deviation (right) of the RF beat note f_b between the comb and the cw reference at 1532.8 nm (red dots) compared to that of the repetition rate (black squares), data in (a) with 5 mHz HPF (red cross circle), synthesizer signal with external reference (blue up-triangles), and GPS-disciplined Rb oscillator (green down-triangles).