Compact optical fiber laser frequency comb based on acetylene-filled photonic microcell

We demonstrated a compact optical fiber frequency comb stabilized to an acetylene-filled photonic microcell. The erbium fiber oscillator with 109 MHz repetition frequency achieved self-start, and days of stable mode-locking based on a figure-9 configuration. The laser was assembled into a 270 × 280 × 45 mm³ box with precise temperature-control around 25 ℃. The CW reference was stabilized to an acetylene-filled hollow-core photonic crystal fiber (HC-PCF) in a sealed box with dimensions of 385 × 260 × 86 mm³ . The 19-cell HC-PCF has a core diameter of 30 μm and length of 3.9 m filled with 100 ~ 200 mTorr (13.3 ~ 26.7 Pa) acetylene gas. A sub-Doppler absorption feature of P (13) line from the 𝜈1+𝜈3 overtone transition was obtained based on saturated absorption spectroscopy. The optimized sub-Doppler feature had a narrow linewidth of 16 MHz and a signal-to-noise ratio (SNR) of ~ 40. The short-term fractional instability of 𝑓𝑟 was 3.47 × 10^-12 at 1s gate time, and 1.81 × 10^-13 at 100s gate time, with a standard deviation of 0.3 mHz. The RF beatnote (𝑓_𝑏) exhibited a fractional instability of 1.53 × 10^-11 at 1s gate time for a Four-hour measurement in laboratory environment. The overall size of the comb system is expected to be 443 × 270 × 131 mm³ including all the locking electronics. Our approach of acetylene-stabilized fiber comb offers a feasible solution to help fiber combs move from laboratory to outdoors applications such as fiber sensing, laser stabilization and spectrum analysis.