Multipass-assisted dual-comb gas sensor for multi-species detection using a free-running fiber laser

We demonstrated a multipass-assisted dual-comb gas sensor for the simultaneous and sensitive detection of C₂H₂ and NH₃ near 1.5 μm using a single free-running fiber laser. Two pulse trains generated from the shared cavity in the passively mode-locked fiber laser ensure the mutual coherence and common-mode noise rejection. One of the comb pulse trains passed through an open-path multipass cell with an effective path length of 4 m. A synchronized acquisition technique was adopted for more efficient interferogram acquisition compared with the conventional continuous acquisition method. At an acquisition time of 1.5 s, we obtained a signal-to-noise ratio of more than 600 by 1500 averages of the acquired spectrum of 0.1% C₂H₂. Pressure broadening of C₂H₂ and NH₃ in the frequency range of 6503–6530 cm⁻¹ between 0.1 and 1 bar were captured by the dual-comb sensor. Finally, the free-running fiber laser-based dual-comb sensor successfully analyzed the C₂H₂/NH₃ mixtures, enabling real-time gas sensing without using complex optical and electronic feedback systems.