Broadband (3.1-5.4 µm) mid-IR dual-comb molecular spectroscopy with sub-MHz resolution.

We demonstrated a broadband mid-IR spectroscopy with sub-MHz resolution using a dual-comb spectroscopy (DCS) system based on two subharmonic optical parametric oscillators (OPOs) pumped by two phase-locked, tied to Rb clock, 1.93-µm thulium (Tm) combs. Each OPO spectrum consists of 350,000 spectral modes with 115 MHz intermodal spacing covering a wavelength range of 3.1-5.4 µm. Each mode linewidth and the uncertainty of the absolute frequency referencing (optical lock of Tm-combs to the 3kHz-linewidth 'RIO' laser at 1.56μm) do not exceed 50 kHz, which determines the spectral resolution. Massively parallel simultaneous detection of 22 molecular species with 115 MHz spacing was demonstrated earlier. Here, scanning the RIO reference laser, we continuously tune both combs with 50 kHz steps within 115 MHz interval, covering the entire 3.1-5.4 µm range at 50 kHz resolution, potentially providing up to 10⁹ spectral data points in a single spectrum. The technique was verified by measuring the ν₁+ν₃ band of the CS₂ molecule in 4.5–4.7 µm range with 15 MHz step, sufficient to resolve 981 strongest Doppler-broadened transitions of the CS₂ molecule and its isotopologues. Most of the line intensities, accurately measured for the first time, are now included in HITRAN2020 database.