Massively Parallel Sensing of Trace Molecules and Isotopologues with Subharmonic Mid-IR Frequency Combs

We introduce a new platform for mid-infrared dual-comb spectroscopy, based on a pair of broadband subharmonic optical parametric oscillators pumped by two phase-locked thulium-fibre combs. Our system provides fast (7 ms for a single interferogram), moving-parts-free, simultaneous acquisition of 350,000 spectral data points, spaced by 115-MHz intermodal interval over 3.1-5.5 μm spectral range. Parallel detection of 22 trace molecules in a gas mixture, including isotopologues containing such isotopes as ¹³C, ¹⁸O, ¹⁷O, ¹⁵N, ³⁴S, ³³S and deuterium, with part-per-billion sensitivity and sub-Doppler resolution has been demonstrated.