Dual-comb MIR Fourier transform spectroscopy
Mid-infrared spectroscopy offers unparalleled sensitivity for the detection of trace gases, solids and liquids, based on the existence of strongest tell-tale vibrational bands in the 3-12 µm band. The technique of frequency-comb Fourier transform spectroscopy [1-2], and especially dual-comb spectroscopy [3-6] is capable of extremely fast data acquisition combined with superior spectral resolution and broadband spectral coverage. The development of the dual-comb spectroscopy in the mid-IR was not as dramatic as in the near-IR, because of lack of sufficiently broadband and mutually coherent sources.
In our group, we pioneered a new platform for mid-infrared dual-comb Fourier-transform spectroscopy. It is based on a pair of ultra-broadband subharmonic optical parametric oscillators (OPOs) pumped by two phase-locked thulium-fibre combs. Such OPOs operating at degeneracy are ideal coherent frequency dividers whose output is inherently frequency- and phase-locked to the pump [7-12]. The ‘instantaneous’ spectrum of frequency combs produced via subharmonic generation can be well above an octave . Our dual-comb spectroscopy 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 the whole 3.1–5.5 µm spectral range. Parallel detection of 22 trace molecular species 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 . The technique also features:
- absolute optical frequency referencing to atomic clock,
- high degree of mutual coherence between the two mid-infrared combs with a relative comb-tooth linewidth of 25 mHz
- possibility of obtaining mode-resolved spectra with a finesse of 4,000
- capability of coherent averaging of > 400,000 interferograms
- feasibility for kHz-scale spectral resolution in a comb-tooth scanning mode