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 telltale vibrational bands in the 3-12 µm band. The technique of frequency-comb based 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. Recently, we demonstrated a 1.5-octave -wide frequency comb operation of a subharmonic GaAs optical parametric oscillator (OPO) centered at 4 µm and demonstrated its high degree of coherence with respect to the pump source [7-9]. Our present dual-comb system is based on a pair of such GaAs OPOs that are pumped by two fully stabilized phase-coherent thulium-fiber laser combs at λ≈2 µm. The sub-Hertz relative lenewidth between the two OPO combs allows coherent averaging of the interferogram over seconds. We demonstrate sensitive and massively parallel detection of a mixture of molecular species (CO2, 13CO2, N2O, CH4 and H2O) in the whole range of 3.2 to 5.3 µm (frequency span 1240 cm-1) with mode-resolved (115 MHz spacing) accuracy. We were able to acquire 300 000 spectral points with intermodal resolution in few seconds.