LAUSR

Significance Each molecule leaves characteristic absorptions, which enable one to unambiguously identify and quantify it, and to understand its structure and dynamics. Such fingerprints, strong in the mid-infrared region of the electromagnetic spectrum, can be accessed by measuring the intensity of light transmitted through a medium as a function of its frequency. Here, we describe a spectrometer for mid-infrared spectroscopy based on two frequency comb generators, light sources emitting narrow evenly spaced laser lines over a broad spectral bandwidth. Our instrument without moving parts simultaneously measures broad high-resolution spectra with calibration of the frequency scale directly to an atomic clock and precise spectral line shapes. These features are obtained from the experimental data without any need for corrective processing. Mid-infrared high-resolution spectroscopy has proven an invaluable tool for the study of the structure and dynamics of molecules in the gas phase. The advent of frequency combs advances the frontiers of precise molecular spectroscopy. Here we demonstrate, in the important 3-µm spectral region of the fundamental CH stretch in molecules, dual-comb spectroscopy with experimental coherence times between the combs that exceed half an hour. Mid-infrared Fourier transform spectroscopy using two frequency combs with self-calibration of the frequency scale, negligible contribution of the instrumental line shape to the spectral profiles, high signal-to-noise ratio, and broad spectral bandwidth opens up opportunities for precision spectroscopy of small molecules. Highly multiplexed metrology of line shapes may be envisioned.