Mid-infrared dual-comb spectroscopy with electro-optic modulators

Absorption spectroscopy of fundamental ro-vibrational transitions in the mid-infrared region provides a powerful tool for studying the structure and dynamics of molecules in the gas phase and for sensitive and quantitative gas sensing. Laser frequency combs permit novel approaches to perform broadband molecular spectroscopy. Multiplex dual-comb spectroscopy without moving parts can achieve particularly high speed, sensitivity and resolution. However, achieving Doppler-limited resolution in the mid-infrared still requires overcoming instrumental challenges. Here we demonstrate a new approach based on difference-frequency generation of frequency-agile near-infrared frequency combs that are produced using electro-optic modulators. The combs have a remarkably flat intensity distribution, and their positions and line spacings can be freely selected by simply dialing a knob. Using the proposed technique, we record, in the 3-mu m region, Doppler-limited absorption spectra with resolved comb lines within milliseconds, and precise molecular line parameters are retrieved. Our technique holds promise for fast and sensitive time-resolved studies of, for example, trace gases.