Time-domain mid-infrared frequency-comb spectrometer

A novel type of Fourier-transform infrared spectrometer (FTIR) is demonstrated. It is based on two Ti:sapphire lasers emitting femtosecond pulse trains with slightly different repetition frequencies. Two mid-infrared beams—derived from those lasers by rectification in GaSe—are superimposed upon a detector to produce purely time-domain interferograms that encode the infrared spectrum. The advantages of this spectrometer compared with the common FTIR include ease of operation (no moving parts), speed of acquisition ($100 \mathit{\mu }\mathrm{s}$ demonstrated), and not-yet-shown collimated long-distance propagation, diffraction-limited microscopic probing, and electronically controllable chemometric factoring. Extending time-domain frequency-comb spectroscopy to lower (terahertz) or higher (visible, ultraviolet) frequencies should be feasible.