Abstract

Optical-optical double resonance spectroscopy (OODR) using a continuous wave (CW) pump and a frequency comb probe is a powerful tool for measuring and assigning hot-band transitions with high selectivity and accuracy [1]. In the first demonstration of the technique, we detected transitions in the 3ν₃ ← ν₃ range of methane with ~1.7 MHz accuracy, using a 3.3 μm CW pump and 1.67 μm comb probe, and a liquid-nitrogen-cooled single pass cell containing the sample [2]. Later on, we introduced a room-temperature enhancement cavity to increase the absorption sensitivity for the comb probe and reach sub-MHz accuracy on probe transition frequencies [3]. However, the transmitted comb probe coverage was limited by the dispersion of the cavity mirrors and the offset frequency (f_ceo) was monitored but not actively stabilized, which complicated the data averaging process.

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