Cavity ring-down spectroscopy at 2-μm wavelength assisted by a comb-locked optical parametric oscillator

We report on a comb-locked cavity ring-down spectrometer developed for high-precision molecular spectroscopy at 2 μm. It is based on the use of an external-cavity diode laser that is offset-frequency locked to the signal output of a singly-resonant optical parametric oscillator. This latter acts as a reference laser, being locked to a self-referenced optical frequency comb, which in turn is stabilized against a GPS-disciplined Rb-clock. The performance of the spectrometer is investigated by probing a pair of N₂O transitions belonging to hot vibrational bands. One of these has never, to the best of our knowledge, been observed before, and is included in the N₂O line list of the ExoMol database. Absolute center frequencies are retrieved with a 1-σ global uncertainty of 108 kHz.