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 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, never observed before, 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.