A comb-referenced difference-frequency spectrometer for cavity ring-down spectroscopy in the 4.5 micron region

We present a unique spectrometer combining absolute-frequency and high-sensitivity capabilities for precise investigation of molecular ro-vibrational spectra in the mid-IR. A coherent source based on a difference-frequency generation non-linear process in a periodically poled LiNbO3 crystal is referenced to an optical frequency-comb synthesizer. The generated radiation is coupled to a high-finesse Fabry-Perot cavity and the transmitted cavity ring-down signal is sensitive to very small gas absorbance. We tested the performance of our set-up on CO2 transitions.