Dual-etalon frequency-comb cavity ringdown spectrometer

We have demonstrated a spectroscopic technique for simultaneously obtaining broad spectral bandwidth and high frequency resolution absorption measurements, with 5 μs temporal resolution, continuously for tens of microseconds in an apparatus with no active stabilization. The technique utilizes two passive air-gap etalons to imprint two frequency comb patterns onto a single pulsed light source. The air-gap etalons also serve as cavity ringdown cells increasing the sensitivity of the absorption spectroscopy by increasing the interrogation path length. Here, we demonstrate the operation of the spectrometer utilizing a ∼0.15 cm −1 bandwidth pulsed dye laser and two nearly identical 300 MHz free-spectral range confocal air-gap etalons each with a finesse of ∼1 × 10 5 , to investigate the (1,1,3) overtone of water and the R(7) transition of the O 2 \documentclass[12pt]{minimal}\begin{document}$b^1 \Sigma _g ^+ \leftarrow X^3 \Sigma _g^-$\end{document} b 1 Σ g + ← X 3 Σ g − (2,0) band with high spectral resolution.