Fast recording of weak absorption spectra in optical cavity using tunable laser

Abstract

The possibilities of new spectral technique R-ICOS are demonstrated by an example of weak absorption spectra of methane. The technique uses the combination of the intensities of laser beams transmitted through and reflected by the gas-filled resonant optical cavity. Such an approach leads to damping signal fluctuations due to mismatching of laser and cavity modes. With cavity mirrors of moderate reflectivity of 0.8–0.99, the spectra of the quality enough to measure the trace methane concentrations in atmosphere can be recorded within 320 μs. For the acquisition time of 20 s, the sensitivity to absorption is α = 2 × 10−8 cm−1 (1σ), which is 40 times better than needed for the background methane monitoring in standard atmosphere.