Demonstration of record sensitivity for water vapor detection by means of comb-locked cavity ring-down spectroscopy

Abstract

We report on the development, characterization, and test of a comb-locked cavity ring-down spectrometer (CL-CRDS) operating in the spectral region around 1.39 µm. The system is based on the use of a hemispherical optical resonator with a finesse as high as ∼507000, which gives an empty-cavity ring-down time of about 285 µs. An Allan-Werle analysis on repeated acquisitions of the ring-down time at a fixed laser frequency suggests a minimum detectable absorption coefficient of 2×10−12cm−1 for the optimum integration time of 45 s. This limit can be exceeded by adopting the strategy of long-term spectral averaging. Taking advantage of the frequency stability guaranteed by the optical frequency comb, the CL-CRDS spectra were averaged over more than two days, thus removing efficiently the effect of mechanical, acoustic, and thermal noises. As a result, we could achieve a minimum detectable absorption coefficient as low as 3.7×10−13cm−1, which corresponds to a limit of detection for H2O in N2 of nine parts per trillion and a H2O partial pressure of 2×10−8 Pa (or 2×10−10 mbar). The potentialities of our approach are demonstrated by recording the absorption features of HD16O and HD18O in flows of ultra-high-purity N2 and ambient air, respectively.