Demonstration of record sensitivity for water vapor detection by means of comb-locked cavity ring-down spectroscopy
A. Castrillo, M. A. Khan, Eugenio Fasci, Vittorio D’Agostino, Stefania Gravina, L. Gianfrani
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 −12 cm −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 −13 cm −1 , which corresponds to a limit of detection for H 2 O in N 2 of nine parts per trillion and a H 2 O 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 HD 16 O and HD 18 O in flows of ultra-high-purity N 2 and ambient air, respectively.