Frequency-stabilized cavity ring-down spectrometer for high-sensitivity measurements of water vapor concentration

We present a portable spectrometer that uses the frequency-stabilized cavity ring-down spectroscopy technique capable of high-precision measurements of trace water vapor concentration. Measuring one of the strongest rovibrational transitions in the nu(1)+nu(3) water vapor combination band near (v) over tilde =7181.156 cm(-1), we compare spectroscopic and thermodynamic determinations of trace water vapor in N-2, and find systematic differences attributable to water vapor background effects and/or uncertainties in line intensities. We also compare the frequency-stabilized ring-down method with other cavity ring-down approaches that are based on unstabilized probe lasers and unstabilized ring-down cavities. We show that for the determination of water vapor concentration, the frequency-stabilized cavity ring-down method has the minimum measurement uncertainty of these techniques. The minimum noise-equivalent absorption coefficient of the spectrometer was 1.2x10(-10) cm(-1) Hz(-1/2) , which further corresponds to a minimum detectable water vapor mole fraction equal to 0.7x10(-9) for an absorption spectrum of 10 minutes duration.