Wavelength-meter controlled cavity ring-down spectroscopy: high-sensitivity detection of trace moisture in N2 at sub-ppb levels

•We developed a measurement system for trace moisture using cavity ring-down spectroscopy (CRDS).•We developed a simple technique to efficiently couple a laser to a high-finesse cavity in CRDS.•The frequency of the laser is controlled using a high-resolution wavelength meter.•A minimum detectable H2O in N2 of 0.012 nmol/mol (0.012 ppb) in mole fraction is attained.•The measurement error in the wavelength meter owing to residual moisture inside the interferometer is observed.

We present a relatively simple technique to efficiently couple a laser to a high-finesse cavity in cavity ring-down spectroscopy (CRDS). The frequency of the probe laser is coupled to a resonant frequency of the cavity using a high-resolution wavelength meter. The repetition rate of measurement of the ring-down events obtained is 150 Hz. We attain a noise equivalent absorption coefficient of 5.2 × 10−11 cm−1 Hz−1/2 and a minimum detectable absorption coefficient of 6.7 × 10−12 cm−1 (after 70 s averaging). We find significant deviations in the indications (readings) of the wavelength meter, probably because of dispersion due to residual moisture inside the interferometer. The dew point of the residual moisture is estimated to be −13.9 °C. By correcting the deviations on the basis of experimentally obtained data, a near-infrared (1393 nm) spectrum of H2O is efficiently recorded in the range of 7179.537 cm−1–7183.330 cm−1 (containing 539 spectral points) in 15 min. Analysis of the average of the eight spectra acquired in 2 h shows a baseline noise (one standard deviation) of 1.3 × 10−11 cm−1, corresponding to 0.012 nmol/mol (0.012 ppb) of H2O in N2 in amount-of-substance fraction (mole fraction).