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

Abstract

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 150Hz. We attain a noise equivalent absorption coefficient of 5.2×10−11cm−1Hz−1/2 and a minimum detectable absorption coefficient of 6.7×10−12cm−1 (after 70s 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 (1393nm) spectrum of H2O is efficiently recorded in the range of 7179.537cm−1–7183.330cm−1 (containing 539 spectral points) in 15min. Analysis of the average of the eight spectra acquired in 2h shows a baseline noise (one standard deviation) of 1.3×10−11cm−1, corresponding to 0.012nmol/mol (0.012ppb) of H2O in N2 in amount-of-substance fraction (mole fraction).