Wavelength-meter controlled cavity ring-down spectroscopy: high-sensitivity detection of trace moisture in N-2 at sub-ppb levels

摘要

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 x 10(-11) cm(-1) Hz(-1/2) and a minimum detectable absorption coefficient of 6.7 x 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 degrees 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 x 10(-11) cm(-1), corresponding to 0.012 nmol/mol (0.012 ppb) of H2O in N-2 in amount-of-substance fraction (mole fraction). (C) 2016 Elsevier B.V. All rights reserved.