Quantitative Moisture Measurement with a Cavity Ring-down Spectrometer using Telecom Diode Lasers

Moisture measurement is of great needs in semiconductor industry, combustion diagnosis, meteorology, and atmospheric studies. We present an optical hygrometer based on cavity ring-down spectroscopy (CRDS). By using different absorption lines of H₂O in the 1.56 and 1.36 μm regions, we are able to determine the relative concentration (mole fraction) of water vapor from a few percent down to the 10^-12 level. The quantitative accuracy is examined by comparing the CRDS hygrometer with a commercial chilled-mirror dew-point meter. The high sensitivity of the CRDS instrument allows a water detection limit of 8 pptv.