Abstract
We introduced frequency-control and temperature-control systems in wavelength-meter-controlled cavity ring-down spectroscopy. The frequency-control system shifted the wavelength of the probe laser from 1393 nm to 696.5 nm where no strong absorption lines of water exist, and therefore, it could avoid measurement errors in laser frequency due to residual moisture in the built-in Fizeau interferometer of a wavelength meter. We verified the hypothesis that the nonuniform reflectivity of the mirror surface contributes to fluctuations in the ring-down time observed with multi-transverse-mode CRDS signals. The fluctuations due to this effect were greatly suppressed by the temperature-control system. Using this system, we could improve the minimum detectable absorption coefficient by three times on average and also improve the experimental standard deviations of the averages by nine times compared with those without the system. We measured near-infrared spectra of residual moisture in dry nitrogen at an approximately 1 nmol/mol (1 ppb) level and performed least-squares fitting of the averaged spectrum. The standard deviation of the residuals of the fitting was 6.6 × 10−12 cm−1, corresponding to a mole fraction of water of 6.3 pmol/mol (6.3 ppt).
Highlights
The demand for accurate measurement of trace moisture in high-purity industrial gases at a level of 10 nmol/mol (10 ppb) or lower has been increasing in the semiconductor industry
One issue in using a high-finesse cavity is that the frequency of the probe laser must be accurately and precisely controlled to match the resonant frequency of the cavity; Cavity ring-down spectroscopy (CRDS) can perform measurement only when the frequency of a laser matches the resonant frequency of the cavity
In wavelength-meter-controlled CRDS, the frequency of the laser is controlled to match a resonant frequency of the cavity on the basis of the frequency measured by a wavelength meter
Summary
The demand for accurate measurement of trace moisture in high-purity industrial gases at a level of 10 nmol/mol (10 ppb) or lower has been increasing in the semiconductor industry. Parts-per-trillion sensitivity for trace-moisture detection using wavelength-meter-controlled cavity ring-down spectroscopy
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