The accurate measurement of trace water vapor (trace moisture) in gases has been increasingly important recently. Among many methods developed to detect the trace moisture, it appears that cavity ring-down spectroscopy (CRDS) is a suitable method to reliably measure trace moisture in gases with high sensitivity. In CRDS, high reflection cavity mirrors are used to enhance detection sensitivity. However, if we use higher reflectivity mirrors, it will be more difficult to efficiently couple the laser light into the cavity. To solve this problem, the Pound-Drever-Hall (PDH) technique has been successfully implemented for locking laser to the cavity in CRDS. However, the PDH method requires complex experimental setup. Considering such circumstances, we developed a relatively simplified system with which we could efficiently couple laser light to high-finesse cavity modes by controlling the laser frequency using a wavelength meter. Using this system, we were able to measure the trace moisture stably. The tentative analysis of spectra recorded using this system showed that a noise level (one standard deviation) of approximately 7 pmol/mol (7 ppt) was already achieved.
Koji Hashiguchi received his Ph.D. degree in applied physics from the University of Tokyo in 2012.
He has been a researcher at the NMIJ/AIST since 2013, working on the development of a measurement system for trace moisture in gases using cavity ring-down spectroscopy.