We developed a measurement system for trace moisture using CRDS with two diode lasers (laser1 and laser2). The two lasers were connected to an optical switch using optical fibers, and the output laser light from the optical switch was introduced to a ring-down cavity. The ring-down cavity was filled with a standard gas mixture of trace moisture in N2 generated using the NMIJ magnetic suspension balance/diffusion-tube humidity generator. The frequency of the laser1 was controlled to the peak position of a water absorption line and that of the laser2 was to an off-peak position (baseline). The laser1 and laser2 were used to measure the moisture content and correct the baseline drift, respectively. By repeatedly switching the probe laser for CRDS between the laser1 and the laser2, we were able to correct the baseline drift in real time and reduce its effect on the measurement results. Using this technique, we attained a noise standard deviation of 100 pmol/mol (100 ppt) or less for real-time measurement.
Hisashi ABE received his Ph.D. degree in physics in 1996 from Kanazawa University. After working at the National Institute for Advanced Interdisciplinary Research and the National Institute for Resources and Environment as a postdoctoral fellow, he joined National Metrology Institute of Japan(NMIJ)/AIST in 2001. He has been responsible for calibration services for the high and low
humidity as well as trace moisture. He is the Chief Senior Researcher of the Gas and Humidity Standards Group.