The residual water vapor in gas, even in ppbv (parts per billion by volume) will have an effect on the quality of the product, so the moisture content in the high purity gas is a key parameter in the semiconductor industry. Gas has a more complex characteristic line in the mid-infrared region, therefore it is necessary to detect the water vapor content in the region. Meanwhile, the rapid development of quantum cascade lasers, which have a wider tuning range, a higher output power, and a narrower line width, have advanced the progress of the infrared spectroscopy detection technology of the mid-infrared region.

Based on the continuous optical cavity ring-down spectroscopy, the research team from the Institute of Optics and Electronics (IOE, CAS) used a 5.2 µm tunable quantum cascade laser to establish a trace water vapor detection device in the mid-infrared region for the first time, and a trace water vapor test was carried out. The optimal number of times of optical cavity ring-down signal was determined to be 602 by analyzing the system noise level with the Allan variance. The water vapor absorption spectrum near 1918 cm^-1 was measured at atmospheric pressure and room temperature, then the measurement of trace water vapor concentration in high purity nitrogen was consistent with the nominal value.  When the reflectivity of the cavity is 99.93%, the detection sensitivity of water vapor could be 24.8 ppbv. The results show that the trace gas detection technology with high sensitivity in the mid – infrared region has a good prospect in industrial monitoring, environmental testing and medical diagnosis.

Related research results published in the "Spectroscopy and Spectral Analysis".