Abstract：Trace water vapor detection plays an important role in semiconductor chip manufacturing, atmospheric model research, high-purity gas preparation, etc. Aiming to accurately and rapidly measure trace water vapor concentration, a scheme of real-time online measurement of trace water vapor in high-purity nitrogen by TDLAS technology based on a 5 020 nm mid-infrared laser is proposed. The mid-infrared band has the characteristics of high selectivity and strong absorption capacity in gas absorption. The trace water vapor detection in this band has the advantages of good selectivityand short response time. According to the spectral parameters in the HITRAN database, the water vapor absorption line is simulated. The simulation results show that H2O has strong absorption at 5 020.36 nm and no other gas interference. In this paper, the spectral line is selected to measure water vapor in high-purity nitrogen. Then, an interband cascade laser (ICL) with a central wavelength of 5 020 nm is selected as the laser source to build a TDLAS experimental system. The laser output laser is divided into two channels. One is a reference optical path to obtain the background water vapor absorption outside the absorption cell; the other is the measurement optical path. The water vapor absorption in the gas to be measured is obtained through the Herriott absorption cell, and the optical path Lin the absorption cell is 10 m. The detector converts the received optical signal into an electrical signal, amplified by the amplifying circuit, which is collected and transmitted to the computer. The sampling frequency is 25 MHz. Baseline fitting and Voigt linear fitting are performed on the reference and measurement optical path signals to obtain water vapor's absorbance curve and integral absorbance. The trace water vapor concentration in high-purity nitrogen gas is obtained by calculating and deducting the background water vapor absorption in the measurement optical path. The algorithm of the above inversion process is embedded in the software platform. The collected transmission light intensity data is directly inverted to obtain the water vapor concentration value in the high-purity nitrogen in the absorption cell in real-time, and the response time is 12 s. After the water vapor concentration value is stable, the average of the 30 measurement results is taken as the experimental result of the water vapor concentration of high-purity nitrogen in each batch. The experimental results show thatthe deviation between the measured trace water vapor concentration of each batch of high-purity nitrogen and the water vapor concentration in high-purity nitrogen required by the national standard is less than ±5%, and the maximum deviation is -3.04%. The error analysis of the experimental results shows that the maximum uncertainty of the water vapor volume fraction is 0.159×10-6. The experimental device can accurately and reliably measure the concentration of trace water vapor in high-purity gas in the mid-infrared band, which provides a feasible scheme for online monitoring of trace gas in the mid-infrared band.
Key words：Mid-infrared; Absorption spectroscopy; Trace water vapor; Online measurement
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