Study on dual-optical paths for simultaneous measurement method of water vapor absorption spectrum in 1 μm band
Authors: Zheng Jian-Jie , Zhu Wen-Yue , Liu Qiang , Ma Hong-Liang , Liu Kun , Qian Xian-Mei , Chen Jie , Yang Tao ,   Key Laboratory of Atmospheric Optics, Anhui Institute of Optics and Fine Mechanics, Hefei Institutes of Physical Science, HFIPS, Chinese Academy of Sciences, Hefei 230031, China,   Science Island Branch of Graduate School, University of Science and Technology of China, Hefei 230026, China,   Advanced Laser Technology Laboratory of Anhui Province, Hefei 230037, China,   School of Electrical Engineering and Intelligent Manufacturing, Anqing Normal University, Anqing 246133, China,   Key Laboratory of Atmospheric Optics, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China
Published: June 2021

Based on the gas multi-pass absorption cell with dual-optical paths (long optical path: 72.46 m; short optical path: 36.23 m), a measurement method of simultaneously detecting water vapor absorption spectra is advanced. Combining with a narrow line-width external cavity diode laser and a high-precision Fabry-Perot etalon, a high-resolution simultaneous measurement device with dual-optical paths for water vapor absorption spectra in 1 μm band is developed. Since the external cavity diode laser has excellent polarization characteristics which could be combined with a half-wave plate and a polarization beam splitter to implement the laser transmissions in dual-optical paths simultaneously. Both the multi-pass absorption cell and the Fabry-Perot etalon in the measurement device have pressure and temperature control units, which are utilized for achieving ambient stability. The free spectral range of Fabry-Perot etalon is accurately measured by the method of optical comb frequency. Corresponding free spectral range with a deviation of only 0.02 % from the theoretical value is obtained to be a value of 749.52 MHz, and the influence of temperature on the frequency shift of etalon is less than 1 % of the measured value. The stability of the pressure and the temperature in the dual-optical path gas multi-pass absorption cell in the system are evaluated in detail, and the calculated relative errors are not more than 0.03 % and 0.02 %, respectively. At a temperature of 300 K, the system is used to measure the absorption spectra of water vapor at 9152.53 cm<sup>–1</sup> from 400 Pa to 2000 Pa on dual-optical paths, then the integrated absorbance and Lorentzian line-width of water vapor for long optical path and short optical path are inverted by fitting absorption spectra with Voigt profile respectively. The absorption line intensities and self-broadening coefficients are acquired by performing linear fitting to the integrated absorbance and Lorentzian line-width under different pressures. And the relative deviations of the average values of the dual-optical path absorption line intensities (converted to the reference temperature of 296 K) and the self-broadening coefficients and the corresponding data of the HITRAN2016 database are 0.78 % and 3.8 %, respectively. Consequently, the feasibility of the dual-optical path simultaneous measurement method and the reliability of the measurement device are demonstrated by the results.