Measurement and Analysis of CO2 Hyperspectral Absorption Spectroscopy at High Pressure in the 2 μm Band

CO2 is one of the important products of combustion processes. In industrial production, , food processing, , clinical medicine and other application scenarios, , monitoring the concentration of CO2 is of great significance. Absorption spectroscopy is a common method based on resonant absorption of molecular energy levels on the laser transmission path to invert flow field parameters. It has the advantages of high sensitivity, , fast response, , and small perturbation to the flow field, , and has been widely applied in trace gas analysis and environmental monitoring. Compared with tunable diode laser absorption spectroscopy, , hyperspectral absorption spectroscopy can cover a wider spectral range, , which is beneficial for achieving more accurate environmental parameter measurements. In contrast with communication bands, , the CO2 spectral line intensity in the 2 mu m band can be increased by two orders of magnitude, , and the selection of spectral lines is more abundant. With the help of the broadband tunable thulium-doped fiber laser self- developed by our group, , the hyperspectral absorption signals of CO2 in the 2 mu m band are measured within the pressure range of 152 kPa to 932 kPa, , and compared with the theoretical spectra calculated based on the HITRAN database. At 152 kPa, , the peak matching between the measured spectrum and the theoretical spectrum is completed to achieve wavelength calibration, , which can verify that the broadband tunable laser has fine frequency linear scanning characteristics. At the same time, , the baseline extraction under high pressure environment is a difficult problem in data processing, , which directly affects the inversion results of absorption spectral line shape. The commonly used baseline extraction method of extracting the weak absorption part of the original signal by polynomial fitting is only a rough calculation method, , which will lead to a low intensity of baseline fitting when the pressure increases, , introducing large errors into the subsequent absorption spectrum inversion. Therefore, , considering using the simultaneous fitting method. Through synchronous fitting of the original signal baseline and absorption spectrum, the spectrum inversion accuracy of the spectral line can be improved. The specific operation is to describe the original signal as a theoretical model under the joint action of baseline and absorption spectrum, with pressure, temperature, CO2 mole fraction, baseline distribution fitting coefficient as the variable parameters of nonlinear curve fitting, and taking the minimum residual as the goal, using optimization algorithm to calculate the baseline parameters and gas environmental parameters under the optimal matching of theoretical model and measured data. Through the experimental measurement of CO2 hyperspectral absorption data at 932 kPa, it can be found that the CO2 absorption spectrum is greatly broadened, and the overall spectral line tends to be homogenized. Meanwhile the baseline profile of the original signal completely disappears, and it is difficult to obtain the baseline distribution by conventional means without a reference optical path. However, by using the simultaneous fitting method to synchronously fit the baseline and absorption spectrum, this problem can be solved to some extent. From the experimental results, the measured spectra in the pressure range of 152 kPa to 932 kPa are highly matched with the theoretical spectra, and the residuals of the full-band spectral fitting are below 0.04, corresponding to the highly consistent environmental parameter inversion results with the experimental settings. In addition, by comparing the 1 965.98 nm absorption line area retrieved from the narrowband absorption spectrum, the reliability and superiority of the hyperspectral absorption spectrum under high pressure are confirmed. Under the condition of approximately constant temperature and gas mole fraction, the absorption area retrieved from the hyperspectral absorption spectra maintains a good linear relationship with pressure within the measurement range. Overall, the hyperspectral absorption technology has a wide range of environmental applicability and strong reliability. Combined with the advantages of high-intensity absorption spectra in the 2 mu m band, the use of simultaneous fitting data processing method can significantly improve the inversion accuracy of spectral line parameters, which can better achieve real-time detection and quantitative analysis of environmental parameters and CO2 gas concentration under wide operating conditions.