Research On Spectral Detection Of Methane Based On Mid-infrared DFG Laser Source

The difference frequency generation?DFG?mid-infrared laser has been widely and importantly applied in the field of high sensitivity trace gas detection because it has lots of advantages,such as simple structure,easy tuning,room-temperature operation and no threshold restriction.In the thesis,a 1550nm tunable DFB laser and a 1060nm semiconductor laser are used as fundamental light sources to generate mid-infrared light source through the second-order non-linear effect DFG of MgO:PPLN crystal.On the basis,the absorption spectrum and the second harmonic detection signal of the standard concentration CH₄ gas molecule at the absorption peak of 3028.751 cm^-11 is obtained since the methane gas?CH₄?direct absorption and second harmonic spectrum detection system was constructed through scanning or modulating the output wavelength of a 1550 nm tunable DFB laser.The specific research work is as follows:1.Firstly,the background and significance of CH₄ gas detection are introduced,the common detection methods of CH₄ gas are summarized,and the advantages and disadvantages of several common detection methods are compared and analyzed.The advantages of TDLAS gas detection technology are briefly introduced.The research status of direct absorption detection technology and wavelength modulation detection technology at home and abroad is analyzed in detail.2.Based on the basic wave equation and coupled wave equation,the three-wave mixing equations are deduced,and then the mid-infrared DFG three-wave mixing equations are obtained.The principle of DFG and QPM technology are introduced.The temperature and wavelength dispersion characteristics of crystal are studied.The temperature and period tuning characteristics of mid-infrared DFG are studied by simulation.Through experiments,the conversion efficiency of DFG light source system is 1.246 mW/W²,and the fluctuation of light source is less than 4.6%.3.The energy level structure,absorption spectrum,absorption line strength and line type of gas molecule are introduced.The effects of temperature and pressure on the three types of lines are systematically studied by simulation.The results show that the amplitude of Lorentz line and Voigt line is less affected by air pressure.The amplitude of Gauss line shape is greatly influenced by air pressure.The amplitude of the three lines is affected by temperature,that is,the higher the temperature is,the smaller the amplitude of the absorption spectrum is.The technology and principle of direct absorption detection and modulation absorption detection are described.4.Based on mid-infrared DFG light source,the direct absorption experiment of CH₄ gas was carried out.The absorption spectrum of CH₄ gas molecule at the base band of 3028.751cm^-1 was obtained.The relative error between theoretical and experimental values of the calculated transmittance is only 2.29%.Based on this experiment,concentration inversion,spatial distance measurement and corresponding error analysis are carried out.5.The internal structure and functional characteristics of SR830 PLA are introduced.Secondary harmonic detection platform is designed and built.The CH₄ gas is measured by experiment and the original second harmonic measurement signal is obtained.Then the original measurement signal is deducted from the background,and the second harmonic detection signal is obtained by smoothing filter processing.