Research On Trace Gas Detection Technology Based On 2f/1f Wavelength Modulated Spectrum

Our country is a developing country with a large population.With the development of economy,the advancement of science and technology,and the significant improvement of people’s living standards,people have begun to generate greater demand for gas monitoring in many aspects such as air pollution,industrial waste gas,coal mine production safety,and daily living environment.In this paper,we have conducted in-depth theoretical and experimental research on absorption spectroscopy based on Lambert-Beer law.Using wavelength modulation spectroscopy and harmonic detection technology,we derive the fundamental（1f）signal and second harmonic of transmitted light and absorbed light（2f）Signal and second harmonic ratio fundamental wave（2f/1f）signal expression,analyzes the difference between tunable laser diode absorption spectroscopy（TDLAS）technology and photoacoustic spectroscopy technology,and proposes a 2f/1f-WMS（Wavelength modulation spectroscopy technology）photoacoustic spectrum self-calibration technology.Finally,the TDLAS gas detection system and the improved photoacoustic gas detection system were built,and the detailed detection of acetylene gas was completed.The specific research work and results in this paper are as follows:First,the principle of laser absorption spectroscopy is deeply studied,and the signal generation process of absorbed light and transmitted light is analyzed,and it is expressed in formulas at the same time,and then build a model of TDLAS and photoacoustic spectroscopy on the MATLAB&Simulink platform.The ideal situation of TDLAS technology and photoacoustic spectroscopy technology were analyzed in detail,the influence of modulation parameters was analyzed,and the response to gas concentration was verified,and the correctness of the formula was verified.Second,the theory of photoacoustic effect of solids is analyzed,and the photoacoustic signal generated by solid materials is superimposed on the photoacoustic signal of gas.A self-calibration technique of photoacoustic spectrum based on 2f/1f-WMS method is proposed.The graphene material was selected and the existing photoacoustic cell structure was improved,and then the structure was simulated using COMSOL Multiphysics software to analyze its photothermal and thermoacoustic processes.Third,based on the HITRAN database,LabVIEW software was used to directly process the absorption line data of acetylene,and the time-domain signal and frequency-domain signal were analyzed.The 1f and 2f signals in TDLAS and photoacoustic spectroscopy were obtained and the accuracy of theoretical analysis has been verified.Fourth,a TDLAS gas detection system and a photoacoustic spectrum gas detection system were established,and acetylene gas was detected.First,the optimal modulation parameters of the TDLAS gas detection system and the photoacoustic spectrum gas detection system are selected.Then,the relationship between the intensity of TDLAS signal and photoacoustic signal to the concentration of C₂H₂ gas and the optical power of laser was studied,and the C₂H₂gas concentration was calibrated according to the response of gas concentration.Finally,the C₂H₂ gas was continuously measured.The detection limits of the TDLAS gas detection system and the photoacoustic spectrum gas detection system were 0.20 ppm and 0.42 ppm,respectively.