Research On Methane Detection System Based On Quartz Enhanced Photoacoustic Spectroscopy

In recent years,trace gases detection has gone deep into various fields,such as environmental detection,underground operation,medical treatment,automobile industry and aerospace.Methane is the main component of gas and greenhouse gas.Methane detection is of great significance for the safety of industrial production and the life and health of personnel.Quartz enhanced photoacoustic spectroscopy（QEPAS）,as a new and rapid development technology,has become a hot spot in trace gas detection.Different from direct absorption spectroscopy,it detects concentration by photoacoustic effect,and it can overcome the shortcomings of traditional photoacoustic spectroscopy that microphone and cantilever is easy to be interfered by environmental noise.It has the advantages of short response time,high sensitivity and stability.The work is mainly divided into the following four parts:Firstly,the research on QEPAS at home and abroad is described.The related principles of the technology,such as infrared absorption spectroscopy and photoacoustic spectroscopy,are introduced.The characteristics of quartz tuning fork（QTF）and spectrophone are explained.At the same time,the expression of photoacoustic signal is derived by combining it with wavelength modulation spectroscopy,and the relationship between modulation coefficient and second harmonic signal is obtained by using MATLAB.Secondly,the theoretical models of QTF and resonance tube are established to analyze the mechanical vibration,piezoelectric response and the optimal length of resonance tube.At the same time,the numerical model of the spectrophone is established,and the traditional on-beam and off-beam spectrophone are simulated by using the finite element software COMSOL.The optimal excitation position of the light source,the optimal distance between the resonance tube and the QTF,and the optimal length of the resonance tube with different inner diameters are calculated.Based on the theory and experiment,the optimal parameters are selected to design the spectrophone.Then,the on-beam configuration methane detection system based on QEPAS is built.A QTF test system based on lock-in amplifier SR830 and signal generator AFG-2225 is established and QTF is tested by Lab VIEW program.At the same time,the data acquisition program is compiled to extract the second harmonic signal.The absorption line of methane is selected.When the average time is 5 s,the 1σdetection limit of the system is 43.37 ppmv,and the calculated normalized noise equivalent absorption coefficient（NNEA）is 7.2×10^-8 W·cm^-1·Hz^-1/2.Finally,evanescent wave technology is introduced to build on-beam configuration methane detection system based on QEPAS,using the tapered fiber instead of the block optical collimating structure to construct the system.In the beginning,the two-dimensional light field distribution of tapered fiber is simulated by COMSOL,and the evanescent field proportion is calculated,and the influence of the taper waist diameter on the system performance is clarified.At the same time,the preparation method of tapered fiber is introduced,and the tapered fiber fusion platform is designed to manufacture the tapered fiber tip of 1.3μm diameter.The experiment was carried out with methane,the detection limit of the system is 105.6ppmv,and NNEA is 1.6×10^-7 W·cm^-1·Hz^-1/2.