Study On Trace Gas Detection Based On Absorption Enhanced Photoacoustic Spectroscopy

Trace gas usually means that the gas with a volume concentration less than part per million.The detection of trace gas has very important application in many fields.For example,in the medical field,the detection of carbon monoxide concentration which is from the breath of human body can diagnose some respiratory diseases.In the electrical field,the detection of acetylene in circuit can determine the low-voltage discharge fault.Real-time monitoring of flammable,explosive and toxic gas produced or used in the industrial field can prevent some dangerous accidents.Therefore,improving the sensitivity of trace gas detection will also promote the development of these application technologies.Photoacoustic spectroscopy is mainly used in trace gas detection because of its advantages of excellent detection performance,good system stability and large dynamic range.This technology obtains the concentration information of gas by detecting the intensity of photoacoustic signal which is generated by gas after absorbing the optical energy.Under the same gas concentration,the more laser energy gas absorbs,the stronger intensity of photoacoustic signal is,of course the detection sensitivity of the system is also higher.So according to this feature,trace gas detection based on absorption enhanced photoacoustic spectroscopy is studied in this paper.First of all,this paper introduces the correlative theory of photoacoustic spectroscopy.Starting from the engery composition of gas molecules,it analysies the principle and conditions of generation of gas molecules infrared absorption,and then expounds the optical energy absorption law of gas and the choice of linear functions.Finally,it introduces the principle of photoacoustic spectroscopy(PAS)and PAS based trace gas detection system.Then,this paper makes the silulation study of the wavelength modulation technology in the photoacoustic spectroscopy system and the sound field distribution inside the photoacoustic cell used in the project.The relationship between the harmonic signal and the modulation coefficient is obtained through the formula derivation,and the second harmonic signal with different modulation coefficients is drawn by silulation software.Finally,the optimal value of the modulation coefficient is obtained.In the simulation study of the sound field distribution inside the photoacoustic cell,the internal structure of the photoacoustic cell and the condition of the acoustic resonance in the one-dimensional resonance tube are introduced.The sound field distribution in the photoacoustic cell is calculated by using COMSOL software.According to the simulation results,the standing wave field mode of the acoustic wave in the photoacoustic cell is analyzed and the optimum detection position of the photoacoustic signal is verified.At last,this paper makes the experimental study of trace gas detection based on absorption enhanced photoacoustic spectroscopy.The trace gas detection system based on doubel path absorption,high power laser with multi path absorption and mid-infrared quantum cascade laser are designed and built,respectively.Each system enhances the laser engery absorbed by gas through using the methods of increasing gas absorption length,improving laser power and detecting gas in the mid-infrafed spectral region which is the fundamental absorption band of gas,respectively.These methods improve the performance of the detection system,and it is also verified that each system has a good concentration linear response with the experiments.