| As human beings enter the industrialized society,environmental pollution has becoming gradually prominent.Meanwhile,the detection technology of trace gas in coal mine safety production detection,transformer equipment fault gas analysis and medical exhaled gas diagnosis has become more and more important.Demand is becoming more and more important.Among many gas detection technologies,photoacoustic spectroscopy is favored by people as a detection technology with many advantages such as high sensitivity,no background noise and real-time detection.As an absorption spectrum technology without background noise,PAS has the advantages of high detection sensitivity,high detection limit and simultaneous detection of multiple gases.The photoacoustic detector in PAS technology determines the detection limit of trace gases.The photoacoustic detector is composed of a photoacoustic cell and an acoustic sensor together,the photoacoustic cell is the important place where the photoacoustic signal is generated in PAS,while the sensitivity of the acoustic sensor plays a crucial role in the strength of the detected photoacoustic signal.At present,the limitation of photoacoustic spectroscopy gas trace detection technology is that the sensitivity of microphone still needs to be improved.Current capacitive microphones are susceptible to electromagnetic interference,have low sensitivity and can not be well matched with photoacoustic cell.In order to improve the sensitivity of photoacoustic detector,this thesis designed a kind of the resonant photoacoustic cell,based on photoacoustic cell resonance frequency design a cantilever beam diaphragm type fiber optic acoustic sensor,the cantilever beam fiber optic acoustic sensor is high sensitivity,resistance to electromagnetic interference characteristics,cantilever beam diaphragm first order resonance frequency and photoacoustic cell resonance frequency are similar,so that the two frequencies can form a resonance.The major work is as below.1.A cantilever beam diaphragm fiber optic acoustic wave sensor was designed and a resonant photoacoustic cell was prepared with a resonant frequency of 1410 Hz.Firstly,the resonant cavity size of the photoacoustic cell was determined by simulation to be 8mm in diameter,120 mm in length and 1410 Hz in resonant frequency.Finally,as the first-order resonant frequency of the cantilever beam microphone needs to match the resonant frequency of the photoacoustic cell,the first-order resonant frequency of the cantilever beam fibre optic acoustic wave sensor was tested at 1450 Hz and the cantilever beam acoustic wave sensor was tested at 1500 Hz.mV/Pa and a signal-to-noise ratio of66.72 dB,which is 21.68 dB higher than the signal-to-noise ratio of an electronic microphone in the same situation.2.A photoacoustic spectroscopy trace gas detection system is built based on a selfmade cantilever acoustic wave sensor.The system uses a Tunable Erbium-Doped Fiber Laser(TEDFL)as the excitation light source and an Erbium-Doped Fiber Amplifier(EDFA)to amplify the power of the TEDFL,and the output power of the light source is300 m W after amplifying the power.The resonance frequency of the resonant photoacoustic cell is matched with the cantilever optical fiber acoustic wave sensor to detect the absorption peak of acetylene gas with a wavelength of 1532.59 nm.The experimental results prove that the system,it has a good response to acetylene gas in the concentration range of 0-100 ppm,with a detection sensitivity of 145.808 μV/ppm and a detection limit of 24.23 ppb.In addition,allan-Werle variance analysis shows that when the average time of the system is 150 s,the minimum detection limit of the system is 0.6ppb. |
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