Research On The Enhancement Of SNR Of QEPAS System Based On Resonant Tube Structure Optimization And Frequency Stabilization

Trace gas detection plays an increasingly important role in industrial process control analysis,Mining Industry and other fields.Nevertheless,the primary detection methods,reported so far,were Electrochemical Sensing Technology,which unable to realize gas detection accurately and real-timely.Optical fiber gas sensing technology has got the extensive research and application with its flexible,heat-resistant and anti-radiation,Quartz-enhanced photoacoustic spectroscopy(QEPAS)is one of the representative technology,which using acoustic micro-resonator(Am R)with quartz tuning fork to enhance the photoacoustic signal,and thereby improve sensitivity of the system.There are two kinds of traditional acoustic micro-resonator(Am R),coaxial configuration and off-axis configuration.Coaxial configuration has a higher requirement for laser beam,which is difficult to adjust,while the off-axis resonance configuration can ably avoid the problem of laser beams alignment,but the traditional single off-axis configuration has a low improvement on the detection performance of the system.Therefore,optimizing the structure of acoustic resonance to improve the SNR of the system and realizing the real-time detection of sensor has become a heated subject in the academia.In this paper,the QEPAS sensor was proposed based on the structure of multi off-axis resonance tube,which can enhance the photoacoustic signal and lock the center absorption wavelength to shorten the time of detection and thereby reduce the noise,Compared with the traditional detection system,the QEPAS sensor has more advantages,such as short detection time,high detection sensitivity and so on.In the research,the QEPAS system was designed and built based on the principle of photoacoustic spectrum and basic circuit knowledge.two new schemes of off-axis resonance structure were proposed according to the technical characteristics of off-axis photoacoustic spectrum,one was single tube double side resonance structure,the other was double tube double side resonance structure.Through the theoretical analysis and characteristic research of the micro acoustic resonance tube,the optimal parameters of the two resonance structures were determined.In this paper,water vapor was used as the sample gas to test the performance of two new structures,and the photoacoustic signals obtained from the test were compared and analyzed.The results show that the signal of single tube double side resonance structure was 19.38 times higher than the bare tuning fork,while the signal of double tube double side resonance structure was 26.8 times higher than the bare tuning fork,which greatly improves the detection sensitivity of the system.In order to solve the detect of photoacoustic system with long detection time,an effective method named frequency stabilization method was adopted,which lock the center absorption wavelength based on Maximum absorption intensity of the gas,and improve the SNR by reducing the bandwidth of phase-locked amplifier and adopting Cumulative average algorithm.In this paper,frequency stabilization method is proved to be feasible through the experiment,compared with the Constant current method,What's more,compared with the traditional scanning method,the SNR of the system is increased above 11 times.These results illustrated that the frequency stabilization method has great potential in the optoacoustic spectrum system Power.