Dual-pass Off-beam Quartz-enhanced Photoacoustic Spectroscopic Gas Sensor
The detection of trace gases is of great significance in the fields of fire alarm,industrial production,biomedicine and so on.Long-term and stable monitoring is needed for the explosive,toxic and harmful gases.Acetylene(C2H2),as a characteristic gas produced in transformer overheating and discharge,is an important parameter to monitor transformer faults.Optical gas sensors based on Lambert-Beer law are more suitable for real-time and long-term gas detection because of their advantages such as high sensitivity,small drift and non-invasive.In 2002,Quartz Enhanced Photoacoustic Spectroscopy(QEPAS)based on Quartz Tuning Fork(QTF)was proposed.Quartz tuning fork has the advantages of high-quality factor,stable resonant frequency and narrow resonant band width,so it has higher sensitivity and stronger anti-environmental noise ability than traditional PAS.In addition,small size and low cost of QTF make a QEPAS system small,portable and easy to integrate.To further improve the detection sensitivity of QEPAS system,methods such as using acoustic micro-resonator(AmR),customizing special-size QTFs and multi-channel detection were widely used.Sensor systems based on QEPAS shows excellent performance in gas detection and provide an excellent scheme for real-time and long-term monitoring of trace gases.However,there is still a problem that part of the laser power will lose due to partial irradiation on the QTF surface,which is not conducive to improving the detection sensitivity.In order to reduce the difficulty of system assembly and optical path adjustment,and avoid the photothermal effect caused by laser beam irradiation on the surface of the tuning fork,off-beam resonant tube configuration is used to isolate the detection beam from the quartz tuning fork,and two resonant tubes with central grooves are placed on both front sides of the tuning fork.To further improve the detection sensitivity,the optical path is equipped with a right-angle prism to realize dual-pass measurement.Optical fiber collimator,resonant tube,QTF,right angle prism and stainless steel gas chamber are integrated,and the overall module size is 5.4 cmX4.0 cmX 1.5 cm.Taking C2H2 as the target gas,the performance of the sensor system is evaluated,and the reliability of the micro-miniature QEPAS gas sensor system is verified.Firstly,the theory of laser absorption spectroscopy,the principle of wavelength modulation technology and the principle of photoacoustic spectroscopy are analyzed,and the relationship between photoacoustic signal and gas concentration is obtained.To derive the optimal resonant tube size and laser beam excitation position,the acoustic finite element method was used to simulate and optimize the resonant tube size and laser beam excitation position in COMSOL.According to the simulation results,a stainless steel capillary tube was micro-machinized,and the tube was 0.6 mm in diameter and 8.8 mm in length,and it was assembled at 0.6 mm away from the top of the QTF.The grooves cut in the center of the resonant tube are equal in width to the interfinger gap of the QTF,so that the sound field in the tube leaks.The two resonant tubes are placed on both sides of the front of the QTF,and the grooves are aligned with the interfinger gap of the QTF for sound detection.The optical path is equipped with a right angle prism,and the incident light of the collimated fiber is reflected twice through the right angle prism to achieve dual-pass measurement,which enhances the sensitivity of the system.Compared with the traditional on-beam configuration,the difficulty of system assembly is greatly reduced,the photothermal effect is completely avoided,and the utilization rate of the laser is further improved.The optical fiber collimator,resonant tube,QTF,right angle prism and stainless steel gas chamber are integrated,and the overall size of the acoustic detection module is 5.4 cm×4.0 cm×1.5 cm,which meets the miniaturization standard.The experimental results show that there is a good linear relationship between the photoacoustic signal and the gas concentration,and the fitting goodness is 0.991.The minimum detection limit and normalized noise equivalent absorption coefficient of system are 11.6×10-6 and 6.7×10 9 W·cm-1·Hz-1/2Finally,the Allan variance analysis was carried out.When the averaging time is 0.5 s,the minimum detection limit of the system is 10.22×10-6,and when the averaging time is 50 s,the minimum detection limit is 1.20X 10-6.These results verify the reliability and stability of the miniature QEPAS gas sensor system,and provide a new idea for the development of portable C2H2 sensor system.
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