Research And Application Of Highly Sensitive CO₂ Gas Sensor Based On Quartz Enhanced Photoacoustic Spectroscopy

Carbon dioxide（CO₂）gas is the main product of the combustion process and human activities,and it is also the most important greenhouse gas.The monitoring of CO₂ is of great significance for global environmental control,agriculture,industry,medical and other fields.Quartz enhanced photoacoustic spectroscopy（QEPAS）technology uses a quartz tuning fork（QTF）to replace the microphone in traditional photoacoustic spectroscopy（PAS）as an acoustic transducer to achieve highly sensitive detection of weak photoacoustic signals.QEPAS technology has been widely used in the detection of trace gases because of its compact structure,low cost,strong anti-noise ability,and no wavelength selectivity.In this paper,according to the principle of photoacoustic spectroscopy,the absorption line of the target gas molecule and the resonant frequency of the detection element are respectively improved,so as to improve the detection sensitivity of the CO₂ gas sensor based on QEPAS technology.This paper firstly explained the importance and significance of detecting CO₂ gas,and described the domestic and international status quo of CO₂ gas sensors based on QEPAS technology.The relevant theories of QEPAS technology were introduced,including the basic theory of gas molecular absorption,the principle of photoacoustic spectroscopy and the piezoelectric effect and characteristics of the quartz tuning fork.By taking the traditional QEPAS gas sensor as an example,its working principle was described.Aiming at the problem that wavelength-modulated light sources cannot be used in gas sensing system based on QEPAS technology,a fast and wideband electro-mechanical light modulator was proposed.The QTF was driven by sinusoidal signal to cause its vibrating prongs to block the laser beam periodically,so as to realize the amplitude modulation（AM）of laser wavelength.By optimizing the position relation between laser beam and QTF,the modulation efficiency of about 100%was obtained.Taking 1.57μm laser as an example,the experimental measurement results showed that the detection limit of the QEPAS gas sensor based on AM technology was 50 ppm for CO₂ gas in the integral time of 1 s.In order to solve the difficulties of assembling the photoacoustic detection module in traditional QEPAS gas sensing system,an all-solid-state photoacoustic detection module with fiber coupling was designed.The module integrated the fiber coupling module,acoustic resonator,preamplifier module and gas photoacoustic pool,which had the advantages of high stability,easy beam alignment,small size,and so on.Combined with the DFB laser with the central wavelength of 2004 nm,the CO₂ gas absorption line located at 2μm was selected as the target detection line,and the absorption line intensity was about two orders of magnitude higher than 1.57μm.A set of QEPAS-CO₂ gas sensor based on fiber coupling all-solid-state photoacoustic detection module was established.The results of Allan variance analysis showed that when the integral time was 1123 s,the minimum detection limit（MDL）reached 1.34 ppm,and the detection sensitivity fully met the detection requirements of ambient atmosphere and indoor CO₂ concentration.The gas absorption theory of photoacoustic spectroscopy indicates that the photoacoustic signal is inversely proportional to the resonant frequency f.In order to further improve the detection sensitivity of the CO₂ gas sensing system based on QEPAS technology,a QTF with a frequency of 28 k Hz was selected as an acoustic transducer to detect weak photoacoustic signals.The experimental results showed that the QEPAS signal intensity of the 28 k Hz QTF was twice that of the 32 k Hz QTF,which verified that the low frequency QTF is more favorable to the improvement of QEPAS signal.As a slow relaxation gas molecule,the relaxation rate of CO₂ gas molecules was promoted by humidifying the gas flow in the gas path.The results showed that the QEPAS signal can be increased by about 22.8%by humidifying the gas flow.Finally,a set of portable CO₂ gas sensing system based on QEPAS technology was built.When the integration time was 125 s,the MDL reached 0.7 ppm.The weight of the whole system was less than 10 Kg.The system was placed in a greenhouse in Guangzhou for continuous measurement of CO₂ gas concentration for 24 hours,which verified the stability and reliability of the system.