Quartz Tuning Fork Enhanced Photoacoustic Spectroscopy System Based On Lens-mirror Combination

Quartz-enhanced photoacoustic spectroscopy technology,as a relatively mature trace gas detection technology,has the advantages of high sensitivity,low loss,anti-interference,low power consumption and low cost,and is very suitable for miniaturized detection of various trace gases.The application and significance of photoacoustic spectroscopy(PAS)technology in trace gas detection is introduced in this paper,and then introduces the off-beam quartz enhanced photoacoustic spectroscopy system(off-beam LR-QEPAS system)based on the lens-reflector combination.In theory,the gas absorption law,spectral line intensity,linear function,the principles of wavelength modulation,and characteristics of the quartz tuning fork(QTF)are described;then the proportional the QTF and its single-arm model are simulated by finite element analysis software COMSOL,including the influence of various parameters of the QTF on the eigenmode frequency of the QTF,and the relationship between the laser power and the strain and potential difference of the QTF are simulated in the ideal environment and the actual experimental environment.Finally,an off-beam quartz enhanced photoacoustic spectroscopy system based on a lens-reflector combined structure was built.This structure can make the laser beam reciprocate four times in the resonance tube and further increase the laser power in the current QEPAS technology.We described the composition,placement,construction and working principle of the lens-reflector combined structure in detail.In order to increase the accuracy of the experiment,the traditional off-beam QEPAS system and two double-pass QEPAS systems were introduced in the experiment as a reference group to evaluate the performance of the off-beam LR-QEPAS system.We optimized the parameters of the modulation depth and studied the relationship between the photoacoustic signal and the number of times the laser passes through the resonance tube.The off-beam QEPAS system based on the lens-reflector combination uses water vapor as the trace gas to measured,and the performance of the system is research at a room temperature of 25°C under one atmospheric pressure.The result shown that the normalized noise equivalent absorption(NNEA)coefficient reaches 3.46×10-9 cm-1W·√Hz.In order to determine the minimum detection limit and long-term stability,the Allan variance is used to process the signals that obtained by continuous acquisition through 1500 s,and finally the lowest detection limit obtained 75.4 ppbv with the integration time of 64 s.Compared with the traditional off-beam QEPAS and double-pass QEPAS systems,the application of lens-reflector combinations in the off-beam LR-QEPAS system significantly improves the detection sensitivity.The main research results and innovations of this article:It is the first time to introduce a lens-reflector combination into the quartz tuning fork enhanced photoacoustic spectroscopy system.Commonly,the laser beam passes through the resonance tube twice at most in the research of the traditional multi-pass QEPAS system,but lens-reflector combined structure realizes the laser beam reciprocating through the miniature resonance tube four times.Compared with the double-pass QEPAS technology,the off-beam LR-QEPAS system has better photoacoustic signal enhancement。In the current research,the application of QTF in QEPAS technology is more extensive,but there are relatively few reports on simulation.In this paper,COMSOL finite element analysis software is used to simulate and simulate the quartz tuning fork and its single-arm model.And under ideal and experimental conditions,the changes in potential difference and stress displacement of the quartz tuning fork and its single-arm model when the laser power changes are simulated.The experiment introduced the off-beam QEPAS and two double-pass QEPAS systems as the reference group to evaluate the performance of the off-beam LR-QEPAS system.The off-beam LR-QEPAS technology is studied for the measurement of 1.2% water vapor in the open atmosphere.At room temperature and one atmospheric pressure,the off-beam LR-QEPAS system reaches 75 ppbv with integrate time of 65 s,which corresponds to the NNEA coefficient is 3.46×10-9 cm-1W·√H.