Theoretical research on quartz enhanced photoacoustic spectroscopy base on the resonance in an elliptical cavity

Abstract

As a new optical detection technique,quartz enhanced photoacoustic spectroscopy (QEPAS) has been widely used in the field of trace gas detection,which has an outstanding performance because of its advantages of extremely high sensitivity,high selectivity and compact absorption detection module.The most important factor of the detection sensitivity for QEPAS sensor is the acoustic wave enhancement.For increasing the acoustic enhancement,great effort has been devoted to the investigations by increasing laser power,employing tube resonators and using custom-made acoustic transducers.However,less attention has been paid to the elliptical cavity enhancement photoacoustic spectroscopy.In this work,novel quartz enhanced photoacoustic spectroscopy based on an elliptical cavity is proposed,which employs two quartz tuning forks and an elliptical cavity to further enhance the acoustic wave.The analysis and optimization of the elliptical cavity are also demonstrated. For the elliptical cavity QEPAS sensor,the acoustic enhancement properties can be influenced by resonant modes, coupling between laser and acoustic wave,and dimension of the cavity.Based on the Helmholtz wave equation,the acoustic modes and corresponding resonance frequency can be quantized.To further investigate the acoustic wave resonance inside the cavity,the model of the cavity is established in Comsol Multiphysics software with finite element method.The acoustic pressure,quality factor can be obtained numerically by the software.With the model,parameters of the spectrophone are investigated,including the resonant modes,laser incidence angle and dimension of the elliptical cavity.As a result,the (2,1) resonant mode is selected as the enhancement mode in the cavity,in which the maximum acoustic pressure is achieved at the ends of the long axis.By changing the incidence angle of the laser beam from 0 to 90,the performance of the sensor is analyzed,which indicates that the laser incidence angle has little influence on acoustic properties except for 30.This is due to the interaction of other resonant modes at this incidence angle.With the length of half-long axis varying from 4.8 mm to 5.2 mm,eccentricity from 0.5 to 0.8 and the cavity height from 0.4 mm to 0.8 mm,the resonance frequency,acoustic pressure and quality factor are studied.It reveals that there is an optimal length of half-long axis for a fixed eccentricity,and a relative large height is beneficial to enhancing the acoustic pressure.On the whole,a set of parameters is identified for the optimal sensor performance. By optimizing and designing the spectrophone,the experiment is conducted,in which a laser (1578 nm) and H2S as the sample gas are used.The detection limit of H2S gas of 6.3 ppm is achieved and the corresponding Normalized noise equivalent absorption coefficient (NNEA) is 2.0210-9cm-1W/Hz1/2.Finally,several H2S detection results of other QEPAS methods are listed and compared for demonstrating the high detection sensitivity of the sensor.This work may contribute to the research of high sensitivity photoacoustic detection.