基于椭圆腔共振的石英增强光声光谱理论研究

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

石英增强光声光谱技术作为一种新型的光学检测技术,已被广泛应用于痕量气体检测场合.其中声波共振增强性能是决定检测灵敏度的重要因素.为提高光声光谱检测系统的信噪比和检测极限,提出一种新型的椭圆腔共振石英增强光声光谱检测方法,建立了其声学特征模型并利用有限元分析方法对光声腔内部声学特性进行仿真研究.研究结果表明,椭圆腔的特征模态在(2,1)模态下长轴两端声压达到最大值.通过对椭圆腔的尺寸和形状进行优化,建立实验装置,得到目标气体硫化氢检测极限为6.3 ppm(parts per million),相应的归一化噪声等效吸收系数为2.02×10^(-9)cm^(-1)W/Hz^(1/2).

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 from0.4 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 H_2S as the sample gas are used. The detection limit of H_2S gas of 6.3 ppm is achieved and the corresponding Normalized noise equivalent absorption coefficient（NNEA） is 2.02 × 10^（-9）cm^（-1）W/Hz^（1/2）. Finally, several H_2S 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.