Yuan Chang-Ying, Liu Xian-Yong, Meng Gui, Zhao Liang
College of National Defense, Southwest University of Science and Technology, Mianyang 621000, China.
Guang Pu Xue Yu Guang Pu Fen Xi. 2010 Apr;30(4):879-82.
Photoacoustic detection of trace concentrations of gases is one of the most sensitive techniques of infrared absorption spectroscopy. High-sensitivity photoacoustic detectors apply an acoustic resonator for the amplification of the weak photoacoustic signal. If the modulation frequency coincides with one of the resonance frequencies of the chamber, a standing acoustic wave is excited and the system works as an acoustic amplifier. The amplification of the resonator relies on the acting mode, quality factor, nature of microphone, and the coupling between electromagnetic radiation and the stand wave resonance mode. With different incidence orientation of the modulated IR laser relative to acoustic chamber, the sound pressure magnitude of resonance mode varies. The influence of different laser incidence orientation on the coupling coefficients of radial resonance mode of cylindrical photoacoustic cells was investigated by both theoretical deduction and numerical computation method. It is concluded that the coupling coefficients have two zeros and two maximums when the laser incidence angle varies from 0 to pi/2. When the incidence angle is 0 or tan(-1) (0.859 2 X 2R/L), the coupling coefficients are zeros and the radial resonance is invalid. When the incidence angle is tan(-1) (0.556 8 X 2R/L) or tan(-1) (2R/L), the coupling coefficients are the maximums and the radial resonance is the strongest. Here R is the radius and L the length of the cell. The results therein before give some theoretical guidelines for photoacoustic cell designing, optimizing, installing and adjusting, and for improvement of detection sensitivity in trace gas detectors through maximal excitement of radial modes in cylindrical acoustic cells.
痕量气体的光声检测是红外吸收光谱中最灵敏的技术之一。高灵敏度光声探测器采用声学谐振器来放大微弱的光声信号。如果调制频率与腔室的共振频率之一重合，就会激发驻波，系统就如同一个声学放大器。谐振器的放大作用取决于其工作模式、品质因数、传声器的特性以及电磁辐射与驻波共振模式之间的耦合。随着调制红外激光相对于声学腔室的入射方向不同，共振模式的声压幅值也会变化。通过理论推导和数值计算方法研究了不同激光入射方向对圆柱形光声池径向共振模式耦合系数的影响。得出结论：当激光入射角从0变化到π/2时，耦合系数有两个零点和两个最大值。当入射角为0或tan⁻¹(0.859 2×2R/L)时，耦合系数为零，径向共振无效。当入射角为tan⁻¹(0.556 8×2R/L)或tan⁻¹(2R/L)时，耦合系数最大，径向共振最强。这里R是池的半径，L是池的长度。上述结果为光声池的设计、优化、安装和调整以及通过最大限度激发圆柱形声学池中的径向模式来提高痕量气体探测器的检测灵敏度提供了一些理论指导。
