光纤倏逝波型石英增强光声光谱技术

采用块状光学准直聚焦透镜组的传统石英增强光声光谱(QEPAS)技术存在体积难以缩减,结构稳定性不佳,无法适应空间狭小、振动复杂的特殊环境等缺点.基于此,将光纤倏逝波技术与QEPAS技术相结合,提出了一种新型微纳结构光纤QEPAS痕量气体检测技术.实验中,为了提高QEPAS系统信号幅值,优化了石英音叉与激光束的空间位置、激光波长调制深度,同时对比了两种不同共振频率的石英音叉,最终采用共振频率较低的30.720 kHz石英音叉作为声波探测元件,获得的检测极限为6.25×10~(-4)(体积分数),归一化噪声等效吸收系数为4.18×10~(-7)cm~(-1).W·Hz~(-1/2).

Fiber evanescent wave quartz-enhanced photoacoustic spectroscopy

In a conventional system of quartz-enhanced photoacoustic spectroscopy (QEPAS), the size of block-like optical collimation focusing lens group is difficult to reduce, and the structural stability is poor, which makes it hard to adapt itself to some special conditions, such as narrow space and vibrating circumstance. Based on this situation, in this research the fiber evanescent wave technique is combined with QEPAS. Therefore, trace gas detection for acetylene (C₂H₂) based on an all-fiber structural QEPAS system is developed. To obtain the characteristics of fiber evanescent wave, the optical distribution of micro structural fiber is simulated and the evanescent wave power ratio is calculated based on the COMSOL Multiphysics software. In order to increase the QEPAS 2f signal amplitude, the optical path between fiber taper and quartz tuning fork (QTF) and the laser wavelength modulation depth are optimized. In addition, two kinds of QTFs with different resonant frequencies are optimized. Finally, a QTF with a lower resonant frequency of 30.720 kHz is adopted as the acoustic wave transducer, and a minimum detection limit (MDL) of 6.25×10^-4 (volume fraction) is obtained with a laser wavelength modulation depth of 0.24 cm^-1. To investigate the evanescent wave power of micro structural fiber, the fiber taper diameter is measured by a scanning electron microscope. Subsequently, by combining the diameter of fiber taper with the theoretical calculation results, we determine an evanescent wave power of 455.9 μW, and the normalization of noise equivalent absorption (NNEA) which indicates the sensor sensitivity is 4.18×10^-7 cm^-1·W·Hz^-1/2.