基于全光型石英增强光声光谱的痕量气体探测系统研究

Study on the Trace Gas Detection Sensor Based on an All-optical Quartz-Enhanced Photoacoustic Spectroscopy

提出一种应用于乙炔气体探测的全光型石英增强光声光谱探测系统,采用具有正交相位点自稳定特性的外腔式光纤珐珀干涉仪作为光声信号解调单元,从而有效提高系统探测灵敏度和精度﹒基于波长调制技术,分析了调制深度系数对2次谐波信号的影响,并选择6 534.36 cm-1作为实验用乙炔吸收线,实验得到了该系统最佳调制系数为2.2,对应的最佳调制深度为0.179 5 cm-1;实验还得到了光声信号与10~300 ppmv乙炔浓度的线性关系,其线性度为0.997;进一步得到该系统的探测极限为584 ppbv,对应的归一化等效噪声吸收系数为2.4×10-7 cm-1?W?Hz-1/2,而在相同条件下,采用传统电解调方式得到的归一化等效噪声系数为1.3×10-6cm-1?W?Hz-1/2﹒结果表明,相对于传统QEPAS系统,该系统的探测灵敏度提高了4.4倍﹒最后,采用Allan-Werle方差对系统的长期稳定性进行分析,由此得到系统的最佳平均时间为102 s时,系统的最小可探测极限降低到136 ppbv﹒

An all-optical quartz-enhanced phtoacoustic spectroscopy（QEPAS） system for acetylene（C2 H2） detection was proposed. The extrinsic optical fiber Fabry-Perot interferometric demodulation unit with quadrature point self-stabilization was used as the sensor for the QEPAS signal to improve the detection sensitivity and precision. The influence of modulation depth coefficient on the second harmonic QEPAS signal was achieved based on the wavelength modulation theory. Experimentally, the optimum modulation depth coefficient of 2.2 was obtained for the C2 H2 6 534.36 cm-1 absorption line, corresponding to the modulation depth of 0.179 5 cm-1. The linear relationship between QEPAS signal and C2 H2 concentration in a range from 10 to 300 ppmv is obtained and the correction coefficient is 0.997. The minimum detection limit of 584 ppbv is obtained, corresponding to the normalized noise equivalent absorption（NNEA） coefficient of 2.4×10-7 cm-1？W？Hz-1/2. Comparing with NNEA of NNEA coefficient of 1.3×10-6 cm-1？W？Hz-1/2 obtained by a conventional QEPAS system, it enhanced detection sensitivity by a factor of 4.4. Finally, the long-term stability of the system was evaluated by the Allan-Werle deviation analysis, the minimum detection limit of 136 ppbv can be achieved for the optimum averaging time of 102 s.