基于2.004μm的离轴石英音叉增强型光声光谱测量CO2的研究

Research on Detecting CO With Off-Beam Quartz-Enhanced Photoacoustic Spectroscopy at 2.004μm

CO2是大气的重要组成成分,也是现代化工业社会过多燃烧煤炭、石油和天然气的产物。一方面大量的人源排放CO2进入大气是引发温室效应最主要因素,另一方面, CO2是窒息性气体,在封闭环境积累过高的CO2会导致窒息等安全问题。因此发展小型化、高灵敏度的CO2检测技术在大气环境探测、封闭环境工作区域安全监测等方面具有重要意义和应用需求。利用近年来快速发展的小型化石英音叉谐振增强光声光谱技术,采用相对简单的离轴结构方案,开展了探测CO2的研究。离轴石英音叉增强型光声光谱技术具有探测模块体积小、灵敏度高、抗干扰、成本低、功耗低,对激光器要求低等优点,在发展低功耗便携式气体传感器方面具有巨大的潜力。近年来,尤其是随着近红外激光器技术的逐渐成熟,为离轴石英音叉增强型光声光谱技术提供质量更好、能量更高的激励光源,使得离轴石英音叉增强型光声光谱检测技术具有更高的探测灵敏度,实现了在低浓度下对气体进行精确的检测。通过HITRAN 2012分子光谱数据库筛选出适合探测的谱线,选择2.004μm近红外分布反馈式半导体激光器作为激励光源,通过波长调制方式来激发CO2光声信号,并采用二次谐波检测技术实现光声信号的探测。实验中通过对进样CO2气体加湿、优化调制振幅等方式提高检测性能,实现了空气CO2的探测。在常压下,通过配气仪配置不同浓度的CO2样品,开展了浓度与信号的响应特性研究,获得了良好的线性响应结果。同时也开展了相同浓度CO2样品在不同压力下的信号测量研究,并用Allan方差对系统性能进行评估。结果表明,当平均时间为1 000 s时,系统的探测极限为4×10^-3μL·L^-1,在压力150 Torr时可获得最佳的测量信号,常压下系统对CO2的最小探测灵敏度为15μL·L^-1,相应的归一化噪声等效吸收系数为7.33×10^-9,在150 Torr下最小探测灵敏度为6μL·L^-1。

CO2 is an important component of the atmosphere and a product of excessive combustion of coal, oil, and natural gas in modern industrial societies. On the one hand, elevated concentrations of CO2 in the atmosphere can cause a greenhouse effect, which is mainly due to human activities. On the other hand, CO2 is a suffocating gas, and excessive accumulation of CO2 in a closed environment can lead to safety problems such as suffocation. Therefore, the development of miniaturized and highly sensitive CO2 detection technology has important significance and application requirements in the detection of atmospheric environment and safety monitoring of closed environmental work areas. In this paper, based on the rapid development of miniaturized quartz-enhanced photoacoustic spectroscopy technology, the research on CO2 detection has been carried out with a relatively simple off-beam structure scheme. Off-beam quartz-enhanced photoacoustic spectroscopy has the advantages of small size, high sensitivity, anti-interference, low cost, low power consumption and low laser requirements, and has great potential for developing low-power portable gas sensors. In recent years, especially with the gradual maturity of near-infrared laser technology, it provides better quality and higher energy excitation light source for this technology, Off-beam quartz-enhanced photoacoustic spectroscopy has a higher detection sensitivity and enables accurate detection of low concentration gases. The HITRAN database 2012 is used to screen out the suitable absorption line(at 4 989.97 cm^-1), and the 2.004 μm near-infrared distributed feedback semiconductor laser is selected as the excitation source. The CO2 photoacoustic signal is excited by the wavelength modulation method, and the second harmonic detection technology is used to detect the photoacoustic signal. In the experiment, the detection performance is improved by humidifying the injected carbon dioxide gas and optimizing the modulation amplitude, and the detection of air CO2 is realized. Under normal pressure, different concentrations of CO2 samples are arranged by gas distribution machine, and the response characteristics of concentration and signal are studied by preparing different concentrations of CO2 samples thought gas distribution machine, and a good linear response result is obtained. At the same time, the signal offixed concentration of CO2 sample under different pressures is measured, and Allan variance was used to evaluate the system performance. The results show that when the average time is 1 000 s, the detection limit of the system is 4×10^-3 μL·L^-1. The best 2f signal is obtained at a pressure of 150 Torr. The minimum detection sensitivity of the system for CO2 is 15 μL·L^-1 at atmospheric pressure, and it is reduced to 6 μL·L^-1 at 150 Torr.