基于功率增强型QEPAS技术的二氧化碳气体高灵敏检测研究

摘要

Carbon dioxide, the major constituent of the atmosphere and burnt gas, has great significance in high sensitive detection in physics and chemistry as well as in the life sciences applications. The existing CO2 detection methods have some defects, which makes the detection difficult to meet the need of the national defense scientific research, energy and chemicals as well as the clinical human breath analysis. Quartz-enhanced photoacoustic spectroscopy (QEPAS) technology invented lately has the advantages of high selectivity, compactness and immunity to environmental acoustic noise. Based on the fact that the QEPAS detection sensitivity scales linearly with excitation laser power, a power boosted QEPAS sensor for CO2 detection is developed. The sensor is based on QEPAS with an erbium-doped fiber amplified 1 572 nm distributed feedback (DFB) laser. In order to reduce the sensor background noise to the thermal noise of the quartz tuning fork, wavelength modulation spectroscopy and the harmonic detection technique were employed. In order to optimize the sensor performance, the laser wavelength modulation depth was optimized at normal atmosphere. A 3.5 ppm detection limit was obtained in the condition of 1 495 mW laser power, 0.33 cm-1 modulation depth and 0.833 Hzdetection bandwidth. The corresponding normalized noise equivalent absorption coefficient was 1.01×10-8 W·cm-1·Hz-1/2.%二氧化碳 (CO2) 是环境大气以及燃烧废气的主要成分, 同时也是重要的化工原料, 对其浓度进行高灵敏度检测在物理、生物、化学等众多学科中均有重要的应用.传统检测方法已经无法满足国防科研、能源化工、医疗诊断等科技前沿领域中对CO2浓度检测的需求.石英增强光声光谱 (QEPAS) 技术是近年来发展迅速的一种激光检测技术, 具有高分辨率、小体积、对环境噪声免疫等优点.基于QEPAS技术探测灵敏度与激励光功率成正比的特性, 以中心波长为1 572 nm的窄线宽分布反馈式半导体激光器为激励光源, 将掺饵光纤放大器 (EDFA) 与QEPAS技术联用, 提出了功率增强型QEPAS技术, 实现了光声信号的大幅度提高.此外, 通过波长调制技术、谐波解调技术以及电调制相消技术的使用, 成功将装置的整体噪声压制在音叉式石英晶振的理论热噪声水平.激光波长调制深度对装置信号幅度的影响也通过实验在一个标准大气压下进行了研究.结果显示, 对6 361.25 cm-1处CO2气体吸收线, 当激光功率为1 495 mW, 调制深度为0.33 cm-1, 系统探测带宽为0.833 Hz时, 功率增强型QEPAS装置对CO2的探测灵敏度为3.5 ppm, 归一化等效吸收系数为1.01×10-8 W·cm-1·Hz-1/2.