基于离轴腔增强吸收光谱双组分CH4/H2O高灵敏度探测研究

High Sensitivity Detection of Two-Component CH4/H2O Based on Off-Axis Cavity Enhanced Absorption Spectroscopy

H2O和CH4在气候变化过程中起着关键作用,实时在线测量H2O和CH4浓度一直都是国内外学者研究的热点问题之一。利用1.653μm可调谐半导体激光器作光源,结合反射率为99.9976%的两片高反射镜组成离轴腔增强吸收光谱装置,开展了H2O和CH4的高灵敏度测量研究。离轴腔增强系统的有效吸收光程通过吸收面积-浓度关系法来标定,吸收面积-浓度关系法的可行性首先通过已知光程的光学吸收池进行验证,确定有效后用于标定离轴腔增强系统的有效光程。结果表明,基长为21cm的离轴腔增强系统的有效吸收光程达到了8626.3m。当谐振腔内压力为5.06kPa时,利用7组不同浓度的CH4标准气体(0.2~1.4μmol·mol^-1)对系统进行了线性响应标定测试,得到了CH4吸收的积分面积与浓度拟合关系曲线。系统的稳定性、可实现的最小探测灵敏度等信息通过Allan方差进行分析,结果表明系统对探测CH4的最佳平均时间为100s,最小可探测浓度极限为7.5nmol·mol^-1;系统对探测H2O的最佳平均时间为200s,最小可探测浓度极限为55μmol·mol^-1。对提高系统测量精度的数据处理方法也进行了分析研究,结果表明相比于多次平均方法,Kalman滤波能显著的提高测量精度,而且缩短了系统的响应时间。最后,利用搭建的离轴腔增强实验系统结合Kalman滤波数据处理方法对实际大气中CH4和H2O浓度进行了连续两天的测量,CH4每天平均的浓度分别为2.1和2.08μmol·mol^-1,H2O每天平均的浓度分别为11515.6和11628.6μmol·mol^-1,由此可知建立的离轴腔增强吸收光谱装置能够用于大气CH4和H2O的测量,另外建立的系统也可用于相关工业领域的高灵敏度CH4和H2O监测。

H 2O and CH 4 play key roles in the process of climate change,then real-time online measurement of H 2O and CH 4 concentrationshas always been one of the hot issues of domestic and foreign scholars.In this paper,an off-axis cavity enhanced absorption spectroscopy device was establishedwith two high reflectivity mirrors of 99.997 6%combining a tunable semiconductor laser operating around 1.653μm as the light source,andthe high sensitivity measurement of H 2O and CH 4 was carried out.The effective absorption optical path of the system was calibrated by the absorption area-concentration relationship.The feasibility of the absorption area-concentration relationship was first verified by an optical absorption cell with a known optical path,and it was used to calibrate effective optical path of the cavity enhancement system.The results showed that the effective absorption path of the cavity enhancement system with the base length of 21 cm reached 8 626.3 m.The linear response calibration test was carried out with 7 groups of CH 4 standard gases of different concentrations(0.2~1.4μmol·mol^-1)when the pressure was 5.06 kPa,and the fitting relationship curve between the integrated area of CH 4 absorption and the concentration was obtained.The stability of the system and the minimum detectable sensitivity were analyzed by Allan variance.The results showed that the optimal average time for detecting CH 4 was 100 s,and the minimum detectable concentration limit was 7.5 nmol·mol^-1.The optimal average time for detecting H 2O was 200 s,and the minimum detectable concentration limit was 55μmol·mol^-1.The data processing method for improving the measurement precision of the system was also analyzed.The results showed that Kalman filtering could greatly improve the measurement precisionand reduce the response time of the systemcompared to the multiple averaging method.Finally,the experimental system of off-axis cavity enhanced absorption spectroscopy device combining with Kalman filtering technology was used to measure the CH 4 and H 2O concentration in real atmosphere for two days.The average daily concentration of CH 4 was 2.1 and 2.08μmol·mol^-1,respectively.The average daily concentration of H 2O was 11 515.6 and 11 628.6μmol·mol^-1,respectively.It can be seen that the experimental device of off-axis integration cavity can be used for atmospheric CH 4 and H 2O detection,andthe established system can also be used for high-sensitivity CH 4 and H 2O monitoring in relevant industrial fields.