腔增强吸收光谱技术中的腔镜反射率标定方法研究

Study on the Calibration of Reflectivity of the Cavity Mirrors Used in Cavity Enhanced Absorption Spectroscopy

腔增强吸收光谱技术具有实验装置相对简单、灵敏度高、环境适应性强等特点,是高灵敏吸收光谱技术的重要分支之一,在其应用过程中,腔镜反射率是影响其测量准确性的重要因素。利用2.0μm可调谐二极管激光器作为光源搭建了一套腔增强吸收光谱测量系统,使用两片反射率为99.9%的高反镜作为腔镜,以CO_(2)气体在5001.49 cm^(-1)处的吸收谱线作为研究目标,对两种简单实用的腔镜反射率标定方法进行了对比研究。第一种标定方法利用已知程长多通池作为参考池,通过测量谐振腔和多通参考池的输出吸收信号,比较二者的吸收率推导出腔增强系统中的有效吸收路径,然后通过镜片反射率和有效吸收路径的关系对腔镜反射率进行标定;第二种标定方法根据理想气体状态方程得到气体分子数密度,并结合数据库中的谱线线强值,实现了对腔镜反射率进行标定。结果表明,方法一中积分腔与参考池测得信号的积分吸收面积之比为10.5,经过多次测量并计算得到积分腔的有效吸收路径与镜片的反射率分别为302.65 m和99.85%,得到大气中CO_(2)气体的浓度为0.0373%,与实际大气CO_(2)的含量相符合,验证了此方法的准确性;该方法的优点是不受样品浓度影响,但因引入新的参考池,需要两池中气体的压强和温度都保持一致,此方法适用于开放式的腔体结构。方法二中测得大气中CO_(2)分子位于5001.49 cm^(-1)处吸收光谱,并结合大气中CO_(2)气体的分子数密度N为9.099×10^(15) molecule·cm^(-3),Hitran数据库中该条谱线线强为3.902×10^(-22) cm·molecule^(-1),计算得到镜片反射率约为99.84%;此方法优点是结构相较前一种方法更简单,但需要已知被测气体的分子数密度,因此在配置气体的过程中浓度、压力的误差会影响腔镜反射率的标定。由此可见两种镜片标定方法均可精确实现对腔镜反射率的标定,根据两种方法的特点,在实际应用中可选取相应适合的方法作为参考。

Cavity Enhanced Absorption Spectroscopy(CEAS)technology is an important part of high sensitive spectroscopy,which has the advanced characteristics of relatively simple apparatus,high sensitivity,and strong environmental adaptability.Using a 2.0μm tunable diode laser as the light source,combined with CEAS,a device for measuring the reflectance of the lens was built,and the absorption spectrum of CO_(2) gas at 5001.49 cm^(-1) was used as the research target.The path length absorption cell and the known concentration of absorption gas have been calibrated for the reflectivity of the same pair of high reflectivity lenses.The first calibration method uses a multi-pass cell with a known path length as a reference cell.In this calibration,the absorptions from the resonant cavity and the multi-pass reference cell are measured simultaneously and compared to deduce the effective absorption path of the cavity enhancement system.Then the mirror reflectivity is obtained from the relationship between the mirror reflectivity and the effective absorption path.In the second calibration method,the integrated absorbance of a CO_(2) transition in a mixture with known concentration are measured for calibration.The mirror reflectivity is calculated by the relation of the integrated absorbance,the gas molecule number density and the line intensity of the selected CO_(2) transition.Finally,the two methods are compared for the calibration results of the lens reflectivity.The results show that the ratio of the integrated absorption area of the signal measured by the integrating cavity to the reference cell in the first method is 10.5,and the effective absorption path of the integrating cavity and the reflectivity of the lens are 302.65 m and 99.85%,respectively.The concentration of CO_(2) gas in the atmosphere is 0.0373%,which is consistent with the actual atmospheric CO_(2) content.The advantage of this method is that it is not affected by sample concentration,but because of the introduction of a new reference cell,the pressure and temperature of the gas in the two cells need to be kept the same,so this method is suitable for an open cavity structure.In the second method,the absorption spectrum of CO_(2) is measured at 5001.49 cm^(-1),the molecular number density N of CO_(2) gas is 9.0991015 molecules·cm^(-3),and the line intensity of the line in the Hitran database is 3.902×10^(-22) cm·molecule^(-1),the reflectance of the lens is calculated to be 99.84%.The advantage of the above method is that the structure is simple,but the molecular number density of the gas needs to be known,so the error of concentration and pressure will affect the calibration of the reflectivity of the cavity mirror.These two methods can accurately calibrate the reflectivity of the cavity mirror,and the corresponding suitable method can be selected as a reference in actual application.