近共心腔气体拉曼光谱增强方法研究

Raman Signal Enhancement for Gas Detection Using a Near Concentric Cavity

对海水中溶存气体(甲烷、二氧化碳等)的探测是海洋环境监测和资源探测的重要环节,由于拉曼光谱技术可实现多组分同时探测的优势,被视为发展海洋溶存气体探测技术的首选,而探测灵敏度的提高则是推动该项技术实用化的关键。针对提高拉曼光谱气体探测灵敏度这一问题,设计并搭建了一套基于近共心腔拉曼信号增强系统,开展了信号收集方向和激发光多次散射模式对信号强度和信噪比影响的研究。研究发现,信号收集方向与腔镜光轴之间的夹角越小,所收集的信号越强、信噪比越大,当夹角为30度时,信噪比是垂直方向的16倍;近共心腔直线型多次反射模式,与共心腔模式相比信噪比增强了近三倍。采用最优化实验条件,该系统与常规拉曼系统相比,信号强度和信噪比增强效果均在70倍左右。根据该系统对空气中CO_2的相应强度(空气中CO_2的浓度为0.648mg·L^(-1)),以三倍于噪声强度计算检测限,估算出该系统对CO_2的探测灵敏度约为0.19 mg·L^(-1),依据CO_2与CH_4拉曼散射截面比为1/6,估算该系统对甲烷的检测限约为11.5μg·L^(-1)。

The detection of dissolved gases in seawater plays an important role in ocean observation and exploration.Raman spectroscopy has a great advantage in simultaneous multiple species detection and is thus regarded as a favorable choice for ocean application.However,its sensitivity remains insufficient,and a demand in enhancements is called!for before putting Raman spectroscopy to actual use in marine studies In this work,we developed a near-concentric cavity,in which laser beam could be trapped and reflected back and forth,for the purpose of intensifying Raman signals.The factors that would influence Raman signals were taken into account.The result show that the smaller angle between collection direction and optical axis of reflection mirror,the stronger the signal and signal to noise ratio （SNR）is.With a collection angle of 30°,our Near-concentric Cavity System managed to raise the SNR to a figure about 16 times larger than that of common methods applying 90°.Moreover,the alignment pattern in our system made it possible to excel concentric cavity with a 3 times larger SNR.Compared with the single-pass Raman signal,the signal intensity of our near-concentric cavity was up to 70 times enhanced.According to the obtained results of CO2 measurement,it can be seen that the new system provides a limit of detection（LOD）for CO2 about 0.19 mg·L-1 using 3-σcriterion standard,and the LOD of 11.5 μg·L-1 for CH4 was evaluated with the theoretical cross section values of CO2 and CH4 .