金属镀层毛细管增强受激拉曼散射增益光谱气体分析

Raman spectroscopy is a widely applied analyzing method. To overcome the shortcoming of low signal level of gas samples, cavity-enhanced Raman analysis and hollow-fiber-enhanced Raman analysis (in which hollow-core photonic crystal fiber HC-PCF or metal-lined capillary is used as sample cell) are studied by the researchers. But there are still problems left for these routes of spontanuous Raman spectroscopy: for the cavity-enhanced one, consumption of sample is large; for the hollow-fiber-enhanced one, the interference of Raman/fluorescence background from fiber wall can not be neglected. In this project, Analyzing of trace gas by stimulated Raman gain spectroscopy enhanced by metal-lined capillary is proposed. Stimulated Raman scattering is realized in metal-lined capillary of which the core size may be as 10 times large as that of HC-PCF with narrow-linewidth excitation source of diode-pumped solid state laser or semiconductor laser and widely tunable probe source of semiconductor laser, by which detection of trace gas with high sensitivity, high resolution and fast renewing of gas sample will be possible. The propose of this project is characterized by fast speed of sample renewing, compact size of experimental setup, and high feasibility for field application. The results of this project will be helpful for the developing of high performance Raman gas analyzers for field applications.

拉曼光谱是一种宽适应范围的检测手段。为应对气体拉曼散射信号微弱的缺陷，研究者对腔增强、空芯光纤（包括空芯光子晶体光纤HCPCF和金属镀层毛细管）增强的自发拉曼散射气体分析进行了探索，但仍存在腔增强拉曼散射测试样品需求量大、空芯光纤纤壁拉曼/荧光背景干扰严重等缺陷。本项目提出金属镀层毛细管增强的受激拉曼增益光谱微量组分气体分析的研究方案，利用二极管泵浦固体激光DPSS或窄线宽半导体激光作激发光源，宽波长调谐范围半导体激光作拉曼增益探测光源，在金属镀层毛细管内（芯径可在HCPCF的10倍以上），实现受激拉曼散射增益增强，抑制毛细管管壁拉曼/荧光背景的干扰，完成对微量组分气体的高灵敏度、高分辨的快速检测，具有样品更新快速、装置体积小巧，更适合现场应用的特点。项目的研究结果，对现场应用的高性能拉曼光谱气体分析仪器的开发有重要价值。

连续谱背景信号是影响拉曼气体分析灵敏度和应用范围的重大因素。项目提出利用金属镀层毛细管/空芯光纤约束气体拉曼散射信号，利用拉曼增益/损耗谱探测规避器壁拉曼荧光干扰的方案，提高气体分析的检测限，推广拉曼气体分析的应用范围。.分析发现，金属镀层毛细管的高分子保护层以及金属膜层本身的吸收是连续波背景的重要来源，毛细管气体拉曼散射的信背比与芯径成反比，拉曼信号增强与高信背比对毛细管的结构要求相反，不易协调获得理想的信号增强。在金属镀层毛细管/微结构空芯光纤对比研究结果的基础上，选择最近发展出来的芯径30 um的单层反谐振空芯光纤作为拉曼信号增强的主要气体样品池。.进行拉曼增益/拉曼损耗光谱等受激拉曼光谱技术的研究，需要有可调谐的激光器。本项目利用光栅外腔结构，对多种不同波段的半导体激光器，实现了可调谐的窄线宽输出，无跳模调谐范围，可达到20 GHz以上。.采用自制光栅外腔半导体激光器、532 nm半导体激光泵浦的固体激光器、或者染料激光器的组合，获得了多种气体的连续波激光激发的拉曼增益谱、拉曼损耗谱、CARS光谱；采用峰值功率约20 kW的波长532 nm的脉冲激光器，获得了宽范围的级联受激拉曼散射，这些结果体现了微结构空芯光纤明显的拉曼散射增强作用。研究发现，连续波激光的拉曼增益谱/损耗谱、CARS光谱是一种高分辨光谱，信号强度与气体分子的振转能级结构密切相关；空芯光纤增强的拉曼增益光谱/损耗光谱最适合的应用范围是气体主要成分拉曼散射光谱的精细结构；而空芯光纤增强的受激拉曼散射，为新型的拉曼气体激光器提供了可能。.利用微结构空芯光纤低背景信号的特点，进行了自发拉曼散射气体分析的研究。200 mW的532 nm入射激光功率，60 sec积分时间，CH4, C2H2, C2H4, C2H6等多种C – H气体的3σ检测限达到接近 1 ppm-Bar的水平。这一结果，为拉曼散射气体分析在微量气体成分分析方面的应用提供了基础。