拉曼光谱作为一种激发光谱, 采用激光作为激发光源, 在气体检测中可以激发所有气体分子的拉曼信号。 由于气体的分子密度低、 透光度高、 拉曼散射截面小, 导致激发光能量的利用效率低； 拉曼信号散射向四周立体空间而常规收集方法只能收集较小的空间立体角, 从而造成检测限较差而不能广泛应用于气体的检测。 提出了一种拉曼直角反射共焦腔用来提高气体等透明样品的拉曼检测的检测灵敏度。 拉曼直角反射共焦腔利用直角反射镜将入射光反射回原方向但是光路具有空间偏移的特点, 采用两个相对放置、 互相平行的直角反射镜, 将光束直径为0.7 mm的激光在工作直径为25.4 mm的共焦腔内10次来回反射, 并采用共焦点相对放置的两个透镜将激发光聚焦到焦点, 从而提高激发光能量的使用效率。 拉曼散射向激光传输方向的信号被直角反射镜反射向原方向, 经过透镜聚焦到焦点后和拉曼散射向激光入射方向的信号一起经过长通滤光片后传输向拉曼光谱仪, 从而提高了拉曼散射信号的收集效率。 以空气作为测试对象进行实验, 300 s内可以获得清晰的CO2的拉曼光谱和N2, O2的精细拉曼光谱并对其强度比进行了分析, 其中N2的2 332 cm-1, O2的1 557 cm-1, CO2的1 388 cm-1的拉曼峰的峰高比是785∶257∶1。 拉曼直角反射共焦腔在常规拉曼散射激发收集光路的基础上增加了两个直角反射镜和一个聚焦镜, 具有体积小, 结构简单, 易于调节的特点。 拉曼散射向周围空间的信号强度分布与入射光的入射方向有关, 在沿入射光方向及其相反方向散射信号强度最大, 拉曼直角反射共焦腔设计的收集散射信号的角度与散射信号强度分布最强方向一致, 并且利用了光学景深的优势, 最大化的提高了拉曼散射信号收集效率。 拉曼直角反射镜腔可以拓展拉曼光谱技术在气体检测中的应用, 例如用于气相化学反应的原位监控、 发动机燃烧过程及排放物检测、 未知污染物气体分析等气体成分复杂的领域。

As an excitation spectrum, Raman spectroscopy uses laser as an excitation source to excite Raman signals of all gas molecules. Due to the low molecular density, high transmittance of light and low Raman Scattering Cross Section, the utilization efficiency of the eycitation light energy is low, and the Raman signal scatters to space around focus, only fraction of signal can be collected by collecting system. As a result, the detection limit is poor and cannot be widely applied to the detection of gas. In this paper, a Raman right angle reflection cavity was proposed to improve the detection limit of Raman detection of transparent samples such as gases. The Raman right angle reflection cavity used a right angle mirror to reflect incident light back to the original direction but the optical path had a spatial offset. Two parallel to each other, oppositely placed right-angle mirrors were used, and the laser with a beam diameter of 0.7 mm had a working diameter of 25.4 mm. The exciting laser was reflected back and forth 10 times in the cavity, and two lenses were placed in opposite direction around focus which were used to focus the excitation light to the same focus, thereby improving the use efficiency of the exciting laser energy. The Raman scattering signal transmitted along with the direction of incident laser was reflected back by the right-angle mirror to the right about, after being focused by the lens to the focus, with the Raman scattering signal scattered to the laser incident direction all passes through the long-pass filter and collected by Raman spectrometer, thereby improving the collection efficiency of Raman scattering signals. The experiment was carried out with air as the test object. The Raman spectrum of clear carbon dioxide and the fine Raman spectrum of nitrogen and oxygen were obtained within 300 s and the intensity ratio was analyzed, including 2 332 cm-1 of nitrogen and 1 557 cm-1 of oxygen. The peak height ratio of the 1 388 cm-1 Raman peak of carbon dioxide was 785∶257∶1. The Raman right angle reflection cavity added two right-angle mirrors and one focusing mirror compare to the conventional Raman scattering excitation collecting system, and had the characteristics of small volume, simple structure and easy adjustment. The signal intensity distribution of Raman scattering to the surrounding space was related to the incident direction of the incident light, and the maximum of Raman signal accorded with the direction of the incident light and the reverse direction. The Raman right angle reflection cavity was designed to match the Raman signal intensity distribution, and along with the advantages of optical depth of field was utilized to maximize the Raman scattering signal collection efficiency. The Raman right-angle mirror cavity can extend the application of Raman spectroscopy in gas detection, such as in-situ monitoring of gas phase chemical reactions, engine combustion processes and emissions detection, and unknown pollutant gas analysis.