为精确测量氮气中痕量甲烷(CH4)浓度, 设计并搭建了基于三角环形腔的连续波光腔衰荡光谱(CW-CRDS)测量装置。 衰荡腔为自主加工设计, 由一片曲率半径为1 m直径为25 mm的凹面镜和两片直径为12.7 mm的平面镜围成, 腔内光路总长为410 mm, 腔体材料为殷钢。 首先, 对系统的基线损耗进行了测量, 随后利用氮气作为混合气配置了五种不同浓度的CH4与N2的混合气, 利用CH4 在1 653.7 nm的吸收峰(CH42ν3带R5支)对气体进行检测, 并根据吸收谱线特性, 利用洛伦兹线型函数通过最小二乘法拟合出衰荡时间常数τ并计算CH4浓度, 所搭建装置对甲烷浓度(体积分数)的检测灵敏度可达54×10-9(五点四亿分之一)。 最后对浓度为510×10-9(五千一百万分之一)的CH4在6 046.7~6 047.2 cm-1范围内的吸收光谱进行了测量, 并将测得数据依照腔自由光谱范围(FSR)分组后分别拟合出τ和吸收系数, 将所得CH4吸收系数与数据库中数据相比, 其最大误差低于1.2×10-9 cm-1, 最高精度达8.8×10-11 cm-1。

In order to accurately measure the trace methane (CH4) concentration in nitrogen, a continuous wave-Cavity Ring-Down Spectrometer (CW-CRDS) measuring device based on the triangular annular cavity was designed and built. The ring-down cavity is designed for autonomous processing. It consists of a concave mirror with a radius of curvature of 1 m and a diameter of 25 mm and two plane mirrors with a diameter of 12.7 mm. The total length of the optical path in the cavity is 410 mm, and the cavity material is Invar. First, we measured the baseline loss of the system, and then used nitrogen as the mixture to configure a mixture of five different concentrations of CH4 and N2. The gas was detected by the absorption peak of CH4 at 1 653.7 nm (CH4ν3 with R5). According to the absorption line characteristics, the attenuation time constant τ is calculated by the least square method using the Lorentz linear function and the CH4 concentration is calculated. The detection sensitivity of the built device to the methane volume concentration can reach 54×10-9 (540 million). Finally, the absorption spectrum of CH4 with a volume concentration of 510×10-9 in the range of 6 046.7~6 047.2 cm-1 was measured, and the measured data was determined according to the cavity free spectrum. The range (Free Spectral Range, FSR) is fitted to the τ and the absorption coefficient respectively. The obtained CH4 absorption coefficient is compared with the data in the database. The maximum error is less than 1.2×10-9 cm-1, and the highest precision is 8.8×10 -11 cm-1.