A Wide-Range and Calibration-Free Spectrometer Which Combines Wavelength Modulation and Direct Absorption Spectroscopy with Cavity Ringdown Spectroscopy

Abstract

A wide-range, calibration-free tunable diode laser spectrometer is established by combining wavelength modulation and direct absorption spectroscopy (WM-DAS) with continuous wave cavity ringdown spectroscopy (CW-CRDS). This spectrometer combines the benefits of absolute concentration measurements, wide range, and high speed, using WM-DAS with enhanced noise reduction in CW-CRDS. The accurate baseline ringdown time, τ0, is calculated by the absorption peak (measured by WM-DAS) and the ringdown time containing gas absorption information (measured by CW-CRDS at the center wavelength of the spectral line). The gas concentration is obtained without measuring τ0 in real time, thus, greatly improving the measuring speed. A WM-DAS/CW-CRDS spectrometer at 1.57 μm for CO detection was assembled for experimental validation of the multiplexing scheme over a concentration ranging from 4 ppm to 1.09% (0.1 MPa, 298 K). The measured concentration of CO at 6374.406 cm−1 shows that the dynamic range of this tunable diode laser absorption spectrometer is extendable up to five orders of magnitude and the corresponding precision is improved. The measurement speed of this spectrometer can extend up to 10 ms, and the detection limit can reach 35 ppb within 25 s.

Monitoring of ambient carbon monoxide concentrations based on wavelength modulation direct absorption spectroscopy

“…其中，参数 S、P、T、X 和 φ(v)定义与公式( 6)相同，c 为光速，τ0 为空腔衰荡 时间，其值取决于腔内镜面透射、颗粒散射、杂质吸收等导致的损耗，在较窄的 波长范围内可认为是常数， 因此吸收光谱的拟合实际只需对测量的(cτ) -1 拟合即可。 文献 [37][38][39] 详细描述了腔衰荡光谱系统具体测量过程，由于本文采用了更高反射率 的镜片，在实验中通过降低压电陶瓷扫描速率至 5~10 Hz 以提升纵模光强，进而 在浓度稳定时，各种浓度下两种方法测量的吸收系数如图 8 所示。CRDS 吸 收系数拟合残差标准差~5×10 -10 cm -1 ，WM-DAS 为~5×10 -9 cm -1 (5×10 -5 ) 。在 0.44…”

The Spectroscopic Characterization of Halogenated Pollutants through the Interplay between Theory and Experiment: Application to R1122

“…However, it should be noted that some halogenated ethenes are considered to be potential threats to human health (see, for example [ 8 , 9 , 10 ] and references therein), so the assessing of their presence in real-time is clearly desirable, both for experimentally studying their atmospheric chemistry as well as for quantifying their concentrations. The recent advances in high-resolution infrared techniques make them very efficient in monitoring and detecting gaseous pollutants in real-time and with a very high sensitivity [ 11 , 12 , 13 ], provided that accurate spectroscopic data are available [ 14 ]. Generally, these data are obtained by the ro-vibrational and line-shape analysis of high-resolution spectra, which are often affected by several resonances [ 15 ]; ab initio calculations are nowadays able to yield accurate predictions for both anharmonic and Coriolis couplings, thus greatly assisting the whole process.…”