Optimization of parabolic cell for gas Raman analysis

Abstract

Raman spectroscopy is a widely practicable technique in gas analysis, but the application in simultaneous detection of multiple‐trace gas is still a challenge for its weak signal level. To extend the application of Raman gas analysis to detection of trace gas constituents, a closed parabolic sample cell proposed in our previous work is optimized by optical analysis and experimental verification. The optical analysis shows that for a parabolic reflector with rim diameter 6p (p, semi‐latus rectum), the collecting solid angle can be as large as 90% of full space (4π) and the power‐collecting efficiency can be as high as 80%. The variation of the Raman signal level with the position of collecting aperture and the parameter p of parabolic reflector was verified by the experimental results. At the optimized structure parameters, we get a signal‐to‐background ratio of 122 and a signal‐to‐noise ratio of 305 in the Raman spectra of ambient air with exposure time of 1 s. From the results of standard analytic gaseous samples, the limits of detection of 33 ppm‐bar for H2, 38 ppm‐bar for CO, 19 ppm‐bar for CH4, 27 ppm‐bar for C2H6, 23 ppm‐bar for C2H4, and 20 ppm‐bar for C2H2 were obtained with exposure time of 200 s. These results indicate that the parabolic sample cell is a competitive Raman collector, which showing good balance in signal enhancement and background level.

Multiple‐pass enhanced Raman spectroscopy for fast industrial trace gas detection and process control

“…For environmental or industrial applications involving natural gas mixtures, atmospheric gases, gases separated from transformer oil, and other nonhazardous gas samples, a slight different setup based on the current system can be used. In fact, the two mirrors (M5 and M4) comprising the small multiple‐pass cell can be set inside a gas cell instead of having a gas cell between them, as in the configuration adopted by Hippler and other groups . The potential of this setup is first investigated using the current setup without the gas cell and will be reported in detail later in a separate article.…”

Recent advances in linear and nonlinear Raman spectroscopy. Part XIV

“…Their approach overcomes fundamental technical limitations of conventional charge‐coupled device operation and is of particular benefit to mobile Raman systems used in the field in the presence of dynamically changing background light conditions. Yu et al [ 202 ] described the optimization of parabolic cells for Raman gas analysis. Their results indicate that the parabolic sample cell is a competitive Raman collector that shows good balance between signal enhancement and background levels.…”

Optical Feedback for Sensitivity Enhancement in Direct Raman Detection of Liquids

“…us, we realized an enhancement factor of 5.6 in the Raman spectroscopic measurement on ethanol. Compared with the reported methods [12,13,18,19], our "F+2f" scheme has a simplest design and low-cost configuration, which produces even higher enhancement. Although we employed pure ethanol and R6G in water as the analytes in this experimental work, the most important motivation is to find effective and practical approaches for improving the sensitivity of Raman spectroscopy, which can be potentially utilized in the direct detection of pollutant molecules in environmental water.…”

“…However, Raman-scattering signal is very weak, which has an intensity lower than (10 −6 ) that of the excitation. erefore, approaches for enhancing the Raman-scattering signals are always important for improving and realizing the related detection techniques [11][12][13].…”

“…ese approaches may include single-channel multiplex optrode [21,22], liquid core waveguides [23,24], and fiber probes [25]. Parabolic reflection mirror and multipass schemes have been reported for the Raman spectroscopic systems, which enabled large enhancement of the detection sensitivity [12,13,18,19].…”