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Fiber‐enhanced Raman spectroscopy is applied actively for gas analysis. Hollow‐core anti‐resonant fibers (HC‐ARFs) with low loss and low spatial overlap between core and cladding modes show the potential of low background. We integrated an HC‐ARF into the Raman system, collected the forward scattering radiation with a lens‐coupled imaging spectrometer, and filtered most of the background through selection of region of interest. The limits of detection (LODs) of this system were determined as 1.2 ppm·bar for CH4, 2.9 ppm·bar for C2H6, 1.6 ppm·bar for C2H4, 2.7 ppm·bar for C2H2, 13.8 ppm·bar for H2, and 16.7 ppm·bar for CO with the excitation laser of 200 mW and the exposure time of 60 s. In addition, the LOD was better than 2.5 ppm·bar for H2S. By the detection of gas samples with the critical detectable concentration, it was proven that the estimated values of LOD from the preceding experimental results are reliable and the system had practical detectability. The gas samples we tested are relevant for multiple fields such as industrial process control and environment monitoring, showing the diversified application prospects of the Raman gas analysis system based on HC‐ARF. [ABSTRACT FROM AUTHOR]

We demonstrated high‐resolution continuous‐wave (CW) coherent anti‐Stokes Raman spectroscopy of gaseous samples in a single ring hollow‐core photonic crystal fiber. Pump beam and Stokes beam were provided by CW single longitudinal mode lasers. These narrow linewidth lasers brought the benefit of a spectral resolution of 99 MHz. The high resolution allowed a measurement of a linewidth 0.3 GHz of the ν1 transition of CO2 in a gas mixture with partial pressure of 0.15 MPa and total pressure of 0.9 MPa. Pump beam from a tunable dye laser and Stokes beam from an external cavity diode laser were coupled into the fiber containing gaseous CO2. Fundamental mode phase matching was achieved by tuning gas pressure. The long interaction length provided by the hollow fiber yielded a 60‐dB increase in the anti‐Stoke signal. The attained signal to noise ratio of the spectrum of ν1 component of 0.6‐MPa CO2 was ~25,000 with 152‐mW pump power and 1.4‐mW Stokes power. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]