Raman Gas Analysis Based On Hollow-Core Anti-Resonant Fiber

As a method of analysis,spontaneous Raman spectroscopy technology can detect almost all gases with a single laser excitation source.The Raman signal is proportionate to the number density of molecules or chemical bonds which is much smaller in the case of gases than that in the cases of solids and liquids,so the Raman scattering of gases is very weak.The researchers are focusing on finding ways to enhance Raman signal,one suitable way is the applying of hollow-core fibers which can be used as not only gas sample cells but also effective collectors of Raman radiation.As the accumulated Raman radiation increases with the fiber length,the Raman signal is enhanced significantly.The hollow-core anti-resonant fibers are advanced fibers that have the advantages of low transmission loss,wide transmission range,and low space overlap ratio between core mode and cladding mode,which will be helpful for the Raman spectra of high signal level and low background.This study proposed a Raman gas analysis system whose signal level was enhanced by a hollow-core anti-resonant fiber(HC-ARF).The exciting laser beam was coupled into HCARF,the resulted forward scattered light was collected by a lens-coupled imaging spectrograph.Based on the spatially resolved spectrum of the imaging spectrograph,the appropriate integral domain was selected to separate most gas signals from the non-target Raman and fluorescent background,which is called digital space filtering.From the theoretical analysis and experimental research,it can be concluded that the background of forward Raman scattering is lower than that of backward Raman scattering.Both the digital space filtering and forward scattering collecting reduce the background of the spectra and significantly improve the signal-to-noise ratio.The spatial and angular distributions of Raman radiation in HC-ARF were recorded and analyzed.It is found that the spatial distribution of the Raman gas signal is different from the background.Low-order modes account for a large proportion of the Raman signal level,and high-order modes also contribute to the Raman signal level but they are mixed with the background.The aperture of lens was adjusted to match the collecting angle,which further improved the signal-tonoise ratio.Under the basic experimental conditions(200 m W laser power,60 s exposure time),limits of detection(LODs)of this system were determined: CH4 of 1.2 ppm·bar,C2H4 of1.7 ppm·bar,C2H2 of 2.6 ppm·bar,C2H6 of 2.9 ppm·bar,H2 of 13.8 ppm·bar,and CO of16.7 ppm·bar.The dynamic range of the Raman gas analysis system based on HC-ARF covered 1~106 ppm.The hydrocarbon,H2,and H2 S gases at the partial pressure of 2.5ppm·bar were detected by the system,which indicated that the calculation results of LODs are reliable and this system has actual capability of ppm-level gas analysis.Besides the confirming of LODs,the practicability of the system was assessed in these following ways:ppm-level gas analysis under the interference of high concentration gases,and the inflating and deflating processes of gases in HC-ARF.These experimental results show that the HCARF enhanced Raman gas analysis system has capability of ppm-level gas analysis,and it can be applied to the fields such as industrial monitoring,scientiffic sensing,and so on.