Development of a gas‐phase Raman instrument using a hollow core anti‐resonant tubular fibre

Abstract

Versatile and flexible gas analysis for compositional identification and quantification is a demand found in a variety of diverse sectors. As such, a compact, deployable instrument exhibiting both high specificity and sensitivity is a highly attractive proposition for a wide range of applications. In this paper, we describe a gas phase Raman spectroscopy-based device using state-of-the-art anti-resonant (tubular) hollow core micro-structured optical fibre (HC-MOF).This fibre architecture allows the use of lengths that are typically longer than have been demonstrated previously, allowing substantially enhanced interaction lengths between the pump laser and the gas sample to be achieved, addressing the sensitivity challenges typically observed in gas-phase Raman measurements and enabling application for remote sensing in hazardous environments. We describe the successful development of a compact, fibreintegrated instrument and present results obtained during a test campaign at an industrial laboratory; marking a milestone in gas-phase Raman spectroscopy. The unique properties of the MOF used allowed a 20-m length to be utilised, representing a new record length for gas phase Raman measurements. The identification and quantification of a variety of gas species, ranging from simple homonuclear diatomic gases to heteronuclear organic gas species were achieved, and, building on previous studies, the instruments stability, gas concentration linearity response, and the hollow core fibre filling and purging characteristics were investigated. K E Y W O R D Sfibre-enhanced gas Raman, hollow core anti-resonant tubular fibre, industrial application, multi-species detection

A Review: Application and Implementation of Optic Fibre Sensors for Gas Detection

“…Because of this, t authors do not intend to provide a detailed analysis, but rather to provide a pathway readers to obtain more information. All three sensors have a commonality, which is th they have a comb-like, periodic structural variation in the refraction index within the fib core that induces a coupling action between the core mode and other modes support The second class of sensors in this category is classified as surface Raman spectroscopy (SRS), and there are a number of variants [49][50][51]. The underlying principle of operation is inelastic scattering of incident photons by the gas, some of which induce excitation of vibration states of gas molecules generating phonons and detectable energy dissipation.…”

Analysis of CH4, C2H6, C2H4, C2H2, H2, CO, and H2S by forward Raman scattering with a hollow‐core anti‐resonant fiber

“…At present, several signal-enhancing devices have been proved to be effective: passively locked cavities, [1][2][3][4] where the work of Ohara et al showed a limit of detection (LOD) of 100 ppm for methane with 1-s exposure time and 40-W intra-cavity power, [2] and the work of Frosch et al showed a signal enhancement of 6 orders of magnitude [3] as well as LODs between 150 and 350 ppm in the process monitoring of biogas production [4] ; actively locked cavities, [5][6][7] where Hippler's work arrived a LOD of 1 mbar in 1 bar of total pressure (1000 ppm) with a diode laser of 10 mW and integration time of 30 s [5] ; multipass cells, [8][9][10] where the work of Velez et al recorded a LOD of methane below 1 ppm with a multi-mode diode laser of 6 W and exposure time of 5 s [10] ; functionalized waveguides, where the work of Holmstrom et al showed an enhancement over 9 orders of magnitude with a strong dependence on analytes [11] ; and hollowcore fibers (HCFs). [12][13][14][15][16][17][18][19][20][21][22][23][24][25] Among these, HCF is an excellent component, which can be used not only as gas sample cells with long optical path but also as effective collectors of Raman radiation.…”

“…Along with the development of microstructure fiber technology, many types of HCFs had been applied for fiber-enhanced Raman spectroscopy (FERS) of gaseous samples, such as metal-coated capillaries (MCCs), [13,14] hollow-core photonic bandgap fibers (HC-PBFs), [15][16][17][18][19] kagome HCFs, [20,21] and hollow-core anti-resonant fibers (HC-ARFs). [22][23][24][25] The advantage of MCCs is that the gas exchange rate is fast because of the large core diameter, but its enhancement of Raman scattering is not as good as other types of fiber. [12] In the previous studies, the best LOD of FERS system based on HC-PBF was reported as 0.2 ppm for methane (CH 4 ) with the excitation laser power of 2 W and the gas pressure of 20 bar, [15] and the best LOD of FERS system based on HC-ARF was reported as 3 ppm for CH 4 , 49 ppm for hydrogen (H 2 ) with the excitation laser power of 1.3 W and the gas pressure of 4 bar.…”

“…The investigations on the Raman scattering of gas samples in HC‐ARF will be reported, where a forward collecting Raman system was designed and built with the objective to explore the LOD of FERS technology. Compared with the published FERS systems, [ 12–25 ] the meaningful improvements in the present paper have been made in the collection direction and collection method of Raman scattering radiation. On the one hand, signals were obtained from the forward Raman scattering instead of backward one, collecting of the scattering, fluorescence, and Raman radiation from the optics and filters, and the inlet terminal was avoided, which lowered the background and the noise level; on the other hand, the forward Raman scattering radiation was collected by a lens‐coupled imaging spectrometer, and the Raman gas signals were separated from the background by selecting the specific region of interest related to gas Raman signals (the digital spatial filtering method).…”

“…As our samples are non‐absorbing gases, the consideration of the analyte extinction coefficients for UV radiation as Forsch et al [ 29 ] is not needed, and the HC‐ARFs applied here have much lower loss coefficients than that [ 17 ] of HC‐PCFs; will be much longer. Brooks et al applied a HC‐ARF of 20 m in their development of a gas‐phase Raman instrument; unfortunately, the LOD they got for ethane is only 400 ppm with an exciting laser of 200 mW and accumulations of s [ 25 ] ; the background occurred in the collected signal of backward Raman scattering might be the main reason.…”