High sensitivity, multispecies raman gas sensing in hollow core fibres

Abstract

Conventional optical fibres, which have a higher refractive index core, guide light via total internal reflection. Total internal reflection is not possible with fibres that have an air core. Over the past two decades, hollow core microstructured optical fibres have emerged where light can be transmitted with low loss within a hollow core. Light is confined within this core by a carefully engineered glass microstructured cladding. The unique properties of these fibres enable for gas-light interactions over extended lengths providing an excellent laser-based trace gas detection platform. This thesis investigates the application of hollow core anti-resonant fibres for high sensitivity, multispecies gas-phase Raman spectroscopy. Analysing a gas mixture of an unknown composition is difficult. Various analytical methods such as gas chromatography coupled with mass spectrometry are often used which are expensive, bulky and time consuming. In recent years, spectroscopic methods have become more commonplace for gas sensing applications. Raman spectroscopy is one such example which uses the scattering of laser light to analyse the vibrational and rotational modes of molecules, providing information about their chemical composition and structure. Raman spectroscopy is challenging for gases due to their inherently weak Raman-scattering signals, which require sophisticated enhancement techniques to overcome limitations in sensitivity. Hollow core fibres enable a high-intensity laser beam to travel a significant distance while coexisting with a small-volume of gas in the hollow core. By using the hollow core fibre as a compact sampling cell, the weak Raman signals from gases can be compensated for. The aim of this work, carried out in collaboration with IS Instruments who are the industrial sponsor of this PhD, is to work towards the development of a robust gas-phase Raman instrument using hollow core anti-resonant tubular fibre. The fibres used in thesis are designed and fabricated at the Optoelectronics Research Centre, Southampton. A number of key areas where IS Instruments wish to deploy this technology include: the oil and gas industry for monitoring hydrogen blending within natural gas pipelines, and also in the nuclear fusion sector, where it is necessary to track the concentration of hydrogen isotopologues. During the course of this PhD, changes in the optical properties of hollow core antiresonant tubular fibres were witnessed when filling them with pressurised gases. Subsequent numerical modelling and experimentation revealed that the refractive index change induced with as little as 1 bar of gas pressure is comparable to the separation in effective indices between the core and cladding modes of the fibre. Differential gas pressures between the core and cladding act to alter the optical characteristics of the fibre. This is important to consider for gas sensing applications.

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Identifiers

ORCID for Natalie Wheeler: orcid.org/0000-0002-1265-9510

ORCID for Peter Horak: orcid.org/0000-0002-8710-8764

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