High Resolution Distributed Sensing Using Exposed-Core Fibres

Abstract

Distributed fibre sensors were proposed more than 40 years ago, with their primary applications in physical sensing for civil infrastructure monitoring. Based on optical frequency domain reflectometry (OFDR), this thesis extends distributed sensing to biomedical applications, with a focus on the detection of small particles. A high-resolution distributed sensing method has been proposed, in which an exposed-core fibre (ECF) is used to detect micro-scaled sized particles. While the ECF has been reported for chemical detection in the past with high sensitivity due to the axial evanescent field along the entire fibre length, this thesis further reveals spatial distributed information of the micro-scaled particles by using the OFDR technique. The proposed scheme offers a compact and all-in-fibre approach, which avoids bulky instruments and external collection optics. Based on both theoretical analysis and experimental results, spatial distributions of micro-scaled particles in one dimension to two dimensions are detected using the few-mode property of the ECF. The size characterisation of micro-scaled particles has also been demonstrated using machine learning algorithms based on the multimode property of the ECF. The ECF OFDR platform is also demonstrated as a useful tool for whispering gallery mode (WGM) excitation of spherical particles, where both Fano resonance and multi-particle (integrated in a single ECF) WGM excitation are demonstrated. The multi-particle WGM excitation opens opportunity to build multi-point sensors or lasers, and to study coupling effects resulting from multiple closely spaced resonators. Overall, this thesis reports the novel applications of using ECFs for distributed detection of small particles with a detectable size down to 10 μm and two-dimensional spatial resolution of several microns. The particle detection with both spatial distributions and size characterisation using fibre optics bridges the gap between fibre sensing and imaging, which brings insights towards in vivo cellular studies in the future.