Optical Coherence Tomography through Fibre Bundles by Angled Divergent Illumination

Researchers have made numerous attempts to develop a fibre bundle Optical Coherence Tomography (OCT) system for endoscopy. However, in the 20 years since initial attempts [1], no approach has been successfully applied to clinical practice. In this thesis, a novel method of performing OCT with a fibre bundle is presented. A compact adapter, that can transform a fibre bundle into a depth resolved imaging instrument, originally conceived as a concept in patent US11717154B2 "Imaging apparatus and method" [2], is experimentally realized, with initial results proving the validity of the concept. The adapter enacts common-path fibre bundle OCT, utilizing angled divergent beams for the object and reference paths. These are delivered from a pair of single-mode-fibres, placed off-axis, making angled illumination onto the sample and the bundle face at 14◦. At this large angular incidence of the reference beam onto the bundle, the light coupled within cores was not suitable for OCT, mainly due to a large degree of coupling into higher order modes, and a low overall coupling efficiency. Placing a thin diffuser at the bundle face achieved close to on-axis injection, enabling OCT to be performed. A phantom consisting of metal and polymer features was imaged by the method of Full-Field Swept-Source OCT (FF-SS-OCT). It was observed that an apparent sample tilt could occur in the OCT images, owing to the superposition of angled divergent beams. However, if the two fibres are correctly aligned, the respective path lengths across the two beams effectively cancel out, and the true shape and orientation of the sample can be secured. B-scans and en-face slices identified features of the phantom, which were corroborated with scans obtained from a bench-top OCT system. With Fast Fourier Transform (FFT) processing, we secured a Signal-to-Noise Ratio (SNR) of 32 dB, and an axial resolution of 34 µm. Image processing by the Complex Master Slave (CMS) method was also explored. A significant improvement in axial resolution, reaching 20 µm, was achieved. However, the FFT method is currently preferred, as the CMS images had a poorer SNR of 27 dB, and suffered from artefact streaks. In summary, the thesis has presented a new method of fibre bundle OCT. Rudimentary OCT imaging was demonstrated from a phantom, validating a new avenue of research towards the goal of achieving a forward-viewing Endoscopic OCT (E-OCT) system suitable for clinical enrolment. Several avenues for further research are suggested, that if perused may lead to clinical application of the new method.