Fibre bundle optical coherence tomography using angled divergent illumination

Fibre imaging bundles, along with chip-on-tip cameras, form the basis of conventional forward-viewing endoscopes, which do not currently have depth-scanning capability. Clinically deployed endoscopic Optical Coherence Tomography (OCT) devices are typically side-viewing, with two-dimensional scanning achieved by rotation and pull-back, which is suitable for imaging along tubular, tract-like organs, such as the airways. Forward-viewing endoscopic OCT could be useful for many other applications, but such devices are difficult to miniaturise due to the need for two-axis scanning at the fibre-tip. Fibre bundles are a potential scanner-free alternative for image relay, but previous work has delivered images with poor signal-to-noise ratio, particularly when the bundle is in only one arm of the OCT interferometer. In this paper, we present an adapter that provides an interferometer at the distal tip of a fibre bundle, such that the bundle is in a common-path configuration, with sample and reference fields relayed together. The sample is illuminated by an angled divergent beam directly from a single-mode fibre, and imaged by a 4f telescope onto the bundle. A reference beam is similarly delivered onto the face of the bundle from a second single-mode fibre, with a diffuser placed flush with the bundle face to improve power injection under angled illumination. This allows for full-field swept-source OCT, delivering a forward-view, in a compact cylindrical package in-line with the bundle. The adapter could potentially enable conventional endoscopes to perform depth-resolved imaging. The approach is verified by imaging a phantom at a volume rate of 0.5 Hz, demonstrating the feasibility of acquiring OCT images through a fibre bundle using the compact adapter.