Adaptive optics correction for bent multimode fibres for use in biological imaging

Thesis

Multimode optical fibres show great promise for in vivo imaging due to their thin profile, limiting invasiveness. However, propagation through such a fibre leads to scrambling of the light, which requires adaptive optics for compensation in order to create a focus at or near the output facet. Furthermore, changes in conformation due to bending lead to a change in transmission properties, limiting the ability to generate a focus at the output facet without recalibration. Hence, a requirement for effective imaging though an optical fibre is that one can adaptively change the calibration as the fibre conformation changes.

The thesis proposes an approach to correct for such changes in transmission and regain the ability to focus through a bent fibre, without access to the distal facet, as the system is intended for use during imaging. The method presented in this report uses a sensorless adaptive optics approach in concomitance with a Walsh mode representation of the modulated input optical field to correct for a bent fibre and increase the intensity of a desired focus. Then, with the use of memory effect, the area in which the correction is achievable is increased. However, the area in which a desired correction of at least 70% of the focus intensity generated with a fully calibrated fibre is limited. Thus, the approach shows that by using multiple correction targets, the entire facet of the fibre can be accounted for. Furthermore, the focus plane is not limited to only the distal facet, but it can be set at different axial depths, through the use of free-space propagation.

The correction procedure is dependent on the use of reflectors for the sensorless adaptive optics optimisation step. Thus, the thesis also proposes a novel approach for the simulation of Bragg gratings in multimode optical fibres, which can provide a tool in choosing the desired reflector to be used with the correction approach.

Authors: Sutu, E

• DOI: 10.5287/ora-domdykjnb
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: fluorescence microscopy; adaptive optics; walsh-hadamard modes; multi-mode optical fibre; correction; bent fibre; memory effect; point-scanning microscopy; Bragg grating
• Subjects: Fiber optics; Bragg gratings; Optical memory effect; Walsh-Hadamard modes