Manipulation of mode coupling in optical fibres

Abstract

In recent years, as the technology that underpins optical fibre fabrication and development has matured, the scope for potential uses of these fibres has rapidly expanded. Initially developed for the telecommunications industry, optical fibres have found their uses across a diverse range of subjects including medical diagnostics, guided wave nonlinear optical processes such as supercontinuum generation, light collection in astrophotonics, and of course the ubiquitous telecommunications industry. The results of all of these applications are, in part, governed by how light is confined and is able to propagate along the optical axis in these structures, in what is termed a mode. Depending on the transverse structure of the fibre, different modes with different properties may be confined. Changes in the transverse structure along the propagation length can allow light to couple from one optical mode to another. These changes, for example induced by post-processing the fibre on a custom-built fibre tapering rig, must satisfy certain adiabatic criteria in order to be low loss, but also can take on a variety of different spatial profiles to fit the given task. If instead of a single core, a fibre contains multiple cores, all of which are able to confine electromagnetic radiation, light input into a mode of one core may couple across to the other local modes of the surrounding cores. In some applications, such as multi-core imaging fibres, it is important to suppress this effect in order to achieve better imaging. In other cases, such as the fibres for sensing or telecommunication, the coupling can give useful information about the state launched at the input facet of the fibre.

This thesis presents the design, and fabrication of different optical fibres where the realisation of mode manipulation through changes in the fibre scale along the propagation axis can achieve specific useful functionalities. The work contained in this thesis focuses on three particular applications: medical imaging, telecommunications and astrophotonics. First, a novel multi-core imaging fibre was developed for medical imaging in the distal lungs. These imaging fibres provide low core to core coupling for high imaging quality across visible wavelengths. Fibres are also fabricated for the telecommunications industry to allow longitudinal structural changes over arbitrary length scales without the detrimental effects of loss. Finally, fibres for astronomy are presented. The first post-processed fibre presented for this application can sense the incoming wavefront of a beam of light, and the second device is a re-formatter that can convert the circularly symmetric mode of an optical fibre to a pseudo-slit shape that is compatible with on-sky spectrographs. The wavefront sensor presented in this thesis is a proof of concept and is only capable of sensing 1D tilts. The linear pseudo-slit output structure achieves a diffraction limited pattern in one direction, potentially able to reduce spurious intensity variations in spectroscopic systems.

Date of Award	19 Jun 2019
Original language	English
Awarding Institution	University of Bath
Supervisor	Tim Birks (Supervisor) & Jonathan Knight (Supervisor)