Boundary element method: An application to the fluid dynamics of optical fibres

Abstract

Hollow Core optical Fibres (HCFs) are optical fibres which guide light in a hollow core surrounded by a delicate microstructure. It is the precise microstructure geometry which is responsible for their optical properties. New geometries can be designed with optimised optical properties but the method to create these may be unknown. It would be costly and time consuming to explore these experimentally, as such, modelling the fibre draw process is preferable. Current models are limited in their application to certain geometries, so a general model is required.

In this thesis, we aimed to create the basis of a general fibre draw model that can handle any arbitrary fibre geometry and incorporate, or be extendable to incorporate, all key properties of a fibre draw. Towards this aim, a surface based numerical method – the Boundary Element Method – was chosen as the model base, as HCFs have a large surface to volume ratio. It was tested for applicability to heat transfer, and then a code basis was established and created for testing the applicability to fluid flow. This fluid flow model was later extended to incorporate an iterative scheme to determine unknown boundary conditions, and this was extended further with an iterative scheme to determine the location of a free surface boundary.

The fluid model was then combined with an axial flow system to create the base model for a fibre draw. This was successfully compared to experimentally validated results for an axisymmetric geometry, and good agreement was found between the model and experimental results for low deformation in a non-axisymmetric geometry. Looking forward, the equations to extend the model to variable properties were also derived, this would allow the impact of heat transfer effects and multi-material fibres to be included in future work.

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Identifiers

ORCID for Francesco Poletti: orcid.org/0000-0002-1000-3083

ORCID for Gregory Jasion: orcid.org/0000-0001-5030-6479

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