Direct and inverse design of microstructured optical fibres

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Abstract

Microstructured optical fibres, where an arrangement of air holes running longitudinally along the fibre guides light in either a solid or a hollow core, have created new opportunities in diverse areas of science and technology. Applications range from the generation of supercontinuum light to optical sensing, nonlinear telecom devices and the generation and delivery of extremely high optical powers.

This thesis concerns the modelling of such fibres with the finite element method, with the multiple purpose of: acquiring a clearer insight and understanding of the physical mechanisms the fibres are based on; designing optimum fibres for a range of applications; understanding experimentally observed phenomena; identifying fundamental limits and design rules.

Optical maps are proposed as a simple, yet effective way to understand the potential and limitations of hexagonally arranged index guiding fibres, and are used to design an optimised fibre for the generation of a broad and spectrally flat supercontinuum and to engineer tapered fibres with a high nonlinear figure of merit. More specific inverse design techniques are also applied to the optimisation of fibres with a larger number of free-parameters. With this approach, the dispersion of silica and compound glass fibres is optimised for applications in nonlinear telecoms devices.

Photonic bandgap fibres, allowing light guidance in a hollow core, are also extensively studied. The main issues preventing accurate simulations of the properties of fabricated fibres are identified and addressed. An ideal, accurate representation of a realistic fibre is then proposed and employed to obtain fundamental scaling rules and to study the interactions between air guided and surface guided modes. Anticrossings between these modes in slightly asymmetric structures are identified as the cause for the unusual polarisation effects experimentally observed in these fibres. And finally, guidelines for fabricating fibres with the widest possible operational bandwidth possible are developed and presented.