Non-destructive characterisation of antiresonant hollow core optical fibres

Antiresonant hollow core optical fibres (ARFs) are an exciting new technology, with the potential to overcome the limitations faced by conventional solid-core silica fibres. In these ARFs, light is confined to a central hollow air or vacuum core by its interaction with a surrounding glass microstructure. Unlocking the exciting properties of these fibres, such as low latency, low optical non-linearity and even low loss, requires extremely precise control of this delicate microstructure at all points along the length of the fibre. The fabrication of these fibres is complicated by the fact that there is currently no way of monitoring this microstructure without cutting a fibre sample and viewing it under a microscope. There are numerous disadvantages of this, among the most penalising of which is the severe reduction in the yield of fibre that can be drawn. In this thesis, a technique to non-destructively and accurately characterise this microstructure is presented, the so-called Lateral Optical Interrogation of Tubular Element Radii (LOITER). In LOITER, broadband light is incident on the side of an ARF, whose microstructure scatters this light. By analysing a portion of this scattered light, it is possible to make inferences about the geometry of the microstructure. A geometrical model used to calculate the various paths travelled by the light around the microstructure is presented, and we show that it is possible to use LOITER to accurately measure the microstructure of two highly promising ARF geometries: the NANF and DNANF (nested and double-nested antiresonant nodeless fibre respectively). Following demonstrations of non-destructive measurements on various fibre samples in the lab, we present results from inline monitoring of NANF and DNANF microstructure during fabrication using LOITER, showing that we are able to capture variations in microstructure over long lengths of fibre and provide useful real-time feedback on adjustments made to the draw parameters