Development of novel photonic crystal fibre fabrication techniques and applications

Soft-glasses (e.g., heavy-metal oxide, fluoride or chalcogenide) have been attracting great interest for the fabrication of novel optical fibres, such as photonic crystal fibres (PCFs). This is because they offer extended ultraviolet (UV) to mid-infrared (mid-IR) transmission and higher Kerr nonlinearities than the commonly used fused silica glass. Lead-germanate and tellurite oxide glasses, if carefully synthesised, can have extremely low water content and impurities, and so offer low absorption over their entire transmission window, making them ideal for generating multi-octave-wide supercontinuum (SC) light. Both glasses are fabricated in an in-house melting and casting facility, making it possible to specifically tailor their optical and mechanical properties by changing their composition. The fibre fabrication is based on the ''stack-and-draw'' method, which is the most highly developed and flexible technique for fabrication all kinds of microstructured fibres. To obtain the rods and tubes required to stack of a preform of the fibre, we implement an extrusion process using specifically designed 3D-printed dies, thus enhancing design flexibility and precision of the fibre structures. Precise control of the fibre cross-section on a sub-micrometre level provides a unique possibility to adjust the modal dispersion over a wide range of wavelengths, making the generation of new frequencies more efficient. Two types of fibres, a suspended-core microstructured fibre and a solid-core PCF, were fabricated from germanate and tellurite glasses. To optimise the spectral properties of the generated SC, the material and fibre designs were investigated using finite element modelling. In both fibres, the strong confinement of the light in the glass and the high nonlinear susceptibility of the material allow generation of a broadband SC ranging from 0.54 up to 3.5 μm. For both fibres, the experimental results are in good agreement with the numerical simulations. A long-standing challenge in SC generation is to generate broadband light in a single circular polarisation state. In a conventional PCF, the polarisation state of the SC varies over its spectrum due to random fluctuations in the weak linear birefringence, the coupling between orthogonally polarised components induced by modal instability and the vectorial nature of soliton fission. This thesis shows that an elegant solution is the use of a chiral PCF which exhibits circular birefringence. The fibre was made from fused silica and the permanent twist was created by spinning the fibre preform during fibre drawing. The theory and experiment confirm that the twisted (circular birefringent) PCF significantly outperforms the untwisted (linear birefringent) PCF in terms of polarisation properties without compromising the spectral bandwidth. The values of the Stokes parameters show the purity circular polarisation state across the measured spectrum