Novel fibre optics : antiresonant guidance and twist-induced nonlinear propagation
This thesis deals with the novel designs of polygonal-core hollow fibres based on antiresonant guidance. A triangular-core hollow fibre is demonstrated to allow a low leakage loss notably below 0.1 dB/m and a dispersion only ∼ 2.3 ps/(nm km) at the Er:YAG wavelength. The jacket tube width has been verified to play a role in preventing light escaping from fibre core. In contrast, another square-core hollow fibre designed for 1.55 µm realises a propagation loss 0.056 dB/m while the same design for 2.94 µm can guide with a loss only 0.023 dB/m. In this square-core design, both fibres assure that their losses of the fundamental modes almost approach less than 1/1000 of those of higher-order modes. An important universal geometrical ratio 0.845 between the effective circle-radius of cladding elements and the core radius is explained to better guide the single-mode design. Besides, this thesis depicts a general method to derive the nonlinear Schr¨odinger equation of nonlinear propagation in twisted fibres. The third-order susceptibility tensor χ (3) in helical coordinates are involved. The feature of circularly-polarised propagation in twisted fibres is discussed. Coordinate transformation proves that the pulse amplitudes remain the same in both the laboratory and helical regimes. Nonlinear Schr¨odinger equation shows strong reliance on the twist-induced orbital and spin angular momenta.