Soft-glass microstructured optical fibres for nonlinear applications.

Abstract

The objective of this thesis is to explore the potentials of using soft-glass (non-silica) based microstructured optical fibres (MOFs) for nonlinear optical applications. The high linear and nonlinear refractive indices of soft glasses and the ability to tailor the dispersion properties of MOFs have made them an ideal platform for developing nonlinear optical applications. Soft glasses also provide the possibilities of developing waveguides with mid-infrared transmission capability. In addition, the advances in fabricating MOFs with sub-wavelength structures and high refractive index contrast mean that nonlinear processes in these fibres can no longer accurately explored using conventional theories (namely scalar theories). A full vectorial theory is thus required, and this thesis drives forward some applications of recently developed generalised nonlinear theory. In this work, the author studies soft-glass MOFs from two aspects. The first aspect is to develop soft-glass MOFs for new frequency light generation using the scalar theories. The second aspect is to explore new nonlinear phenomena in sub-wavelength scale high-index-contrast waveguides, which includes soft-glass MOFs, using a full vectorial theory. The goal of studying these two aspects is develop a general theory that can explain and accurately predict all nonlinear effects in all types of waveguides. Progress towards two novel nonlinear light sources is described, namely a femtosecond near-transform-limited tunable light source based on a fibre optical parametric oscillator (FOPO) and a broadband highly coherent supercontinuum (SC) source in the mid-infrared. The advantages of soft-glass MOFs have never previously been employed to enhance the performance of FOPOs. Also the bandwidth and coherence of SC sources have never been optimised simultaneously, utilising the advantages of soft-glass MOFs. In this work, a genetic algorithm based fibre design approach is developed and applied to design and optimise fibres for these devices. The extrusion technique is used to fabricate fibre preforms. Simulations and experiments are performed to demonstrate SC generation and new frequency light generation in the final fibre. The Kerr nonlinearity and nonlinear polarisation interactions are studied in a full vectorial framework. A continuous self-phase-modulation method was used to measure the Kerr nonlinearity of soft-glass MOFs, which confirmed the validity of the full vectorial theory. After that, new polarisation behaviours including polarisation self-switching were discovered and studied. The outcomes of the works demonstrate the efficiency of using soft-glass MOFs for nonlinear applications. As described in Part I, for the FOPO, only a few millimetres of fibres were used to achieve near transform limited output with high conversion efficiency across a large frequency separation. For the supercontinuum source, a broadband continuum spectral output that was only limited by the transmission window of the glass and highly coherent was obtained. The supercontinuum experiments indicated the fabricated fibres had behaved as expected with the consideration of fabrication distortion. As described in Part II, the work on the nonlinear effects with the full vectorial theory indicates that the Kerr nonlinearities of soft-glass MOFs can be significantly larger than they were previous predicted. New nonlinear polarisation behaviours including polarisation self-switching can take place in soft-glass MOFs with milliwatt level of power which indicates significant differences in the nonlinear processes that involved in FOPO and SC light sources should be observed with soft-glass MOFs. Combining the full vectorial theory with FOPOs, supercontinuum sources and other nonlinear applications will lead to a better understanding the physics of the nonlinear processes behind these applications as well as further increase the efficiency of using soft-glass MOFs for these nonlinear applications and discover new applications.