Abstract
The subject of this thesis is the design, analysis, fabrication and characterisation of third order Bragg grating optical filters on silicon-on-insulator (SOI) rib waveguides. New design guidelines for small cross sectional SOI waveguides have been proposed and described to address the issue of satisfying polarisation independence and single mode conditions simultaneously. This waveguide design will be used as a building block for the realisation of Bragg grating filters. The reflection spectral response of the deep Bragg grating operating in a third diffraction order on a single mode rib SOI waveguide has been studied theoretically using Floquet-Bloch Theory (FBT) developed in Politecnico di Bari, Italy in comparison with optical modelling software utilising Coupled Mode Theory (CMT). A series of Bragg gratings with different grating etch depths and lengths were fabricated at Southampton University to investigate the agreement between experimental results with theoretical predictions. The wavelength tuning capability of these Bragg grating filters in SOI waveguide structures were also investigated and implemented using the thermo-optic effect, through Joule heating of thin film aluminium heaters situated on top of the rib structure. The SOI rib waveguides with 1.5mum height are designed to exhibit polarisation independence and single mode operation. The Bragg grating filter is designed to operate at a wavelength of 1.55mum with a grating period of 689nm. The less rigorous fabrication tolerance of third order grating in comparison with that required by 228nm first order gratings is highly desirable only at the expense of slightly lower maximum reflectivity. The maximum reflectivity measurements of approximately 0.42 for third order grating are in agreement with theoretical prediction by FBT. The Bragg grating filters were thermally tuned to shift the Bragg resonance wavelength by up to 3.5nm with heater power of approximately 190mW. The tuning range of the filter is inhibited by the short lifetime of the heater caused by electromigration. At the time this work was carried out, this is the first demonstration of thermo-optic tuning through an integrated heating element, of third order Bragg grating filters based on small cross sectional SOI waveguide. The temperature sensitivity of the Bragg grating filters was analysed using a 2-D finite element method (FEM) and was consistent with the experimental results.