Abstract
The subject of the thesis is the modelling, design, fabrication and characterisation of single-stage and multi-stage resonators on Silicon-on-Insulator (SOI) strip and rib waveguides. The devices have been investigated with the aim to produce small and efficient wavelength selective elements that could one day be used in multiplexers, filters and other components of integrated optical circuits. Due to the complexity of devices and very often requirements of advanced simulation packages, most of the devices have been modelled (lambda = 1.55mum) through separate analyses of the components forming the filters. The study starts with the modelling of rib and strip waveguides aiming at the single-mode and zero-birefringent regime of operation, followed by the analysis of a directional coupler. The modelling suggests that the cross-sectional rectangular area of a strip waveguide should be smaller than 0.10mum2. Similarly, rib waveguides with a height of 1.35mum, and a waveguide width of 0.8mum or 1.0mum, could be used as basic single-mode and zero-birefringent elements for building relatively large rib waveguide based devices. The analysis of a directional coupler on strip waveguides has shown that a near-polarisation-independence regime is possible for waveguide separations below 0.20mum and waveguide widths in range 0.29 - 0.40mum, when a waveguide height is chosen to be 0.29mum or 0.34mum. Simplified z-transform models of filters have been employed to calculate values of the most relevant figures of merit such as Free Spectral Range (FSR), Full Width at Half Maximum (FWHM), Finesse (F) and Q-factor, and also to quantify the sensitivity of the transfer function to the changes of geometric parameters, coupling issues and thermal effects. Based on the modelling and information from test chips of previous students, 4 main designs grouped in 6 test chips have been proposed and fabricated in collaboration with the Intel Corporation Photonics research groups from San Jose and Jerusalem. Two designs were based on rib waveguide type devices and two on strip waveguide type devices. The goals in all cases were; polarisation insensitivity, single-mode behaviour, improvement of the FSR, shaping response by using various geometries, the possibility of tuning response by thermal means etc. Experimental results have shown improvement in the FWHM and FSR as expected for both strip and rib designs. An additional stage of multi-level, serially coupled racetrack resonator in rib waveguides has resulted in a decrease of the FWHM by more than 30% (6pm). Polarisation independence by using identical multiple serial-coupled rib racetracks has also been demonstrated. The FSR above 60nm have been reported for small strip resonators (radius of l.5mum) with good polarisation characteristics for rings which radius is near 3mum. To the author's knowledge this is the largest FSR yet reported for a silicon based ring resonator. There is also improvement of the spectral response of multiple Vernier rings, which, with some corrections in terms of side lobes appearing in the spectrum, may be used for designing devices with the FSR as large as 70nm.