Microphotonic silicon waveguide components

This thesis describes the design, simulation, fabrication and characterisation of microphotonic silicon waveguide components on silicon-on-insulator (SOI) substrates. The focus is on approximately 10 µm thick and single-moded (SM) silicon rib waveguides. In particular, simulation results are given for straight and bent Si waveguides, directional couplers (DCs), thermo-optically (TO) modulated Mach-Zehnder interferometers (MZI), and waveguide gratings. A new analytical SM condition for Si rib waveguides is proposed and the development of a new grating simulation algorithm is reported. The theoretical part of the work also involves inventions relating to multi-step patterning of Si waveguides, modulation of interferometric devices, and measurement of polarisation axes from waveguides and polarisation maintaining (PM) fibers. Clean room processing of waveguide chips is briefly described. Main process steps are photolithography, electron-beam lithography, thermal oxidation, oxide deposition, oxide dry etching and Si dry etching. Post-processing of the chips is also reported, including dicing, polishing, anti-reflection (AR) coating, fiber pigtailing and wire bonding. The development of fabrication processes for multi-step patterning, waveguide gratings and photonic crystal waveguides is reported, although the optical characterisation of devices based on these three processes is not included in the thesis. Experimental results are given for Si rib waveguides with different thicknesses (H) and widths (W). The minimum fiber coupling loss with H = 9 µm was 1.3 dB/facet without an AR coating. The AR coating reduced the coupling loss by 0.7-0.8 dB/facet. Minimum propagation loss for a 114 cm long waveguide spiral with H = 9 µm and W = 7 µm was 0.13 dB/cm. With H = 9 µm, the birefringence varied from 0.00063 to <0.0001 depending on the cladding material, and the maximum polarisation extinction ratio for straight waveguides and directional couplers was >15 dB. Furthermore, fast modulation with 15 dB extinction ratio (ER) is reported for TO MZI switches by using both traditional (10 kHz) and novel (167 kHz) modulation methods. Rise and fall times for single switching operations were pushed below 750 ns with 9 dB ER. The setups and methods used in measurements are described in detail, including a novel method for measuring the polarisation axes of waveguides and PM fibers.