Fabrication and Characterisation of Silicon Waveguides for High-Speed Optical Signal Processing

This Ph.D. thesis treats various aspects of silicon photonics. From the limitations of silicon as a linear and nonlinear waveguide medium to its synergy with other waveguide materials. Various methods for reducing sidewall roughness and line edge roughness of silicon waveguides are attempted. The methods include enhancements of etch mask roughness as well as etch isotropy and direct reduction of already present sidewall roughness. Although promising roughness assessments were made based on electron microscopy images it did not translate into significantly lower propagation loss in fabricated silicon waveguides. As an alternative to crystalline silicon waveguides for nonlinear optical applications, amorphous silicon was explored using RF sputtering potentially allowing for low density of detrimental hydrogen content in the final material. Unfortunately, the linear optical loss in the material was too high for any practical applications. It is speculated that the attempt at creating a material with low density of dangling bonds was unsuccessful. Nevertheless, linear losses of 2.4dB/cm at 1550nm wavelength in the silicon waveguides remained sufficiently low that high speed nonlinear optical signal processing could be demonstrated. This includes four wave mixing based wavelength conversion of a 320Gb/s Nyquist OTDM signal and cross phase modulation based signal regeneration of a 40Gb/s OTDM signal. Finally, a new type of low loss electrically driven optical modulator in silicon and silicon nitride is demonstrated. The device is an attempt to bridge the gap between the low loss platform of silicon nitride and the electrical capabilities of silicon on insulator. In this hybrid waveguide device, light is modulated via evanescent coupling from a nitride strip waveguide to a charge carrier based PiN modulator in an thin silicon slab. The device is demonstrated in conjunction with a coupling modulated ring resonator, a device which benefits from the low loss characteristics of this type of this hybrid waveguide phase shifter.