Grating couplers using silicon-on-insulator

In integrated optics, grating couplers are used when conventional end-fire methods are cumbersome and less efficient in coupling light in and out of thin-film waveguides. Our Im is to fabricate an high efficiency grating coupler for integrated optics applications at infra-red wavelengths and for thin-film waveguides which can be used for sensor applications. In this paper, theoretical output efficiencies of silicon (Si) rectangular, ideal right-angled blazed and non-ideal trapezoidal gratings are presented. Using perturbation theory, Si rectangular gratings with optimum grating heights exhibit a maximum predicted output efficiency towards the surface at the order of 80% and Si right-angled blazed gratings have predicted efficiencies approaching 100%. The fabrication method consists of using electron beam lithography and reactive ion etching. Ion beam milling is also considered, with the aim of creating blazed profiles by tilting the silicon-on-insulator (SOT) wafer. In our work, smart cut SOI Unibond wafers are used as the base material for fabricating the grating couplers as they offer good flexibility in choosing the guiding layer and buried layer thickness'. These waveguides are chosen to have a Si film thickness of 0.92 mu m and a SiO2 buried layer thickness of 0.67 mu m in order to use the transverse resonance effect to improve the output coupling efficiency. Si rectangular with various grating heights, designed at the first order of diffraction, were fabricated and characterised. The highest efficiency grating yet reported in SOI was produced, having the coupling efficiency in excess of 70%.