Dual grating-assisted directional coupler in silicon-on-insulator.

In recent years, there has been a trend to fabricate smaller photonic devices. One very serious difficulty emerges from the move to smaller dimensions, however, which is the problem of coupling light from an optical fibre to small optical waveguides. This is a particular problem in semiconductors, since the refractive index of the silica fibre is very different from that of a semiconductor waveguide. There have been several published methods of achieving such coupling, but none are sufficiently efficient whilst being robust enough for commercial applications. In this work, a novel device, called a Dual-Grating Assisted Directional Coupler (DGADC), has been presented. Several theoretical methods have been used in the design stage and for the analysis of the experimental results. Those include the Transfer Matrix Method, Floquet Bloch Theory and the Finite Element Method. The coupler has been fabricated in Silicon-on-Insulator (SOI) using standard microelectronic technology. It enables coupling to thin (~250nm) silicon waveguides. The coupler consists of a 5mum thick surface silicon oxynitride waveguide, which allows efficient coupling with an optical fibre, a 500nm thick silicon nitride waveguide, and a silicon waveguide, two silicon oxynifride separation layers (150-200nm in thickness) and two shallow gratings fabricated on silicon nitride and silicon waveguides. The most important layer is the silicon nitride since it bridges the gap between the fibre and silicon waveguide both in dimensions and refractive index. It has been shown that the theoretical and experimental results agree to within 5% with the maximum experimental efficiency being 55% and maximum theoretical coupling efficiency being 60%. The principle of coupling by this novel method has been successfully demonstrated, and the coupling efficiency measured is, to the best of author's knowledge, the highest experimental coupling efficiency achieved for any grating based directional coupler, and among the best achieved for coupling to thin semiconductor waveguides by any method. Keywords: grating, grating coupler, directional coupler, silicon on insulator, silicon photonics, coupling efficiency