High efficiency Silicon-on-Insulator grating coupler based on a poly-Silicon overlay

A high efficiency broadband grating coupler for Silicon-On-Insulator waveguides was designed. The grating coupler is defined by locally adding a poly-Silicon layer on top of the existing waveguide layer structure prior to grating etching. Adding this poly-Silicon layer reshapes the grating structure which changes its diffraction properties. Coupling efficiencies as high as 78% at a wavelength of 1.55 mum are calculated and the optical 3dB bandwidth of the device is about 85 nm. The device layout is compatible with standard CMOS technology processing.