Adiabatic mode coupling between SiGe photonic devices and SOI waveguides

We describe the coupling between optical modes of silicon-on-insulator SOI waveguides and Ge/SiGe quantum well modulators using an eigenmode expansion method. Laterally tapered features in the epitaxial layers are investigated for adiabatic optical coupling, and we find that there is a critical width range of the Ge/SiGe structure of 200-300 nm, where the taper angle should be minimised. We identify optimised taper profiles, which, for 1-μm-wide waveguides, allow the length of an adiabatic taper to be reduced from 250 μm for a simple linear profile to 40 μm for the optimised structure.