Designing short tapered waveguide adapters for Ge lasers and Ge/SiGe modulators integrated with SOI waveguides

We study optical mode propagation in vertically-coupled lateral tapers, which act as mode adapters between SOI waveguides and epitaxial Ge or SiGe layers for applications in laser cavities and optical modulators. We find that the taper length at which the loss tends to zero is strongly dependent on the thickness of the epitaxial layer, and we describe designs for modified tapers, which act as efficient mode adapters and are significantly shorter than linear tapers.