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In recent years, Ge/Si avalanche photodiodes (APDs) have shown significant potential in high-sensitivity and high-speed photodetection for optical communications. Waveguide-coupling can further increase the bandwidth-efficiency product of these devices by separating the optical absorption path from the carrier collection path. Here we present a step-coupler design for efficient vertical coupling between Si waveguides and Ge/Si APDs. The step-coupler approach can achieve >;70% absorption with a 10-μm-long Ge absorption region at λ = 1550 nm, 5 X reduction in device length compared to the conventional waveguide coupling. The significant increase in coupling efficiency is due to vertical multimode interference (MMI) that enhances light scattering towards the Ge active region and creates mirror images of optical modes close to the Ge layer.