Germanium Quantum Well QCSE Waveguide Modulator With Tapered Coupling in Distributed Modulator–Detector System

Optical interconnections (interconnects) have been proposed as solutions to the ever-increasing bandwidth requirements and energy consumption in communication systems. Among possible photonic modulation strategies, the Ge quantum well (QW) based quantum-confined Stark effect (QCSE) stands out, as its strong electro-absorption effect allows for potentially lower power consumption and smaller device sizes compared to other modulation mechanisms. Here, we experimentally demonstrate a thin buffer layer Ge QW QCSE waveguide modulator that evanescently couples to and from an Si waveguide through an adiabatic three-dimensional (3D) taper. Simulations confirm that this 3-D taper yields higher coupling efficiency and improved maintenance of the fundamental mode after coupling compared to a 2-D taper. We also demonstrate that this geometry could potentially work in an integrated modulator-detector system. The combination of thin SiGe epitaxy (i.e., the buffer and device layers) with Si waveguides paves the way to easier integration of Si photonic integrated circuits.