Overlapping multimode interference couplers as a building block for folded interferometric circuits

In standard generic multimode interference couplers (MMIs), splitting behavior does not depend on the exact lateral position of the inputs and outputs, but on their symmetry around the center of the multimode region. In this work, we demonstrate that this insight allows several independent MMI functions to be spatially overlapped within a single multimode cavity. We design and experimentally demonstrate a combined MMI (Co-MMI) on an InP platform, integrating two 50/50 splitters in a 4×4 device with low imbalance and low leakage. As an application, we implement the device in a double-pass Mach-Zehnder modulator, achieving more than 50% reduction in Vπ and an improved extinction ratio.