Breaking the Size‐Tolerance Trade‐off in Vertical Couplers via Adjoint‐Based Inverse Design

Heterogeneous integration of III–V lasers on silicon is pivotal for scaling next‐generation photonic systems, yet it faces a fundamental bottleneck: the inherent trade‐off between coupler compactness and misalignment tolerance. In this work, we break this longstanding barrier by combining multi‐start adjoint optimization with a physics‐guided strategy that places dense control points in near‐phase‐matched regions. This approach produces a more extended phase‐near‐matched region that spatially overlaps with the region of controlled odd supermode excitation, which is the key condition for simultaneously achieving compactness and tolerance. Our design achieves 98.11% coupling efficiency within a footprint of just 20 μm. Remarkably, it sustains over 80% efficiency under substantial misalignments up to ± 1.1 μm and waveguide width variations of ± 0.4 μm—representing the highest tolerance per unit length reported for vertical III–V‐on‐silicon couplers. This achievement decouples the traditional conflict between size and robustness, offering a critical solution for high‐yield micro‐transfer printing and dense photonic integration. The successful extension of our optimization framework to the lithium niobate‐on‐silicon nitride platform confirms its generality, positioning it as a versatile and transformative design strategy for a broad range of heterogeneous photonic platforms.