Demonstration of a compact and broadband silicon nitride multimode interference coupler enabled by guided inverse design.

2 × 2 multimode interference (MMI) couplers are basic building blocks for photonic integrated circuits. However, conventional MMIs exhibit limited bandwidth, leading to substantial cumulative loss in cascaded systems. In this Letter, we experimentally demonstrate a compact and broadband silicon nitride MMI coupler enabled by guided inverse design. Leveraging the effective-width-reduction principle, the multimode-region boundary is parameterized by smooth cubic splines and optimized directly for broadband low excess loss and balanced 3-dB splitting. The optimized device achieves a compact footprint of 41 × 5.1 μm2 while maintaining sufficient access waveguide separation. The fabricated device exhibits uniform splitting and an experimental 0.5-dB bandwidth of 180 nm over 1460-1640 nm. A passive MZI composed of two optimized couplers further demonstrates broadband interferometric operation. These results indicate that the proposed MMI is a promising compact broadband building block for large-scale silicon nitride photonic circuits.