Design of a multimode waveguide bend with an arbitrary angle based on high degree of freedom curves

A compact and low-loss multimode waveguide bend plays a significant role in multimode channels and high-density on-chip optical interconnection architectures, and it has become a key component of photonic integrated chips. Here, we propose an inverse design method based on staged optimization of high-order Bezier curves with high degrees of freedom, which effectively overcomes the optimization limitations of traditional geometric curve design. Using this approach, we demonstrate an ultra-compact 90° multimode waveguide bend on a 220 nm silicon-on-insulator (SOI) platform, featuring an effective bending radius as small as 9 μm and supporting four TE modes. Furthermore, the bend is extended to arbitrary angle interconnects, with 60°, 120°, and 180° configurations as examples, significantly enhancing the flexibility and adaptability of on-chip compact multimode interconnections. Simulation results show that 90° bend exhibits excellent performance at 1550 nm with excess losses below 0.038 dB and crosstalk below −30 dB . The proposed design was further fabricated and experimentally characterized. The maximum measured excess loss is 0.13 dB, and the inter-mode crosstalk is all below −25 dB at 1550 nm. This device combines ultra-compact footprint, low loss, and excellent scalability, suitable for high-density on-chip interconnects.