Ultra-compact multimode waveguide bends with arbitrary bending angles

Achieving compact waveguide routing with arbitrary configurations is essential for on-chip mode-division multiplexing (MDM) systems to enable higher integration density and more flexible layouts. However, current multimode waveguide bends (MWBs) are limited to 90°, restricting design flexibility. In this paper, we propose, for the first time to our knowledge, ultra-compact MWBs with arbitrary bending angles. As a proof of concept, we design and experimentally demonstrate four different MWBs with bending angles of 15°, 30°, 45°, and 60° on a standard silicon-on-insulator (SOI) platform, supporting the first three TE modes. Dual Bezier contours are employed for the MWBs' trajectory optimization. For all MWBs, the largest footprint is only 7.5 × 5.1 µm². Measurement results show insertion losses below 0.417 dB and inter-mode crosstalks below -16.6 dB within the wide wavelength range of 1500-1600 nm for all three modes. Furthermore, these MWBs can serve as building blocks for arbitrary-angle MWBs and S-bends with various displacements, paving the way for ultra-compact on-chip MDM systems.