Ultra-compact bent multimode silicon waveguide with ultralow inter-mode crosstalk.

We propose and experimentally demonstrate a sharply bent multimode silicon waveguide by introducing a pair of mode converters. The principle of the multimode bend is based on the eigenmode conversion between straight and bent waveguides, and the particle swarm optimization is adopted to engineer the geometry. The device is fabricated on the standard silicon-on-insulator platform using one lithography/etching, and no additional steps are required. The measured results show that the insertion loss is <0.2  dB, and the inter-mode crosstalk is <-22  dB with a bend radius of 5 μm, from 1500 to 1600 nm. A comparable performance can be achieved using a conventional scheme with 40 μm radius.