Broadband and Compact Silicon Multimode Waveguide Bends Based on Hybrid Shape Optimization

Multimode waveguide bend (MWB) is one of the essential components for achieving high-density integration of on-chip mode-division multiplexing (MDM). This paper proposes a low-loss and compact MWB utilizing the hybrid shape optimization (HSO) method. The HSO combines particle swarm optimization (PSO) and the adjoint approach to optimize the shape of MWB efficiently. Two types of MWBs are designed, with effective radii of 6 $\mu$ m and 10 $\mu$ m, supporting three modes and four modes, respectively. Fabrication of these MWBs requires a single lithography step and exhibits excellent fabrication tolerances. Experimental results show that the MWB supporting three modes has an insertion loss of less than 0.6 dB over a broad bandwidth of 100 nm (1500-1600 nm), with crosstalk of below -20 dB between all modes. Similarly, the MWB supporting four modes has an insertion loss of less than 0.8 dB over the same bandwidth, with inter-mode crosstalk of below -20 dB. The proposed MWB exhibits the best overall performance considering footprint, loss, inter-mode crosstalk, and operation bandwidth.