Ultra-sharp silicon multimode waveguide bends based on double free-form curves

Mode-division multiplexing(MDM)can greatly improve the capacity of information transmission.The multimode waveguide bend(MWB)with small size and high performance is of great significance for the on-chip MDM integrated system.In this paper,an MWB with high performance based on double free-form curves(DFFCs)is proposed and realized.The DFFC is a combination of a series of arcs optimized by the inverse design method.The fabrication of this MWB only needs one-step lithography and plasma etching and has a large fabrication tolerance.MWBs with effective radii of 6μm and 10μm are designed to support three modes and four modes,respectively.The proposed method gives the best overall performance considering both the effective bending radius and the transmission efficiency.The fabricated MWB with four mode channels has low excess losses and crosstalks below-21 dB in the wavelength range from 1520 to 1580 nm.It is expected that this design can play an important role in promoting the dense integration of multimode transmission systems.