Inverse Design of Ultra‐Compact Multimode Waveguide Bends Based on the Free‐Form Curves

On‐chip mode‐division multiplexing (MDM) can increase the transmission capacity of a single optical bus waveguide. It is of great significance for the MDM systems to design the multimode waveguide bends (MWBs) with compact sizes. A method of designing the MWB using the inverse design algorithm is presented in this paper. Ultra‐compact MWBs with high performances are realized by changing the curvature radius of the waveguide curve, and the fabrication process only requires a single step photo‐lithography and plasma etching. The three‐mode bent waveguide with an effective radius of only 9.35 µm is designed and tested. The theoretical excess losses of TE 0 , TE 1 , and TE 2 mode are less than 0.04 dB in a wide spectral range of 1500 to 1600 nm, and the crosstalks between all guided modes are all less than −29 dB. For the fabricated 90° waveguide bend the measured excess losses are 0.17, 0.04, and 0.16 dB at the central wavelength of 1550 nm, respectively, and the crosstalks between all three modes are less than −21 dB in the wavelength range from 1520 to 1600 nm. The proposed design method can be also extended to more mode‐channels.