Design of an ultrasharp silicon multimode waveguide bend assisted by multiregional subwavelength grating structures

Sharp multimode waveguide bends are beneficial to improve the integration density of the on-chip mode-division multiplexing (MDM) systems, which can be used to increase the optical information transmission capacity. A method of designing a multimode bending structure based on subwavelength gratings is proposed in this paper, through the mode conversion and matching between the straight and bending waveguide. The proposed bending structure is composed of a 90-deg arc-bend with a radius of 5 μm combined with the shallow-etched grating structures on the top, and two-mode converters on the input and output straight waveguide realized also with the help of nonuniform grating-like structures, achieving a total effective bending radius of 8.25 μm and high transmission efficiencies of the first three TE modes (TE0 − TE2) in a wide wavelength range of 1500 to 1600 nm. At the center wavelength of 1550 nm, the calculated excess losses of the three modes TE0, TE1, and TE2 are 0.21, 0.19, and 0.59 dB, respectively, and the crosstalks for all modes are less than −26.58 dB. More importantly, we add a non-uniform grating converter in the straight waveguide region which pre-deviates the mode fields inside the straight waveguide to help further reduce the size of the bending part to only 5 μm, which will be helpful for large scale integration when a great number of bends are cascaded.