Ultra-Compact Inverse Designed Multimode Waveguide Bend Based on Levelset Method

Mode-division multiplexing (MDM) technology can expand transimission capacity of a communication system and multimode waveguide bend (MWB) is a vital component. However, the reported MWBs either occupy large footprints or require complex fabrication steps. In this letter, the levelset method and adjoint method are combined to achieve an ultracompact three-mode MWB with an effective radius of 4.4
$\mu {\mathrm{ m}}$
. The MWB structure can be defined by levelset method which projects a three dimensional (3D) surface into a two dimensional (2D) curve. Moreover, the efficient evolution of the MWB structure are guaranteed by the adjoint method, this method only requires two simulations to derive the derivative of the figure of merit (FOM) with respect to design parameters. The simulation results of our MWB reveals that the average insertion losses for TE0, TE1 and TE2 modes are 0.04 dB,0.06 dB and 0.07 dB, respectively. In addition, the intermode crosstalks are all below −21dB in a wide spectral range from 1500 nm to 1600 nm. The MWB is also fabricated on a commerical silicon-on-insulator (SOI) wafer, the experimental spectra show that the insertion losses for three modes are 0.28 dB, 0.25 dB and 0.18 dB, the crosstalks between those modes are less than −13.4 dB across the wavelength range from 1525 nm to 1600 nm. The design methods can be applied not only to various nanophotonic components but also to other material platforms.