Multimode waveguide bend is one of the key components for realizing high-density mode-division multiplexing systems on chip. However, the reported multimode waveguide bends are either large, bandwidth-limited or fabrication-complicated, which hinders their applications in future high-density multimode photonic circuits. Here we propose a compact multimode waveguide bend supporting four TE modes simply by shape-optimizing with transformation optics. The shape of the waveguide is optimized in the virtual space with gradient distribution of the refractive index, so that the scattering loss and intermode cross talk are well suppressed. After conformal mapping back into the physical space, a compact (effective radius of 17 μm) multimode bending waveguide is obtained. Simulations show that the proposed multimode waveguide bend has little loss (<0.1 dB) and low cross talk (<−20 dB) throughout an ultrabroad wavelength range of 1.16–1.66 μm. We also fabricated the shape-optimized multimode bending waveguide on a silicon-on-insulator wafer. At 1550 nm wavelength, the measured excess losses for the four lowest-order TE modes are less than 0.6 dB, and the intermode cross talks are all below −17 dB. Our study paves the way for realizing high-density and large-scale multimode integrated optical circuits for optical interconnect.

中文翻译：

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通过使用变换光学器件优化形状的紧凑型宽带多模波导弯曲

多模波导弯曲是实现片上高密度模分复用系统的关键部件之一。然而，报道的多模波导弯曲要么很大，要么带宽受限，要么制造复杂，这阻碍了它们在未来高密度多模光子电路中的应用。在这里，我们提出了一种紧凑的多模波导弯曲，只需通过变换光学进行形状优化即可支持四种 TE 模式。在折射率梯度分布的虚拟空间中优化波导的形状，从而很好地抑制了散射损耗和模间串扰。在共形映射回物理空间后，获得了紧凑（有效半径为 17 μm）的多模弯曲波导。仿真表明，所提出的多模波导弯曲损耗很小（<0. 1 dB) 和低串扰 (<-20 dB) 在 1.16–1.66 μm 的超宽波长范围内。我们还在绝缘体上硅晶片上制造了形状优化的多模弯曲波导。在 1550 nm 波长处，四种最低阶 TE 模式测得的超额损耗小于 0.6 dB，模间串扰均低于 -17 dB。我们的研究为实现用于光互连的高密度和大规模多模集成光路铺平了道路。