Abstract Manipulating on-chip optical modes via components in analogy with free-space devices provides intuitional light control, and this concept has been adopted to implement single-lens–assisted spot size conversion using integrated device. However, the reported schemes have been demonstrated only for fundamental mode, while high-order or irregular modes are preferred in specific applications. The 4-f system is widely used in Fourier optics for optical information processing. Under the inspiration of the 4-f system and the beam expander in bulk optics, a spot size converter (SSC) with two metamaterial-based graded-index waveguides is proposed and demonstrated. The proposed device is capable of widening an arbitrary mode while preserving its profile shape. Compared with conventional SSC using adiabatic taper, the footprint can be reduced by 91.5% under a same intermode crosstalk. Experimentally, an expansion ratio of five is demonstrated for regular modes. Furthermore, for an irregular mode, the functionality is numerically verified without structure modification. This work offers a universal solution to on-chip spot size conversion and may broaden the on-chip application prospects of Fourier optics.

中文翻译：

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片上任意模式光斑尺寸转换

摘要 通过与自由空间器件类比的元件来操纵片上光模提供了直观的光控制，并且该概念已被采用以使用集成器件实现单透镜辅助光斑尺寸转换。然而，报告的方案仅针对基模进行了演示，而在特定应用中更喜欢高阶或不规则模式。4-f 系统广泛用于傅立叶光学中的光学信息处理。在体光学中 4-f 系统和扩束器的启发下，提出并演示了具有两个基于超材料的渐变折射率波导的光斑尺寸转换器 (SSC)。所提出的设备能够在保持其轮廓形状的同时加宽任意模式。与使用绝热锥度的传统 SSC 相比，占地面积可减少 91。在相同的模间串扰下为 5%。实验证明，常规模式的膨胀比为 5。此外，对于不规则模式，功能在没有结构修改的情况下进行了数值验证。这项工作为片上光斑尺寸转换提供了一种通用的解决方案，并可能拓宽傅立叶光学的片上应用前景。