Mode-division multiplexing is a promising and cost-effective way to increase the communication capability of integrated photonic circuits, for both classical and quantum information processing. To construct large-scale on-chip multimode routing systems, the multimode waveguide crossing is one of the key components. However, there have been only a few dual-mode waveguide crossings reported, which can support merely two waveguide modes or two crossing channels. This severely limits the density and capacity of multimode routing systems. Here we demonstrate for the first time, to the best of our knowledge, a universal multimode waveguide star crossing based on transformation optics, which can handle, in principle, any number of waveguide modes and any number of crossing channels as well. The structure is transformed from a Maxwell fisheye and can realize aberration-free imaging for each waveguide mode. A gray-scale electron-beam lithography is adopted to fabricate it on a commercial silicon-on-insulator wafer. The proposed multimode waveguide star crossing has little loss and low crosstalk throughout an ultra-broad wavelength range of $\sim 400\mathrm{nm}$. Our study paves the way for realizing highly integrated and large capacity on-chip multimode routing and communication systems.

中文翻译：

---

基于变换光学的通用多模波导交叉

模分多路复用是一种有前途且具有成本效益的方式，可以提高集成光子电路的通信能力，从而实现经典信息处理和量子信息处理。为了构建大规模的片上多模路由系统，多模波导交叉是关键组件之一。但是，仅报道了很少的双模波导交叉，它们只能支持两个波导模式或两个交叉通道。这严重限制了多模式路由系统的密度和容量。在这里，我们就我们所知，第一次展示了基于变换光学的通用多模波导星交叉，它原则上可以处理任意数量的波导模式和任意数量的交叉通道。该结构是从麦克斯韦鱼眼镜头改造而成的，可以实现每种波导模式的无像差成像。采用灰度电子束光刻技术在商用绝缘体上硅晶片上制造它。拟议的多模波导星形交叉在整个超宽波长范围内损耗很小，串扰低。$〜400\mathrm{纳米}$。我们的研究为实现高度集成和大容量的片上多模路由和通信系统铺平了道路。