As silicon photonics technology advances toward high-density integration scenarios—such as large-scale matrices, optical phased arrays, and optical neural networks—single-layer waveguide routing encounters severe topological challenges, rendering waveguide crossings indispensable fundamental components for constructing complex on-chip interconnect networks. As photonic hubs bridging distinct functional regions, the insertion loss, crosstalk, and bandwidth of these crossings directly dictate the signal integrity and transmission capacity of optical links. This paper systematically reviews recent research progress and key technologies concerning silicon-based waveguide crossings. Initially, the mechanism of scattering loss in direct crossings is elucidated, followed by a detailed examination of three mainstream design paradigms for loss mitigation: multimode interference (MMI) structures based on the self-imaging principle, adiabatic transformation structures relying on mode evolution, and medium engineering structures utilizing sub-wavelength gratings and metamaterials. Furthermore, the application of algorithm-driven inverse design in overcoming the constraints of traditional physical configurations is discussed. Crucially, addressing the urgent demand for ultra-high transmission capacity in the post-Moore era, this review highlights functional crossings capable of polarization division multiplexing (PDM) and mode division multiplexing (MDM), analyzing the design challenges and breakthroughs associated with multi-dimensional light field manipulation. Finally, this paper presents prospects for the future development trends of the waveguide crossing junction.
Ni et al. (Sat,) studied this question.