Topological Multi-Mode Photonic Crystal Fiber

Topologically-protected optical fiber is thought to significantly reduce the backscattering effects caused by the defects or impurities during the light propagation along the fiber length. The Zak phase is a topological invariant that characterizes the phase change of the wave function over one period in a periodic system. Here, we realize the topological transition of two-dimensional Zak phases by tuning the distribution of scattering columns at the propagation constant kz > 0, and demonstrate the corner states formed by the coupling of edge states as the guiding modes of the fiber. This topological guiding mode exhibits a strong localization with an ultra-small mode-field area of 0.3 μm2 (or high nonlinearity) and low confinement loss of ∼ 10-7 dB/km at 1.55 μm. Moreover, the multi-mode nature of this topological fiber is demonstrated by the realization of corner state modes located in the high-order topological bandgap at kz > 0. The high-order corner state modes perfectly match with the results obtained by the mode analysis based on finite element method. This topological multi-mode fiber holds a promise in short-distance backscattering-immune propagation and the nonlinear applications.