Broadband coupled resonator optical waveguides with robust topological coupler

Coupled resonator optical waveguides (CROWs) are widely used for filtering, storing, and enhancing light-matter interactions. However, their bandwidth, which characterizes the operation wavelength range, is fundamentally limited by the tradeoff between the free spectral range (FSR) and finesse. To address this limitation, we introduce topological coupler into CROWs, significantly enlarging their bandwidth. By leveraging the topology and symmetry of topological coupler, the finesse of CROW is independent of its FSR, which efficiently relieves the tradeoff. Our topological CROW achieves a bandwidth of 9.0 nm. By cascading such CROWs, we realize a topological two-channel add-drop filter with a bandwidth of over 4.7 nm, even with dimensional errors up to 20 nm. Furthermore, high-speed data transmission at 170 Gb/s is realized in the topological filter. Our broadband topological CROWs and filter offer new opportunities for robust optical buffering, broadband modulation, and novel polariton applications.