A basic model of topological insulators, the Su-Schrieffer-Heeger (SSH) model, has been applied in various photonic systems for novel optical effects and related applications. Here, we show that odd-numbered SSH chains may enable extremely broadband waveguide couplers. A special feature of odd-numbered SSH chain is that there is always a zero-energy localized state regardless of its bulk topological invariant value. We utilize this unique feature in adiabatic photonic waveguide systems for spectrally robust optical power dividers or combiners as opposed to the conventional interferometric components such as directional couplers and multimode interference couplers. We demonstrate broadband edge-to-edge and 1N couplers with their power balance persisting over the entire optical telecommunications bands. We provide detailed theory, experimental results, and remaining challenges.
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