On-chip mode (de)multiplexer utilizing sandwich valley-topological edge waveguides

Valley-topological photonic crystals have emerged as a promising solution for on-chip mode multiplexing communication, attributable to their unique advantages in device integration. However, the inherent fixed waveguide width associated with the topological valley-kink state leads to significant challenges, including severe mode field leakage and distortion, which obstruct mode compatibility and result in untargeted radiation modes during coupling and separation processes. These issues render them currently unsuitable for effective mode (de)multiplexing. To address these limitations, we propose a photonic crystal waveguide with a sandwich structure, incorporating a tunable number of Dirac photonic crystal layers as interlayers. This design allows for the adjustment of the valley-topological edge waveguide width. The tunable waveguide width, in conjunction with strong optical-matter interaction modulation, facilitates adiabatic mode evolution among width-dependent mode fields via evanescent wave coupling. As a proof-of-concept, we have designed and fabricated a bi-mode valley-topological photonic crystal mode (de)multiplexer with a compact footprint of 55.08 × 10.9 μm². This device exhibits minimal mode field leakage, with crosstalk levels below −18.52 dB. Furthermore, the (de)multiplexer demonstrates effective mode field compatibility, successfully transmitting 1.875 Tbit/s QPSK-OFDM signals with bit error rates below the forward error correction threshold. By leveraging topological protection and unidirectional excitation, this approach not only advances on-chip mode division multiplexing but also enhances stable channel connectivity, thereby improving the resilience of integrated optical communication networks.