Topologically protected dense wavelength division multiplexer based on valley photonic crystal slabs.

On-chip dense wavelength division multiplexing (DWDM) significantly boosts transmission capacity; the denser the channels, the stronger the information processing capability, establishing it as a cornerstone of high-density optical interconnects. However, conventional on-chip DWDM devices, such as arrayed waveguide gratings (AWGs), typically suffer from large footprints and high insertion losses and are highly susceptible to fabrication imperfections and environmental perturbations, leading to performance degradation. Consequently, realizing a high-density, low-loss, and robust on-chip DWDM device remains an open challenge. Here, we report the design of a topologically protected multiport tunable filter on a silicon photonic platform by cascading topologically protected contra-directional couplers based on valley photonic crystals. In particular, the device achieves a narrow filtering bandwidth of 1.56 nm across the communication band and a high transmittance of 0.97, while exhibiting resilience against manufacturing defects. We further demonstrate that the output wavelength of every port is programmable via integrated micro-heaters. Our study paves the way for the realization of robust, ultra-compact optical interconnects and can have important applications in the field of next-generation elastic optical networks and quantum communication systems.