  Wavelength division multiplexing (WDM) devices are key elements of Photonic
integrated circuits (PICs). Conventional WDM devices based on silicon
waveguides and photonic crystals have limited transmittance due to high loss
introduced by the strong backward scattering from defects. In addition, it is
challenging to reduce the footprint of those devices. Here we theoretically
demonstrate a WDM device in the telecommunication range based on all-dielectric
silicon topological valley photonic crystal (VPC) structures. We tune its
effective refractive index by tuning the physical parameters of the lattice in
the silicon substrate, which can continuously tune the working wavelength range
of the topological edge states, which allows designing WDM devices with
different channels. The WDM device has two channels (1470 nm-1523 nm and 1548
nm-1609 nm), with contrast ratios of 22.4 dB and 24.9 dB, respectively. The
principle of manipulating the working bandwidth of the topological edge states
can be generally applied in designing different integratable photonic devices,
thus it will find broad applications.
