Ultra-low-loss and broadband wavelength demultiplexer on LNOI for 780/1550  nm using SWG-assisted directional coupling

The wavelength division demultiplexer (WDD) is an indispensable component for separating and routing photons to spatially distinct detection nodes in practical quantum key distribution (QKD) systems. In this work, we propose an ultra-low-loss and broadband WDD based on lithium niobate on insulator (LNOI), designed and optimized through comprehensive three-dimensional simulations, utilizing subwavelength grating (SWG)-assisted double directional coupling for the separation of 780 and 1550 nm wavelengths. The SWG waveguide enables both a compact footprint and low insertion loss (IL) over a broad wavelength range, making it well suited for QKD applications. The optimized device, with a total length of 45 µm, achieves ILs of 0.04 dB at 780 nm and 0.22 dB at 1550 nm, along with extinction ratios of 22 and 32 dB, respectively. The 0.5 dB bandwidth exceeds 140 nm for 1550 nm and 230 nm for 780 nm. Furthermore, the structure exhibits strong tolerance to fabrication-induced variations, supporting its suitability for robust and scalable photonic integration.