Low-loss broadband i-line lithography-compatible thin-film lithium niobate edge coupler interfaced with single-mode fiber

Thin-film lithium niobate (TFLN) provides an outstanding platform for exploring nonlinear optics and electro-optic effects, where efficient coupling between TFLN waveguide and a fiber is always required. In this work, we present an i-line lithography-compatible edge coupler, featuring a vertically stacked silica (SiO₂) cladding layer atop a wedge-shaped TFLN waveguide with a linewidth larger than 600 nm. Theoretically, the proposed structure exhibits a coupling loss of 1.64 dB/facet for the TE₀ mode, whereas the experimental measurement yields a coupling loss of 2.80 dB/facet. Across the 1510-1630 nm wavelength band, the edge coupler maintains a coupling loss of less than 3 dB/facet. Due to the 600 nm minimum linewidth in the coupler, it can be produced using i-line lithography machines.