Mass-producible edge coupler with wedge taper and SiON stack for efficient broadband interfacing with single-mode fiber on thin-film lithium niobate

Thin-film lithium niobate (TFLN) emerges as an integrated photonics platform capitalizing on superior electro-optic and nonlinear performance. However, efficient light coupling remains challenging due to significant mode-field mismatch between single-mode fibers (mode field diameter, MFD =10.4 μm @1550 nm) and TFLN waveguides (MFD ∼1 μm). To address this critical limitation, we propose a lithography-compatible edge coupler featuring a vertically stacked silicon oxynitride (SiON) layer and silicon dioxide (SiO₂) layer on a wedge-shaped TFLN waveguide. Our design achieves a low coupling loss of 0.38 dB/facet for the TE₀ mode and 0.86 dB/facet for the TM₀ mode at the wavelength of 1550 nm. The coupler demonstrates a ± 3.5 μm fiber alignment tolerance and maintains <3 dB loss over a 300 nm bandwidth. Furthermore, the 600 nm minimum linewidth makes this approach compatible with the mass production of TFLN wafers using i-line stepper lithography.