Efficient broadband polarization-insensitive edge coupler on a thin film lithium niobate platform for single-mode fibers

We propose a broadband, polarization-insensitive, and fabrication-friendly edge coupler (EC) optimized for efficient coupling between a 600-nm X-cut thin-film lithium niobate (TFLN) platform and a standard 9.2-μm single-mode fiber at 1550 nm. A single-layer silicon nitride (Si₃N₄) waveguide structure and a three-stage adiabatic taper are employed to enhance mode-field matching and minimize propagation losses. The optimized EC achieves coupling losses of -0.546/-0.501 dB (TE/TM), polarization-dependent loss (PDL) of 0.035 dB, a 1-dB bandwidth exceeding 100 nm, and robust alignment tolerances (±2.0 μm/±2.2 μm laterally and ±1.7 μm/±1.9 μm vertically for TE/TM modes), making it highly suitable for scalable TFLN photonic integration.