Ultra-Compact and Highly Efficient Inverse-Designed Taper for Fiber-to-TFLN Nanowaveguide Coupling

Edge coupling between optical fibers and thin-film lithium niobate waveguides is fundamentally constrained by modal mismatch and the length requirements associated with adiabatic tapering. Here, a fabrication-aware, adjoint-based inverse design methodology is employed to develop a high-efficiency bilayer taper. This optimized structure achieves non-adiabatic mode transformation within an interaction length of only 25 μm, drastically reducing the device footprint compared to conventional designs. The simulation results predict a total coupling loss of approximately 0.42 dB at 1.55 μm wavelength and demonstrate stable, low-loss broadband performance across the 1.5 μm to 1.6 μm range, providing a scalable solution for high-density photonic integration.