Design of Low-loss Thin-film Lithium Niobate Coupling Structure Based on Bi-level Taper

The primary objective of this study is to design and experimentally validate a low-loss edge coupler based on a bi-level taper structure on the thin-film lithium niobate platform， specifically targeting efficient coupling with standard single-mode fibers having a mode field diameter of 6 μm at the 1 310 nm wavelength， to address the critical challenge of high coupling losses between Thin-film lithium niobate waveguides and optical fibers that hinder practical applications in integrated photonic systems such as fiber optic gyroscopes， aiming for a solution compatible with mass production via deep ultraviolet lithography while maintaining low optical loss and high alignment tolerance.To achieve this， the coupler design incorporates two sequentially fabricated inverted tapers—a fully etched lower taper and a partially etched upper taper—within a silicon dioxide cladding， and the structural parameters were rigorously optimized using three-dimensional finite-difference time-domain simulations to maximize the mode field overlap integral between the waveguide and fiber， where key geometric variables including the taper lengths L1 and L2 （both set equal to L for simplicity）， the lower taper tip width W1， the upper taper tip width W2， the lower taper output width W3， and the straight waveguide width W4 were systematically varied to assess their impact on coupling efficiency， with initial simulations determining an optimal taper length L of 300 μm to satisfy adiabatic conditions and avoid excessive transmission loss or fabrication-induced defects， followed by width optimizations revealing optimal values of W1 at 0.2 μm， W2 at 0.2 μm， W3 at 3.5 μm， and W4 at 1.2 μm for single-mode operation， while additional analysis of the silicon dioxide upper cladding thickness indicated that a minimum thickness of 3 μm is essential to confine the mode effectively and prevent vertical leakage， with negligible efficiency variation between 3 and 6 μm.Fabrication involved electron beam lithography for rapid prototyping and validation， where X-cut Thin-film lithium niobate with a 300 nm thick lithium niobate layer on a 4.7 μm silicon dioxide substrate was patterned using two Electron beam lithography steps followed by inductively coupled plasma-reactive ion etching to define the upper and lower tapers sequentially， and a 6-micrometer silicon dioxide cladding was deposited via a combination of low-pressure chemical vapor deposition and plasma-enhanced chemical vapor deposition to ensure density and minimize hydrogen-related losses. Experimental characterization employed a 1 310 nm superluminescent diode source and single-mode fibers in a cut-back method setup to measure total insertion loss， yielding a coupling loss of approximately 2.3 dB per facet for the optimized design， with the straight waveguide propagation loss separately quantified as 0.47 dB per centimeter， and scanning electron microscopy imaging confirmed the fabricated structure but indicated an approximate 110-nanometer lateral misalignment between the upper and lower tapers， though subsequent Finite-difference time-domain simulations demonstrated that such misalignments up to ±300 nm introduce negligible additional loss below 0.03 dB， affirming high fabrication tolerance. Further experimental sweeps of parameters W1 and L showed consistent trends with simulations， where coupling loss decreased with increasing L up to 300 μm and increased with W1 above 0.2 μm， and comparative analysis with existing coupler designs highlighted that this bi-level taper structure achieves a favorable balance between performance and process complexity， requiring only two etching steps and avoiding reliance on costly ultra-high numerical aperture or lensed fibers， thus enabling practical， low-cost deployment in fiber gyroscope applications.In conclusion， the proposed bi-level taper edge coupler successfully demonstrates low coupling loss， robust manufacturability with substantial alignment tolerances， and full compatibility with standard single-mode fibers and deep ultraviolet lithography mass production， thereby providing a viable pathway for scalable integration of Thin-film lithium niobate based photonic devices.