Strip-Loaded Overlay SH0 Waveguide Based Directional Couplers in Thin Film Lithium Niobate on Insulator

This work presents the design, simulation, and experimental validation of a strip-loaded SH0 mode-based acoustic directional coupler on Y-propagating X-cut thin-film lithium niobate on insulator (LNOI). Key supporting structures, including straight, tapered, and bent waveguides are optimized to achieve efficient coupling with minimal insertion loss. Through finite element method (FEM) simulations and experimental characterization, we systematically investigate the impact of taper length, coupling gap <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(1-2 \mu \mathrm{m})$</tex>, and bend radius <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$( 150-300 \mu \mathrm{m})$</tex> on device performance. Experimental results demonstrate a propagation loss of 8 dB/mm for straight waveguides and sinusoidal coupling trends for couplers as a function of length, aligning with theoretical predictions. The optimized directional coupler exhibits a coupling coefficient of up to 27.79% at 709 MHz, enabled by a simplified one-mask fabrication process with 100 nm gold strip loading. These findings may be used towards the development of compact, low-loss acoustic wave devices for radio frequency (RF) systems, such as phased arrays and all-acoustic signal processing.