A 4.3 GHZ Lithium Niobate on Insulator Wideband Surface Acoustic Wave Delay Line with Multi-Mode Composition

In this study, we present a novel technique for expanding the bandwidth of surface acoustic wave (SAW) delay lines by leveraging the multi-mode behavior of thin-film acoustic waveguides. Through numerical analysis, we show that a wide and sharp "multi-mode passband" can be achieved by merging the responses of adjacent acoustic modes in an acoustic delay line (ADL) via a carefully optimized thickness-to-wavelength ratio of the waveguide. To validate this concept, we designed the ADL on a silicon-based thin-film lithium niobate-on-insulator platform (LN/SiO2/Si) equipped with single-phase unidirectional transducers (SPUDTs). Three waveguide surface topographies are experimentally analyzed to optimize the passband. With a wavelength (λ) of 1 μm, the proposed multi-mode passband ADL achieves a minimum insertion loss (IL) of 8.58 dB, a wide 3-dB fractional bandwidth (FBW) of 9.55%, and a large group delay (GD) of 35 ns at 4.3 GHz. The competitive performance of the proposed design, compared to state-of-the-art ADLs, underscores the significant potential of the multi-mode design concept for wideband radio frequency (RF) signal processing.