High performance B-spline multimode waveguide bends in lithium niobate on insulator.

Lithium niobate on insulator (LNOI) has emerged as a transformative platform for integrated photonics, combining the exceptional material properties of lithium niobate with the compactness and scalability of modern photonic technologies. While traditional waveguide bend designs for LNOI face challenges such as bending losses, mode mismatch, and fabrication complexity, this study introduces an approach leveraging B-spline curves for multimode waveguide bend optimization. B-spline curves offer unparalleled design flexibility, enabling precise control of curvature profiles, smooth transitions, and simultaneous optimization of insertion loss (IL) and mode crosstalk (CT). Experimental results demonstrate that 90° B-spline-based waveguide bends achieve ultra-low insertion losses of 0.05 dB, 0.10 dB, and 0.29 dB for TE, TE, and TE modes, respectively, with crosstalk below -16.71 dB across all modes for cascaded bends. These results suggest that B-spline-based designs hold promise for enabling high-performance multimode waveguides, offering a potential solution to key challenges in LNOI photonic integrated circuits.