Inverse-Designed Lithium Niobate Waveguide Crossing with Broadband Operation, Low Loss, and Compact Footprint

Waveguide crossings are fundamental building blocks of high-density photonic integrated circuits and on-chip optical routing systems. However, realizing a high-performance, compact waveguide crossing on a lithium niobate-on-insulator (LNOI) platform remains challenging because conventional designs often rely on larger device dimensions to meet stringent performance requirements. Furthermore, narrow single-mode waveguides provide strong mode confinement but are more sensitive to fabrication-induced sidewall roughness due to stronger interaction between the guided mode and the waveguide sidewalls. In contrast, increasing the waveguide width can reduce the sensitivity to sidewall roughness but may introduce higher-order modes. Here, we propose and experimentally demonstrate an inverse-designed waveguide crossing on the LNOI platform. To enable greater flexibility in manipulating light within a compact footprint, the waveguide crossing is optimized using particle swarm optimization (PSO) combined with finite-difference time-domain (FDTD) simulations to enhance TE0-mode transmission. Meanwhile, to reduce sidewall-induced scattering while preserving predominantly TE0 mode transmission, 2 μm wide multimode input and output waveguides are adopted. The fabricated crossing occupies a footprint of 14.55 μm × 14.55 μm and exhibits an insertion loss below 0.1 dB and crosstalk below −29 dB over the wavelength range of 1480–1640 nm. The demonstrated device provides a favorable trade-off among insertion loss, crosstalk, operating bandwidth, footprint, and fabrication simplicity compared with representative previously reported LNOI waveguide crossings. The PSO-assisted framework is extendable to other LNOI devices, enabling dense photonic integration.