Inverse-designed compact waveguide bend on X-cut TFLN

Compact, low-loss waveguide bends are essential for high-density thin-film lithium niobate (TFLN) photonic integrated circuits, yet tight curvature increases radiation loss and fabrication-induced scattering, making it challenging to achieve a small footprint, low loss, and manufacturability simultaneously. In this work, an inverse-designed waveguide bend on an X-cut TFLN platform is demonstrated using a generalized inverse-design framework that rigorously accounts for material anisotropy, occupying 18.5 × 18.5 μm², with cascaded measurements extracting a per-bend loss of −1.23 dB at 1550 nm. Fabrication tolerance is quantified against dimensional bias and sidewall-angle variations, and additional devices with ±10 nm and ±20 nm geometric offsets are fabricated for experimental verification. The measured spectra remain highly consistent across bias conditions, with negligible loss variation under ±20 nm bias (ΔLoss<0.01 dB), while simulations show only minor degradation for sidewall angles from 60° to 90° (ΔLoss<0.1 dB). The residual mismatch between simulation and experiment is mainly attributed to sidewall roughness. These results establish a manufacturable and scalable bend building block for dense on-chip routing and complex passive photonic networks on X-cut TFLN platforms.