Ultra‐Wideband Fabrication‐Tolerant Anti‐Bound‐State‐in‐Continuum Polarization Filter on Directionally Controlled Lithium Niobate‐on‐Insulator

As an anisotropic material, X‐cut lithium niobate‐on‐insulator (LNOI) suffers from birefringence‐induced polarization crosstalk, demanding low‐loss and broadband on‐chip polarization management. In this work, we propose and demonstrate an ultra‐wideband quasi‐TE pass polarization filter on the X‐cut LNOI platform based on waveguide direction control. The fabricated device exhibits the quasi‐TE excess loss (EL) above −0.15 dB and the quasi‐TM extinction ratio (ER) < −20.16 dB at wavelengths of 1064 nm, 1310 nm, and 1440 nm—1640 nm. This covers the O, E, S, C, L, and U communication bands and the 1064 nm laser. Owing to propagation along the Z‐axis of lithium niobate (LN), the polarization directions of both the bound TM mode and the continuum TE mode experience the ordinary refractive index n o . This results in highly parallel dispersion curves for their effective refractive indices, enabling quasi‐phase matching that is extremely insensitive to wavelength variations. To date, this work demonstrates the highest‐bandwidth polarization filter reported on the LNOI platform. Additionally, clear eye diagrams at 120 Gbaud indicate the potential of the device for high‐speed optical communications. This work provides a polarization management solution featuring ultra‐wide bandwidth and low loss for the multi‐band X‐cut LNOI platform.