X-Cut Lithium-Niobate-On-Insulator Polarization Beam Splitter with Anisotropic Subwavelength Perturbation

Lithium niobate on insulator (LNOI) is increasingly recognized as a versatile platform for photonic integrated circuits, owing to its superior electro-optic and nonlinear properties. However, developing polarization beam splitters (PBSs) on the X-cut LNOI presents challenges due to its inherent low birefringence, intrinsic anisotropy, and vertical asymmetry. In this study, we introduce an approach utilizing anisotropic subwavelength perturbations on an in-plane anisotropy platform, facilitating effective control over polarization-dependent mode fields. By engineering side-wall subwavelength structures to selectively manipulate the evanescent coupling of quasi-TE0 and quasi-TM0 modes, we identify an exceptional coupling point that enables highly asymmetric coupling behavior. For the proposed design, the fabricated device exhibits peak extinction ratios of ∼40 dB and ∼36 dB and ultralow excess losses of 0.1 dB and 0.3 dB for quasi-TE0 and -TM0 modes, respectively. Further bandwidth enhancement is achieved using an adiabatically tapered design, which extends the bandwidth to preserve high extinction ratios across the entire C-band. The devices are fabricated via a single-step lithography process, highlighting their scalability and simplicity. This approach is broadly applicable to other anisotropic low-birefringence platforms and offers a promising route toward dense polarization-sensitive photonic integration.