Silicon rib-loaded LiNbO3 waveguide polarization beam splitter based on bound state in the continuum design

Over the last years, LiNbO3 on insulator (LNOI) has emerged as one of the most important platforms for integrated photonic applications because of its large refractive index contrast, outstanding electro-optical and nonlinear optical properties. Rib-loaded waveguide technology is an efficient approach to avoid high surface and sidewall roughness and to relax the difficulty in construction of sub-wavelength structures with steep sidewalls in case of dry-etched LNOI on-chip nanowires. The main issue for the development of rib-loaded LNOI waveguides is the intrinsic TM loss which is caused by the mode conversion between different polarizations at rib boundaries. In this work we show that the intrinsic TM loss can be eliminated by precise control of the waveguide cross section, and a silicon rib-loaded LNOI polarization beam splitter (PBS) is proposed theoretically based on a symmetric directional coupler design. The PBS is designed with a compact size of 42-μm coupling length and relatively large bandwidth of ∼90 nm for extinction ratio above 15 dB. It is, to the best of our knowledge, the first PBS proposed on rib-loaded LNOI waveguide. We believe this work paves the way for exploration of functional integrated devices based on rib-loaded LNOI waveguides.