Compact and efficient polarization rotator using laterally asymmetric rib waveguides on a...

Photonic devices based on a lithium-niobate-on-insulator (LNOI) are current research hotspots; however, owing to the high refractive index contrast of the LNOI platform and inherent birefringence of lithium niobate itself, such photonic devices are generally polarization sensitive, affecting their further wide application. This paper proposes a simple, compact, and efficient polarization rotator (PR) based on a laterally asymmetric rib waveguide by depositing a layer of semi-infinite silicon nitride dielectric material on one side of the rib waveguide. The results show that a PR with a polarization rotation region length of 15.77 µm is achieved, and the polarization extinction ratio (PER), insertion loss (IL), and polarization conversion efficiency (PCE) are 38.57/68.95 and 0.2/0.22 dB, and 99.99%/almost 100%, respectively, for the fundamental transverse electric mode (T E ₀) and transverse magnetic mode (T M ₀) at a 1.55 µm wavelength. The operation bandwidth is around 120 nm for the T E ₀ mode and T M ₀ mode when the PER, IL, and PCE are greater than 20 dB, less than 0.32 dB, and more than 99%, respectively. Fabrication tolerances to the key structural parameters are investigated in detail. In addition, the evolution fields of the T E ₀ mode and T M ₀ mode along the propagation direction through the proposed device are presented.