Broadband and compact polarization rotator based on cascaded-stair waveguide with low insertion loss

Given that silicon-on-insulator (SOI) has a high refractive index contrast and intrinsic birefringence, photonics devices based on SOI are typically polarization-sensitive. To address this issue, a novel broadband mid-infrared polarization rotator (PR) based on cascaded stepped waveguides was put forward. It can achieve polarization conversion between the fundamental TM₀ and TE₀ modes through mode hybridization formed in asymmetric waveguides. The finite-difference time-domain (FDTD) method was employed to explore its polarization rotation characteristics and optimize the device structure. Simulation results demonstrate that at the central wavelength of 2.53 µm, the maximum polarization extinction ratio (PER) can attain 40.02 dB, the polarization conversion efficiency (PCE) exceeds 99.8%, and the insertion loss (IL) is as low as 0.12 dB. Moreover, the operating bandwidth is expanded to 490 nm (spanning from 2.28 to 2.77 µm). Meanwhile, the device length is merely 16.4 μm. Furthermore, tolerance analysis indicates that the device has good manufacturing tolerance. Owing to its high PER, large bandwidth, and small footprint, the proposed PR has significant application potential in mid-infrared photonic integrated circuits (PICs).