Inverse design of an ultra-compact low-loss single-step-etched polarization rotator based on adjoint method staged optimization

On-chip polarization control plays a crucial role in photonic integrated circuits implemented on silicon-on- insulator platforms, with key components including polarization rotators. Here, by combining boundary shape optimization and topology optimization in the adjoint method for staged optimization, we designed a polarization rotator enabling polarization conversion from TM0 to TE0 via a polarization rotation part and a mode conversion part. Simulation results indicate that using the staged optimization method can further improve the performance of the polarization rotator while maintaining device compactness. With a compact footprint of 1.05 x 12.00 mu m(2), the polarization rotator exhibited low insertion loss (< 0.44 dB) and high extinction ratio (> 24 dB) in the wavelength range from 1530 nm to 1570 nm. The device has a simple structure, requiring only a single-step etching, and it exhibits a certain tolerance to fabrication imperfections.