High-performance and scalable polarization splitter-rotator based on photonic crystal nanobeam reflection.

The polarization splitter-rotator (PSR) is a key device for polarization processing in polarization diversity systems, which has wide applications in achieving polarization independence and mixed multiplexing. However, it remains a significant challenge to simultaneously achieve a better balance in bandwidth, crosstalk (CT), polarization extinction ratio (PER), and compact footprint of the PSR. In this article, a photonic crystal nanobeam (PCN) structure is introduced to PSR for large bandwidth and compact size, with a device length of only 104 µm. Additionally, to achieve lower CT, a bridge waveguide is introduced for primary filtering. Simulation results show that the insertion loss (IL) is less than 0.55 dB, CT less than -35 dB, and PER greater than 35 dB within a bandwidth exceeding 110 nm, while maintaining a large process tolerance. Furthermore, the proposed PSR design breaks through the limitations of traditional schemes by extending its functionality effectively. To further improve integration, a novel approach to PSR using mode hybridization followed by spatial beam splitting is proposed. By controlling the phase-matching condition of various modes in different waveguides, the designed spatial beam splitting achieves lower CT and better compactness. Simulation results verify that the IL of the improved scheme is less than 1 dB, CT less than -24 dB, and PER greater than 22 dB within an 85 nm bandwidth, while reducing the overall length to less than 20 µm.