Robust silicon waveguide polarization rotator with an amorphous silicon overlayer

We propose a robust polarization rotator based on the mode-evolution mechanism. The polarization rotation in a silicon wire waveguide is achieved by forming an amorphous silicon (a-Si) overlayer and an SiO_2 spacer on top of the waveguide. A strip pattern of a constant width is designed to be etched through the overlayer at a specific angle with respectto the Si waveguide. The asymmetry in the a-Si overlayer affects the waveguide mode by rotating the modal axis. This polarization rotator design is amenable to comparatively simple fabrication compatible with standard silicon photonic processing for integration. The length ofthe rotation section is 17 µm, and the broadband operation is achieved with a rotation efficiency higher than 90% for a wavelength range exceeding 135 nm. A maximum polarization rotation efficiency of 99.5% is predicted by calculation.