Contents 1. Introduction
Photonic integrated devices on the silicon-on-insulator (SOI) platform are attractive due to the complementary-metal-oxide-semiconductor (CMOS)-compatible fabrication process. The high refractive index contrast between the silicon core and the cladding enables compact photonic devices with small footprints. However, it also results in large polarization-dependent dispersions or losses [1]. Polarization splitter-rotators (PSRs) for splitting two orthogonal polarization modes and rotating one of them by 90° are key components in polarization-diversity schemes to eliminate the polarization sensitivities [2]. Various types of the PSRs were reported based on asymmetric directional coupler [3], Y-branch splitter [4], multi-mode interferometer [5], etc. However, these PSRs show limited polarization extinction ratios (PERs < 20 dB) and are sensitive to fabrication errors. Fabrication-tolerant PSRs based on tapered directional couplers and taper-etched waveguides were proposed, at the cost of large footprints and complex fabrication process [6]–[8]. Recently, a compact PSR based on an asymmetrical directional coupler with a subwavelength grating (SWG) was numerically proposed [9]. By using the SWG structure, fabrication tolerances of the PSR were substantially enhanced. However, no experimental demonstration of such a SWG-based PSR was reported.
