Ultracompact silicon polarization splitter-rotator using a dual-etched and tapered coupler

The polarization splitter-rotator (PSR) plays a significant role in telecom, Datacom, and quantum circuits to meet significant requirements for polarization processing and multiplexing. We design an ultracompact PSR based on a silicon asymmetrical directional coupler (ADC). One waveguide of the ADC is gradually etched in two levels along the coupling region and the S-bend. A trapezoidal taper on the top of the dual-etched waveguide is first formed to satisfy the phase-matching condition for the TM-TE mode conversion, such that the TM mode is cross-coupled from the input waveguide without etching. Then, the etching width gradually increases in the S-bend and the following to achieve a single-mode waveguide for further removal of the residual TM mode. In this way, a high extinction ratio can be achieved for the polarization mode splitting in an ultracompact silicon ADC. For the designed PSR with a total length of about 24 µm, the simulation results reveal that the minimum extinction ratio is greater than 30, 20, or 15 dB within the bandwidth of 33, 100, or 150 nm, respectively, while the maximum polarization conversion loss is less than 0.4, 0.9, or 1 dB.