Fabrication tolerant and broadband polarization splitter and rotator based on a taper-etched directional coupler.

We propose a fabrication tolerant polarization splitter and rotator (PSR) on the silicon-on-insulator platform based on the mode-coupling mechanism. The PSR consists of a silicon wire waveguide coupled to a taper-etched waveguide. Compared to previously reported PSRs based on directional couplers which are sensitive to fabrication variations, the partially etched taper structure can compensate for fabrication inaccuracies. In addition, the taper-etched geometry breaks both the horizontal and vertical symmetries of the waveguide, introducing an additional degree of design freedom to accommodate different upper cladding layers. The proposed PSR can be readily integrated in a planar waveguide circuit using e.g. SiO(2) cladding, making it compatible with typical metal back-end-of-line processes. Our simulation results show that the PSR has a low TM-to-TE polarization conversion loss of -0.09 dB in the C-band (or a conversion efficiency of 98%). A low TE-to-TE through insertion loss (-0.07 dB) and a very low polarization crosstalk (-30 dB) over a wide wavelength range exceeding 160 nm with a large fabrication tolerance (>50 nm) are numerically demonstrated.