High coupling efficiency polarization splitting waveguide grating couplers.

We propose and validate a new, to the best of our knowledge, approach for high coupling efficiency polarization splitting waveguide grating couplers (PSWGCs). The high coupling efficiency is achieved by using a polysilicon overlay grating, which is shifted with respect to the underlying silicon grating. The PSWGC is designed to couple the two orthogonal polarizations in different directions. The design simulations predicted the coupling efficiency to be -1.46 dB for transverse electric (TE) polarization and -1.88 dB for transverse magnetic (TM) polarization with a single grating coupler, while the experimentally measured coupling efficiency was -2.37 dB and -2.51 dB for TE and TM polarizations. The high coupling efficiency GC is achieved without using bottom metal reflectors, and the design of the PSWGC is fully compatible with large-volume manufacturing using photolithography typically available in commercial silicon photonic foundries.