Optimizing grating couplers for silicon photonics

Grating couplers realized on a Silicon-on-Insulator (SOI) platform are an efficient means of coupling light from a single-mode optical fiber into silicon photonic waveguides. Grating couplers can be 1D, coupling light with a fixed polarization into a single waveguide, or 2D, in order to couple light with unknown polarization into a pair of silicon waveguides. In this work we review computational approaches, based on the finite-difference time domain (FDTD) method, that allow optimizing 1D and 2D grating couplers in order to maximize the coupling efficiency with respect to the various structural parameters. In general, we find that the gap between the performance of 1D and 2D grating couplers can be reduced with proper optimization. Relevant factors like polarization-dependent loss (for 2D couplers), bandwidth, tolerances, and fabrication constraints are addressed.