Si-wire two-dimensional grating coupler with polarization-dependent loss of lower than 0.3 dB over a 60-nm-wide spectral range

Two-dimensional grating couplers (2D-GCs) that operate with an arbitrary polarization typically exhibit a polarization-dependent loss (PDL) due to the fiber angle tilted to reduce the back reflections at the fiber/grating interface. We numerically analyzed the dependence of the PDL of a 2D-GC in terms of the scatterer arrays and shapes using the three-dimensional finite difference time domain (3D-FDTD) method. The reduction in the PDL of the 2D-GC was realized by slanted arrays and cross-shaped scatterers. We experimentally demonstrated a PDL < 0.3 dB over a 60-nm-wide spectral range and the maximum coupling efficiency (CE) of −3.1 dB in the O-band spectral regime. This relationship of the PDL characteristic was maintained even when the half-etching depth of the 2D-GC deviated in a 200 mm silicon-on-insulator (SOI) wafer.