Focusing Grating Couplers in Unmodified 180-nm Silicon-on-Insulator CMOS

The design and demonstration of photonic grating couplers with improved efficiencies monolithically integrated in IBM\´s 7RF silicon-on-insulator (SoI) complementary metal–oxide–semiconductor (CMOS) technology offering are reported. The grating couplers are fabricated using two standard layers available in 7RF SoI CMOS, where focusing circular grating structures and a small-footprint tapered slab are fabricated on the poly-silicon and crystalline-silicon (c-Si) layers, respectively. The coupling efficiencies for 1.3- and 1.55- $\mu{\rm m}$ grating couplers are measured to be ${-}{\rm 2.8}$ and ${-}{\rm 3.8}~{\rm dB}$ , respectively. These results are at least $2\times$ more efficient than the best previously reported grating couplers in this technology, which were fabricated using c-Si layers only. Furthermore, the devices exhibit 1-dB bandwidths of ${\sim}{40}$ and 52 nm for 1.3 and ${\rm 1.55}~\mu{\rm m}$ , respectively.