Single-etch grating coupler for micrometric silicon rib waveguides

Grating couplers are widely used as an efficient and versatile fiber–chip coupling structure in nanometric silicon wire waveguides. The implementation of efficient grating couplers in micrometric silicon-on-insulator (SOI) rib waveguides is, however, challenging, since the coupler waveguide region is multimode. Here we experimentally demonstrate grating couplers in $1.5\mathrm{\mu m}$-thick SOI rib waveguides with a coupling efficiency of $-2.2\mathrm{dB}$ and a $3\mathrm{dB}$ bandwidth of $40\mathrm{nm}$. An inverse taper is used to adiabatically transform the interconnection waveguide mode to the optimum grating coupler excitation field with negligible higher order Bloch mode excitation. Couplers are fabricated in the same etch step as the waveguides using i-line stepper lithography. The benefits of wafer-scale testing and device characterization without facet preparation are thus attained at no additional cost.