Normal-incidence grating couplers in Ge-Si

We report a normal-incidence grating coupler that is monolithically integrated with a Ge(x)Si(1-x) rib waveguide. The design, fabrication, and testing procedures are described for a 0.33-microm-period grating coupler integrated with a Ge(0.04)Si(0.96) waveguide. Also, a theoretical analysis based on coupler-mode theory has been carried out to predict the coupling efficiency and its dependence on the angle of incidence of light. For normal incidence the measured efficiency of 6.5% was found to be in good agreement with the theoretical prediction of 6.9%. The measured FWHM of the acceptance angle about the normal was 5 degrees , compared with the theoretical prediction of 3 degrees .