NORMAL-INCIDENCE GRATING COUPLERS IN GE-SI

We report a normal-incidence grating coupler that is monolithically in tegrated with a GexSi1-x rib waveguide. The design, fabrication, and t esting procedures are described for a 0.33-mum-period grating coupler integrated with a Ge0.04Si0.96 waveguide. Also, a theoretical analysis based on coupler-mode theory has been carried out to predict the coup ling 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 measure d FWHM of the acceptance angle about the normal was 5-degrees, compare d with the theoretical prediction of 3-degrees.