Effect of beam defocus on the efficiency of planar waveguide grating couplers

Results of theoretical calculations and experimental measurements of variation of grating coupling efficiency with beam-defocus position are presented. Our results indicate that (1) coupling efficiency is not necessarily maximum at the beam waist, (2) coupling efficiency as a function of defocus position shows oscillatory behavior particularly for a tightly focused beam. The effect of beam size alone cannot account for the observed behavior of variation of coupling efficiency with defocus position. The results of measured angular dependence of coupling efficiency for various defocus conditions are also presented.