Modeling and optimization of non-periodic grating couplers

The higher degree of freedom available for non-periodic gratings (as compared with their periodic counterparts) is investigated. These non-periodic structures may be employed to design novel light couplers with increased functionality. Optimizing such devices requires a complex search in a huge parameter space. The success in the solution of this task depends on the availability of a fast forward solver and a reliable search algorithm. Here, a fast forward solver based on the multiple multipole (MMP) method together with a near-to-far field transformation and a multiple scattering calculation is presented. Thanks to the efficiency of our approach, non-periodic gratings are evaluated with a speed comparable to commonly used periodic grating approximations. This allows our solver to be combined with a heuristic global search scheme, namely an evolutionary algorithm. The procedure is demonstrated with the optimization of a non-periodic grating output coupler that suppresses an unwanted second diffracted order.