| Non-periodic grating couplers in dielectric waveguides (2007) | |||||||||||||||||
Abstract | |||||||||||||||||
| Grating couplers are fundamental building components for the design of optical circuits. The proper treatment of a device that couples the guided modes of a waveguide with the radiation in free space is particularly difficult because of the continuum nature of the radiating modes. The desire of high-density integration poses additional problems due to the fact that the simplest used coupling structures (shallow periodic gratings) need to be quite large. In order to reduce the size of the device we study non-periodic non-shallow structures, for which additional degrees of freedom are available. It is known that the analysis of this type of structures requires [1] a rigorous method for the solution of the field equations. We employ the Multiple Multipole (MMP) Method [2] for the computation of the scattered fields. The design of a waveguide coupler, which fits optimally to certain given specifications, is equivalent to the solution of an inverse problem. The combinatorial nature of a nonperiodic grating, the huge number of possible grating profiles and the highly different performance of similar grating structures, suggest [3, 4] the use of evolutionary algorithms for this optimization problem, where classical search procedures would fail. The evaluation of the quality of each structure proposed by the evolutionary algorithm, necessitates the solution of a diffraction problem. The size of every such problem impedes [5] its direct computation. Therefore, we partition the whole scattering problem in smaller subproblems. A near-to-far-field transformation [6] allows us to obtain the scattered far field produced by each single subproblem. Finally, by addition of those single scattering far fields, the approximate solution of the whole problem is found. | |||||||||||||||||
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