Optimizing silicon-on-oxide 2D-grating couplers

3D finite difference time domain calculations are used to optimize the coupling between a single-mode telecom fibre and a silicon-on-oxide 2D-grating coupler. By simultaneously varying the hole-radius and grating-pitch of the photonic crystal array that makes up the active region of the coupler, the maximum in coupling efficiency is tuned to 1550nm and optimized. Guided mode expansion indicates that an optically-active resonant guided mode in the photonic crystal array is responsible for the coupling. The results of the mode expansion calculation closely correspond to those of the significantly more computationally intensive finite difference time domain calculations. Using guided mode expansion to provide initial parameters that are later fine-tuned by 3D finite difference time domain calculations may open the way for truly globally optimized 2D-grating couplers that offer significantly improved coupling efficiencies and performance over current designs.