Fast design method for a flat-top arrayed-waveguide-grating demultiplexer using the reciprocity theory

A fast design method, based on the reciprocity theory, is presented for the optimal design of a special input structure (e.g., a parabolic waveguide horn) connected at the end of the input waveguide for flattening the spectral response of an arrayed-waveguide-grating (AWG) demultiplexer. The spectral response is calculated by reversal of the propagation direction of light in the AWG demultiplexer. Light is transmitted from the output waveguide and finally focused at the interface between the first free-propagation region (FPR) and the special input structure. When the special input structure in the optimization is adjusted, the distribution of the focused field (in the first FPR) excited by the modal field of the output waveguide remains unchanged and needs to be calculated only once. The optimal design of a parabolic waveguide horn for a flat-top AWG demultiplexer is given as a numerical example to illustrate the fast design method.