Fraunhofer propagation using Fourier optics.

% Fraunhofer propagation
% Based on Computational Fourier Optics by Voelz
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% Assuming uniform sampling and presents reflections on the boundaries
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% PARAMETERS
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% u1 - Complex Amplitude of the beam at the source plane
% L1 - Sidelength of the simulation window of the source plane
% lambda - Wavelength
% z - Propagation distance
% u2 - Complex Amplitude of the beam at the observation plane
% L2 - Sidelength of the simulation window of the observation plane
% X2 - X coordinates on the observation plane
% Y2 - Y coordinates on the observation plane