Efficient high-order accurate Fresnel diffraction via areal quadrature and the nonuniform fast Fourier transform

We present a fast algorithm for computing the diffracted field from arbitrary binary (sharp-edged) planar apertures and occulters in the scalar Fresnel approximation for up to moderately high Fresnel numbers (less than or similar to 10(3)). It uses a high-order areal quadrature over the aperture and then exploits a single 2D nonuniform fast Fourier transform to evaluate rapidly at target points (on the order of 10(7) such points per second, independent of aperture complexity). It thus combines the high accuracy of edge integral methods with the high speed of Fourier methods. Its cost is O(n(2) log n), where n is the linear resolution required in the source and target planes, to be compared with O(n(3)) for edge integral methods. In tests with several aperture shapes, this translates to between two and five orders of magnitude acceleration. In starshade modeling for exoplanet astronomy, we find that it is roughly 10(4)x faster than the state-of-the art in accurately computing the set of telescope pupil wavefronts. We provide a documented, tested MATLAB/ Octave implementation. An appendix shows the mathematical equivalence of the boundary diffraction wave, angular integration, and line integral formulas and then the analysis of a new non-singular reformulation that eliminates their common difficulties near the geometric shadow edge. This supplies a robust edge integral reference against which to validate the main proposal. (C) The Authors.