Algebraic corrections for paraxial wave fields

Asymptotic analysis of the angular spectrum solution for diffraction is used to establish the validity of a standard, formal series for nonparaxial wave propagation. The lowest term corresponds to the field in the Fresnel approximation, and this derivation clarifies some of the remarkable aspects of Fresnel validity for both small and large propagation distances. This asymptotic approach is extended to derive simple, generic algebraic corrections to the field estimates found by using the paraxial model, i.e., the Fresnel approximation. Contour maps of the field errors associated with the diffraction of collimated beams–both uniform and Gaussian–in two and three dimensions demonstrate the effectiveness of these corrections.