Numerical Fresnel models of Fresnel zone plates for plane wave at angle of incidence.

Diffractive optical elements (DOEs) are increasingly used in electro-optical systems due to their design flexibility, which offers advantages over refractive optics. Although determining the point spread function (PSF) is straightforward when the plane wave is parallel to the surface normal of DOE, a comprehensive method is needed for the plane waves with an angle of incidence. Therefore, this paper proposes a general approach in order to determine the PSF under non-normal illumination of plane waves. Although the numerical PSF is specifically proposed for the Fresnel zone plate (FZP) diffractive lens, the method is adaptable to any DOE. The simulation results were validated against the Zemax optical design software for angular incidences ranging from - 60° to + 60⁰, and the results were shared. Additionally, the paraxial approximation of the PSF was proposed and verified using ZEMAX for angular incidences between -10° and + 10°. Finally, a simplified approximation for the Fresnel number, applicable to FZPs was proposed. Consequently, this research presents a novel method that can be applied to all types of DOEs for determining the numerical PSF non-normal illumination of plane waves. The proposed approach also allows for the calculation of the modulation transfer function (MTF) for diffractive optical elements in such scenarios. By incorporating the angle of incidence into PSF analysis, this work makes a significant advancement in the field of diffractive optics.