Aberrations of a Gaussian laser beam focused by a zone plate

We discuss the Seidel-type wavefront aberrations which degrade the quality of the Gaussian beam focused by a zone plate. First we examine the diffraction of a complex-source-point spherical wave through the zone plate, while the terms of up to fourth order in aperture variables are under consideration. Then we find an explicit formula for the diffracted beam that is represented in terms of Seidel-type wavefront aberrations. We also evaluate the effects of aberrations on the quality of the diffracted beam through the zone plate. If the axis of the incident beam is inclined slightly with respect to the optic axis, both spherical aberration and field curvature affect the beam quality in the sagittal section of the focal plane, whereas the beam quality in its meridian section is under the influence of all Seidel-type aberrations. The fourth order theory of propagation of a slightly inclined Gaussian beam as described here may be applied to evaluate the degradation in the quality of the Gaussian beam focused by a zone plate.