Fresnel and Fraunhofer diffraction of a laser Gaussian beam by   fork-shaped gratings
  Expressions describing the vortex beams, which are generated in a process of Fresnel diffraction of a Gaussian beam, incident out of waist on a fork-shaped gratings of arbitrary integer charge p, and vortex spots in the case of Fraunhofer diffraction by these gratings are deduced. The common general transmission function of the gratings is defined and specialized for the cases of amplitude holograms, binary amplitude gratings, and their phase versions. Optical vortex beams, or carriers of phase singularity with charges mp and -mp, are the higher negative and positive diffraction order beams. The radial part of their wave amplitudes is described by the product of mp-th order Gauss-doughnut function and a Kummer function, or by the first order Gauss-doughnut function and a difference of two modified Bessel functions, whose orders do not match the singularity charge value. The wave amplitude and the intensity distributions are discussed for the near and far field, in the focal plane of a convergent lens, as well as the specialization of the results when the grating charge p=0, i.e. the grating turns from forked into rectilinear. The analytical expressions for the vortex radii are also discussed. 
Ljiljana Janicijevic and Suzana Topuzoski