Diffraction of light by a spiral phase plate with piecewise-continuous microrelief

Using a Huygens-Fresnel principle and the FDTD method we show that an optical vortex generated by a refractive spiral plate with a relief step has an asymmetric profile. The annular diffraction pattern in the optical vortex beam cross-section is found to be disturbed at a distance of several Fresnel lengths not only for the near-field diffraction but also for the middle-field diffraction. For a spiral phase plate with topological charge m = 3, the asymmetry of the optical vortex has been shown experimentally.