Properties of Fraunhofer Diffraction by an Annular Spiral Phase Plate for Sidelobe Suppression

A simple and accurate method for calculating the optimal width of an annular spiral phase plate (SPP) to generate optical vortices with sidelobe suppression is proposed. The sidelobes can be sharply suppressed when the ratio of inner and outer radii of an annular SPP is equal to that of the principal ring and the first sidelobe diffracted by a circular SPP with the same topological charge n. Moreover, the ratio of the inner and outer radii of the optimal annular SPP depends only on the topological charge n and is not affected by the incident wavelength or the size of the SPP.