Far-field diffraction pattern of an optical light beam with orbital angular momentum through of a rectangular and pentagonal aperture

Abstract

We report some experimental results and numerical simulations of the Fraunhofer diffraction of light fields with integer topological charge (TC) by rectangular and pentagonal apertures. We show that Fraunhofer diffraction by a rectangular aperture provides information about the absolute value of the TC but none about its sign. However, for large values of the TC, it becomes difficult to define an appropriate intensity threshold in the diffraction pattern of a rectangular aperture that yields the absolute value of the TC. We show that this is due to the radial dependence of the phase gradient that determines the variations of the optical current in the azimuthal direction. We then analyse the diffraction pattern of OAM waves by a non-equilateral pentagonal aperture, which presents series of quasi linear interference patterns with well-distinct maxima and minima. We calculate the intensity profiles along a locus line that crosses all maxima in a series and obtain both the magnitude and sign of large TC values. Diffraction through a non-equilateral pentagonal aperture almost doubles the range of measurable TC values reachable by diffraction through a triangular aperture.