Cosine-type apodized spiral zone plate to handle the topological charge of a vortex beam

Abstract

By taking advantage of spiral petal-like zone plates, we have recently demonstrated that a spiral element's geometric shape impacts its charge. Keeping it in mind, we aim to control the topological charge of a vortex using phase modification of a spiral zone plate without transforming its geometrical shape. This is achieved by apodizing a spiral zone plate having p spiral zones with a radial grating with spatial frequency m. Having implemented some gratings with different m, we realized that an optical vortex is produced whose topological charge and its handedness depend on p and m. Consequently, a constant phase replaces on-axis screw dislocation under the situation that p takes odd multiples of m. Studies are carried out for large and small pairs of (p, m) and various gaps between the couple.