Capability of topological charge-based orthogonal zone plate in generating sharp focal spot and ring-like beam

Abstract

We aim to present an orthogonal symmetric-based diffractive lens which has unique focusing properties. Structure of the element is based on the framework of orthogonal zone plate which is constructed by overlaying two perpendicular linear zone plates with opposite topological charges. We illustrate that the proposed element has sharper focal spot as well as longer focal depth than square and circular zone plate. Apart from having smaller spot size, it also generates a ring-like pattern at a given distance from the focus which depends on topological charge. Maintaining its shape in a distance equal to focal depth may be considered as an interesting feature of the generated annular beam. It is also demonstrated that numerical simulations are verified by experiments.