Detecting orbital angular momentum in optical vortex based on ptychographic imaging

Abstract

A new orbital angular momentum (OAM) model detection approach is proposed by extending the coherent diffraction ptychographic imaging (CDPI) approach. A beam carrying an OAM is a vortex beam which has a doughnut-shape intensity and a helical phase with a topological charge same as the mode index of OAM. To recover its phase helix, a vortex beam radiates a template region-by-region, in grid fashion or scanning way, and the adjacent radiated regions should be overlapped partially. The exit wave field of each radiated region produces a far-field diffraction intensity pattern at the back focal plane of a lens. After recording the intensity patterns, the wavefront of the incident beam can be reconstructed from the patterns via ptychographic iterative engine (PIE). In this paper, we first determine the diffraction intensity recording requirements by analyzing diffraction features of a vortex beam. We second derive the spatial sample requirements in ptychographic OAM detection, for a convergence to the non-ambiguity solution of a helical wavefront, involving two case, with and without a prior knowledge about the template function. Finally, numerical and optical experiments confirm that the helical wavefront of a vortex beam can be accurately recovered by conventional PIE when template is treated as reference. And using a modified regularized PIE (rPIE), the wavefront of incident vortex beam and the unknown transmission function of template can be reconstructed simultaneously.