Measuring the topological charge of a helico-conical vortex beam via the lens phase.

Helico-conical vortex (HCV) beams are a kind of orbital angular momentum-carrying beam whose phase profile has a non-separable azimuthal and radial dependence. In this Letter, we introduce a lens phase into the helico-conical phase to measure the topological charge carried by the helico-conical beam. The focal-field intensity distributions of the beam obtained from the superimposition of the helico-conical phase and the lens phase are analyzed theoretically and experimentally. The experimental findings are in good agreement with the simulation results. The results demonstrate that a certain number of elliptical dark regions are embedded in the intensity patterns and the number of dark regions is equal to the amount of topological charge carried by the HCV beam. Meanwhile, the tilt direction of the elliptical dark regions can determine the sign of the topological charges. This method will have potential applications in the field of light field modulation. [ABSTRACT FROM AUTHOR]