Precision Measurement of Fractional Orbital Angular Momentum

Optical vortices bearing orbital angular momentum (OAM) with fractional topological charge have many applications in particle guiding and transport, anisotropic edge enhancement, high-dimensional quantum entanglement, and free-space optical communication. A fractional optical vortex (FOV) breaks OAM orthogonality, though, making precise measurement complex and difficult. This article shows how to measure the charge of a FOV by using a two-dimensional multifocal array of vortices with different integer charges. With an error of FOV detection below 2.5%, this technique is sufficient for a wide range of real-time requirements for using FOV in optical manipulation, imaging, or quantum optics.