Local topological charge analysis of electromagnetic vortex beam based on empirical mode decomposition.

The topological charge of an electromagnetic vortex beam depends on its wavefront helicity. For mixed vortex beams composed of several different coaxial vortices, it is significant to investigate the local topological charges. Fourier transform based methods are restrained by the uncertainty principle and cannot achieve high angular resolution and mode resolution simultaneously. In this paper, an analysis method for local topological charges of vortex beams is presented based on the empirical mode decomposition (EMD). From EMD, the intrinsic mode functions (IMFs) can be obtained to construct the bases of the electromagnetic wave, and each local topological charge can be respectively defined. With this method the local value achieves high resolution of both azimuth angle and topological charge, meanwhile the amplitudes of each orbital angular momentum (OAM) modes are presented as well. The simulation and experimental results confirm the validity of the EMD based method.