Topological charge measurement of concentric OAM states using the phase-shift method.

We proposed a method to measure orbital angular momentum (OAM) states by using a multi-zone pure phase grating with the phase-shift technique. In free-space optical communication systems, the topological charges (TCs) of concentric OAM beams are usually detected for decoding, since the diameter of the OAM beam relates to the transmission distance. Two typical concentric OAM beams, concentric Laguerre-Gaussian beams and perfect vortex beams, were generated as incident beams, and the diffraction patterns can be separated by implementing a pure phase grating in the Fourier plane of a 4-f system. By counting the corresponding diffraction fringes, we can identify the TCs of the two OAM states. The simulation results show that the diffraction angles of the concentric OAM beams can be controlled and two concentric OAM states can be measured simultaneously.