Diffraction characteristics of optical vortices using annular radial grating with gradually changing period

Using a grating to measure the orbital angular momentum (OAM) of a vortex beam is a straightforward and economical approach. We devised an annular radial grating with a gradually changing period to characterise a vortex beam. By utilising the different off-axis positions of incidence, the spot distribution law for the far-field diffraction pattern can provide information regarding the topological charge magnitude and sign of the incident vortex beam. Our findings demonstrate that the fringe number and orientation of the diffracted spot can be employed to measure the vortex beam. Furthermore, we simulate the highest measurable OAM order, reaching +170. This measurement technique displays good tolerances for beam alignment and parameter selection. Our study offers a vital approach to the verification of high-order OAM beam generation and demultiplexing and wavefront correction in OAM multiplexing communication systems. These applications have significant value for large-capacity free-space optical communications and OAM holography.