Measurement of orbital angular momentum spectra of multiplexing optical vortices.

Optical vortex beams carrying orbital angular momentum (OAM) are widely investigated for their unique performance in recent years. They can be used to extend the capacity of optical communications system due to the orthogonality of different channels. In the receiver side of a multiplexing optical vortices system, verifying the OAM spectrum is of great importance. A new kind of diffraction element called Dammann vortex grating can distribute energies among different diffraction orders equally. Based on this unique characteristic, we reported a new algorithm to analyze the spot of each diffraction order. The OAM spectrum in the receiver side can then be obtained. In the experiment, the OAM spectrum measurement of at most six-channel multiplexing optical vortices is realized. The experimental results illustrate that the OAM spectrum gained by this approach is highly consistent with the theoretical value.