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Efficient multiplexing and demultiplexing of orbital angular momentum beams

In the near-field regime, the number of spatial modes that a free-space communication system can efficiently use is given by the product of the Fresnel numbers of the transmit and receive apertures. It can be advantageous to decompose the field into modes that have rotational symmetry or definite orbital angular momentum (OAM modes). A key challenge to using OAM modes as parallel channels in a practical communication system is efficient multiplexing of single-spatial-mode transmitters to the orthogonal OAM modes, and demultiplexing the combined beam into single-spatial-mode receiver arrays. Previous approaches have utilized modes of different OAM, but ignored the radial coordinate, leading to inefficient use of the Fresnel number. We identify a method, using lenses and holographic phase plates, to efficiently and reversibly convert concentric Laguerre-Gauss OAM beams into an array of separated Gaussian beams.