Radial-phase enabled OAM multiplexing independent of beam divergence

As a promising solution to expanding the capacity of free-space optical (FSO) communications, orbital angular momentum (OAM) multiplexing faces the inherent diffraction issue. As the number of multiplexed OAM modes increases, higher-order modes are inevitably employed, whose stronger intrinsic diffraction leads to more pronounced beam divergence and thus reduced received optical power. To address the challenges, we propose a novel, to the best of our knowledge, divergence suppression approach for multiplexed OAM beams. By controlling the radial-phase distribution of multiplexed OAM beams, the beams can propagate along a pre-designed trajectory and maintain the received beam size independent of OAM modes. We experimentally implement the radial-phase controlled multiplexed (RPCM) OAM beams and demonstrate its effectiveness of enabling more multiplexed OAM modes and longer transmission distance. Compared to conventional multiplexed (CM) OAM beams, RPCM OAM beams can increase the average received power by approximately 25 dB and 30 dB for turbulence strengths D/r₀=1 and D/r₀=4, respectively.