  Spatial mode (de)multiplexing of orbital angular momentum (OAM) beams is a
promising solution to address future bandwidth issues, but the rapidly
increasing divergence with the mode order severely limits the practically
addressable number of OAM modes. Here we present a set of multi-vortex
geometric beams (MVGBs) as high-dimensional information carriers, by virtue of
three independent degrees of freedom (DoFs) including central OAM, sub-beam
OAM, and coherent-state phase. The novel modal basis set has high divergence
degeneracy, and highly consistent propagation behaviors among all spatial
modes, capable of increasing the addressable spatial channels by two orders of
magnitude than OAM basis as predicted. We experimentally realize the tri-DoF
MVGB mode (de)multiplexing and shift keying encoding/decoding by the conjugated
modulation method, demonstrating ultra-low bit error rates (BERs) caused by
center offset and coherent background noise. Our work provides a useful basis
for next generation of large-scale dense data communication.
