A Novel Orbital Angular Momentum Coding System Using High Order Azimuthal Indices of Laguerre Gaussian Beams

Orbital angular momentum in optical vortex beams offers a new dimension of space mode because coaxially propagating optical vortex beams with different azimuthal orbital angular momentum states are mutually orthogonal. The optical vortex beams can be efficiently multiplexed and demultiplexed to increase the capacity of the optical communication system. Therefore, the azimuthal and radial indices of the transverse spatial mode have been investigated many times. This paper focused on the azimuthal indices of the Laguerre Gaussian beams and a novel orbital angular momentum coding system under the influence of turbulence and noise with high order azimuthal indices was established through simulations of an optical communication system in free space. At the transmitter, a series of holograms of Laguerre Gaussian mode with different azimuthal indices were used to achieve coaxial transmission of multiplexed beams, where each beam represented a data bit. At the receiver, superposition of the conjugate light field corresponding to the transmitted beams was used to demultiplex the multiplexed beams. The combination of the phase shift factor and the centroid algorithm were realized for simultaneous detection of the azimuthal indices in each channel and efficient decoding of the information. In this article, for the first time, multiplex and demultiplex were investigated by using the centroid algorithm, keeping the radial index constant and taking different positive and negative values of the azimuthal index. Simulation results show that using four orthogonal LG modes (M = 4) achieves a MIMO capacity of 18.802 bits/s/Hz at 20 dB SNR, providing about a threefold increase over single-mode transmission, while turbulence reduces mode purity to −6.02 dB to −1.25 dB and induces an average crosstalk of −4.77 dB.