Separation and detection of orbital angular momentum states of composite vortex beams in atmospheric turbulence channels

The separation and detection of compound orbital angular momentum modes are the basis for achieving high-speed and high-capacity communication, but the atmospheric turbulence causes distortion of the phase fronts of vortex beams, which hinders the recognition of orbital angular momentum modes. To solve this problem, in this work, we propose and investigate a joint scheme of combining the Gerchberg-Saxton algorithm and the phase of addition and subtraction model that can achieve high recognition accuracy and a wide range of orbital angular momentum modes of compound perfect optical vortex beam under long-distance turbulence environment. At the receiving end, the Gerchberg-Saxton algorithm and phase addition and subtraction are used to compensate and modulate the incident vortex light field to obtain the pre-processed light field, and then the orbital angular momentum is detected by the coordinate transformation method. The simulation results show that: The detection range of the orbital angular momentum of the 3 optical path detection model for the vortex beam transmitting 1000m reaches -37~+37 in the vacuum environment, and the mode purity of -37~+37 can reach more than 0.3 when the turbulence intensity is Cn2=2×10-15m-2/3. The detection range of the orbital angular momentum of the vortex can be further expanded by increasing the optical path. This study provides a new method for the separation and detection of orbital angular momentum of composite vortex beams.