Alleviating Orbital-Angular-Momentum-Mode Dispersion Using a Reflective Metasurface

Electromagnetic waves carrying orbital angular momentum (OAM) have attracted much interest due to their capacity to enhance communication capabilities. In this work, the OAM-mode-dispersion (OMD) phenomenon is analyzed by decomposing the vortex wave into a series of plane waves based on the spectral-domain method. Specifically, the OMD phenomenon induced by off-axis transmission is analyzed in detail, which clearly demonstrates that the vortex wave with a single OAM mode is spread to multiple OAM modes after off-axis propagation. To avoid the performance degradation of the OAM system due to OMD (e.g., interchannel crosstalk and signal-to-noise ratio reduction), a method for alleviating the OMD based on the reflective metasurface is proposed to transmit or receive the vortex waves with multiple OAM modes. Theoretical formulas for the phase distribution are developed in order to construct the metasurface. As proof-of-concept demonstrations, two metasurface prototypes are designed, fabricated, and measured to verify the proposed method in the radio-frequency range. The experimental results agree very well with the full-wave simulation results, validating the design methodology.