Research On OAM-Frequency Multiplexed Waves Based On Spatio-temporal Metasurfaces

In recent years,the continuous growth of mobile communication terminals has put higher requirements on the spectrum utilization of wireless communication systems.The vortex electromagnetic waves carrying orbital angular momentum(OAM)bring new multiplexing dimensions to wireless communication due to their orthogonal characteristics of different modes.Theoretically,they can achieve an infinite number of independent multiplexing channels at a single frequency point,thereby greatly improving the utilization of spectrum resources,and thus becoming a hot research topic at home and abroad.Currently,the use of electromagnetic wave beams with OAM generated by metasurfaces has been widely studied due to its advantages of simple equipment and low cost.The mainstream research includes using reconfigurable metasurfaces to generate OAM waves with different modes from a single device,and using the temporal and spatial modulation of metasurfaces to control the frequency domain to generate multi-frequency OAM waves.However,the use of programmable metasurfaces and spatiotemporal modulation metasurfaces to replace the continuous helical phase of OAM waves with discrete phase points may affect the mode purity of OAM beams,which may affect the multiplexing effect.This area is still blank.In addition,the ability of spatiotemporal modulation metasurfaces to independently control multiple frequency points can be combined with OAM to achieve multi-dimensional multiplexing.And there is currently no research on this aspect.To solve the above problems,the work of this paper is as follows:(1)The law of generating OAM interference modes with phase discontinuity of digital metasurfaces was studied.Starting from the receiving principle of OAM beams,the OAM mode distribution contained in the receiving aperture of discrete and continuous phases was calculated and a mathematical relationship was established.The mathematical expression of the interference mode law was obtained through theoretical analysis and derivation,and it was verified in electromagnetic simulation software.The results are widely applicable and instructive for OAM multiplexing research based on reconfigurable metasurfaces.(2)Based on the relevant theory of spatiotemporal modulation metasurfaces,a coding theory for multi-frequency independent control was proposed.Combined with the research results in the first point,a feasible scheme for generating OAM-frequency synthesis multiplexed beams using spatiotemporal modulation metasurfaces was achieved.The scheme can freely generate two different modes of OAM waves at two frequency points.Simulation results show that the scheme meets the expected design,and the relevant principles are widely applicable and easy to upgrade and expand.