A Method Based on Meta-surface Bessel Lens for Converging OAM Beams

OAM beams generated by circular phased antenna array have large divergence angles, which restricts the effective transmission distance. To alleviate this drawback, in this paper, a planar Bessel lens composed of $21 \times 21$ multilayer meta-surface units is proposed. The meta-surface unit consists of four metallic layers of the same “quasi-cross” type copper and three dielectric layers. An ample 360° phase shift can be achieved with a good transmission coefficient through changing the length of the quasi-cross patch. The simulation results show that the proposed Bessel lens not only reduces the central empty area but also improves the gain. The divergence angle of OAM mode $l=+1$ falls 4° and the gain improves 2.5 dBi while the divergence angle of OAM mode $l=+2$ drops 8° and the gain increases 0.5 dBi, which increases the feasibility of practical applications using OAM beams.