Millimeter-Wave Holographic Flat Lens Antenna for Orbital Angular Momentum Multiplexing

In this article, a holograph-inspired flat lens antenna capable of simultaneously transmitting two independent coaxially propagating orbital angular momentum (OAM) beams operating at 60 GHz is proposed. In the flat lens design, holographic theory including the processes of recording and reconstruction are developed to obtain the desired compensation phase distribution of the flat lens. Novel wideband
$C$
-shaped polarization-conversation transmit array element and broadband magneto-electric (ME) dipole antenna are designed as the building block and feed source of the flat lens, respectively, resulting in the wideband performance of the whole antenna. The ME dipole sources do not carry OAM modes while the flat lens can transform the spherical wavefront from the two ME dipole sources into transmitted vortex waves with different OAM numbers
$\ell =-1$
and +1 simultaneously. For validation, an antenna prototype was fabricated and the radiation performances including return loss, isolation between the two OAM channels, near-field, and far-field performance are experimentally characterized. Both numerical and experimental results validate that the designed flat lens antenna can successfully demonstrate OAM mode multiplexing. The proposed OAM-multiplexing antenna shows promising potential in millimeter-wave high-speed wireless communications.