Comparison of lighting strategies for generating high purity OAM vortex beams with metasurfaces in millimeter waves

In recent years, there has been a notable advancement in technology, particularly in the domain of wireless communications. Consequently, the telecommunications industry has progressed rapidly from 4G to 5G, and it is anticipated that 6G will emerge within the next few years. Furthermore, it is already well established that the preferred frequency range for the development of the latter two is the millimetre wave band. In this context, orbital angular momentum (OAM) multiplexing is being proposed as one of the key emerging technologies to satisfy novel requirements and enlarge the capacity of wireless systems. Consequently, in this work, we will not only design a metasurface that generates an OAM, but we will primarily focus on examining the impact of different illuminations on the purity of the OAM vortex generated by the metasurface. To this end, we have initially designed a metasurface capable of generating an OAM vortex beam with mode l = 1, which works in transmission at 86 GHz. The initial stage is to ascertain whether the selected unit cell possesses the requisite characteristics, in conjunction with the implementation of the Pancharatnam-Berry principle to modulate the phase. In order to proceed with the design, it is first necessary to obtain the theoretical phase that is to be implemented in the metasurface through numerical calculations and then synthesize it. Once the metasurface has been designed, we will proceed to carry out an analytical study of how the illuminations, namely uniform circular, Gaussian, Hamming, Hanning, Blackman, Flattop and a real antenna illumination, affect the purity. In order to achieve this, the analytical study has been carried out using the Huygens-Fresnel method implemented in Matlab. Subsequently, the purity of the various OAM vortex beams generated, has been calculated with consideration given to variables such as distance, illumination and the methodology employed in calculating the purity. The findings will be substantiated through simulations carried out using the CST Suite Studio simulation software, which will be conducted on the most intriguing illuminations. Finally, the study demonstrates that the quality of the generated OAM vortex can be enhanced by selecting an appropriate metasurface illumination.