Plane spiral orbital angular momentum wave and its applications

The applications of orbital angular momentum (OAM) carried beam are limited by its on-axis phase singularity and wide-angle directivity. In this paper, a new form of OAM wave termed as plane spiral OAM wave is proposed. Different from general OAM wave propagating along the axial direction, plane spiral OAM wave has the distinguishing characteristic of transverse propagation. Hence it has no singularity along the propagation path and divergence problem no longer exists. A traveling-wave circular slot antenna excited by a 90° hybrid coupler is used to generate plane spiral OAM waves. Due to the ring horn outside the antenna, energy is focused on the radial plane and the generated waves are propagating in the transverse direction. The measured results of a fabricated prototype with OAM states l=±3 for 10 GHz operation validate its good performance and characteristic of plane spiral. The potential applications of plane spiral OAM waves, e.g., plane spiral OAM based multiple-in-multiple-out (MIMO) system and plane spiral OAM based beam-forming method, are also detailedly discussed.