Stable divergence angle control of OAM vortex beams with different modes

Abstract

In this letter, we propose a stable divergence angle control method for orbital angular momentum (OAM) beams with different modes. Typically, OAM beams with different mode numbers exhibit distinct divergence angles even under the same antenna aperture. This greatly limits its long-distance communication capability since receiving the OAM beams of different modes with varying divergence angle is difficult when the receiving antenna is usually fixed. Based on the directivity function of OAM beam generated by the uniform concentric circular array (UCCA), local extremum value can be found at the divergence angle. Hence, we can calculate the derivative of directivity functions to obtain the required excitation amplitude distribution. By changing the excitation amplitude of each ring in the UCCA without adjustment of the antenna aperture, the OAM beams with modes l = 1, 2 and 3 can reach the same divergence angle. Both simulated and measured results verify the correctness of the proposed method and it can provide references for applying OAM waves to the practical long-distance wireless communication systems.