Generation of rotational orbital angular momentum beams in the radio frequency based on an optical-controlled circular antenna array

The orbital angular momentum (OAM) has been widely used in the wireless short-range communication system, but for long-distance communication, the huge difficulty of beam receiving is a great challenge. In this paper, to overcome this challenge, a generation system of radio-frequency rotational orbital angular momentum (RF-ROAM) beams based on an optical-controlled circular antenna array (CAA) is proposed. The ROAM beam is an OAM beam rotating at a certain speed around the beam axis. According to the rotational Doppler effect, the rotation of the OAM beam will induce a frequency shift proportional to the OAM mode and the rotation speed. Thereby, by rotating an OAM beam at a fixed speed scheduled in advance in the transmitting end, the beam can be mode-distinguished by just detecting the frequency shift without receiving the whole wavefront vertical to the beam axis in the receiving end. This provides a partial reception scheme for the OAM-based wireless communication system. The generation system of RF-ROAM beams is proposed and constructed, and the proof-of-concept experiment is performed. In the system, the optical-controlled CAA is constructed to generate the general RF-OAM beam, the optical signal processor (OSP) is employed to control the phase shifts to further control the OAM mode, and the signal with time-varying phase is generated as the rotation factor to control the rotation speed. In the experiment, the RF-ROAM beams with different mode and mode combination are generated and successfully measured by detecting the frequency shift of the signal received in a fixed point.