Extraction of the characteristics of vortex beams with a partial receiving aperture at arbitrary locations

Orbital angular momentum (OAM) has aroused widespread interest in communication systems because it improves spectrum efficiency. However, for the free-space link, OAM-based communications encounter problems of OAM beam divergence and misalignment, which reduces the channel quality. To address these two challenges, we propose an algorithm to detect the OAM beams based on the dynamic mode decomposition (DMD). Only the partial receiving aperture is used to address the challenge caused by the beam divergence, and the output DMD spectrum exactly sorts the OAM indices of superposed modes. Besides, when there is a misalignment, the DMD spectrum is utilized as a feedback indicator to search for the lateral displacement of the mode axis. With the differential evolution, the axis of OAM beams can be located. Compared with existing detection schemes, our work enables the separation of mixed OAM modes with less received information, and the perfect alignment is dispensable as a priori. This decentralized partial aperture receiving scheme could serve as a practical and effective detection technique for OAM-based communication systems.