Orthogonal Chirp Orbital Angular Momentum Beams

In this paper, orthogonal chirp orbital angular momentum (COAM) beams with a non-linear azimuthal spiral phase variation are introduced. The COAM beams have a linear spatial frequency variation in the azimuthal plane similar to chirp signals. This variation contrasts the constant spatial frequency variation in the case of conventional OAM beams. Thus, a COAM beam consists of a spread of multiple OAM modes. This mode-spreading makes the COAM beams resistant to spatial frequency distortions during propagation, which would otherwise adversely affect the mode orthogonality of conventional OAM beams at radio frequencies. It is shown that orthogonal COAM beams having the same linear spatial frequency variation are associated with an additional azimuthal phase variation of integer multiples of 360 degrees. Generation of COAM beams is demonstrated using uniform circular arrays (UCAs) with antenna element phase excitations that vary as the square of the element number. It is also shown that concentric UCAs can generate multiplexed orthogonal COAM beams. Several examples of such beams are generated using UCAs designed at 17.8 GHz operating frequency via full-wave simulations in the CST Microwave Studio software. Such COAM beams could enable robust OAM communications in turbulent propagation conditions.