Optical communication in a turbulent atmosphere via the orbital angular momentum of a laser beam. I. Mode purity of OAM transmission

The functioning of an optical data transmission system operating via an atmospheric channel with data modulation by the orbital angular momentum (OAM) value is simulated numerically. The following method is chosen to determine the OAM: the wave field of the beam is expanded into optical vortices, and then OAM is retrieved from the expansion coefficients (vortex spectrum) measured at the receiver end of the path. In this case, the quality of information transmission depends on how accurately the recorded spectrum reproduces the transmitted spectrum. The quality of a communication channel is assessed by the so-called mode purity. To estimate the mode purity, we propose using the characteristic that we call the effective width of the vortex spectrum. The effective width of the vortex spectrum depends on both misalignment of the transmit-receive system and the intensity of atmospheric turbulence at the path between the transmitter and receiver. It should be kept in mind that in actual practice, receiving is only possible for the finite number of received modes or the finite width of the receiving window. We propose and test two methods for estimating the effective width of the vortex spectrum that can be used for the radiation propagating through homogeneous or randomly inhomogeneous media. It is shown that knowing the effective width of the OAM vortex spectrum makes it possible to estimate the minimal window half-width M required for correct operation of atmospheric data lines. Errors of OAM measurements are estimated as functions of configuration and parameters of the transmit-receive system and turbulent propagation conditions. (c) 2024 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.