The Propagation Properties Of Non-Diffraction Beams Based On Orbital Angular Momentum In Atmosdpheric Turbulence

The vortex beams with orbital angular momentum(OAM)have the helical phase front term exp(ilp),where l denotes the topological charge with arbitrary number.The special structure brings in phase singularity showing as a black hollow spot in transverse plane.These beams carrying distinct OAM modes exhibit orthogonality and form an infinite dimension of Hilbert space.The OAM beams can be used to carry information as carriers for modulation,multiplexing and multicasting,which can enhance the channel capacity without increasing the spectral bandwidth.In addition,it can be also used for optical switching and routing.In recent years,free-space optical(FSO)communication using twisted light for high-data-rate transmission has attracted widely attention and deep research.In this paper,a novel multichannel FSO communication system based on the "Frozen Waves(FWs)" with OAM have been investigated.The propagation model is established and the propagation properties are analyzed.By means of the detection results,the information capacity and secrecy capacity are simulated and computed,respectively.The procedure is finished as follows:Firstly,a longitudinal orbital angular momentum multiplexing(LOAMM)system based on OAM carried by FWs with different longitudinal patterns in propagation axis is proposed and studied in this paper.Secondly,The LOAMM system is simulated under atmospheric turbulence,which is based on split-step Fourier transform method with multiple random phase screens.The random phase screens are generated by power spectrum inversion method.Thirdly,The influences of system parameters on the average normalized OAM probability spectrum are analyzed and discussed based on numerical simulation,such as propagation distance,strength of turbulence,sizes of finite aperture function,OAM mode numbers and source parameters.Fourthly,to verify the accuracy of results obtained from numerical simulation,an analytical example derived by Rytov approximation method in weak fluctuation is used as comparison.The results indicate that the transmission model established by numerical simulation is credible.Fifthly,Based on the analysis of design considerations and properties of the LOAMM system in previous section,the aggregate channel capacity and secrecy capacity of the traditional OAM space division multiplexing(OAM-SDM)system with Bessel beams are compared with the proposed LOAMM system in the same conditions.The results reveal that the LOAMM system avoids crosstalk from adjacent OAM modes and outperforms the OAM-SDM system.