Study On Spreading And Correction Of Orbital Angular Momentum Spectra Of Vortex Beam In Atmospheric Turbulence

Since Allen pointed out that vortex beam has the orbital angular momentum(OAM)in 1992,there is a great deal of interest in the application of vortex beams in various fields.As the OAM quantum number carried by vortex beam can be any integer,each vortex beam in theory can have an infinite number of OAM eigen-modes.In addition,the orthogonality between OAM eigen-modes of vortex beam is beneficial to the use of different modes for high-speed data transmission.In recent years,the researches show that vortex beam has great potential in atmospheric communication.However,OAM may spread seriously due to the effect of the atmospheric turbulence,and this phenomenon further leads to serious crosstalk in communication.In fact,OAM spreading reduces communication quality and limits communication distance.At present a number of effective methods,such as adaptive optics,have been found to improve the communication quality.In general,the atmospheric optical communication based on OAM is at present still in an exploratory stage.The main work of this thesis is as follows:1.The propagation characteristics of a vortex beam named circular beam(CiB)in atmospheric turbulence are studied.The results show that when the parameters of CiB are taken with some special values,CiB can have convergence characteristics and thus the OAM modes present automatic recovery effect in the atmospheric turbulence,which means that the OAM scattering can be reversed.This finding can increase the detection probability of the initial OAM state and further improve the communication quality.2.It is proved that the two-dimensional superposed OAM states may exhibit synergy effect in the atmospheric turbulence.The synergy effect means that the detection probability of the two-dimensional superposed OAM state is larger than that of the corresponding OAM eigenstates.In addition,it is found that the synergy effect is mainly due to the probability that the two OAM quantum numbers of the two dimensional superposed OAM states turn into each other.When such probabilities decrease,the synergy effect becomes weak or even disappear.3.The rotational coherence function in the atmospheric turbulence is discussed.It is found that there is a fixed upper bound on the probability of an OAM eigenstate with arbitrary wave function casting into other OAM eigenstates when it is through the weak floating region of atmospheric turbulence.The upper bound of the probability transition to a particular OAM eigenstate depends only on the OAM number of the initial state.4.In this paper,the properties of entangled OAM photon pairs in atmospheric turbulence are studied.The preparation of entangled OAM photon pairs is derived from the spontaneous parametric down conversion method.In this method,the pump is superposed by OAM modes,and the simulation results show that the detection probability is the largest when the wave function of the receiving state is consistent with the complex amplitude function of the pump.In addition,for the case of entanglement,the detection probability of the two-dimensional superposed OAM states is slightly larger than that of the OAM eigenstates,and the difference is more obvious in the stronger turbulence.