Research On Beam Wander Of Optical Vortex Thtough Atmospheric Turbulence And Compensation Method

Free space optical communication (FSO) attract a lot of attention for its great advantage of high information volume and high rate. Optical vortex beam with orbital angular momentum (OAM) has an intensity null in its core and a phase singularity around which its phase circulates. Based on these particular qualities, optical vortex beam has been widely applied in FSO. It is very obvious that OAM modes span an infinite dimensional basis, which can not only be used to send information in a photon with no limitations but also provides a much higher level of security than the polarization-encoding approach. However, optical vortex is easily affected by atmospheric turbulence, such as beam wander, beam expand and so on, which has bed influence on communication system.This article focus on the impact of atmospheric turbulence on vortex beam and analysis the influence of topological charges in random wandering of optical vortex. Furthermore, a novel adaptive optics technology has been used in phase compensation of vortex beam.First, the wandering behavior of optical vortex with topological charge of1has been analyzed theoretically. We find that the coordinates x and y on the receiver plane are Gaussian random variables. Then the wandering behavior of higher order optical vortex with topological charge of1,5,10,15is obtained by use of multiple random phase screen simulation method and vortex extracting method. It shows that coordinates x and y of optical vortex with topological charge of1follow normal statistics, which is given to support the theoretical results. It means the probability density of vortex position on receiver plane center is the maximum. As topological charge increases, Gaussian fitting curves become broader, the statistics of x and y gradually becomes to random distribution. In conclusion, optical vortex beams carrying high topological charge wander more widely than the beams carrying small ones, that is, high-order vortex beams are more impressionable to atmosphere turbulence.Second, adaptive optics has been applied to make a correction to FSO system. The phase of vortex beam is used to carry information and has comparatively more influence on the dislocation of vortex beams than the magnitude distortion, so we focus on the phase reconstruction. On the help of the iterative algorithm of adaptive optics wavefront reconstruction, we use distorted intensity distribution information to restore the phase. An innovative method is used in GS algorithm to make the first order spiral phase replace the random phase. The phase reconstruction achieve very good results in weak turbulence. These results have potential application in optical vortex communication systems.