Research On Mitigation For Turbulence Distortion In Free Space Optical Communication With Orbital Angular Momentum

As we all know,it is a great potential that the Orbital Angular Momentum?OAM?beam can improve spectrum efficiency and channel capacity of the Free Space Optical?FSO?communication.However,it is inevitable that the quality of OAM beams will be seriously distorted by atmospheric turbulence during transmission,thus the communication link is likely to break down.In recent years,turbulence mitigation technology is a hotspot of current research in OAMs'FSO communication.It is difficult to break through the distortion of beam quality only by the method of signal processing,and this problem can be effectively solved by adaptive optics,due to its real-time and dynamic of beam wavefront correction.But the system structure of adaptive optics is relatively complex and the correction algorithm of implementation is also affected by many factors.So,now the key and difficult problem of the turbulence mitigation technology is to realize a more compact system structure with stronger turbulence adaptability.As for the key techniques of mitigating the turbulence distortion,we present three kinds of turbulence mitigation methods.That is,the GS algorithm with pretreatment of split screen by auxiliary ligtht,the SPGD algorithm with the light field feature extraction and turbulence heterodyne coherent mitigation by common-path auxiliary light.?1?In the GS algorithm scheme.Firstly,the Gaussian beam and the OAM beams are sent to the turbulent channel in a common-path.In the receiver,to obtian the turbulence screen,the a Gaussian filter is used to filter distorted Gaussian beam,then let the OAM beams pass through the conjugate of turbulence screen.Thus,the intensity distribution of OAM beams is obtained and is then used as the initial conditions of the GS algorithm.Finally,the phase recovery can be realized by GS algorithm.In the simulation,this scheme achieve the effective compensation of single-mode OAM beam phase distortion under the condition of unknown sending information.?2?As for the scheme of SPGD algorithm with the light field feature extraction.Firstly,we extract the main profile feature of the OAM modes according to its optical field distribution.Secondly,based on the theory of power in the bucket,the light energy ratio of the distorted OAM modes and the initial OAM modes is used as the performance evaluation function in SPGD algorithm.Finally,the corresponding compensation of wavefront aberration is rezlized by correcting the main profile characteristics.Numerical simulation results show that under moderate turbulence intensity,this scheme can realize the turbulence distortion compensation in the single OAM mode and double-modes links,and in single-mode link,the performance evaluation function of distorted light field before and after compensation has ascended 0.23.in addition,there is still enough room for improvement.?3?According to the disadvantage of adaptive optics,such as,the the high complexity of system structure and correction algorithms.We combine the technology of probe and coherent detection,we put forward a new scheme,that is,turbulence heterodyne coherent mitigation by common-path auxiliary light.In the scheme,the OAM beams after turbulence are used as the signal light,and after matching a phase pattern to the distorted auxiliary light,the distorted auxiliary light is then used as the local oscillation light.Thus,the model of heterodyne coherent detection with common-path auxiliary light is formed.on the basis of above research,multiplex OAM phase distortion coherent counteraction and their demultiplex are studied.And then a machansim of multiple OAMs'transmission and distortion compensation is established.The simulation results show that under the condition of moderate turbulence,the purity of OAM modes can increase 50%and the system BER can maintain10^-5 or less.Finally,two kinds of the beam shape adjustment of input beams are put forward,that is,the OAM beam waist adjustment scheme,Gaussian beam waist adjustment scheme.Both of them has further enhance the distortion compensation effect.In conclusion,the scheme of turbulence heterodyne coherent mitigation by common-path auxiliary light has advantages of low complexity and high resistivity of strong turbulence,it may pave a way to the practical application in phase distortion compensation and the proposed method is promising to reduce OAM mode crosstalk.