The Study On The Propagation Properties Of OAM Beam Through Atmospheric Turbulence

The beams as a carrier of information transmission in free space is the development trend in the future.However,beams propagating in free space are easily affected by the atmospheric turbulence,which leads to a series of phenomena,such as intensity scintillation,beam enpanding and phase distortion These phenomena will affect the quality of the communication system.Therefore,studying the properties of beam propagating in the free space has important significance for improving the effectiveness and reliability of the communication system.Because of having a spiral wavefront,the vortex beam has orbital angular momentum(Orbital Angular Momentum,OAM),which has an infinite orthogonal mode.And the central intensity of the beam is zero.Under the same conditions,vortex beam has a stronger ability to resist the atmospheric turbulence than the common beam,and which as a carrier of information can achieve a better communication quality.So,it has been widely studied by scholars.In recent years,many researchers have focused on the study of free space optical communication(FSO)based on OAM,which can improve the channel capacity,spectral efficiency and security of communication system.However,the beam propagating in the free space is easily influenced by the atmospheric turbulence,which affects the quality of the communication and the topological charges are more difficult to be detected.At present,most studies focus on the propagation properties of free space optical communication based on orbital angular momentum,for example,how to decrease crosstalk and bit error rate.But it is short of systematic study for the vortex beam propagating through the atmospheric turbulence.The Laguerre-Gauss beam(LG)is the representative of vortex beam.LG is taken as the object of study.Then the properties of LG beam propagating in the atmospheric turbulence is studied.And we analyze the influence of transmission distance,topological charge and atmospheric turbulence intensity on the distribution of intensity,phase and spiral spectra of LG beams.We propose a method to measure the topological charge of LG beam by using orbital angular momentum multiplexing.And The root mean square(RMS)based on Zernike polynomials is proposed as an important figure to evaluate the wavefront aberration.First,the angular spectrum of Fresnel diffraction integral is derived by using FFT and step by step transmission.The multiplexed LG beam propagating in vacuum is simulated.It is found that the phase distribution is related to the topological charge of the LG beams.In addition,The numbers of the rotating cycles and the rotation direction of the innermost are the same as the smaller absolute value of topological charge.Moreover,the numbers of the rotating cycles is equal to the difference between the absolute value of topological charge of the two beams.And the rotation direction is of the same as the beam with the bigger absolute value of topological charge.The phase distribution can be used to determine the topological charge of LG beam.It provides a new method to measure the topology.Second,power spectrum inversion method is combined with sub-harmonic method to achieve a more accurate phase screen.LG beams propagating in the atmospheric turbulence is simulated.Through the light intensity distribution,phase distribution and spiral spectra,the relationship among turbulence,distance and the value of topological charge is analyzed.The results show that with the increasing of the distance,the phase distortion of LG beam increases,and the phase singularity is unstable,and the beam expanding and mode dispersion is serious.The smaller topological charge,the samller phase distortion and mode dispersion are.But the intensity of the beam is opposite.This conclusion provides basis for the selection of topological charge.Finally,Zernike polynomials is used to derive the RMS of LG beam's wavefront distortion.RMSs of phase distortions of LG beams with various topological charges and distance are investigated and analyzed.The results is in accordence with the simulation results.It provides a new method to quantitatively analysis the wavefront distortion of LG beams.