Investigation On The Propagation Of Vortex Beams In Random Medium

With the increasing demand of wireless optical communication,laser communication technology has developed rapidly.Because of its incomparable advantages,wireless optical communication technology has made great progress in laser communication,space exploration,radar imaging,aviation measurement,remote sensing and other aspects,and has a very broad application prospect.Because of its unique optical field distribution and phase structure,the vortex beam with spiralling phase wavefront distribution and orbital angular momentum has unique advantages in reducing the impact of turbulent media on beam transmission and can be used for particle acquisition,manipulation and optical storage.The transmission evolution properties of vortex beam in different random media have been widely studied by scholars.Based on the above background,this paper takes the interaction of vortex beam and random medium as the theme,and analyzes and studies on the evolution properties of vortex beam in random medium such as atmospheric turbulence,turbulentocean and plasma.The specific research contents are as follows: 1.Based on the generalized Huygens-Fresnel principle and the high-order moment theory of Wigner distribution function,the propagation properties of partially coherent flat-topped vortex beams through atmospheric turbulence are studied.The effects of turbulence parameters and beam parameters on the Wigner distribution function and kurtosis parameters of partially coherent flat-topped vortex beams in turbulence are analyzed in detail.The propagation properties of vortex and non-vortex beams are also studied and compared.The simulation results show that the Wigner distribution function of vortex beam is smaller than that of non-vortex beam,and the kurtosis parameter of non-vortex beam is a constant when it propagates in turbulence.The kurtosis parameter of vortex beam in turbulent atmosphere becomes larger because of the influence of turbulence medium,but it is smaller than that of non-vortex beam,and the beam profile is smoother than the former.2.The multi-Gaussian Schell vortex beam with special coherent structure is taken as an example,Based on the generalized Huygens-Fresnel principle and the second moment of the Wigner distribution function,the expressions of the angle spread and the propagation factor of the multi-Gaussian Schell vortex beam in the turbulent atmosphere are derived,and the propagation properties of the beam in the turbulence are analyzed.The results show that the propagation properties of the multi-Gaussian Schell vortex beams with special coherent structure are better than those of the conventional ones,and they are more suitable for free space wireless optical communication.3.Based on the generalized Huygens-Fresnel principle,the propagation properties of partially coherent Lommel-Gaussian vortex beams in turbulent atmosphere are studied.The theoretical simulation of the intensity and spectral coherence of the beams in turbulent atmosphere are carried out.The results show that,with the increase of the propagation distance,the distribution structure of the spectral intensity for the beam degenerates into Gaussian distribution due to the influence of the turbulence medium,and the modulation effect of the symmetry factor on the intensity distribution disappears with the increase of the propagation distance.4.Taking the ocean turbulence as an example,the propagation of partially coherent Lommel-Gaussian vortex beams in the ocean turbulence is studied.The expressions of the angle spread and the propagation factor of the beams in the turbulent ocean are derived in detail.The changes of the beams with the beam parameters and turbulent parameters are simulated and analyzed.It is found that in the same case,the propagation properties of partially coherent Bessel-Gaussian vortex beams are the same as that of partially coherent Lommel-Gaussian vortex beams,and the symmetry factor has no effect on the propagation of the beams.Because of its unique intensity distrubution,the partially coherent Lommel-Gaussian vortex beams are more widely used than the former.5.Based on WKB method and paraxial approximation,the propagation model of Laguerre-Gaussian beam in nonlinear plasma is established.The self-focusing propagation law of Laguerre Gaussian beam in the collisional nonlinear mechanism and the ponderomotive nonlinear mechanism is analyzed,and the influence of different parameters on the self-focusing characteristics of the beam is studied.The results show that at the same propagation distance,with the increase of orbital angular momentum,the beam width of the beam changes greatly in the two plasma mechanisms,and the ability of the self-focusing property decreases.