| Since the laser theory was put forward by Einstein in 1917,it has become an indispensable part of the optical theory system.Laser is widely used in laser communication,laser radar,laser weapon,laser ranging,and other projects because of its excellent single frequency and directional property.With the continuous breakthrough of theoretical research,many transformative optical technologies are born and developed gradually.Vortex beams carrying orbital angular momentum can further improve the information capacity and spectral efficiency of existing optical communication technologies.Partially coherent beams with stronger anti-turbulence ability can reduce the adverse effects of atmospheric turbulence such as intensity scintillation.Partially coherent vortex beams combine the advantages of both and have important research significance and strategic value.Based on the theory of multi-phase screen propagation,the turbulence effects of partially coherent vortex beams in atmospheric,such as light intensity scintillation,beam spreading,beam wander,and orbital angular momentum spectrum broadening,are studied in this thesis.The main research contents and achievements of this thesis are as follows:1.Based on the power spectrum inversion method,a random phase screen of atmospheric turbulence is constructed,and the low frequency part of the random phase screen is compensated by the sub-harmonic compensation method,which improves the accuracy of atmospheric turbulence simulation.The equivalent phase screen principle is used to superimpose part of the phase interference dynamic factors based on the vortex beam,and the partially coherent vortex beam is generated.Then,the specific steps of numerical simulation modeling of atmospheric turbulence propagation of partially coherent vortex beams based on atmospheric turbulence multilayer random phase screen method are given.Based on mathematical statistics,the influence of light source parameters and atmospheric turbulence parameters on the scintillation index,beam spreading,beam wander and orbital angular momentum spectrum broadening of partially coherent vortex beams in atmospheric turbulence is discussed.The results show that reducing the coherence length of partially coherent vortex beams can reduce the scintillation index of the beam in atmospheric turbulence,but at the same time it will also lead to stronger beam spreading and beam wander,and increasing the difficulty of detecting orbital angular momentum modes.Although increasing the orbital angular momentum topological charge can reduce light intensity scintillation,beam spreading and beam wander,it will cause orbital angular momentum spectrum dispersion problems,which is not conducive to the detection and application of orbital angular momentum modes.2.Based on the atmospheric turbulence multilayer random phase screen transmission model of partially coherent vortex beams,the one-way path transmission model is extended to two-way path transmission,and the ”transmission-reflection-transmission” process is simulated numerically.There are three typical reflective targets been considered: plane mirror,retroreflector and random rough surface,and their partially coherent vortex beam reflection models are given.Based on mathematical statistics,the influence of light source parameters,atmospheric turbulence parameters and target parameters on the light intensity and phase distribution,scintillation index,beam spreading and orbital angular momentum spectrum broadening of partially coherent vortex beams in atmospheric turbulence is discussed.The existence of the backward enhancement effect when partially coherent vortex beam is reflected by the rough surface is verified numerically.The results show that the hard-edged aperture plane mirror and the Gaussian aperture retroreflector have a serious influence on the light intensity distribution of partially coherent vortex beam,and has a weaker influence on the orbital angular momentum spectrum distribution;the Gaussian rough surface has a weak effect on the beam spreading of partially coherent vortex beams,and has a serious influence on the phase distribution,which causes the relative intensity of the orbital angular momentum mode decrease obviously.In addition,by comparing the scattering intensity distribution of the partially coherent vortex beams in the monostatic mode and the bistatic mode,it is found that the light intensity near the optical axis in the monostatic mode is stronger than in the bistatic mode,which verifies the phenomenon that backscattering enhancement effect of partially coherent vortex beams reflected from rough surfaces.3.Taking into account the uneven distribution of turbulence intensity in the slant atmospheric path with height,based on the atmospheric refractive index structure constant model and the setting scheme of the equal Rytov index-interval phase screen,the interval of the atmospheric turbulence phase screen and the Fried parameter during slant transmission is calculated.Then,the difference of turbulence effects of partially coherent vortex beams in the slant uplink and downlink transmission is compared,and the influence of different zenith angles on light intensity distribution,beam spreading and relative intensity of orbital angular momentum modes is studied.The results show that when the slant transmission height is determined,the zenith angle affects the transmission distance of partially coherent vortex beams,thereby significantly affecting its atmospheric turbulence transmission effect.Due to the strong intensity of atmospheric turbulence near the ground,when the partially coherent vortex beam is transmitted along the uplink,the optical turbulence effect is obviously stronger than that of the downlink.Under the same transmission distance,the impact of the slant atmospheric turbulence link on the laser transmission is significantly weaker than that of the horizontal atmospheric turbulence link.The advantage of partially coherent beams against turbulence disturbance is weak.The disadvantage of the lower coherence length results in the initial relative orbital angular momentum mode will be more prominent. |
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