Propagation Characteristics Of Array Beams Through Turbulence And Adaptive Beam Shaping

Laser beam transmitted through atmosphere/ocean is influenced by turbulence effects.Atmospheric/oceanic turbulence lead to a series of problems,such as beam spreading,beam wander and scintillation,which limit the application of free space optical communication,lidar and other applications.Compared to single beam,beam array can effectively improve the output power of laser.Different combination methods(coherent/incoherent)are suitable for different application scenarios.In this dissertation,beam spreading,beam wander and scintillation characteristics of coherent/incoherent combined Gaussian array beam through oceanic turbulence are studied in detail by theoretical derivation and numerical simulation.Furthermore,based on the coherent combined fiber array system,the adaptive beam shaping based on Stochastic Parallel Gradient Descent algorithm(SPGD algorithm)is proposed.The effectiveness of beam shaping is verified by numerical simulation analysis.This method can be applied to wireless charging of UAV and other fields.The mainly research contents of this dissertation include: Based on the generalized Huygens-Fresnel principle and the Rytov method,the theoretical formula of beam spreading,beam wander and intensity scintillation characteristics of coherent and incoherent combined radial Gaussian beam array through oceanic turbulence are derived.The general approximate expression of beam wander characteristics through oceanic turbulence transmission is discussed,which is helpful to estimate the beam wander and convenient for engineering application.The influence of different initial beam setting(radial radius,beam width of sub-beam),oceanic turbulence parameters(kinetic energy dissipation rate,temperature variance dissipation rate,ratio of temperature and salinity contributions)and different transmission distance on the propagation characteristics are numerically analyzed.Results show that incoherent combined beam array have larger beam spreading radius,while less affected by the beam wander and intensity scintillation.With the increase of the transmission distance,stronger oceanic turbulence(larger temperature dissipation rate,smaller kinetic energy dissipation rate,and the turbulence tends to be dominated by salinity),the turbulence effects on coherent/incoherent combined beam array are both more serious.Considering the time domain characteristics of phase screen method,we introduce the infinite length phase screen method based on phase correlation interpolation and the autoregression method considering time frequency component.The equation of beam spreading,beam wander and scintillation of Gaussian beam propagating through oceanic turbulence are derived follow the theory of beam propagation through random media.According to the phase screen method used for atmospheric turbulence simulation,the rules of phase screen parameter setting used for oceanic turbulence simulation are given.At last,the theoretical computation results and simulation results are compared and the validity range of phase screen simulated oceanic turbulence is testified.Results show that the beam spreading and beam wander characteristics simulated by phase screen method are in good accordance with the theory under different turbulence conditions,while the scintillation index has large deviation under strong fluctuation turbulence conditions.Finally,propagation characsteristics of Gaussian beam array with different distribution(single beam,rectangular array and radial array)through oceanic turbulence are disscussed by using phase screen method.Based on the phase-locked adaptive fiber array technique,the concept of beam shaping is proposed.Beam cleanup,turbulence compensate and beam shaping are realized by piston and/or tip/tilt control based on SPGD algorithm.For cooperative target,the feedback signal is sent by RF signal,and the adaptive shaping optimization function is introduced as the metric to describe far-field beam shaping.For the pseudo-cooperative target,the reflected light is obtained by the reflector distributed on the target,and the total reflected light power is used as the metric to achieve far-field beam shaping.In this study,numerical simulation method is used to compare the adaptive beam shaping effects of coherent and incoherent combination under synchronous/asynchronous control,moving/stationary targets,different target shapes,and different atmospheric turbulence.Results show that coherent combined fiber array based on the SPGD algorithm can effectively realize beam shaping under different atmospheric turbulence,different shape of target and different speed moving targets while the incoherent combined fiber array does not have the ability of beam shaping.