The Propagation Properties Of Laguerre-gaussian Beam In Turbulent Atmosphere

In recent years, Orbital Angular Momentum(OAM)has become a research hot spots in the field of free space optical communication(FSO), and has made a rapid progress. Using the OAM beam for information exchanged, multiplexed, and on this basis, combined with other traditional multiplexed technology, can significantly improve the optical data transmission capacity and spectrum efficiency. However, atmospheric turbulence is the biggest challenges in FSO system, when the OAM beam propagation in free space, the atmospheric turbulence will cause light intensity fluctuation, increase of the bit error rate, and reduce the capacity of communication system channel. Because Laguerre-Gaussian(LG) beam is a linear combination of the base class for all the other kinds of OAM beams, therefore, to study its propagation characteristics has certain representativeness. The evolution properties of such beam in free space and turbulent atmosphere have been studied in detail and comparatively in this thesis.Firstly, the specific diffraction model which described by angular spectrum of vortex beams and the corresponding sampling conditions are derived. Secondly, simulating the atmospheric turbulence using Monte-Carlo phase screen which improved by sub-harmonic method. And then,in order to obtain the propagation properties of LG beams, we are used of MATLAB as a tool for simulating and analyzing the model of LG beams which propagating in turbulence. And then, the light intensity distribution and phase distribution are under numerical analysis, so that, we can obtain its relationship between turbulence distance and OAM modes. Finally, we are compared the turbulence resistance of the Gaussian beam, point source with LG beams by using PIB theory. The research result is shown that there exists big difference between the condition of with and without turbulence: when the beam is propagating in the turbulence, with the propagation distance is increased, the beam will extends,and the focusing ability of beam in the far field decreased at the same time. the ring-shape intensity profile and spiral phase distribution of LG beams serious distortion, as well. The degree of distortion are closely related to the size of the OAM value, turbulence strength, propagation distance. By comparing the PIB curve can found that turbulent flow resistance has the obvious difference in different values of LG beams.A set of practical and feasible method of theoretical analysis and numerical simulation is designed in MATLAB by using the Fourier optics, Split-Step Beam Propagation Method and phase screen. Giving the specific diffraction model which described by angular spectrum and the corresponding sampling conditions to obtain the most accurate simulation results and the relationship between LG beams and turbulence. These methods and conclusions is of great significance to comprehensively understand the communication characteristics of beams in atmospheric, provide the theory for the further research on the optical communication system based on OAM beams and the field of its application.