Experiment And Simulation Study Of The Atmospheric Turbulence Effects In The Satellite-to-Ground Laser Communication

As a result of an increasing data quantity from satellite and a great demand for real-time Transmission, the transmission rate of space-based information system is required up to1Gbps clearly. Compareing to the microwave communication system, laser communication has the advantages of higher speed, lower power consumption, lighter weight and no restriction for frequency use. Recently, satellite laser communication has turned to be a hot topic in the field of laser communication research in the world.In satellite-to-groud laser communications, laser beam which carries the information will propagates in the turbulent atmosphere, which leads to a series of turbulent effects such as intensity scintillation, angle of arrival fluctuation and beam wander. In order to improve the communication quality from satellite-to-groud link, the laser propagation characteristic in the turbulence atmosphere should be researched, and then taking relevant measures when holding the essential rules of turbulent effects. In scientific researches or engineering designs, people have an urgent need of an atmospheric turbulence simulator to evaluate system performance in the laboratory environment. In this paper, the theory of laser propagation characteristics in turbulent atmosphere is researched and numerical simulation methods for turbulent phase screen are discussed. Also an indoor atmospheric turbulence simulator is designed based on liquid crystal space light modulator (LC-SLM), and then intensity scintillation and angle of arrival fluctuation effects are analyzed under different kind of turbulence.Firstly, in this paper, laser propagation characteristic in the turbulent atmosphere is discussed in detail. As a starting point of turbulent statistical theory, Kolmogorov and Non-Kolmogorov spectrum models are introduced, also weak and strong fluctuation theory in laser propagation under those models are discussed respectively. Intensity scintillation and angle of arrival fluctuation effects are emphatically analyzed.Sencondly, numerical simulation methods for static phase screen are presentated, which contains power spectrum method(FFT), Zernike polynomial method and fractal method. The merits and demerits of those methods are comparatively analyzed, from three aspects such as algorithm time complexity, simulation precision and application scope. A general expression for discrete phase screen in Non-Kolmogorov turbulence is derived by using FFT. The demerit of random midpoint displacement (RMD) is introduced. In order to improve simulation accuracy, a new algorithm is presented. Also simulation methods for dynamic phase screen are presentated. Considering the amount of the generated screens is limited by using the traditional turbulence frozen mothod, a implementation method for spline interpolation is given. With this mothod, a long-time dynamic simulation comes true.Thirdly, an indoor atmospheric turbulence simulator is designed based on LC-SLM, and a simulation test bed is build. An analysis of equivalence between indoor test bed and outdoor experiment system is discussed. Also the theory of far-field spot amplification and feasibility of indoor simulation are deduced particularly. According to the principle of lens imaging magnification, a cascade of light amplification system is designed to simulate different links for laser propagation in turbulent atmosphere. The turbulence effects of angle of arrival fluctuation and intensity fluctuation in3.4km link are simulated in the Kolmogorov and Non-Kolmogorov turbulence.Lastly, long distance laser propagation and communication experiments are developed. A long-term experiments is observed over3.4km horizontal link, some crucial parameters such as refractive index structure constant or atmospheric coherent length are acquired during24-hour observation. Also temporal power spectras of intensity fluctuation are tested and analyzed in the3.4km link. A demo system for satellite laser communication terminal is presentated and the link budget is done for this demo system. Laser propagation and communication experiments for40km link are also discussed. Bit error rate versus SNR is deduced theoretically based on Gamma-Gamma turbulence channel. The relations between satellite-to-groud link and long distance horizontal link are discussed from the aspects of atmosphere attenuation and turbulence.This paper provides theoretical foundation for the reaserch of atmospheric turbulence effects in the satellite-to-ground laser link. Also it provides experimental foundation for the design of indoor turbulence simulator and the simulation of far-field laser propagation.