| Free space optical communication(FSO)is a new type of wireless communication method,which uses laser as signal carrier and free space such as atmosphere and seawater as transmission medium.Due to its advantages of large transmission bandwidth,high communication rate,strong anti-interference and flexible deployment,FSO has rapidly become a research hotspot in the field of communication and is widely applied in satellite communication,satellite-ground communication and military fields.However,when the laser is transmitted in the space optical path,on the one hand,because of the interference of atmospheric turbulence,it is easy to cause light intensity flicker,beam drift,wave-front phase distortion and other phenomena;on the other hand,the single-mode fiber with very small mode field radius is usually used for space optical coupling at the receiving end,which further increase the difficulty of laser entering the receiver,and eventually lead to the reduction of the received optical power and seriously affect the stability of the communication system.Therefore,how to compensate the turbulence interference and improve the fiber coupling efficiency has become a problem that must be solved in space optical communication.This paper mainly focuses on the research of adaptive optical technology,and some prospective research on mode diversity receiving technology based on few-mode fiber is carried out,the main works are as follows:1.In this paper,the development of free space optical communication and turbulence compensation technology is reviewed,mainly including:the domestic and foreign research progress of free space optical communication system,the influence of atmospheric turbulence on laser transmission,the current mainstream turbulence compensation technology and so on.2.The theory of laser propagation and the statistical model of atmospheric turbulence in space optical communication system are studied.The simulation of laser transmission in atmospheric channel is realized based on the Split-Step Fourier algorithm;The numerical simulations of static and dynamic turbulence phase screen are realized based on Kolmogorov turbulence power spectral density and Taylor turbulence frozen theory,and correctness of turbulence simulation are verified by the turbulence channel model;In addition,the calculation methods of coupling efficiency of single-mode fiber and few-mode fiber in space optical communication system are introduced respectively.3.The numerical simulation of adaptive optics system based on stochastic parallel gradient descent algorithm(SPGD)is completed,and the SPGD algorithm is used to compensate the wave-front phase distortion caused by static and dynamic atmospheric turbulence in space optical communication;Besides,using few-mode fiber demultiplexing system to replace single-mode fiber for spatial optical coupling in the simulation,and the coupling characteristics of single-mode fiber and few-mode fiber before and after the SPGD algorithm correction are analyzed respectively,the results preliminarily verify the improvement effect of mode diversity receiving technology based on few-mode fiber on fiber coupling efficiency.4.The theoretical modeling and numerical simulation of the adaptive optics system with wave-front sensor are completed.The working principle and mathematical model of each component in the adaptive optics system with wave-front sensor are studied,and the common wave-front reconstruction algorithms are introduced.The reconstruction and correction of the distorted wave-front phase caused by turbulence effect are realized by numerical simulation,and the correction effect of the adaptive optics system on the fiber coupling efficiency and the root mean square of the wave-front phase in space optical communication is discussed and analyzed. |
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