| Free space optical communications system is a new type of communication technology that realizes large-capacity information transmission by using laser as carrier wave and atmosphere as transmission medium.However,various adverse effects of atmospheric channels have been difficult to overcome,which has hindered the practical development of technology.For example,severe weather(such as smog)conditions can cause significant optical power attenuation.Even when the weather is clear,the laser will be affected by atmospheric attenuation,atmospheric turbulence,space loss and background radiation when it is transmitted in the atmospheric channel,especially the atmospheric turbulence is the most obvious.Random changes in atmospheric refractive index(atmospheric turbulence)cause effects such as light intensity flicker,beam drift,angle of arrival fluctuations,and beam expansion.The light intensity flicker refers to the random fluctuation of light intensity in time and space under the influence of atmospheric turbulence.This random fluctuation is also called channel fading,which causes the light intensity of the received signal to fall below the detectable threshold,causing The bit error rate rises,and communication is even interrupted.In atmospheric laser communications,studies have shown that atmospheric turbulence and attenuation characteristics vary with regions and weather conditions.Xi'an is located in the middle of the Loess Plateau and the Qinling Mountains.The city is large in scale and has many buildings,resulting in frequent haze in Xi'an in the autumn and winter,which makes the turbulent atmosphere in the area more complicated.To this end,this paper builds an external laser transmission link in Xi'an area to measure the light intensity distribution and flicker characteristics in different weather.The main works of the thesis are as follows:(1)The generation mechanism of atmospheric turbulence is introduced,and several common atmospheric turbulence simulation methods and statistical optical turbulence characteristic parameter calculation methods are described.(2)Laser transmission experiment with a link length of 2500m was carried out in Xi 'an area at night.Which studied the statistical characteristics of irradiance scintillation in different weather.The intensity distribution law of the spot was analyzed through the method of Three-dimensional pseudo-color visualization.The results show that:The intensity of the spot obeys the Gauss-like distribution.Near geometric center of light spots,the steepness of light spots is the largest in sunny days,followed by cloudy days,while rainy days and snow days are relatively gentle.The maximum light intensity of light spots decreases gradually in the order of sunny days,cloudy days,smoggy days,rainy days and snow days.(3)In this paper,a system for measuring the structure constant of atmospheric refractive index Cn2 using a scintillation method is designed.The structure constant of atmospheric refractive index and fluctuation degree decreases gradually in the order of sunny days,cloudy days,smoggy days.At the same time,the statistical characteristics of light intensity in different meteorological conditions by comparing the nonlinear fitting results of the logarithmic normal,Gamma-Gamma and exponential Weibull distribution were studied.Affected by the average effect of the aperture,the probability distribution of light intensity flicker under this experimental condition was closer to the exponential Weibull distribution.The analysis of the turbulent trend at night and the discussion of the probability distribution of light intensity flicker in some sense are valuable for studying the channel of free space optical communications system in Xi'an area. |
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