Experimental Investigation Of Light Intensity Fluctuation And Attenuation Characteristics In Lanzhou Area

When laser is transmitted in atmosphere channel,it is affected by space loss,atmospheric attenuation,atmospheric turbulence and background radiation,especially the influence of atmospheric turbulence and attenuation is most pronounced.Atmospheric turbulence cause a random fluctuation in the refractive index of the atmosphere,leading to distortion of the laser wavefront,causing fluctuations in light intensity,beam drift,beam expansion and beam jitter.Atmospheric turbulence and atmospheric attenuation cause the wireless optical signal to be severely disturbed or even cause the signal to be off target,resulting in a large bit error rate and short-time communication interruption,which seriously affects the stability and reliability of laser communication.In atmospheric laser communication,studies have shown that the turbulence and attenuation characteristics of the atmosphere show different laws depending on the geography and weather conditions.However,at present,the study on the fluctuation of light intensity and attenuation characteristics in our country mainly focuses on the eastern region,and most of experiments are conducted on sunny day.The semi-arid region of the Loess Plateau has a special climatic background and a markedly uneven underlying surface,leading to more complicated turbulence than other parts of the country.Due to the frequent occurrence of dust storms in the Loess Plateau,the composition and distribution of aerosol is quite different from other places.Therefore,in this dissertation,we study the light intensity fluctuation and its attenuation characteristics under different weather through practical experiment.The main work of this paper is as follows:(1)Combined with the theory of light intensity fluctuation,a wireless optical link in length of 610 m is established in Lanzhou area to investigate the light intensity distribution of Gaussian laser under different weather conditions.The three-dimensional pseudo-color transform method is utilized to analyze the spatial distribution of light intensity.The results show that the intensity distribution of the main lobe within the light spot follows the Gaussian distribution.The steepness of light intensity distribution decreases and the attenuation of light intensity increases according to the order of sunny day,cloudy day and overcast day in turn.(2)The atmospheric scintillation index,Rytov variance,atmospheric refractive index structure constant and probability density distribution are analyzed by using the measured light intensity data.The results show that in sunny day,haze day and dust day,the light intensity fluctuation belongs to weak fluctuation at most of the time and there is a small part of the time it belongs to moderate fluctuation.However,the light intensity fluctuation in overcast day belongs to weak fluctuation in all time.The light intensity fluctuation in sunny day and dust day are greater than overcast day and haze day.We can conclude that the probability distribution of light intensity in different meteorological conditions is closest to the exponential weibull distribution by comparing the nonlinear fitting results of the logarithmic normal,gamma-gamma and exponential Weibull distribution.(3)Based on the Mie scattering theory,the relationship between efficiency factor of water solubility aerosol,dust aerosol and soot aerosol and particle size was simulated and analyzed.On this basis,the relationship between aerosol particle transmittance and particle concentration was simulated and analyzed.Then,by conducting actual experiments,the atmospheric attenuation under different weather conditions was studied.The results show that the intensity of attenuation in sleet day and dust day greater than sunny day and overcast day,and the light-intensity attenuation value is the smallest in the sunny days with an average of 43 d B/km.Finally,experiments have verified that the transmittance gradually decreases as the particle concentration increases.The research results of this dissertation provide some guidance for laser communication in Lanzhou.