| Due to the Free Space Optical(FSO) communication applications have the advantages of low cost, flexible networking, convenient installation and dispense with frequency permission, it has become a major technology in the present information technologies, such as laser communications, laser rangefinder, laser radar, laser weapons and laser energy transportation. The random variation of laser transmission caused by turbid media and turbulence in the atmosphere severely reduced the use of different laser systems performance, and even determine the technical feasibility of the laser system. Therefore, the research of influence on laser signal caused by turbid media and turbulence in the atmosphere has important theoretical and applied significance.For the random characteristics of the atmospheric channel, this topic researches the effect of laser signal transmission caused by atmospheric random channel. First, the topic analyzes the representation of the total amount of laser signal attenuation in the atmospheric medium transmission, the absorption and scattering of air molecules on different wavelengths of laser signal as well as the different aerosol particles. Then Analysis on the statistical characteristics of the atmospheric turbulence and the impact of intensity fluctuation, atmospheric scintillation, beam shift and expansion on laser signal transmission in atmospheric turbulence. For the impact of turbidity particles on the laser signal, the topic start with the scattering of single particle, then combines with the size distribution, calculate the amount of attenuation according to Mie formula at last.Detailed summary of the derivation and development of atmospheric refractive index formula and atmospheric structure constant formula is made in this article. According to the atmospheric temperature, atmospheric pressure, content of carbon dioxide and water vapor pressure, the International Geodetic Association modified and simplified the formula of group refractive index and phase refractive index. On the basis of the season, weather, location and altitude characteristics, Post gives a good description formula of the variation of the atmospheric structure constant. In accordance with the strength of the turbulence, the topic made a detail analysis of atmospheric scintillation statistical characteristics and scintillation index expression.Based on Mie scattering theory and raindrop size model, the laser signal attenuation expression in rain medium is given, the relationship of the signal attenuation and rainfall rate is determined. The results show that the attenuation role of small raindrops particles is larger than big raindrops particles, laser attenuation in the drizzle is bigger than in heavy rain.Laser signal attenuation in fog is also use the expression of Mie attenuation formula and it is measured by atmospheric visibility. The atmospheric visibility is relates to droplet size, droplet concentration or water content related. From the above aspects, it is find out that the visibility in fog increases with the fog droplet concentration and water content. The change of small droplet concentration has a big affect of the visibility, when the water content of the same, large amount of small droplets become little amount of big droplets on collision course, the visibility improved.In a sandstorm, the sands absorb the air moisture and adhere sulfides and water soluble substances in the transmission process, so it should consider the combined effect of the water and sulfide when calculate the laser signal attenuation in sandstorm. Due to the gravity of the dust, scale distribution at different heights is different. After considering these factors, use visibility to measure the laser signal attenuation.The article combines actual measured data and by theoretical studies it explains the attenuation conditions of laser signal in atmosphere, provides a theoretical basis to the actual application of the laser communication system. |
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