Research On The Measurement Of Atmospheric Coherence Length In Xi'an Area

Atmospheric coherence length is an important parameter for evaluating the effect of adaptive phase correction in laser transmission,so there is a high research value to measure and analyze the atmospheric coherence length.Differential image motion method is widely used in engineering because of its high accuracy and good stability.In this thesis based on the differential image motion method,in Xi'an,we aim at measuring the atmospheric coherence length by the means of constructing laser transmission line and adopting an adaptive threshold algorithm,which can extract useful information from differential motion images quickly.The main works of the thesis are as follows:1.Introducing the mechanism of production and describing several common simulation methods of atmospheric turbulence,as well as presenting the calculation method of characteristic parameters of statistical optical turbulence.And quantitatively analyzing the effects of wavelength,waist radius and atmospheric refractive index structure constant on spot drift and beam expansion in laser transmission.2.Simulating the random phase fluctuation in atmospheric turbulence according to the theory of laser atmospheric transmission and Fresnel diffraction.Besides,simulating the transmission of Gauss beam going through the different intensity atmospheric turbulence channel models combining with power spectrum model,and analyzing and contrasting the spot distribution on receiving surfaces of different contrasting position.3.Introducing various measurement methods and basic equations of atmospheric coherence length.In this thesis,the author uses differential image motion method to measure atmospheric coherence length,in the mean while,determining the sub-pupil aperture,the distance between two sub-pupils and the exposure time of CCD,taking advantage of simple four-hole differential image motion measurement instrument in the field experiments,at the same time,collecting data as well as comparing them with the theoretical models.4.Proposing a fast algorithm detecting multi-target spot based on the characteristics of symmetry and high signal-to-noise ratio of differential motion images.Experiments show that the algorithm can adaptively select the threshold under different noise interference,and it can effectively and steadily segment the spot areas to achieve fast detection of multi-target spot.In the Xi an area,the experiment results show that the variation trend all day of atmospheric coherence length in the outdoor experiments calculated by equations designed by this articleis in accordance with the basic theoretical theory.The overall variation trend is almost the same although the data of atmospheric coherence length obtained in different directions is different,and the variation at night is relatively stable than the varnation in the daytime.