Atomspheric Turbulence Measurement With Scintillation Lidar

Atmospheric turbulence causes effects of irradiance fluctuation,phase fluctuation and so on,which seriously affect the application of optical engineering.Accurate acquisition of turbulence intensity by Lidar,which is an active optical detection technology,is of great significance in the study of optical propagation in random media and the application of optical engineering.In this paper,the basic theoretical research and influence factors of scintillation effect in the transmission of Residence Turbulence Scintillation lidar(RTS lidar)are carried out.Further,the echo signal is denoised and atmospheric turbulence intensity in the horizontal and vertical direction is achieved.The main works and conclusions are as follows:(1)The scintillation effect of double-pass in the detection of RTS lidar is analyzed.For an approximation of single partical scattering,based on the Huygens-Fresnel principle and Rytov approximation,the scintillation effect of double-pass is analyzed when considering the influence of atmospheric turbulence on double-pass laser propagation and aperture smoothing effect.The conclusion shows that when the RTS lidar meets the condition of large aperture,scintillation index of double-pass path is that of spherical wave caused by uplink path in the long distance detection of turbulence.If the condition of point reflector is not satisfied,scintillation index obtained by RTS lidar is scintillation index on the target plane averaged by aperture of diffuse reflector.(2)By using three turbulence power spectrum models-Kolmogorov spectrummodified Hill spectrum and improved Rytov mode,we obtain the relationship between scintillation index and atmospheric refractive index structure constant on detection path of RTS lidar.For the horizontal path,changes of turbulence intensity under difference inner scales and influence of inner scale on the detection of turbulence by RTS lidar are analyzed according to theory and experiment.The results show that the refractive index structure constant of finite inner scale is deviated from situation of not considering inner scale,which is related to propagation distance and magnitude of inner scale.Therefore,inner scale must be considered in the detection of RTS lidar.(3)The statistical characteristic of intensity distribution of echo signal is analyzed.Probability distribution of actual irradiance intensity is fitted with maximum likelihood probability distribution.By comparing with normal distribution,we can draw a conclusion:maximum likelihood probability distribution of the lowest four order moment can describe the actual distribution of logarithmic intensity of backscatter signal.Furthermore,the analysis of skewness and steepness shows that although the actual probability density distribution of logarithmic intensity is slightly different from normal distribution,but basically obeys normal distribution.Fitting results of exponential Weibull distribution show that there is a certain deviation between fitting results and results of calculation,however it is basically consistent.The accuracy of exponential Weibull distribution calculated in vertical direction depends largely on the accuracy of the inversion of Cn2.(4)Detection of Cn2 on different paths is realized under finite inner scale by using RTS lidar.The wavelet threshold method and EMD(Empirical Mode Decomposition)method are used to denoise echo signal of lidar.The contrast experiment with Scintillometer on horizontal path shows that the size and trend of Cn2 under finite inner scale are basically same as that of Scintillometer.And the feasibility of EMD denoising method is proved.On vertical path,iterative algorithm is used in retrieval of Cn2 profile.By comparing with the Cn2 profile model in summer of Hefei shows that:The results of inversion are in accordance with the overall change trend of model,but the turbulence intensity is relatively small under finite inner scale.The reason is that:In the process of inversion,owing to inability to obtain the change of inner scale with height,the inner scale of whole detection path is assumed to be constant,which causes measurement error.Thus lead to the inaccuracy of Cn2 profile under finite inner scale.