Detection Methods Of Atmospheric Turbulence Profile Based On Differential Light Column Lidar

Refractive structure constant is an important parameter for characterizing atmospheric turbulence strength,which its altitude distribution function is called atmospheric turbulence profile.The atmospheric turbulence profile is a key factor for the design,optimization and performance correction of adaptive optics.It is also of great importance to the transmissions of free space optical communication.This thesis has mainly discussed the retrieval methods and noise reduction methods of differential column image motion lidar(DCIM lidar).Furthermore,the theory of two-aperture differential scintillation method is proposed to acquire atmospheric turbulence profile,which it provides the theoretical foundation for differential column scintillation lidar.1.Improved retrieval method of turbulence profile based on generalized HV model is proposed for DCIM lidar.Aiming at the large retrieval error in high-altitude turbulence of current Levenberg-Marquardt inversion algorithm,a novel inversion model with inequality path constrained is developed and then the penalty function method is used to handle this model,thereby avoiding unphysical solution by adding the information of high-altitude turbulence.Furthermore,in order to weaken the current algorithm dependency on initial value and priori knowledge,a new optimization strategy of initial value based on genetic algorithm is presented to locate initial value of current algorithm in global variable space.Typical atmosphere turbulence profiles are simulated using modified algorithm and current algorithm.The measured lidar data in Hefei is also analyzed.Numeric simulation and experiment results show that modified algorithm can enhance the global search capability of iteration process and perform strong robustness against measurement noises,improving the retrieval precision and accurate quantification of high-altitude turbulence effectively.Moreover,the modified algorithm accelerates rapidity of convergence.2.A two-stage regularization-based inversion method which does not require a priori model of Cn2 for DCIM lidar is proposed.Based on the integral equation between atmospheric coherence length ro of spherical wave and refractive index structure constant Cn2,the ill-posedness and ill-conditioning of the integral equation are analyzed,and the necessity of regularization is discussed.In the two-stage regularization-based method,the first stage uses a regularized least squares QR-factorization(LSQR)method to retrieve the general shape of Cn2 profile.To get the stable solution of the final Cn2 profile,the second stage is related to an adaptive selecting algorithm to smooth out the artificial oscillations adaptively.Both the simulation and experiment illustrate the validity of the presented approach.It is shown that the algorithm can be applied to estimate reliable Cn2 profile from the ground level up to 15km in the presence of noises.3.Three methods of singular value decomposition(SVD),wavelet threshold(WT)and complete ensemble empirical mode decomposition(CEEMD)are used to reduce the noises of DCIM lidar.In order to improve the retrieval precision of atmosphere turbulence profiles,it is very necessary to implement the denoising process for the measured atmospheric coherence length profiles of DCIM lidar.The impacts of denoising methods on original r0 profiles and recovered Cn2 profiles are studied.The denoising effects of the three methods are compared.The results show that all the three methods can improve the signal-to-noise ratio(SNR)of atmospheric coherent length profile and reduce the recovered error of the atmospheric turbulence profile.Among the three methods,the WT method has the best ability of denoisng and SVD method is superior to CEEMD method.4.The two-aperture differential scintillation method is proposed for monitoring of the vertical profile of atmospheric optical turbulence strength,which it provides a theoretical basis for differential column scintillation lidar.A concise expression was proposed for irradiance fluctuation structure function with differential aperture in the Rytov approximation under a weak fluctuation regime based on the cross-path theory.The two-aperture differential scintillation method is then developed.The effects of bacon aperture and bacon altitude on path weighting function of this method were analyzed for Kolmogorov turbulence.The experiments were set up to compare the two-aperture differential scintillation method and single-aperture scintillation method in atmospheric boundary layer over 2 km horizontal single-passage path.It shows that the results of atmospheric refractive index structure constant deduced from the two methods are in good agreement under different weather and turbulence conditions.For the measurements of atmospheric coherence length of spherical wave corresponding to the two methods,they indicate a linear correction factor(R2)of 0.96.Feasibility and effectiveness of two-aperture differential scintillation method are thus verified by experiments.The novel method can separate single-passage scintillation information of active bacon double-passage propagation,thereby providing an accurate technique to measure atmospheric turbulence for active beacon.