Characteristics Of Reflected Wave From Targets Illuminated By Laser Beam In Turbulent Atmosphere

The characteristics of reflected laser wave by a target in turbulent atmosphere aresignificant for the application of laser beam in Lidar system, laser guiding andcountermeasure, and so on. Based on the extended Huygens-Fresnel principle，theRytov Method and the theory of effective parameter, the dissertation presents theoreticalstudies on the second-and fourth-order moments of the reflected wave from targetssuch as fully diffused plates, rough plates, retroreflectors or retroreflector arrays, whichare illuminated by Gaussian beams or Gaussian-Schell Model (GSM) beams inturbulence. The main results are as follow:First, a high-frequency fitering function is introduced into the Slip-Step Method,which greatly lowers the noise in the simulations of highly divergent beams. Then themethod is used to simulate the single-pass and double-pass propagation of GSM beamsin atmospheric turbulence along the horizontal and slant path. Based on the simulatedsamples of the propagating field, the Mutual Coherence Function (MCF) andscintillation index are calculated and the results agree with those of Rytov method.Effects of the properties of the source and target on the statistical moments of thesimulated beams are analyzed. The differences between the upgoing and downgoingbeams along the same slant path as well as those between the monostatic and bistaticcases during the double-pass propagation are discussed.Second, characteristics of the reflected wave in atmospheric turbulence are studiedby using the extended Huygens-Fresnel principle in two cases, one is a plane orspherical wave illuminating a fully diffused plate with a hard edge, another is a finitebeam illuminating a unbounded fully diffused plane. In the first case, statisticalmoments of the reflected beam such as MCF, mean intensity, backscatteringenhancement coefficient, covariance, variance of intensity and the residual scintillationindex are calculated considering the backscattering enhancement. The effects of thetarget radius, turbulence strength and zenith angle of the path on the reflected wave arediscussed. In the second case, the coherence length and intensity variance of thereflected beam are calculated by ignoring the backscattering enhancement, and theeffects of properties of the source, for example, the degree of coherence, radius, focallength, are discussed.Third, the theory of the effective parameter is extended to research thecharacteristic of reflected wave from a rough plate. The analytic expressions for thecoherence length and scintillation index of the reflected wave from a rough plate illuminated by a GSM beam in turbulence are developed. The effects of the beam radius,degree of coherence of the source, target radius, target roughness on the characteristic ofthe reflected wave are discussed. An expression of on-axis scintillation index of thereflected wave from a rough plate under any turbulence strength is derived bycombining the theory of the effective parameter and the Modified Rytov theory. Thechanges of scintillation index of GSM beam with the strength of turbulence arereserached, which shows that the scintillation index of the scattered wave from a roughtarget in strong turbulence does not tend to unity.Fourth, based on the Rytov theory and rotationally symmetric ABCD ray matrices,the mean intensity, degree of coherence and scintillation index of wave reflected byretroreflectors are expressed in terms of the basic statistical moments of complexperturbation induced by turbulence, and the changes of these moments with the sourceradius and the target radius are numerically discussed, the corresponding results formirrors are also given as comparisons. The expression of MCF of wave refected byretroreflectors with hard edges is derived by decomposing the hard-edge circularaperture into a sum of complex Gaussian functions, and the numerical results show thatthe second moments of the refected wave oscillate with the increase of the size of theretroreflector. The expressions of coherence length and scintillation index of thereflected wave from retroreflectors illuminated by a GSM beam are derived by using thetheory of effective parameter.Finally, by approximately taking the cell of the retroreflector array as a pointreflector, the mean intensity and the backscattering enhancement coefficient of thereflected wave from a retroreflector illuminated by a GSM beam with any incidentangle on the off-axis position are derived. The speckle field of the retroreflector array issimulated numerically with slip-sted Method, showing that the speckle field of an arrayin turbulence is the nestification of two kinds of speckle with different sizes. Analyticexpressions of the mean intensity, the backscattering enhancement coefficient thereflected wave from an array are derived by using the point refector approximation. Andthe intensity variance of the scattered wave is fumulated by the spherical waveapproximation, while the numerical results show that the intensity variance index canalso be smoothed by the aperture of the array.