Propagation And Target Scattered Characteristics Of Array Beams In Turbulent Atmosphere

Laser beams propagation in the turbulent atmosphere affected by the atmospheric turbulence will produce the turbulence effects of beam spread, beam wander, scintillation and angle-of-arrival fluctuation, by which the laser applications in fields such as free space laser communication, laser radar, laser ranging and laser remote sensing are severely restricted. It is found that changing the shapes of the laser beams (such as beams with special beam profile or partially coherent beams or array beams) can reduce the atmospheric turbulence effects. Thus, investigation on the propagation properties and target scattering characteristics of different types of single or array beams will have great significance and value. Based on the extended Huygens-Fresnel principle and the method of the quadratic approximation of Rytov’s phase structure function, the transmission characteristics of the flattened Gaussian beam and several kinds of array beams such as Gaussian array beams, Gaussian Schell-Model (GSM) array beams, Gaussian array beams, Electromagnetic Gaussian Schell-Model (EGSM) array beams in turbulent atmosphere are investigated in detail.In combination with Goodman’s rough surface scattering theory, the second-order statistical properties for the field of the GSM array beams backscattering from a rough target in double-passage turbulent atmosphere are studied. The main research contents are as follows:Based on oblique propagation theory and the ITU-R Atmospheric refractive index structure constant model, the Rytov’s method is applied to study the optical-wave oblique propagation in non-Kolmogorov turbulence. The expressions of the average intensity,the second order moment beam width, the relative beam width and the expression of the on-axis scintillation index under weak fluctuation condition for the flattened Gaussian beam (FGB) in non-Kolmogorov turbulence are derived. The influences of the sourse size, the beam order, the generalized exponent, the propagation distance and the zenith angle on the transformation characteristics of the FGB in slant non-Kolmogorov turbulent atmosphere are analyzed. The comparison between the propagation characteristics of FGB and that of the Gaussian beam is made.Based on the extended Huygens-Fresnel principle, the expression of the beam width of the Gaussian array beams propagating through non-Kolmogorov turbulence is derived. The expression of the scintillation index under weak fluctuation condition for Gaussian array beams in non-Kolmogorov turbulence is also derived by using the method of the quadratic approximation of Rytov’s phase structure function. The influences of the waist width, the ring radius, the beam number, the generalized exponent and the propagation distance on the beam width, on-axis and off-axis scintillation index are investigated.With the help of the method of the quadratic approximation of Rytov’s phase structure function and the integral transformation, the expressions of the cross spectral density function for the coherent and incoherent superposition GSM array beams are derived. The variations of the characteristics of the spatial coherence and scintillation index with the source parameters, the turbulence intensity and the propagation distance are investigated in detail for both coherent and incoherent superposition GSM array beams. The results show that the incoherent superposition GSM array beams are less affected by the atmospheric turbulence.Based on the unified theory of coherence and polarization of stochastic electromagnetic beams, the analytic expressions of the cross spectral density matrix (CSDM) elements for EGSM array beams in non-Kolmogorov turbulent atmosphere are derived. The transformation characteristics of the average intensity, the spreading of the beam width,the far-field divergence angle, the degree of polarization, the orientation angle and the ellipticity for the EGSM array beams in non-Kolmogorov turbulent atmosphere are studied. The results show that for shorter propagation distance the source initial parameters take the dominant role in changes of transmission characteristics and for longger distance the effects of turbulence will take the dominant role, which are different from that propagating in free space.Based on the extended Huygens-Fresnel principle and Goodman’s rough surface scattering theory, the expression of mutual coherence function for the field of the GSM array beams backscattering from a Gaussian rough target in double-passage turbulent atmosphere is derived. The results can be degenerated to the condition of diffuse target when lcâ†'0. The effects of the source parameters, the target feature and the the atmospheric turbulence on the average intensity and the spatial coherence of the backscattered field are studied in detail.