Propagation And Scattering From Surface Of A Partially Coherent Laser Gaussian Beam Through Turbulent Atmosphere

With the development of laser communication and laser radar, laser propagation and statistics of the scattering from target have important application in areas such as navigation, target detection, laser ranging, guiding and remote sensing in the turbulent atmosphere. Using the extend Huygens-Fresnel principle, the Kolmogorov spectrum of refractive-index and a quadratic approximation for the wave-structure function, the dissertation describes the propagation of a partially coherent beam and scattering from target through random medium. The main results obtained are as follows:Based on the extended Huygens-Fresnel integral and the expression for cross-spectral density, the expressions for average intensity, beam spreading, beam wander, power in the bucke of the GSM beam is derived. Then, we deal with the aperture-averaging factor and intensity variance in the weak-turbulence regime for GSM beam and the average spectral density, temporal mean intensity, temporal broadening and the spectral degree of coherence of a GSMP beam in atmospheric turbulence are in turn examined.Based on a single passage and the extended Huygens-Fresnel principle, we derive the mean intensity, variance, spatial power spectral density, time-delayed covariance expressions at receiver with phase perturbation dominant. The treatment includes the effects of the turbulent atmosphere on the laser beam as it propagates to the target and on the scattering field as it propagates back to the receiver. Considering the forward and return paths are not treated as independent, the result is shown that the off-axis backscattered intensity enhancement factor of the reflected GSM beam is relation with the coherence length of source, the wave-length, and the size of target and conditions of turbulence. The close form expressions can interpret backscattered intensity enhancement of plane and spherical wave scattered from a diffuse target.We develop an arbitrary roughness target model in double-passage atmospheric turbulence that reduces to a totally diffuse target and smooth target. An expression is developed for the mutual coherent function (MCF) of a reflected Gaussian-Schell model beam from a semi-rough target in atmospheric turbulence with the assumption of independent effects of turbulence on outgoing and return paths. Then according to the MCF we derive an expression about the mean intensity and the wavefront coherence length at the receiver. The analysis indicates that the mean intensity is closely related to the ratio of root mean square (RMS) height to the lateral correlation length, in addition, in different turbulence strength, the wavefront coherence length is mainly affected by roughness of target, propagation distance and partially coherence length.Based on the extended Huygens-Fresnel principle and phase perturbation assumption, an expression is developed for the Multiple-Frequency mutual coherence function (MCF) of a reflected pulse beam from rough target in atmospheric turbulence. The results are reduced to one-point, one-frequency and one-point, two-frequency correlation function. Then, we deal with the degree of correlation of pulse scattering from Gaussian homogenous, zero-mean surface. The analysis indicates that with the RMS height of a surface increases, coherent band width decreases or pulse width increases and the lateral correlation length has nothing with coherence bandwidth.Using quadratic approximation, we formulate the average intensity and signal-to-noise of a direction-detection imaging system in turbulent media. We investigate three kinds object the dependence of the average spot size on the imaging lens, the turbulence strength and the object size. The rougher the surface, the smaller the average intensity. Accordingly the SNR of a spatially-incoherent and spatially coherent object is evaluated and the effect of changing the strength of turbulence and the lens radius on the degradation of the SNR is studied. The effect of the lens radius on the SNR for a spatially-incoherent and spatially coherent object is different and other parameter is the same.