| Atmospheric turbulence with its associated random refractive-index inhomogeneities disturbs a light beam which propagates any significant distance through the atmosphere. In the first part of this dissertation, we study how atmospheric turbulence affects the images formed by an imaging system. The extended Huygens-Fresnel principle is used to analyze the average image intensity and Signal-To-Noise Ratio (SNR) of a self-luminous, partially coherent source in a turbulent atmosphere. We have shown that moving the observation plane toward the source results in a reduction of the average image spot size. We have also shown that the analysis of SNR can be considered as a combined effect of the receiver-aperture and transmitter-aperture averaging effect in turbulence. In addition, we have found that, under the narrow-band approximation, the source temporal coherence has little influence on the performance of the composite turbulence-lens imaging system.; Atmospheric turbulence is obviously a disturbing noise factor, but under certain conditions as pointed out by V. U. Zavorotnyy et al the turbulence may lead to an increase in the telescope resolution beyond the diffraction limit in vacuum, usually referred to as superresolution. In the rest of this dissertation, we present a theoretical description of this effect. An analytical expression of the average image spectrum of an extended object has been developed. We have shown that for the occurrence of superresolution, the object size shall be much larger than the correlation length of the turbulence, in this case, each point on the extended object undergoes different turbulence-induced perturbation, it is therefore the turbulence-induced anisoplanatism which gives rise to the characteristics of superresolution. The relationship between various parameters, such as: wavelength, turbulence strength, lens size, etc on the occurrence of superresolution has also been investigated. We have found that superresolution takes place when the turbulence is weak. The degree of illuminating source coherence and the location of the turbulent layer on superresolution have also been analyzed. We have shown that, for the occurrence of superresolution, the illuminating source must possess some degree of coherence, also the turbulent layer shall be as close to the object as possible. |
