Study On The Spatial Correlation Property And Directionality Of Partially Coherent Beams Propagating Through Atmospheric Turbulence

The research of the theory of the laser beam propagation in the atmospheric turbulence is of great importance in the area of laser communication, laser survey, aviation mapping, satellite remote and laser weapons. Actually, the situation of partial coherence always exists in laser beams; and partially coherent beams propagating through atmospheric turbulence are less sensitive to the effects of turbulence than fully coherent beams. Illustrated by the case of partially coherent Hermite-Gaussian beams (H-G) and partially coherent annular beams, partially coherent beams propagating through atmospheric turbulence are studied in this thesis. The main results obtained are summarized as follows:1. Based on the extended Huygens-Fresnel principle, the expressions for the spectral degree of coherence of the partially coherent Hermite-Gaussian (H-G) beams and partially coherent annular beams propagating through atmospheric turbulent are derived by using the quadratic approximation of Rytov's phase structure function, and the influence of atmospheric turbulence on spatial correlation property is studied. It is shown that the oscillatory behavior and phase singularities of the spectral degree of coherence may appear when partially coherent H-G beams and partially coherent annular beams propagate through atmospheric turbulence, which is very different from the behavior of Gaussian Schell-model (GSM) beams. But, the oscillatory behavior becomes weaker with increasing turbulence, and even disappears when the turbulence is strong enough. The smaller the coherence parameter is, the less the spatial correlation property of partially coherent H-G beams is affected by the turbulence; the larger the obscure ratio is, the less the spatial correlation property of partially annular beams is affected by the turbulence. In addition, a comparison between the mean-squared width of the spectral degree of coherence and the mean-squared width of the spectral intensity is also given. Some interesting results are obtained, and are explained physically.2. Based on the extended Huygens-Fresnel principle, the closed-form expression for the angular spread of partially coherent annular beams propagating through atmospheric turbulence is derived by using the integral transform technique. It is shown that, except for equivalent Gaussian Schell-model (GSM) beams, there also exist equivalent partially coherent annular beams, equivalent annular GSM beams, equivalent fully coherent annular beams, equivalent fully coherent annular Gaussian beams and equivalent partially coherent flat-topped beams which may generate the same directionality as the corresponding fully coherent Gaussian beam both in free space and in turbulence.3. Based on the extended Huygens-Fresnel principle, the closed-form expression for the mean squared beam width of annular beams propagating through atmospheric turbulence is derived by using the integral transform technique. The influence of turbulence on the spreading of annular beams is studied. On the other hand, the range of turbulence-independent propagation is given by examining the mean squared beam width, which indicates under what circumstances annular beams will be less affected by turbulence. It is shown that annular beams with larger obscure ratioε, larger beam order M (N), larger wave lengthλ, and smaller outer radius w0 are less sensitive to the effects of turbulence than those with smallerε, M (N),λ, and larger w0 .4. Under the extended Huygens-Fresnel principle, the closed-form expressions for the mean-squared width and the angular spread of general partially coherent beams in turbulence are derived by using the quadratic approximation of Rytov's phase structure function. It is found that under a certain condition different types of partially coherent beams may have the same directionality as a fully coherent Gaussian beam in free space and also in atmospheric turbulence if the angular spread is chosen as the characteristic parameter of beam directionality. On the other hand, it is shown that, the directionality of partially coherent beams expressed in terms of the angular spread is not consistent with that in terms of the normalized far-field average intensity distribution in free space, but the consistency can be achieved in atmospheric turbulence. In comparison with previous work, the results obtained in this thesis are more general.