Influence Of Turbulence On Propagation Properties Of Partially Coherent Decentered Annular Beams And Hermite-Gaussian Beams

It is very important to study the propagation properties of laser beams through turbulence for applications, such as laser communication, satellite remote sensing and tracking. Atmospheric turbulence and oceanic turbulence are two different important laser propagation medium. Therefore, it is very interesting to study the influence of atmospheric turbulence and oceanic turbulence on propagation properties of laser beams. Actually, the atmospheric turbulence may exhibit non-homogeneity and anisotropy. Furthermore, in practice, laser beams may exhibit partial coherence, high order mode, annular distribution, decentered distribution. Taking partially coherent Hermite-Gaussian beams and partially coherent decentered annular beams as typical examples of laser beams, the influence of atmospheric turbulence and oceanic turbulence on propagation properties of laser beams are studied in this thesis. The main works in this thesis are summarized as follows:（1） The regions of spreading of partially coherent Hermite-Gaussian beams propagating through non-Kolmogorov atmospheric turbulence are studied. It is shown that when laser beams propagate through atmospheric turbulence, the beam spreading depends on two mechanisms, i.e., free-space diffraction and atmospheric turbulence, and the influence of turbulence on the beam spreading relates with diffraction. The propagation distance is divided into three regions, in which the beam spreading is dominated by free-space diffraction, free-space diffraction and atmospheric turbulence, and atmospheric turbulence, respectively. The expressions for the turbulence distance and the Rayleigh range are derived, the influence of beam parameters and turbulence parameters on the three regions is studied by numerical calculations, and the relation between the first region and the Rayleigh region is also examined. In addition, the physical explanations of the main results obtained in this paper are given.（2） Changes of the average intensity, the centroid position and the position of the maximum intensity of partially coherent decentred annular beams on propagation are studied. The analytical expressions for the average intensity and the centroid position of partially coherent decentred annular beams propagating through oceanic turbulence are derived, and the propagation equation of the position of the maximum intensity is also given. It is shown that in free space the position of the maximum intensity is nearer to the propagation z-axis than the centroid position when the propagation distance is large enough. The position of the maximum intensity is nearer to the propagation z-axis with increasing the correlation parameter, and far away from the propagation z-axis with increasing the decentered parameter and the obscure ratio. However, in oceanic turbulence the position of the maximum intensity is close to the centroid position when the propagation distance is large enough, and the evolution is speeded with increasing the strength of oceanic turbulence. The influence of the beam coherence on propagation characteristics decreases due to oceanic turbulence. On the other hand, the centroid position is independent of the beam coherence, the propagation distance and the oceanic turbulence. The centroid position is far away from the propagation z-axis with increasing the decentered parameter and the obscure ratio.（3） Changes of skewness and sharpness of partially coherent decentered annular beams on propagation in oceanic turbulence are studied. Based on the Wigner distribution function, the analytical expressions for the high-order moments, the skewness parameter A and the kurtosis parameter K of partially coherent decentered annular beams are derived. The analytical expression for the oceanic turbulence parameter T ’ related to K is also derived, and the characteristic of T ’ is examined. It is found that the behaviors of A and K in oceanic turbulence are quite different from those in free space. In free space, the mass of the intensity distribution may move from one side of the centroid position axis y_c to another side at a certain propagation distance z₀, and z₀ is independent of the correlation parameter t. The mass of the intensity distribution is concentrated on one side of y_c on propagation only for a poorly coherent beam in free space, but it is always this situation for different value of t when oceanic turbulence is not weak. In free space, it takes a leptokurtic profile in the far fied, and a Gaussian profile appears only for a poorly coherent beam. However, in oceanic turbulence it always reaches a Gaussian profile for different value of t in the far field.