Study On The Propagation Properties Of Partially Coherent Annular Beams In Atmospheric Turbulence Along A Horizontal Path And A Slant Path

The propagation of laser beams through atmospheric turbulence is a topic that is considerable theortical and practical interest. The propagation of laser beams along a slant path (i.e., non-homogeneity turbulence) may encounter in many domains, such as optical communications, measuring, and laser weapons. On the other hand, fully coherent laser beams spread quickly duo to atmospheric turbulence. The turbulence-induced spatial broadening of fully coherent laser beams is a limiting factor in most applications. It has been shown theoretically and experimently that partially coherent beams are less sensitive to the effects of turbulence than fully coherent ones. Moreover, high-power laser beams produced by unstable optical resonators are always with annular and non-uniformity prpfile. Therefore, it is very important to study propagation properties of partially coherent annular beams in atmospheric turbulence along a horizontal path and a slant path. The main works are summarized as follows:1. Based on the extended Huygens-Fresnel principle, closed-form expressions for the Rayleigh range and the far-field divergence angle of partially coherent annular beams propagating through atmospheric turbulence are derived by using the Wigner distribution function (WDF). Taking the Rayleigh range and the far-field divergence angle as the characteristic parameters of near-field and far-field spreading respectively, the spreading of partially coherent annular beams both in free space and in turbulence is studied in detail. It is found that the effect of the strength of turbulence and beam parameters (e.g., the spatial correlation length, the waist width and the wave length) on the beam spreading in the near field is in agreement with that in the far field. However, in turbulence the effect of the obscure ratio of annular beams on the spreading is different between in the near field and in the far field. Namely, in turbulence the beam spreading in the near field becomes smaller and the beam spreading in the far field becomes larger as the obscure ratio increases. In particular, the influence of turbulence on the Rayleigh range and the far-field divergence angle is nearly unchanged versus the obscure ratio when the spatial correlation length is small. The main results obtained in this paper are explained physically.2. The model of partially coherent annular beams with linear non-uniformity field profile in the x direction is set up. The analytic expressions for the average intensity and the centre of gravity of partially coherent annular beams with decentered field propagating through atmospheric turbulence along a slant path are derived. The propagation equation governing the position of the intensity maximum is also given. It is found that the beam non-uniformity is amended gradually as the propagation distance and the strength of turbulence increase. The centre of beam gravity is independent of both the propagation distance and the turbulence. However, the position of the intensity maximum changes versus the propagation distance and the turbulence, and is farthest away from the propagation z-axis at a certain propagation distance. When the propagation distance is large enough, the position of the intensity maximum reaches an asymptotic value which increases with decreasing the zenith angle and is largest for the free space case. When the propagation distance is large enough, the position of the intensity maximum is not on the propagation z-axis, and is nearer to the propagation z-axis than the centre of beam gravity. On the other hand, changes in the intensity maximum in the far field are also examined in this thesis.