Influence Of Atmospheric Turbulence On The Beam Guality Of Laser Beams With Distortion And The Propagation Property Of Laser Beams Reflected By A Cat-eye Optical Lens

In high-power laser systems, the gain non-uniformity, gain saturation, aperture diffraction and nonlinear optical effects inevitably result in laser beam distortions on both amplitude and phase. It is important to study the propagation and control of laser beams with distortions. On the other hand, by using the active laser detection technique based on the theory of cat-eye effect of optical lens, a much longer detection distance and a higher orientation precision can be obtained than by using passive detection technique. The active laser detection technique has attracted more and more attention because it is useful in modern war. The effect of turbulence cannot be neglected when high-power laser beams and active detection laser beams propagate through atmospheric turbulence. The main works in this thesis are summarized as follows:1. The beam quality of truncated laser beams with amplitude modulations and phase fluctuations in atmospheric turbulence are studied. Based on the extended Huygens-Fresnel principle, the closed-form expressions for the Rayleigh range zr and the M2-factor of truncated laser beams with amplitude modulations (AMs) and phase fluctuations (PFs) in turbulence are derived by using the Wigner distribution function(WDF). The beam quality is studied by taking the zR and the M2-factor as the characteristic parameters of beam quality. It is found that the M2-factor of truncated laser beams with AMs and PFs is always larger than that of truncated Gaussian beams both in free space and in turbulence. However, in turbulence the beam quality of truncated laser beams with AMs and PFs may be better than that of truncated Gaussian beams if the zR is taken as the characteristic parameter of beam quality. For laser beams with AMs and PFs in turbulence, the beam quality expressed in terms of zR is consistent with that in terms of the M2-factor versus the phase fluctuation parameter a, but not versus the intensity modulation parameter σA-The beam quality of truncated laser beams with AMs and PFs is less sensitive to turbulence than that of truncated Gaussian beams. The laser beams with smaller a and larger σA is less affected by turbulence than those with larger α and smaller σA.The results obtain in this thesis will be usefull for applications of high-power laser beams propagating through atmospheric turbulence.2. The propagation properties of tilted and off-axis partially coherent beams reflected by a cat-eye optical lens in atmospheric turbulence are studied. Taking the partially coherent beam as an active detection laser beam, and taking the Gaussian Schell-model (GSM) beam as a typical example of partially coherent beams, analytical expressions for the return departure, the average intensity, the mean-squared beam width and the far-field divergence angle at the receiver plane are derived. For the small positive defocus of a cat-eye optical lens, the on-axis average intensity reaches its maximum. The positive defocus value becomes small as the focal length of the cat-eye optical lens decreases, but it is independent of atmospheric turbulence and the coherence of active detection laser beams. The less beam spreading may occur with increasing the propagation distance for active detection laser beams with less coherence, which is quite different the behavior of laser beams propagating in free space. Active detection laser beams cannot be reflected by a cat-eye optical lens in the direction of the source if active detection laser beams are tilted and off-axis. The absolute return departure increases as the beam coherence increases. Therefore, partially coherent beams are more suitable as active detection laser beams than fully coherent beams. The absolute return departure decreases greatly due to aperture effect, but the change of the absolute return departure versus strength of turbulence is not monotonic. Furthermore, influences of defocus of a cat-eye optical lens, the beam coherence and the atmospheric turbulence on the mean-squared beam width and the far-field divergence angle at the receiver plane are also examined. The results obtained in this thesis will be useful for applications of active laser dectetion technioue.