Study On The Propagation Properties And Beam Control Of High-power Lasers With Distortions

In designing and building up a high-power laser facility to satisfy practical requirements, both the power/energy scaling scheme and beam quality improvement are of importance. As usual, there are amplitude modulations and phase fluctuations in real high-power lasers. In particular, while a high-power laser propagates in the atmosphere, the thermal effect is induced, which gives rise to the phase aberrations of lasers, and results in the degradation of beam quality. Taking some practical applications into consideration, the purpose of the present dissertation is to study the propagation properties and beam control of high power lasers with amplitude and/or phase distortions. The main results obtained are summarized as follows:1. By using Zernike polynomials to describe the wavefront distortion of beams, the propagation properties of the flattened Gaussian beams (FGBs), super Gaussian beams (SGBs), Gaussian beams and plane waves passing through an aberrated lens or an aberrated annular lens are studied based on the generalized Huygens-Fresnel diffraction integral. In this dissertation, based on the beam propagation equation in the free-space, the hyperbolic-fit mothed is used to calculate the M2- factor of Gaussian beams passing through a spherically aberrated lens for the first time. M2-factor, the power in the bucket (PIB) and ft parameter are chosen as criteria for characterizing the beam quality. The influence of aberration and obscure ratio of lens . on the laser beam quality are studied. It is shown that by a suitable choice of the positive spherical aberration may give rise to a shift of the position of peak intensityaway from the lens, but to a decrement of peak intensity, whereas the negative spherical aberration leads to a shift of the position of peak intensity towards the lens, and to an increment of peak intensity in comparison with the aberration-free case. Thus, focusing without focal shift and with inverse focal shift can appear in use of the positive spherical aberration. On the other hand, focusing beyond conventional diffraction limit can be achieved by using a lens with negative spherical aberration. As compared with aberration-free super-Gaussian beams, the profile of spherically aberrated super-Gaussian beams can be leptokurtic, mesokurtic or platykurtic, depending on the aberration and propagation distance. Spherical aberration reduces the M2- factor, thus degrade the beam quality. There is the same PIB curve of beams at the geometrical focal plane of lenses with the equal absolute value of negative and positive spherical aberrations, whereas their PIB curves at the real focal plane are separate, and the largest PIB value, i.e., the best focusability is achieved at the real focal plane. Moreover, the smaller the values of obscure ratio of lens, the better the focusability of beams demonstrates. A Gaussian beam becomes an astigmatic one after passing through the astigmatic lens, but remains symmetrically at the geometrical focal plane. With the increasing of astigmatic coefficient of a lens the intensity distribution of SGBs becomes flat at the geometrical focal plane. Astigmatism and title don't change M2- factor, but result in decreasing of PIB. From a practical point of view it is better to choose the PIB as the beam quality characteristic parameter for astigmatic and titled beams than the M2- factor.2. The real high-power laser system, such as ICF system contains many hard-edge apertures, which can be regarded as a multi-apertured optical imaging system of B=0. Because as usual it is difficult to derive analytical expressions for laser beams through optical systems with apertures, numerical calculations of straightforward integral of the Collins formula have to be made, which, however, become very time-consuming as the number of apertures increases. In comparison with laser beams without amplitude modulations and phase fluctuations, which can be characterized by the field distribution, laser beams with amplitude modulationsand phase fluctuations are described by the mutual intensity. Cons...