The Study Of The Nonlinear Steady-state Self-focusing

The nonlinear self-focusing effects ane the main nonlinear constrain to the large-scale high power laser facility. The study of nonlinear self-focusing problems is of vital inijxjrtant.In this work, the equations describing circular Gaussian beams propagating inside absorptive Keer medium have been derived from nonlinear paraxial equation. Based on the equations, an evolution equation of beam waist lias been obtained. Calculations indicate that the beam waist volves in two ways: (i) monotonously convergent, and (ii) convergent at first and then divergent. After the threshold beam was defined, the nonlinear focusing cliaracteristics of the beams have been researched, and the threshold intensity and threshold focal length of the threshold beam liave been also been specified.The characteristics of high power laser beams propagating in the amplifier of stacked-slabs(ASS) liave been studied with the Bespalov-Talanov theory. The impacts of the modulation of amplitude and pliase on the beams propagating in the amplifier of stacked-slabs have been addressed. When the B-integral value is a constant and the number of slabs is increased, the low frequency part of power gain spectrum is influenced slightly, while the middle and high frequency part changes considerately. When the modulations of amplitude and phase are white noise, their impacts on the beam propagation are different. The modulation of amplitude results in a growth of the low frequency components and the modulation of pliase leads to chiefly the growth of the middle and high frequency components. The increase of the B-integral value causes cliiefly the growth of the middle and high frequency components when other conditions are kept unchanged. As phase noises at die low frequency can be described by a Gaussian function, the ripples is shown in the power gain spectrum and theoscillating peak value at the middle and high frequency rises considerabely. As the modulation of amplitude is dust particle, the fluctuation and peaks are shown in the power gain spectrum. The peaks at the low frequency part reduce the power gain and that at the middle and high frequency part rise up the power gain. Both the location in the beams and the size of dust particle influence the peaks of power gain.The characteristics of a super-Gaussian beam propagating in the ASS have been studied with numerical simulation. The influence of factors on the laser propagating in the ASS, such as the waist parameters, order number of super-Gaussian beam and free space of laser propagation, have been studied. The intensity dependence of the evolutions of a super-Gaussian beam propagating in the ASS has been analyzed as well.Relative Intensity Noise (RIN) has been adopted to study the intensity distributions of laser beams propagating in the muti-stacked medium in frequency domain. The analytical results of the single rod experiment have shown that nonlinear self-focusing effects basically cause the growth of the middle and high frequency components. In order to suppress the growth of the middle and high frequency components, the size of the pinhole of the filter has been optimized, and the beam qualities improved . When a high power laser beam propagates through a nurti-stage amplifier, the influences of distances separating different stages on the intensity distributions have been analyzed . The analytical results shows that, when the distance is shorter, the RIN at low frequency part reduces slightly and that at the middle and high frequency part increases considerablely.