Research On Spatial Properties And Control Of Nonlinear Propagation Of Intense Laser Beams

The propagation and the control of intense laser beams are core, key technologies for the innovative development of high-power laser drivers, especially for the advance in the energy flux and the running safety of high-power laser system. Thus, the investigation on the spatial properties of high-power laser beams and the control measures of them are of great significance in both theory and practice. In this dissertation, we will investigate the rationale and laws of the spatial properties of laser beams and the control of them by bandwidth and by the unique optical effects of newly-developed metamaterials. Our work and results are mainly follows:Firstly, the numerical simulation and the computer codes for the nonlinear propagation of broadband laser beam are in the exploration stage, though those for the amplification and propagation of narrowband laser beam are mature. This is because of the specialties of broadband beams such as duration and bandwidth. On the basis of the theoretical model for the broadband beam propagation, we have independently developed a computer program to simulate the nonlinear propagation of broadband laser beams, which is proved a dependable tool for the research of the spatial properties and their control of the nonlinear propagation of high-power laser beams. This program is characterized by its interactive graphical interface, the visualization of simulation results, and so on. It can be used for the simulations of the nonlinear propagations of both narrowband and broadband beams.Secondly, broadband laser becomes an important development direction of high-power laser drivers. Because the propagation of broadband beam is greatly different from that of narrowband beam, the formation of hot image in broadband beam needs to be investigated in detail. We have investigated the mechanism and characteristics of the formation of hot images in broadband beams. It is found that the formation of hot image can be suppressed by certain bandwidth. We have revealed quantitatively the laws of controlling hot image by both chirp-type bandwidth and transform limited-type bandwidth.With computer simulation, we have compared the hot image formation in narrowband beams and that in broadband beams. It is found that the location of hot image in broadband beam is almost the same as that in narrowband beam, but that the intensity of the hot image in the former case can be much lower than that in the latter case, which indicates hot image can be well suppressed to by certain bandwidth. Next, we have made clear the influence of bandwidth on the hot image and the beam uniformity by changing the initial temporal chirp and by changing the initial pulse duration. It is found that the image location doesn't change with bandwidth, and that the intensity of hot image and the beam contrast at the hot image decreases as the initially temporal chirp increases but increases as the initial duration increases. Especially, it is proved that the initial pulse duration is the most important factor in those which affect the effects of bandwidth on hot image, and the bigger the initial duration is, the weaker the effect will be. For the second-order hot image formation, similar results are obtained, except for that both positive chirp-type and negative chirp-type bandwidths can suppress second-order hot image when the duration of the incident beam is relatively bigger. Besides, under the broadband circumstance, the influences of the beam power, the characteristics of the scatter and the dispersion of the nonlinear medium on hot image formation are also investigated.Thirdly, left-handed medium is a kind of composite material which has a man-made structure and unique physical properties which are not found in natural media. Left-handed media have many optical characteristics different from those in conventional right-handed media. What is more important is that they have great unique ability in operating light and will a pay important role in controlling the spatial properties of light beam. We have established a theory model for the propagation of broadband beam in nonlinear left-handed media and investigated the basic law of the propagation of Gaussian beams. We have also brought forward and demonstrated the idea to control the spatial properties of intense laser beam by left-handed media.On the basis of the theory for the nonlinear propagation of pulsed beam in conventional media, we have derived the (3+1)-dimensional nonlinear propagation mathematical model in left-handed media by taking the electromagnetic dispersion property of left-handed media into consideration. In this model, originated from the negative dispersive magnetic permeability, there are a term for the anomalous self-steepening effect and terms for nonlinear effects similar to those for high-order frequency dispersion effects. On the basis of this model, we have revealed the anomalous delay of temporal soliton propagation, investigated the nonlinear propagation of Gaussian beams and their self-focusing and the self-defocusing in left-handed media. We have also obtained the expression for the location of the focus of beam self-focusing and investigated tuning effect for self-focusing in left-handed media. Our results show that the beam nonlinear propagation and beam self-focusing and self-defocusing in left-handed media can be strongly tuned by the convergence/divergence of the incident beam and the magnetic permeability of left-handed media. Moreover, the idea to control the spatial properties of intense light beam by using nonlinear left-handed media is presented and theoretically demonstrated. We have investigated the controlling effect of a plane slab of nonlinear left-handed medium on intense Gaussian beams, and found the interesting effect that left-handed media which have positive Kerr nonlinearity can focus Gaussian beams, while those which have negative Kerr nonlinearity can expand Gaussian beams. We have also found that such a controlling effect can be greatly tuned by the power and the convergence/divergence of the incident beam and the physical properties of the slab, and that the beam focusing or expanding may even be converted.