【作者】 胡勇华；

【导师】 文双春；

【作者基本信息】 湖南大学 ， 计算机应用技术， 2008， 博士

【摘要】 激光光束的传输与控制是高功率激光驱动器创新发展的核心关键技术。研究高功率激光非线性传输的空间特性及其控制措施对提高高功率激光驱动器的运行通量、保障系统运行安全等具有十分重要的理论和实际意义。本论文主要探索了利用光束的带宽和最新的左手介质的独特光学特性控制光束非线性传输的空间特性的基本原理和规律,获得了若干创新性的成果,主要体现在如下三个方面:第一,尽管窄带激光脉冲放大和传输的数值模拟和程序编码已较为成熟,但由于宽带光束的脉冲宽度和带宽的特殊性,其非线性传输的数值模拟和算法尚处于探讨阶段。本论文基于宽带光束非线性传输的理论模型自主开发了宽带光束非线性传输的数值模拟程序,为研究高功率激光非线性传输的空间特性及其控制提供了可靠的模拟计算工具。该程序具有图形界面交互式操作、结果可视化等特点,能够模拟窄带和宽带光束的各种线性和非线性传输特性。第二,宽带激光是高功率超强激光驱动器的一个重要发展方向,宽频带激光驱动器中光束的传输与常规的窄带系统有很大区别,而宽带光束热成像这一非线性传输现象尚没有被系统研究过。本论文研究了宽带光束热成像的机理和特点,发现一定的带宽能抑制热像的形成,定量揭示了啁啾型带宽和变换极限型带宽对热像的控制规律。利用数值方法对宽带和窄带光束热成像进行了比较研究,发现宽带光束的热成像位置和窄带光束基本相同,但宽带光束的热像强度比窄带光束低很多,说明一定的宽带能很好地抑制非线性热像的形成。其次,通过改变脉冲啁啾和脉冲宽度两种方式分别定量揭示了带宽对热成像和光束匀滑的影响,发现不同带宽情形下热像的位置基本相同,均处于共轭位置处;热像的光强和热像处光束的调制对比度随着啁啾值的增大而降低、随脉冲宽度的增大而增大。特别是,我们发现,入射光束的时间宽度是影响带宽对热像的控制效果的最主要因素,时间宽度越大,带宽对热像的控制作用越小。对于二级热像,得到了类似结果,但在入射光束时间宽度较大时正、负啁啾型带宽都可以抑制二级热像。此外,还研究了入射功率、散射物的特性和介质的色散对宽带光束热成像的影响规律。第三,左手介质是一种具有人工设计的结构并呈现出天然和常规介质所不具备的超常物理性质的复合材料。左手介质不仅具有很多与常规右手介质完全相反的光学特性,更重要的是它具有很强的独特的操控光的能力,将在控制光束的空间特性方面发挥重要作用。我们建立了宽带光束在非线性左手介质中传输的理论模型,揭示了高斯光束在左手介质中非线性传输的基本规律,提出并从理论上论证了用左手介质片控制强光束空间特性的思路。借助脉冲光束在常规介质中非线性传输的基本理论,并结合左手介质的电磁色散特性,推导出了左手介质中宽带光束(3+1)维非线性传输模型,发现左手介质中色散磁导率的作用导致在传输模型中出现反常自陡峭效应项和类似于高阶色散特性的非线性效应项。基于建立的模型,揭示了一维时间孤子传输的反常延迟,研究了非线性左手介质中高斯光束的自聚焦和自散焦现象,得到了自聚焦焦点位置的表达式和自聚焦效应受调控的规律。结果表明,入射光束的会聚特性和左手介质的磁导率对自聚焦和自散焦等非线性过程有很强的调控作用。此外,提出并从理论上论证了用非线性左手介质片控制强光束传输和空间特性的思路,以高斯光束为例揭示了左手介质片控制强光束传输的规律,发现具有正(负)Kerr非线性的左手介质可以将高斯光束聚焦(扩展)。这种控制作用可以通过入射光束的发散度和功率以及介质片的物理性质来调节,且聚焦和扩展作用甚至可以发生相互转换。更多还原

【Abstract】 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.更多还原