【作者】 邓剑钦；

【导师】 范滇元；

【作者基本信息】 湖南大学 ， 计算机应用技术， 2012， 博士

【摘要】 自聚焦是非线性光学中最常见最基本的物理问题之一,从上世纪六十年代起,自聚焦就一直是非线性光学领域热门的研究课题。从实践的角度来看,自聚焦效应限制了允许通过介质的光功率,因为自聚焦通常会导致光学介质的损伤。在高功率固体激光装置中,自聚焦特别是小尺度自聚焦一直是设计高功率激光系统的限制因素。用于惯性约束核聚变(Inertial Confinement Fusion,简称ICF)的激光驱动器,是高功率固体激光装置最典型的应用,而惯性约束聚变点火工程是《国家中长期科学和技术发展规划》的十六项重大专项之一。对用于ICF驱动的高功率固体激光系统而言,高能量、高效率和高光束质量的输出是实现靶丸快点火必需的要求和目标,而小尺度自聚焦引发的高功率激光多路成丝效应是限制固体玻璃激光器提高能量的主要因素。例如,美国劳仑斯·利弗莫尔国家实验室(LLNL)就在其激光运行报告中指出,小尺度自聚焦一直是用于ICF的激光驱动器总体设计、工程研制和安全运行的重大科学问题。从上世纪七十年代起,大量的研究人员对由小尺度自聚焦所导致的光束分裂成丝产生了浓厚的兴趣。因为,高能量、高效率和高光束质量的输出,是用于ICF驱动器的高功率固体激光系统的要求和目标,而小尺度自聚焦是限制固体玻璃激光器提高能量的主要因素。为突破自聚焦效应对高功率固体激光系统输出能力的限制,寻找和提出有效的抑制方法,一直以来都是高功率激光技术领域的重要研究方向。从某种意义上说高功率固体激光系统的发展过程,主要是解决自聚焦特别是小尺度自聚焦问题的过程。小尺度自聚焦的根源是激光束振幅和相位的不规则分布,所以提高光束质量改善光束空间分布的均匀性一直是抑制小尺度自聚焦的主要方法和措施。包括空间滤波器、软边光阑、以及提高光学元件的制备精度等方法都是改善光束质量抑制小尺度自聚焦的常用方法。近年来,人们又陆续提出了一些新的抑制小尺度白聚焦的方法,包括利用发散光束、部分相干光和宽带激光等。宽带激光被认为是用于突破由小尺度自聚焦效应引起的高功率固体激光系统的功率输出瓶颈的可能的技术路线。与单纵模的窄带激光相比,带宽的引入为激光的传输和控制提供了额外的自由度,这意味着随带宽引入的色散效应打破原本以衍射效应和非线性效应竞争的传输状态,使得色散与衍射一起与以小尺度自聚焦为主导的非线性效应形成竞争。因而,带宽的引入为抑制小尺度自聚焦提高光束近场均匀性提供了新的手段和可能性。因此,宽带激光有利于防止光束近场分布的均匀性因严重的小尺度自聚焦效应而出现明显的下降,从而能够保证最终产生高能量、高质量和高均匀性的三倍频聚焦光斑,提高靶面辐照的均匀性和靶丸的内爆效应。本论文基于高功率固体激光驱动系统中高负载能量和高光束质量的需求,研究了小尺度自聚焦的形成机制和功率条件,探讨了不同类型的宽带激光在传输过程中的对小尺度自聚焦效应的作用规律,以及宽带激光自身的各个参量对小尺度自聚焦效应的抑制条件和抑制规律,获得的主要成果如下：第一、研究了高斯光束发生小尺度自聚焦效应的功率条件。研究发现,高斯光束发生小尺度自聚焦效应所需的功率值与初始调制幅度有关,初始调制幅度越大,则高斯光束因小尺度自聚焦而分裂成丝所需的功率越小；反之,初始调制幅度越小则所需的功率越大。第二、研究了由谱色散匀滑(Smoothing by Spectral Dispersion,简称SSD)装置产生的具有相位调制和光谱色散特性的小宽带脉冲激光(简称谱色散光束)的菲涅耳衍射,以及以衍射场为输入的小尺度自聚焦现象。研究发现,谱色散光束可以用来改善光束的近场均匀性。选择合适的SSD装置的元件参数可以提高光束质量,从而实现对小尺度自聚焦的抑制作用。第三、研究了在宽带啁啾脉冲激光的小尺度自聚焦过程中,利用带宽抑制光束小尺度自聚焦效应的条件。研究发现,对于宽带啁啾脉冲激光而言,带宽可以在特定条件下用来抑制高功率激光系统中的小尺度自聚焦效应。当脉冲宽度一定时,带宽越大对空间小尺度自聚焦的抑制作用越明显,而当带宽一定时,脉宽越大带宽对小尺度自聚焦的抑制作用越有限；在不同脉宽条件下,对小尺度自聚焦实现相同程度的抑制作用,光束必须具有相同的带宽脉宽比,并且带宽脉宽比反映了带宽对小尺度自聚焦效应的抑制程度,比值越大则抑制作用越明显。第四、实验研究了宽带啁啾脉冲激光的小尺度自聚焦效应,验证了利用带宽抑制小尺度自聚焦的相关特性。通过控制调制场的空间频率,利用宽带啁啾脉冲激光正弦调制的增长实验,证明了与单色平面波条件下的B-T理论相比,宽带啁啾脉冲激光的增益谱曲线,整体向高频方向移动。另外,针对宽带啁啾脉冲激光B积分增长曲线的实验研究,其结果表明,与准单色激光的B积分增长曲线相比,宽带啁啾脉冲激光的B积分增长曲线,其峰均比对比度迅速增长的B积分拐点后移,初始噪声缓慢增长的平坦区更长。本文以提高高功率激光系统中激光光束控制能力和高能负载为目标,从不同角度针对不同类型的宽带脉冲激光,讨论带宽和其它相关光束参量在其线性和非线性传输过程中的影响规律,得到了一些新的认识,并提出了一些新的观点。更多还原

【Abstract】 Self-focusing is an intriguing optical phenomenon, when the High-power laser transmission in the Kerr nonlinear medium. Unlike the common lens focusing, self-focusing is a kind of beam "self-acting" effect, which is closely related with the transverse spatial intensity distribution of the beam itself, for example, Gaussian beam tend to collapse into a point due to the whole self-focusing, while the super-Gaussian beams, characterized by a flat-top, it is more likely to collapse into a ring-shaped profile. So we can see that self-focusing can occur only if the power carried by the beam is exceed to the critical power for self-focusing, and the focused beam profile is determined by the spatial distribution of the beam itself. The self-focusing effect can usually be divided into the whole beam self-focusing (also known as the total beam collapse) and small-scale self-focusing (SSSF). Whole beam self-focusing means the beam as a whole collapse into a focus with a very high strength; while SSSF is refers to the high power laser spontaneous split to form many filaments in the self-focusing medium, and the beam profile is also rely on the local intensity or phase modulation distribution.For high-power solid-state laser systems, SSSF is the more common phenomenon. Because, in high-power solid-state lasers, the peak power of the beam is much greater than the self-focusing critical power, and thus lead to a beam collapse into a focus point, before the beam split to numerous filaments. Since the1970s, a large number of researchers became interested in SSSF. Because, the laser output with high-energy, high-efficiency and high beam quality is necessary for the inertial confinement fusion (ICF) high power solid-state laser driver. But, SSSF often leads to damage in optical materials, and is the major limiting factor in the design of high-power laser systems. Therefore, in a sense, the process of the development of high-power laser systems, which is the process to solve the self-focusing, especially SSSF.To break the restrictions of the load capacity and the output power of the high-power solid-state laser system due to the serious SSSF, investigation on find and propose effective suppression method has been one of the focuses in high-power laser technology fields. Owing to the source of SSSF is the non-uniform distribution of the amplitude or the phase, so improve the near-field beam quality, enhance the uniformity of the beam has been the major method to suppress SSSF. Common techniques include:spatial filter, soft-edged apertures, improves the preparation precision of the optical elements.This dissertation focuses on the SSSF formation mechanism and power conditions. On that basis, propose new scheme to improve the beam uniformity and suppress SSSF in the high power solid-state laser system. We obtained the following results:Firstly, we investigated theoretically and numerically to find the critical power of SSSF for the Gaussian beam. We found that the critical power for SSSF decrease with increasing the modulation amplitude, it means that the critical power for Gaussian beam switch from whole beam self-focusing to SSSF, which is relevant to the initial modulation amplitude.Secondly, we investigated theoretically and numerically the Fresnel diffraction of a phase modulated and spectrally dispersed (PM-SD) beam after passing a hard-edged aperture and SSSF. The theoretical analysis shows that, in comparison with the monochromatic SG beam, the diffraction field of the PM-SD beam is more uniform in the Fresnel domain. The results suggested that the PM-SD beam can smooth out the diffraction intensity spikes. The numerical simulation results demonstrated that the PM-SD beam has a slower nonlinear growth than that of the monochromatic SG beam.Thirdly, we investigated the condition for suppression of SSSF of high-power laser beams by spectral bandwidth. And we found that broadband laser can be used to relax the restriction of nonlinear effect and obviously improve the output power of the high-power solid laser systems, because of the spectral bandwidth of such kind of broadband laser can delay the onset of SSSF. We proved that the degree of self-focusing suppression is closely related to the ratio of the bandwidth and pulse duration, in the same pulse width the big bandwidth the more obvious restrain effect, and in the same bandwidth the restrain effect of bandwidth will become weaker with increasing the pulse duration.Lastly, the growth of interference fringes and random noise caused by nonlinear propagation instabilities, which has been experimentally investigated by using a femtosecond pulsed laser. The results of experiment have proved that broadband laser can be used to relax the restriction of nonlinear effect, the whole gain curve moved to the direction of higher frequency, and relative to the narrow-band laser, the growth curve of the time-integrated spatial contrast with B-integral has a longer flat region of slow growth in the broad band condition, and cause the fast growth turning point of the curves significant moving to large B-integral.更多还原