【作者】 王伟；

【导师】 张小民； 赵圣之；

【作者基本信息】 山东大学 ， 光学工程， 2011， 博士

【摘要】 面对日益严峻的能源危机,惯性约束聚变(Inertial Confinement Fusion,简称ICF)为人类解决这一棘手问题提供了一个新的技术途径。以此为牵引,研制输出能力高达兆焦耳级、重频高功率激光驱动器已成为当今强激光科学技术研究的重要方向之一；同时,涵盖极端条件下的凝聚态物理、高亮度X射线和高能粒子束的产生以及实验室天体物理等前沿基础研究的高能量密度科学(High Energy Density Science, HEDS),也急需持续提升高功率激光驱动器的综合性能。概括起来,未来聚变能源以及高能量密度科学对高功率激光驱动器发展趋势主要集中在更高的能量转换效率、更高的光束质量、更高的峰值功率、更高的运行可靠性,以及包括重频运行在内的复合运行体制,简称“四高一复合”。高功率激光驱动器的总体能量转换效率主要决定于主放大器储能效率、系统储能抽取效率和谐波转换效率(包括二次谐波或三次谐波转换)等三大环节。其中,实现高效谐波转换的主要物理因素之一是能否满足波矢匹配条件,在技术层面主要体现在激光带宽、倍频晶体自身的“质量”、光束质量和调谐精度等相关条件的有效控制。为了突破高功率激光驱动器由于非线性效应引入的功率受限(或称B积分受限),有效提高系统输出能力,多年来人们一直积极探索与研究宽带激光脉冲传输放大模型,并开展了部分有意义的基础工作。然而,研究的深度和宽度离实际工程应用仍然有较大的差距,高效宽带三倍频转换仍然是制约宽带激光传输放大模型发展与应用的“瓶颈”问题之一。如何打破这一“瓶颈”,研究与发展宽带激光高效三次谐波转换技术,具有重要的学术价值和应用需求背景。本文立足前人研究的基础,根据未来兆焦耳级高功率激光驱动器发展的总体需求,系统地研究了宽带激光三次谐波转换的相关问题,进一步深化研究了高效宽带三倍频理论模型以及技术或工程关键问题,取得了一系列有意义的研究成果,为宽带激光传输放大模型的工程应用奠定了必要的基础。本论文的主要研究内容和创新点包括：1.建立相位调制匹配高效宽带三倍频理论模型,首次优化设计了正弦相位调制匹配宽带三倍频结构原理图,数值模拟表明,与其它三倍频模型相比,该结构设计在增加晶体接受带宽、提高入射光强动态范围和抑制FM-to-AM强度调制等方面优势明显,能够实现更大输入带宽的高效三倍频转换。在小信号情况下,推导了三倍频过程群速度匹配解析关系式,直观揭示了宽带三倍频的受限条件。为突破这一制约,从相位匹配和瞬时波矢匹配两个角度研究了满足正弦相位调制匹配要求的调制深度参量最佳比例关系,为广义相位调制匹配提供了理论思路。设计了实现正弦调制匹配结构的技术方案,优化了倍频晶体,相位调制器以及谐波分离双色镜等关键器件的相关参数。数值模拟与研究了晶体接受带宽、强度动态范围和FM-to-AM转换等三倍频输出特性,与其它三倍频模型比较有效提升了输入接受带宽；同时,量化了三倍频过程对频率转换晶体角度失谐和双色镜滤波能力灵敏性,为该理论模型的实际应用提供了误差容许量控制精度。2.提出了两块热键合氘化KDP晶体实现宽带三倍频的技术方案,验证了氘含量这一新自由度高效三倍频与角度失谐的等效性。理论模拟和实验结果表明,该技术方案比传统三倍频方案可将接受带宽提高3倍。由于键合消除了空气间隙影响,简化了实际工程应用晶体调节的技术难度以及提升了三倍频单元的结构稳定性。首次建立宽带光两块三倍频晶体之间空气间隙物理模型,分析其影响机制并得到等效空气间隙周期；首次通过解析模型揭示了KDP晶体热光系数是温度导致倍频晶体匹配角变化的关键参量。研究了部分氘化KDP晶体折射率变化特性,利用氘含量这一新自由度优化设计了两块部分氘化KDP晶体热键合三倍频结构模型,验证了氘含量这一自由度与角度失谐的等效一致性,理论和实验研究了该模型的三倍频输出特性,较常规三倍频结构模型,晶体接受带宽增加了约3倍,且更具实际应用前景。对两块混频晶体三倍频结构,建立模型分析了两晶体之间空气间隙对三倍频过程影响的物理机理,得到了与完整相位变化周期等效的4cm空气间隙变化周期；利用窄带等效宽带光源,实验验证了两晶体级联结构该空气间隙周期及其影响特性,首次实验获得了正弦调频宽带1.2nm宽带光机制下空气间隙物理特性,间接预知了理想情况下三倍频输出特性,与理论结果相符合。介绍了两种具有实际可操作性的消除空气间隙影响技术手段,比较了氘化晶体热键合三倍频模型在抑制空气间隙影响,晶体调节和结构稳定性等方面的优势。首次推导了KDP晶体相位匹配角对温度灵敏性的解析关系式,阐明了晶体的热光系数是相位匹配角温度灵敏性的关键因素并进行了实验验证,合理解释了不同结果之间的差异性,为在实际应用中控制温度对频率转换过程影响提供了重要参量指导。3.从瞬时频率角度定性分析了正弦调频宽带三倍频中FM-to-AM效应的物理机制和基本规律,首次得到了基频光相位调制和三倍频强度调制之间调制频率的相关性；首次推导并验证了调制度与角度失谐关系的解析表达式,创新性地将FM-to-AM转换特性运用到三倍频最佳匹配角调试中,比效率调谐曲线方法更快捷方便,精度更高,为进一步工程应用提供了新的调谐方法。深入研究了正弦调频宽带激光三倍频过程中的FM-to-AM强度调制问题,分析了FM-to-AM效应的物理机制,推导出了量化调制度的解析表达式,数值模拟验证了其有效性；应用瞬时频率概念,直观解释了FM-to-AM效应的物理图像,定性地分析了三倍频波形调制特性,首次得到了有无角度失谐两种情况下频率调制与幅度调制的相关性；结合理论导出的角度失谐与波长偏移等效关系,首次推导了调制度与失谐角度的解析关系式并进行了数值验证,创新性地将FM-to-AM效应运用到三倍频最佳相位匹配角调试过程,方便快捷,提高了调试精度。基于单倍单混三倍频基线结构,以正弦调频谱宽0.3nm宽带光为例,理论和实验研究了三倍频过程转换效率,强度调制和光谱输出特性,得到了与窄带三倍频情况相一致的转换效率强度调谐曲线,三倍频光谱稳定且未发生光谱丢失和窄化效应,实验结果与理论符合较好,验证了定性分析结果的可靠性。更多还原

【Abstract】 With the escalation of energy crisis, laser-driven inertial confinement fusion (ICF) technology might provide a new routine to solve such a problem. Therefore, the study of high power laser driver with the output capacity up to megajoule level and high repetition rate has become one of the important research directions within the framework of high power laser science and technology. Meanwhile, there are also strong desires to improve the integrated performance of the high power laser driver in frontier basic research in high energy density science (HEDS), including condensed state physics at extreme conditions, generation of high intensity X-rays and particle beams, and laboratory astrophysics etc. All in all, the technology tendency of high power laser driver for future fusion energy and HEDS will primarily concentrate on the complex laser systems with higher energy conversion efficiency, higher beam quality, higher peak power, higher operation reliability and repetition rate operation, which is the so-called "four high and one composite."The total energy conversion efficiency of high power laser driver is mainly determined by the energy storage efficiency of amplifier, energy extraction efficiency of the system, as well as the conversion efficiency of harmonic waves (such as the second harmonic or the third harmonic generation) etc. For the conversion efficiency of harmonic waves, the key issue is the wave vector matching condition. To achieve this goal in practical engineering, laser bandwidth, doubler crystal "quality", beam quality and tuning precision, etc have to be effectively controlled. In addition, due to nonlinear effects, the power limitation (or B integral limitation) of laser driver has to be overcome to enhance system output capability. Over past decades, investigations have been done with emphasis on the propagation and amplification model of broadband laser and some meaningfully fundamental research has been carried out. However, efficient broadband third harmonic generation (THG) is still one of the bottleneck problems that limited the development and application of the broadband laser propagation and amplification model.How to overcome such a bottleneck and to develop efficient broadband THG technology is of academic and application demand background importance. Based on previous research findings, according to the total demand of future megajoule level high power laser driver development, the relevant issues of broadband third harmonic conversion are studied systematically, and the efficient broadband THG theoretical model and the key issues of technology and project are investigated in depth in this article. We obtained several meaningful achievements, which are expected eventually to pave the way to the project applications of the broadband laser propagation and amplification model.The spotlights of this thesis are as follows:1. An effective broadband THG theoretical model based on phase modulation matched is established and a configuration principle sketch of sinusoidal phase modulation matching is optimally designed. In comparison with the traditional THG methods, the numerical simulations indicate such a design has prominent advantages to dramatically increase the bandwidth acceptance, widely improve intensity dynamic range and substantially suppress the resulting FM-to-AM conversion. More importantly, the model can support efficient THG at much larger input bandwidth.In small signal case, we analytically obtained the group velocity matched relationship of THG and revealed the limitation of the broadband THG process. In order to break through such a limitation, the optimal modulation depth parameter ratio in sinusoidal phase modulation matching is investigated from two aspects of phase matching and instantaneous wave vector matching, providing a theoretical guide for the generalized phase modulation matching. The technical scheme of sinusoidal phase modulation matching configuration is designed and the major device parameters of crystal thickness, phase modulator and dichroic mirror are optimized.The THG output characteristics such as bandwidth acceptance, input intensity dynamic range and FM-to-AM conversion are numerically simulated, leading to larger input acceptable bandwidth in comparison with other broadband THG models. In addition, the sensitivities of the third harmonic laser performance to both the crystal orientations and the filtering capability of the dichroic mirror are quantitatively analyzed, giving a control precision of acceptable tolerance for theoretical model applications in practice.2. An efficient broadband THG technical scheme with two partially deuterated KDP crystals bonded is proposed. New degree of freedom of deuteration level is validated equivalent to angular detune. Both the theoretical and experimental results show the bandwidth acceptance can be increased by factor of 3 in comparison with the traditional THG schemes. The technical difficulty of crystal regulation in engineering applications can be simplified and the stability of THG structural unit can be improved due to eliminations of air gap effects with the thermal bonding technology. The influence of air gap between two cascaded triplers is analyzed and the equivalent cycle of air gap is obtained by a new broadband physical model. It is the first time to exploit that the thermo-optic coefficients of KDP crystal are the primary factors affecting the dependence of phase matching (PM) angle on temperature by our the analytical model.The refractive index change of practically deuterated KDP crystals is studied. Based on such new degree of freedom of deuteration level, an efficient broadband THG configuration model with two partially deuterated KDP crystals bonded is designed. The equivalent function between deuteration level and angular detune is achieved. The THG output characteristics are theoretically and experimentally studied, showing that the bandwidth acceptance is three times larger than that of traditional methods.For THG model with two cascaded mixers, the influence of air gap is analyzed by building a theoretical model and the 4cm full cycle of air gap equivalent to that of phase mismatch is obtained. With narrowband equivalent to broadband source, such air gap cycle and corresponding THG output characteristics are experimentally demonstrated. The output THG performances induced by such air gaps for 1.2 nm bandwidth phase modulation pulse are firstly obtained in experiments, indirectly predicting the optimal output performance via the 4cm air gap, which is in good agreement with the numerical simulations. Two feasible methods of controlling the effects of air gap in practice are introduced, in comparison, highlighting the advantages of bonding crystals in terms of air gap suppression, crystal regulation and structure stability.By deducing the analytical expression of PM angle regarding temperature sensitivity of KDP crystal, we firstly indicate and demonstrate that the thermo-optic coefficients are the primary factors for the dependence of PM angle on temperature. The small errors in thermo-optic coefficients offer a reasonable interpret to the discrepancy among the reported results in previous reference, giving a key parameter for controlling the temperature effects in practice.3. From the point of view of instantaneous frequency, the physical mechanism and basic law of resulting FM-to-AM conversion effects in sinusoidal phase modulation THG are qualitatively analyzed, and consequently, the modulation frequency relationship between the THG intensity modulation and the fundamental phase modulation is firstly obtained. The analytical relationship between modulation degree and angular detune is originally derivated and verified. The resulting FM-to-AM effects are innovatively and positively applied for the perfect PM angle regulation, leading to higher regulating accuracy and fewer steps than the conventional efficiency tuning curve method and adding a new tuning approach in project applications.The resulting intensity modulation in sinusoidal modulation THG process induced by FM-to-AM conversion is investigated in depth and an analytical model is employed to quantify the THG intensity modulation, leading to an excellent agreement with the numerical simulations. By using instantaneous frequency conception, the physical picture of resulting intensity modulation is intuitively interpreted and the relationships between the frequency modulation and intensity modulation are firstly achieved in the case of angular detune or not. Together with the dependence of PM wavelength shift on the angular detune, an analytical expression of modulation degree regarding detuned angle is deduced and verified by numerical simulations. The resulting FM-to-AM conversion is innovatively and positively applied for the perfect PM angle regulation, leading to higher regulating accuracy and more convenient operation.Based on conventional THG baseline scheme, the THG conversion efficiency, intensity modulation and spectrum output characteristics of sinusoidal modulation 0.3nm bandwidth laser is theoretically and experimentally studied, resulting in accordant efficiency intensity tuning curve with that of narrowband THG. Stable THG spectrum without spectrum narrowing and loss is measured, agreeing well with simulations and giving a demonstration of the analytical model.更多还原