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chi20191129上海光机所朱健强557639Owing to rapid development in the chirped pulse amplification (CPA) and optical parametric CPA (OPCPA) techniques, the output power of ultra-intense ultrashort laser systems has grown dramatically, achieving high-output laser intensity in the range of 10^21–10^22W/cm^2. Such an intense laser provides a powerful tool for the experimental study of laser–matter interactions in the relativistic regime. Nowadays, tens of petawatt-class laser facilities had been constructed, and laser facilities with peak power of sub-exawatt (EW) level was proposed. A crucial parameter of ultrashort high peak power laser systems in high field experiments is the temporal contrast ratio, which is defined as the intensity ratio of the main pulse to the background noise. A poor contrast ratio can change the interaction mechanism and fail the physical experiments. For a petawatt laser system, a typical temporal contrast ratio in the range of 10^11–10^12 is required. Unfortunately, without the application of any pulse cleaning technique, the temporal contrast of the focal spot is several orders of magnitude lower than the required level. As a result, the development of contrast improvement techniques is of great importance for the construction of petawatt-class or laser systems with higher peak power. In most high-power laser systems, the beam size is gradually expanded by Keplerian telescope beam expanders along with energy scaling to avoid optical damage and clear unwanted intensity modulation with high spatial frequency. However, lens-based beam expanders unfavorably introduce chromatic aberration, which could tremendously reduce the output intensity of ultrashort high-power laser systems by deteriorating the spatial-temporal focusing performance. A careful compensation of the chromatic aberration greatly contributes both to the optimization of the temporal and spatial beam profile, and consequently, to the optimization of the output intensity. The main contents and innovations of this dissertation are as followed. 1.Starting from the Maxwell's equations, an equation for analyzing ultrashort pulses propagation in nonlinear and anisotropic media was derived. Based on this equation, the Nonlinear Schrodinger Equation and the Master equation were derived, and the three-wave and four-wave parametric amplification equation in space-time domain were obtained. According to this equation, a numerical algorithm and program for analyzing the contrast evolution in DFWM was compiled. The program takes into account the spatial diffraction and walk-off, phase mismatch and dispersion in time domain and nonlinear effects in third-order nonlinear process. 2.A comprehensive study on the potential of PCW generation in femtosecond laser systems, based on the backward DFWM process as a pulse cleaner was presented. The simulations focused on the contrast enhancement capability, the gain bandwidth of the spectrum, and the energy conversion efficiency, as these are all key parameters to evaluate the performance of pulse cleaners in broad-bandwidth laser systems. Specifically, a recursive formula is established to determine the contrast ratio of the PCW directly after the DFWM process, which can numerically study the contrast evolution in other nonlinear processes, such as OPA, XPW, and frequency doubling. A pedestal noise within a duration of few picoseconds or broader was found when the contrast of the PCW after pulse compression was analyzed. The origin and the method of the removal of this pedestal noise were studied. Next, we examined the optimization of the gain bandwidth of the PCW to realize a minimally compressible pulse width. The systematic requirement of the input pulse width for the DFWM process was determined for laser systems with different bandwidths. Additionally, the simulation results proved that the conversion efficiency is mainly affected by the spatial–temporal profile and the pump–signal intensity ratio of the input fields. The numerical calculations verified that a considerably high efficiency bandwidth product and excellent contrast enhancement capacity could be realized by finding a balance between each physical effect and input parameters. The simulation results in this study provide guidance for applying this technique in laser systems with a wide range of bandwidth. 3.A promising chromatic aberration pre-compensation scheme with a wide dynamic compensation range was proposed to overcome the drawbacks of current chromatic aberration compensation methods. The proposed scheme consists of a convex lens, group of concave lenses, and spherical mirror combined with a conventional vacuum chamber. It provides the flexibility to compensate the chromatic aberration of an entire laser system via continuously controlling the amount of PTD induced by the compensator without changing the input and output beam size. The performance of the proposed scheme was experimentally validated in the SG-II 5PW laser system. The pre-compensation scheme is capable to be applied in high-power ultrashort laser systems as a pre-compensator to correct chromatic aberration of the entire system accurately and effectively. 4. Aiming to develop laser facilities with higher energy, narrower pulse width, a nova amplification scheme based on quasi-degenerate four-wave mixing process was proposed to overcome the deficiencies of OPA technique. Compared with technique based on second-order parametric process, this amplification scheme has better contrast ratio and broader spectrum bandwidth. The shortcome lies in the lower energy conversion efficiency and difficulty in optical synchronization and alignment. The potential applications of this scheme in picosecond and femtosecond high power laser was analyzed. Two novel techniques to overcome the deficiency of energy conversion reduction in FWM process was proposed: Four-photon Quasi-parametric Amplification (FP-QPA) and Multi-pass Four-wave Mixing (M-FWM). Based on this scheme, a contrast enhancement unit was designed for SG-II 5PW laser system. In conclusion, a theoretical investigation towards PCW generation via DFWM process was proposed for contrast enhancement in femtosecond petawatt laser systems; a dynamic chromatic compensation scheme was provided and experimentally validated in SG-II 5PW laser system.2020atalunwen2213111821287OPCPA;temporal contrast ratio;optical phase conjugate wave;DFWM;chromatic aberration compensationResearch on high contrast amplification techniques and focuing capability enhancement in Multi-Petawatt Laser System数拍瓦超短脉冲激光系统高对比度放大技术及终端时空性能提升研究随着CPA技术和OPCPA技术的发明,高功率超短脉冲激光技术取得了飞速发展,为探索各种极端条件下的物理现象提供了强有力的研究工具。高功率超短脉冲激光装置始终朝着更大输出能量、更窄脉冲宽度和更高聚焦功率密度的方向发展。目前,拍瓦(PW)级激光装置已日趋成熟,世界范围内正在研制一批输出功率达到十拍瓦量级、朝着亚艾瓦(EW)量级迈进的超高功率激光装置。 在高功率超短脉冲激光装置的研制过程中,脉冲时域对比度是一个重要参数指标,时域对比度的高低在很大程度上决定了相关物理实验的成败。拍瓦级激光系统需要至少10^11-10^12的时域对比度才能满足物理实验的要求,而实际系统在不采取对比度提升手段的前提下无法满足要求。因此,研究如何提升激光系统的时域对比度对于优化拍瓦级激光装置性能、建设更高功率的超短脉冲激光装置起着举足轻重的作用。 此外,色差是制约高功率宽带激光装置聚焦功率密度的重要因素。高功率激光系统通常由大口径透射元件组成的空间滤波器实现对激光的扩束、空间滤波和像传递。而大口径透射元件不可避免的引入色差,造成系统终端聚焦性能的恶化,从而降低系统峰值功率密度。因此,全面补偿系统色差对于改善装置的聚焦能力、提升装置峰值功率密度有着重要的意义。 本论文围绕着制约高功率超短脉冲激光装置性能的两个重要参数指标:脉冲时域对比度和色差展开研究,致力于研究新的时域对比度提升技术、探索高对比度的放大技术、开发高效的系统色差补偿方法。本文的主要内容以及创新点如下: 1、从麦克斯韦方程组出发,详细推导了超短脉冲激光在非线性、各向异性介质中的传播方程。依据此方程,推导出描述超短脉冲在非线性波导中传输的非线性薛定谔方程、研究锁模过程的Ginzburg-Landau方程、描述三波混频和四波混频过程的非线性耦合波方程。讨论了非线性耦合波方程的数值解法,编写了用于分析四波混频过程的时空域三维模拟程序,该程序综合考虑了四波混频所涉及的空间衍射、走离,相位失配、色散以及三阶非线性效应。为本文研究四光子参量过程的中对比度的演化提供有效手段。 2、提出了一种基于简并四波混频产生光学相位共轭光的飞秒宽带激光系统对比度提升技术。以数值模拟为基础,分别从对比度提升水平、增益带宽和能量转换效率三个方面综合评估了相位共轭技术在宽带激光系统中的适用性。建立了一个能够用于数值模拟非线性过程中对比度演化的迭代公式,利用该公式研究了简并四波混频过程中相位共轭光对比度的演化规律。利用该公式可以研究包括OPA、XPW在内的多种非线性过程中对比度的演化规律。在对相位共轭光对比度的研究中,探讨了相位共轭光噪声台阶的来源和相应的抑制方法。研究了简并四波混频过程中入射参数对相位共轭光的光谱宽度和极限压缩脉宽的影响。针对不同带宽的激光系统,研究了如何选择合适的入射激光参数以实现最优化的脉冲压缩。研究了时空域各种物理效应对相位共轭光能量转换效率的影响。研究结果表明:相比现有的技术方案,该方法具有极高的对比度提升能力、极宽的增益带宽和较高的能量转换效率,能够作为一种小口径对比度提升技术广泛应用于飞秒宽带高功率激光系统。 3、研究了高功率超短脉冲激光系统中色差的主要来源以及对系统输出性能的影响。在全面调研现有色差补偿技术方案的基础上,为了弥补现存补偿方案存在的普遍性不足,提出了一种基于像传递结构、具有动态补偿能力的色差预补偿方案。该补偿方案具有成本低、光束质量好、色差补偿动态范围大等优势。基于该方案,为SG-II 5PW激光装置设计并搭建了色差补偿实验平台。实验结果表明,经过色差补偿后系统的聚焦焦斑面积缩小到补偿前的1/20,聚焦功率密度得到了显著提升。经过色差补偿后,在SG-II 5PW装置上开展了质子加速物理实验,获得了超过16MeV的质子加速动能,该实验结果在国内外飞秒级高功率激光装置中位居前列。物理实验结果表明,SG-II 5PW系统已实现稳定运行,具备在拍瓦量级下开展物理实验的能力。本文所提出的色差动态补偿方案有潜力广泛应用于皮秒、飞秒高功率激光系统进行全系统的色差补偿。 4、针对目前OPCPA技术中增益带宽窄化和对比度不足的问题,面向未来建设更大能量、该窄脉宽激光装置的发展方向,探索了基于四光子参量过程的放大技术。相比基于三光子参量过程的OPA技术,基于四光子参量过程的放大技术具有更宽的光谱宽度、更高的时域对比度;其不足之处在于光路调节相对复杂且能量转换效率偏低。针对四光子参量过程的特点,理论分析了该技术在皮秒拍瓦激光系统和飞秒拍瓦激光系统的应用前景。为了能够提高四光子参量过程能量转换效率,将现有OPA技术中抑制能量逆转换和增益窄化的技术推广到四光子参量过程,提出了两种技术手段:四光子准参量放大(FP-QPA)技术和多通四波混频(M-FWM)技术,理论分析这两种技术各自的特点。调研了能够产生高效四光子参量放大的非线性材料。基于该技术,为SG-II 5PW激光装置设计了用于对比度提升的单元模块。 综上所述,本论文系统研究了四光子参量过程在高功率拍瓦激光系统的应用潜力;提出了基于相位共轭光产生的时域对比度提升技术;为SG-II 5PW激光装置设计并搭建了色差补偿系统,使其成为国际上为数不多的具备在拍瓦量级功率下开展物理实验的飞秒激光装置。光参量啁啾脉冲放大;时域对比度;光学相位共轭光;简并四波混频;色差补偿中国科学院上海光学精密机械研究所崔自若光学工程博士
中文题目: 数拍瓦超短脉冲激光系统高对比度放大技术及终端时空性能提升研究
外文题目: Research on high contrast amplification techniques and focuing capability enhancement in Multi-Petawatt Laser System
作者: 崔自若
导师姓名: 朱健强
学位授予机构: 中国科学院上海光学精密机械研究所
答辩时间: 20191129
中文关键词:
光参量啁啾脉冲放大;时域对比度;光学相位共轭光;简并四波混频;色差补偿
英文关键词:
OPCPA;temporal contrast ratio;optical phase conjugate wave;DFWM;chromatic aberration compensation
中文摘要:
英文摘要:
文献类型:学位论文
学位级别: 博士
正文语种: chi
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