【作者】 郑欢；

【导师】 明海； 许立新；

【作者基本信息】 中国科学技术大学 ， 光学， 2009， 博士

【摘要】 本论文的研究工作是在863—804重大专项《前端系统总体技术研究》的子课题《前端系统计算机模拟设计技术研究》和国家自然科学基金《高功率啁啾脉冲在单模光纤中的非线性传输特性研究》等项目的资助下完成的。基于啁啾脉冲堆积技术的新型高功率激光器前端系统是我国为研究惯性约束核聚变而开发的新一代高功率激光器系统的重要组成部分,其主要功能是向后继系统提供数个已整形、具有一定能量(mJ量级)、一定带宽、高信噪比和高光束质量的激光脉冲。本文系统地研究了啁啾脉冲堆积技术中可能存在的物理和工程问题,如啁啾脉冲堆积的时间起伏、光谱特性和稳定性等,为前端系统的研制提供理论依据;完善了前端系统的计算机模拟软件平台,实现了与后端系统模拟软件平台的对接;此外,从理论上提出了一种新型方波发生器的实现方案,本方案对啁啾脉冲堆积技术具有重要的参考价值。本论文主要的研究工作和成果如下:1.通过谱分析方法对线性啁啾高斯脉冲堆积技术中存在的强度起伏问题进行了详细的研究。线性啁啾高斯脉冲堆积技术中堆积脉冲不可避免存在有强度起伏。我们对于线性啁啾高斯脉冲堆积技术中堆积脉冲的强度起伏频率进行了分析。对于多个等幅度线性啁啾高斯脉冲等延迟叠加的情况,堆积脉冲的强度起伏频率呈以一个基频和多个高阶频率组成的多个边带的结构。由于幅度的交叠部分过少,堆积脉冲的强度谱中极高阶频率处的起伏很小。堆积脉冲的时间起伏频率随原脉冲啁啾系数和时间延迟的增加而增大,随原脉冲的脉宽的增加而减小。2.系统分析了线性啁啾高斯脉冲堆积过程中堆积脉冲的光谱特性,指出堆积脉冲的光谱中存在有调制结构,并详细讨论了调制结构的频率特性。堆积脉冲光谱调制频率的分析结论在实验中得到了验证。根据我们的光谱分析,堆积脉冲的时间起伏无法通过幅度滤波器进行滤除。3.分析了线性啁啾高斯脉冲堆积中堆积脉冲的高频起伏对激光与等离子体相互作用的影响,并首次对线性啁啾高斯脉冲堆积和无啁啾高斯脉冲堆积的受环境扰动的稳定性从单路时延扰动和整体时延扰动两方面进行了深入的讨论,指出当堆积子脉冲的时间延迟有扰动时,啁啾脉冲堆积经过接收器接收后相对无啁啾脉冲堆积更加稳定。4.方波脉冲发生器是解决啁啾脉冲堆积中相干噪声问题的有效手段。提出了一种基于非线性光纤放大环形镜的8字腔方波发生器,并对输出脉冲的功率和脉宽进行详细分析。8字腔方波发生器的输出脉冲的功率和脉宽具有一定的调谐特性,给出了该发生器中各器件的具体设计参数。方波脉冲发生器的研制对于解决啁啾脉冲堆积的不稳定性具有重要的意义。5.针对前端系统以及啁啾脉冲堆积过程中的物理和工程问题研究,我们编写了高功率激光器前端系统模拟软件,该模拟软件直观地给出给个位置的实时信噪比、波形、光谱等信息,不仅为前端系统各个部件的设计提供理论指导,而且对前端系统运行中的各部件给出安全工作预警,保证高功率激光器前端系统的正常运转。此外,本软件可以实现与后端系统模拟平台的对接,对于研究惯性约束核聚变具有重要的意义。本论文的创新点主要包括:1.对啁啾脉冲堆积技术中堆积脉冲的时间起伏和光谱特性进行深入的理论分析,并在谱分析的基础上,分析了堆积脉冲经过幅度滤波后的起伏变化,对指导前端系统的研制起到重要作用。2.详细分析了激光等离子体相互作用过程中啁啾脉冲堆积技术的稳定性,通过分析指出啁啾脉冲堆积相对无啁啾脉冲堆积稳定。对开展基于啁啾脉冲堆积技术的前端系统的实验提供理论依据。3.理论上提出一种基于非线性光纤放大环形镜的新型8字腔方波发生器,对于啁啾脉冲堆积方案的实现具有重要的参考价值。4.开发出新型高功率激光器前端系统模拟软件,并实现了与后端系统模拟软件的对接,对高功率激光器系统的整体运作提供软件平台的保障。更多还原

【Abstract】 The research work within this dissertation is supported by 863-804 project "The Simulation and Design of High Power Laser Front-end System" and the National Natural Science Foundation of China "Nonlinear Transmission Characteristics of High Power Chirped Stacked Pulses in Single-mode Fibers".The novel high power laser front-end system,based on the technique of Chirped Pulse Stacking(CPS),becomes an important part of the new high power laser system,which aims for the Inertial Confinement Fusion(ICF).It provides for the subsequent power amplifier system several pulses with the ability of arbitrary shaping, high energy(mili-joule level),bandwidth,high Signal-to-Noise Ratio(SNR) and high beam quality.In this dissertation,we systematically study the physical and engineering problems in the technique of CPS,including the temporal fluctuation, spectral characteristics and the stability of the stacked pulse.We also develop a set of simulation software for the front-end system(SGFSSim),which can communicate with the software for power amplifier system(SG99).Moreover,we propose a new scheme for generating square pulses theoretically,which can eliminate the coherent noise of CPS.The main research works and results are as followings:1.We present a systematic analysis on intensity fluctuation for the technique of Linearly Chirped Gaussian Pulse Stacking(LCGPS) based on spectral analysis.The results show that there are several separated frequency sidebands in the intensity spectrum of the stacked pulse,which is responsible for the intensity fluctuation.The fluctuation frequencies are discussed in detail in multiple LCGPS.In the case of multiple LCGPS with same amplitude and same timing delay,the fluctuations have a fundamental frequency sidebands centered at the frequency which is the same as that of two LCGPS,and high order frequency sidebands which are multiples of the fundamental frequency.We also find that the contributions to intensity fluctuation from high frequency sidebands would be small because of the amplitude overlapping.Generally the fluctuation frequencies increase with the increase of chirp factor and timing delay,and with the decrease of pulse width of the initial Gaussian pulse.2.Spectral characteristics of the stacked pulse in the technique of LCGPS are analyzed systematically.The results show that there are modulation structures in the spectrum of the stacked pulse.Modulation frequencies are discussed in detail.The analysis matches with the experiment results. Our spectral analysis reveals that the temporal intensity fluctuation of the stacked pulse in LCGPS cannot be reduced by applying an optical amplitude filter,but can be reduced by applying an intensity filter.3.We study the LPR in the ICF system and analyze the stability of both LCGPS and NCGPS in the LPR.The LPR can be treated as the process that the stacked pulse is first intensity filtered and then induces the plasma, due to the thermalization time of the plasma.We also discuss and compare the stability of both LCGPS and NCGPS with the thermalization time of the plasma,timing delay and intensity attenuation of the stacked pulse in the LPR.Our results show that the LCGPS is more stable than NCGPS in the LPR with the environment change in the ICF system.We believe this would be helpful for experiments of the ICF front-end systems.4.We propose a novel method for square pulse generation.We prove theoretically the power of the generated square pulse is steady and flat. We point out there would be a stable region around the certain steady power.Shapes and temporal width of the square pulse are analyzed. Multi-square pulse output is mentioned.We also study the power tunability of the square pulse and give a restriction for optical components in order to generate the square pulse.5.A new software(SGFSSim) is developed for studying the physical and engineering problems in the CPS.The design,physical models and functions of SGFSSim are fully introduced.The most important function of the software is to communicate with the software of power amplifier system(SG99).It also has the ability of setting damage threshold for every device in the high power laser front-end system,and showing SNR, waveform,and spectrum information of the lasing pulses.Highlights of the dissertation are as following: 1.The intensity fluctuation and spectral characteristics of the stacked pulse in LCGPS are analyzed systematically.The intensity fluctuation of the stacked pulse after intensity filtering is also discussed based on the spectral analysis.2.Stability of the LCGPS in LPR is studied in detail.Our results show that the LCGPS is more stable than NCGPS in the LPR process.3.A new square pulse generator is proposed based on the Nonlinear Amplifying Loop Mirror(NALM).The figure-eight square pulse generator can eliminate the coherent noise in the technique of CPS.4.SGFSSim is developed for communicating with SG99,which provides a simulation platform for the high power laser system.更多还原