| 题名 | 大口径超短脉冲时空特性分析测量与信噪比提升 |
| 作者 | 朱坪 |
| 文献子类 | 博士 |
| 导师 | 朱健强 研究员 |
| 关键词 | 超短脉冲，时间信噪比，大口径激光系统，时空耦合，时空测量 ultrashort pulse, temporal contrast, large diameter laser system, spatiotemporal coupling, spatiotemporal measurement |
| 其他题名 | Investigation and Measurement of Spatiotemporal Characteristics and Temporal Contrast Improvement of Large Diameter Ultrashort Pulse Lasers |
| 英文摘要 | 高功率超短脉冲随着激光技术不断的创新与完善日益成为各学科各领域中一种重要的研究手段与工具。高功率超短脉冲激光系统输出功率密度极高，一方面能够在实验室创造极端物态条件，在高能物理领域有很高的应用价值，另一方面可以用于开展快点火激光惯性约束核聚变研究，降低点火对激光总能量的需求。同时，如此广泛而重大的应用需求又牵引着高功率超短脉冲激光技术研究的进一步发展，以追求更高聚焦功率密度、更高脉冲时间信噪比。高功率超短脉冲激光系统口径越来越大、脉冲能量越来越高、压缩脉宽越来越短、峰值功率越来越高。然而脉冲时间信噪比始终是一个瓶颈难题，因为为了防止主脉冲前沿噪声导致靶材的预电离，超短脉冲的聚焦功率密度越高，对脉冲时间信噪比的要求就越高。 大口径高功率激光系统包含众多的大口径光学元器件，不可避免地导致大口径高功率超短脉冲的时空畸变，如中高频波前噪声、空间色散分布等。不同于小口径下的超短脉冲，大口径高功率激光系统中的超短脉冲是一种具有复杂时空耦合特性的超短脉冲。因而，影响大口径高功率超短脉冲激光系统输出时间信噪比的主要原因有三类：1. 注入大口径激光系统的小口径超短脉冲本身时间信噪比低；2. 大口径激光系统非理想的时空特性造成超短脉冲时间信噪比退化；3. 脉冲放大过程中在时域上或者光谱域上引入的各类调制与噪声降低了脉冲时间信噪比。前人对第三类原因已经进行了详细的研究，分析与验证了脉冲放大过程中各个噪声源及其对时间信噪比的影响；对第一类原因提出了一些提升小口径时间信噪比的方法，而对第二类原因还少有研究。 因此，本文主要针对上述第一类原因与第二类原因进行研究分析，主要研究内容和创新点为： 1. 本文从麦克斯韦方程组出发，提出一种利用坐标变换分两步FFT傅里叶积分变换方法计算模拟超短脉冲的聚焦传输过程，定量探究超短脉冲在焦斑区域的时空分布与时间信噪比，验证了大口径高功率激光系统空间特性，如中高频波前噪声、像差、光束口径等，通过聚焦传输过程中对焦点处高功率超短脉冲时间信噪比的影响。根据理论计算与实验结果光学系统三个频段的波前畸变控制提出了要求，明确了大口径光学系统中随机波前畸变与信噪比之间的关系，对于大口径高功率超短脉冲系统的设计指标分配与波前畸变控制都具有指导意义。 2. 本文通过建立大口径高功率超短脉冲系统级联空间滤波器-光栅压缩器-聚焦传输模型，数值模拟了大口径高功率激光系统级联空间滤波器引入的色差通过压缩器与聚焦过程对输出超短脉冲时间特性的影响，分析色差造成的离焦色散、传输时间延迟与输出超短脉冲波形与聚焦时间信噪比的定量关系，对大口径空间滤波器消色差后的残余色差提出要求，为高功率超短脉冲系统中宽带空间滤波器的设计与优化给出理论依据，有助于优化超短脉冲激光系统的总体设计。 3. 本文研究并改进了一种名为STRIPED FISH的空间上波长复用全息干涉超短脉冲时空特性测量技术。在了解输出超短脉冲时间信噪比与大口径高功率激光系统的时空特性密切相关后，如何准确的测量时空特性就显得尤为重要。超短脉冲时空全面特性的有效监测是优化大口径高功率超短脉冲激光系统时空特性、防止大口径高功率激光系统输出超短脉冲时间信噪比退化、提升大口径高功率超短脉冲激光系统总体输出性能的关键。空间上波长复用全息干涉测量技术STRIPED FISH是一种有效的向时域扩展的空域测量技术，能够测量出脉冲的三维时空特性。本文针对STRIPED FISH技术的一些不足，提出延迟扫描与时间串联改进方案，并用该方法测量了一组时空耦合的复杂脉冲，验证了该改进方案的可行性与可靠性。 4. 本文从时空二元性角度出发，利用时间透镜的傅里叶“成像”性质设计了一种时间透镜滤波器，用于在有效监控并优化大口径高功率超短脉冲激光系统时空特性的同时，解决影响信噪比的第一类原因，提高注入大口径激光系统的脉冲的时间信噪比。通过对不同噪声调制的滤波模拟展示了其优良的时间信噪比提升能力。 综上，本文针对大口径高功率超短脉冲激光输出时间信噪比较差的问题，从大口径高功率超短脉冲系统时空特性与时间信噪比关系分析、超短脉冲时空特性全面测量、注入激光系统的小口径脉冲时间信噪比提升三个角度，构成了一套大口径高功率超短脉冲激光系统的时空特性与时间信噪比研究体系，对于指导大口径高功率超短脉冲激光系统优化设计，提升高功率超短脉冲激光系统总体性能有着重要的意义。; Thanks to the continuous innovation and improvement of laser technology，the high power ultrashort pulse laser has increasingly become one of the important research methods and tools in various fields and disciplines. The extremely high output power density of the high power ultrashort pulse laser system can create extreme physical conditions in the laboratory, which has a special application value for the high energy physics research. Also, high power ultrashort pulse laser can help inertial confinement fusion research reducing the necessary total laser energy by fast ignition scheme. At the same time, such a wide range of significant applications requires the further development of the high power ultrashort pulse laser technology, with the pursuit of higher focal power density and higher temporal contrast. This makes the high power ultrashort pulse systems have larger diameters, higher pulse energy, shorter compressed pulse width and higher peak power. However, the temporal contrast of the output ultrashort pulse is always a bottleneck problem for the further development of higher focal power density. In order to prevent the target from being pre-ionized by the noise pulses ahead of the main pulse, the requirement for higher temporal contrast must be met in a higher power ultrashort pulse laser system. A large diameter high power laser system contains a large number of large aperture optical components, which inevitably leads to the spatiotemporal distortions of large diameter high power ultrashort pulses, such as high-frequency wave-front distortions and spatial distribution of dispersion. Unlike the small diameter ultrashort pulse laser, the ultrashort pulse in a large diameter high power laser system has complex spatiotemporal coupling characteristics. Therefore, the main reasons for the output temporal contrast degradation in a large diameter high power ultrashort pulse laser system go into three categories: 1. The initial temporal contrast of small diameter ultrashort pulse injected into the large diameter laser system is low; 2. The non-ideal spatiotemporal characteristics of the large diameter laser system cause the temporal contrast degradation of output ultrashort pulses; 3. The various types of temporal modulations and temporal noises are introduced into the pulse during the amplification process degrading the temporal contrast. The third reason has been studied by many researchers, analyzing and verifying the influence of many noise sources in pulse amplification and their effects on temporal contrast and several ways to enhance the small diameter temporal contrast has been proposed for the first reason. Yet, the second reason has not gotten enough attention and the research on the temporal contrast degradation by non-ideal spatiotemporal characteristics of the large diameter laser system has been rarely reported. Therefore, this dissertation is mainly focused on the first and second reasons of temporal contrast degradation and the research contents are: 1. This dissertation introduces a two-step fast Fourier transformation method using a coordinate transformation method from Maxwell’s equations to numerically calculate the propagation and focusing of ultrashort pulses and investigates the spatiotemporal distribution of the ultrashort pulse in the focal spot region. The effect on focal temporal contrast degradation of the spatial characteristics of a large diameter high power laser system, such as high-frequency wavefront noise, aberrations, and beam diameter, are theoretically analyzed and experimentally verified. According to the theoretical calculation and the experimental results, the wave-front distortion control of the three frequency bands of the optical system is given and the relationship between wavefront distortion and focal temporal contrast degradation in large diameter optical system is clarified, which is important for the optimizing design and wave-front control in a large diameter high power ultrashort pulse system. 2. In this dissertation, the cascade spatial filters, grating compressor and focusing simulation models of a large diameter high power ultrashort pulse system are established. The influence of the chromatic aberration introduced by the cascade broadband spatial filters in the large aperture high power laser system on the temporal characteristics of the output ultrashort pulse after the compression and the focusing is numerically analyzed. The output ultrashort pulse shape distortion and the temporal contrast degradation from the defocus dispersion and the propagation time difference introduced by the chromatic aberration of the large diameter ultrashort pulse laser system are calculated. The requirement of the residual chromatic aberration for chromatic aberration compensation is proposed, which is the theoretical basis for the design and optimization of the broadband spatial filters in the high power ultrashort pulse system and helps improve the overall performs of the ultrashort pulse laser system. 3. A spatially wavelength-multiplexing holographic interferometry, called STRIPED FISH, is studied and improved in this dissertation to measure the spatiotemporal characteristics of the ultrashort pulses. The importance of accurately measuring the spatiotemporal characteristics of a large diameter high power laser system arises particularly after understanding the output ultrashort pulse temporal contrast is closely related to the spatiotemporal characteristics. The effective monitoring of the complex spatiotemporal pulse is key to prevent output temporal contrast degradation in a large diameter laser system and optimize the overall output performance of a large diameter high power ultrashort pulse laser system. This spatially wavelength-multiplexing holographic interferometry is a promising temporal extended spatial technology, which can measure the three-dimensional spatiotemporal characteristics of the ultrashort pulses. This dissertation points out some shortcomings of STRIPED FISH technology and proposes an improvement scheme involving delay scanning and time concatenation. Complex spatiotemporal-coupled pulses are measured by this method, in which the feasibility and reliability of the improved scheme are verified experimentally. 4. Last but not least, from the perspective of spatiotemporal duality, this dissertation takes advantage of Fourier "imaging" of time lens and design a time lens filter to clean out the temporal noise to take care the first reason of poor output temporal contrast, while effectively monitoring and optimization of large diameter high power ultrashort pulse laser system. The initial temporal contrast of the pulses injected into the large diameter laser system should be improved for better output temporal contrast. The ability to improve the initial temporal contrast is demonstrated by simulation of different noise modulations. In summary, for the topic of the output temporal contrast degradation in a large diameter high power ultrashort pulse laser system, this dissertation studies the spatiotemporal characteristics and temporal contrast of a large diameter high power ultrashort pulse laser system, comprising the relationship analysis between spatiotemporal characteristics and temporal contrast in a large diameter high power ultrashort pulse laser system, a complete spatiotemporal measurement technology of ultrashort pulses and an initial temporal contrast improvement method. It gives a guidance for designing and optimizing large diameter high power ultrashort pulse laser systems based on output temporal contrast, and it is of great significance to improve the overall performance of high power ultrashort pulse laser systems. |
| 学科主题 | 光学工程 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/30945]  |
| 专题 | 中国科学院上海光学精密机械研究所 |
| 作者单位 | 中国科学院上海光学精密机械研究所 |
| 推荐引用方式 GB/T 7714 | 朱坪. 大口径超短脉冲时空特性分析测量与信噪比提升[D]. |
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