皮秒脉冲激光时间特性测量技术研究
文献类型：学位论文
| 作者 | 孔雪 |
| 文献子类 | 硕士 |
| 导师 | 朱宝强 |
| 关键词 | 高功率激光，超短脉冲，脉宽测量，单次自相关 High Power Laser, Ultrashort Pulse, Pulsewidth Measurement, Single-shot Autocorrelation |
| 其他题名 | Research on Measurement Technique of Temporal Characterization on Picosecond Laser Pulses |
| 英文摘要 | 随着超短超强激光技术的飞跃式发展，激光聚焦功率密度已达到1022W/cm2量级，脉冲宽度达到皮秒(ps)甚至飞秒(fs)量级，在惯性约束聚变(ICF)、快点火、高能密度物理(HEDP)等研究领域具有重要意义。 世界各地相继研究和建造拍瓦级激光驱动器，用于开展快点火、二次辐射光源、强场物理等方面的高能密度物理实验研究。由于拍瓦级激光的输出能量通常在100J左右，甚至1000J以上，放大链路中的热效应因素制约了输出脉冲的重复频率。例如，神光II(SG-II)第九路拍瓦激光系统的输出脉冲频率为1.9×10-4Hz，即1发次/1.5小时。它也被称为单次脉冲。准确测量拍瓦级激光输出的皮秒级单次脉冲的时间特性，是一项国际性技术难题。皮秒拍瓦激光的时间特性测量主要包括皮秒脉冲的宽度测量和信噪比测量。拍瓦激光的脉冲宽度是ICF物理实验的关键参数之一，能直接反映拍瓦激光装置的运行状态和输出性能，并且为光路中关键元件的损伤判断提供重要评估依据。 目前市场上已有的商品化超短脉冲测量设备，无法满足拍瓦级激光系统在线实时测量的复杂应用环境。千焦耳级皮秒拍瓦激光系统与商品化的小型皮秒激光器之间存在诸多不同之处，例如光束口径更大(Φ310mm)，输出能量更高(1000J)，重复频率非常低(1.9×10-4Hz)，光谱宽度(~6nm)大于商业皮秒激光器(0.1nm)。而且，拍瓦激光系统结构复杂，总体光路长，元件数量众多，对皮秒脉冲的实时精密测量提出了更高的挑战。 拍瓦激光系统中激光束状态稳定性，测试系统本身，以及数据处理算法等都会影响脉宽测量的准确性和稳定性。本学位论文致力于皮秒脉冲激光时间特性测量技术研究，主要从激光束状态稳定性角度展开分析，提高神光II拍瓦激光皮秒脉冲宽度测量的可靠性，保证拍瓦激光健康稳定运行，为物理打靶提供可靠的实验参数。基于SG-II拍瓦皮秒脉冲宽度测量平台，掌握了单次自相关法测量原理和过程。本论文的主要研究成果如下： （1）分析了自相关测量法的基本原理及利用双折射晶体非线性效应产生自相关信号的物理过程。结合神光II拍瓦激光装置的测量参数和精度要求，阐述了皮秒脉宽单次自相关测量仪的技术原理和测量过程，并简要介绍了皮秒脉冲信噪比测量方案。 （2）研究了光束指向性对于皮秒激光脉冲宽度测量的影响。主激光指向偏差经缩束单元放大8倍后进入测量系统，会降低时间分辨率和自相关信号强度，产生脉宽测量误差。使用过程中应力释放和蠕变导致的角度偏差可以通过准直技术消除。设计简化实验，验证了光束指向性是影响皮秒脉宽测量准确性和稳定性的一个重要误差来源。 （3）研究了近场正余弦周期调制对于皮秒激光脉冲宽度测量的误差影响。理论分析了近场正余弦周期调制下的自相关信号，在不同调制深度和调制周期下的脉冲宽度测量误差。模拟结果表明，调制深度增大、误差变大，调制周期增加、误差降低。相同调制条件下，镜像结构的自相关光路比普通结构的测量精度更高。另外，进行了近场周期调制实验，将不同周期的调制板放至近场，在与相关工作面正交、倾斜相交和平行三种调制方向下分别测量脉冲宽度。实验结果表明，调制周期增加，误差降低；正交方向调制误差最大，平行方向调制误差最小。 （4）改善了拍瓦激光装置中大尺寸(1025*350mm)光栅的条状保护光阑引入的近场调制问题。实验验证了条状保护光阑是影响近场分布和产生测量误差的重要因素，旋转条状光阑可弥补时间轴缺失，降低近场缺陷导致的测量误差。实验结果表明，光路改进后的脉宽测量结果可靠性明显提高。 （5）设计了一种大量程、高分辨率、低误差的皮秒激光脉冲宽度测量装置，脉宽测量范围1-50ps，时间分辨率0.05ps。采用晶体拼接技术，结合误差范围、现有工艺对引入的误差来源进行定量分析和误差分配，在透镜焦平面设置狭缝光阑，消除拼缝衍射条纹产生的测量误差。在单块20mm口径的BBO晶体下，进行了初步的离线时间分辨率标定实验和在线可调节脉宽范围验证实验。; With the rapid development of ultrashot and ultrastrong laser technology, its focal power density has reached 1022W/cm2 level, and pulse width has reached picosecond (ps) or femtosecond (fs), which is greatly significant in the research field of inertial confinement fusion (ICF), fast ignition, high energy density physics (HEDP). A series of research and construction of petawatt (PW) laser are carried out in various parts of the world for the development of fast ignition, secondary radiation source and strong field physics in HEDP experiments. Since the output energy of the PW laser is usually around 100J, or even more than 1000J, the heat effect factor in the amplifying link restricts the repeat frequency of the output pulses. For example, the output pulse frequency of the 9th road in Shenguang II (SG-II) laser system is 1.9×10-4Hz, namely 1 time /1.5 hours. It's also called a single-shot pulse. It is an international technical problem to accurately measure the temporal characteristic of the single-shot ps pulse produced by the PW laser. The temporal characteristic measurement of picosecond PW pulse mainly include its pulse width and SNR. PW laser pulse width is one of the key parameters of ICF physical experiments, can directly reflect the running state and output performance of the laser device, and provide important basis to judge key optical elements’ damage. At present, the existing commercialized ultra-short pulse measuring equipment can not meet the complex application environment of the on-line real-time measurement of the PW laser system. There are many differences between a kilojoule picosecond PW laser system and a commercialized small ps laser, such as larger beam diameter (Φ310mm), higher energy output (1000J), very low recurrence rate (1.9×10-4Hz), and spectral width (~6nm) greater than the commercialized ps laser (0.1nm). Moreover, the structure of the laser system is complex, the overall optical path is long, the components are numerous, and the real-time precision measurement of ps pulse poses a higher challenge. In a PW laser system, the stability of laser beams, the test system itself, and the data processing algorithm can affect the accuracy and stability of the pulsewidth measurement. This thesis is devoted to research on the temporal characteristic measurement technology of ps pulse laser, which is mainly analyzed from the laser beam stability, to improve the reliability of ps pulsewidth measurement on SG-II-PW, ensuring its healthy and stable operation and providing reliable experimental parameters for physical target. Through SG-II-PW ps pulsewidth measurement system, the measuring principle and process of the single-shot autocorrelation are basically mastered. The main research results of this paper are as follows: (1) The measuring principle of autocorrelation and the process generated by the nonlinear effect of the birefringent crystal are analyzed. Combining measuring parameters and accuracy requirements of SG-II PW laser facility, the principle and process of single-short autocorrelation measurement instrument are stated. This paper also briefly introduces the measurement scheme of ps pulse SNR. (2) The influence of beam pointing on the measurement of ps laser pulse width is studied. The down-collimator magnifies the angular deviation in the main laser 8 times to the measuring system, which reduces the time resolution and autocorrelation signal intensity, leading to a pulsewidth measurement error. The Angle deviation caused by stress release and creep can be eliminated by collimation. A simplified experiment is designed to verify that beam pointing is an important factor that affects the accuracy and stability of the ps pulsewidth measurement. (3) The error of ps laser pulsewidth measurement by sine and cosine modulation in the near field is studied. The autocorrelation signal is theoretically analyzed with the sine and cosine modulation in the near field. The pulsewidth measurement error under different modulation depth and modulation period is also discussed. The simulation results show that increased modulation depth leads to increased error, and increased modulation period leads to reduced error. Under the same modulation condition, the autocorrelation optical path with mirror structure is more accurate than that of ordinary structure. In addition, different periodic boards are put to the near field in modulation experiments, measuring the pulse width in three modulation direction of orthogonal, oblique and parallel with related working surface. The results reflect that increased modulation periods cause decreased error. The error of orthogonal direction modulation is the largest ,while the error of parallel direction modulation is the minimum. (4) The near field modulation introduced by the strip protection diaphragm of the large size (1025*350mm) grating in PW laser facility was improved. Experiments indicate that the strip diaphragm is an important factor affecting the near-field distribution and the error. Rotating the strip diaphragm can make up the loss of the time axis, and reduce the error caused by the near-field defect. The results prove that the reliability of the pulsewidth measurement in the modified optical path were significantly improved. (5) A measurement device is designed to measure the pulse width of ps laser with a wide range, high resolution and low error, which designs that the pulsewidth measurement range is 1-50ps, and the time resolution is 0.05ps. It uses crystal splicing technology. Combined with error range and current technology, the introduced error sources are quantitatively analyzed and distributed. A diaphragm is set up in the focal plane to eliminate the error by the seam diffraction stripes. With the single-block 20mm BBO crystal, the initial off-line time resolution calibration experiment and online adjustable pulsewidth range verification experiment are conducted. |
| 学科主题 | 光学工程 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/31065]  |
| 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 | 孔雪. 皮秒脉冲激光时间特性测量技术研究[D]. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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