Shanghai Institute of Optics and Fine Mechanics,Chinese Academy of Sciences
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Detection of Organic Contamination and Laser Induced Damage in High Power Laser System
No full text光学元件的损伤限制高功率激光驱动器负载能力提升，阻碍了惯性约束核聚变的发展。其中，有机物对光学元件的污染是降低光学元件损伤阈值，诱导光学元件发生损伤的重要原因。研究高功率激光器中有机污染物诱导光学元件损伤的特性，监测系统中污染物在光学元件表面的沉积，有利于有机物污染源的控制，改进装置洁净环境，避免光学元件损伤的发生，增加光学元件的使用寿命，对提高负载能力和运行稳定性具有现实意义。 本论文主要包括激光与聚合物材料相互作用、有机污染物诱致光学元件损伤和分子态污染物在光学元件表面沉积监测技术研究等三部分，具体研究内容如下： 一、激光与聚合物材料相互作用。本论文涉及高功率激光器中常用有机物聚四氟乙烯、真空橡胶、丁腈橡胶、硅橡胶等的基本特性以及激光与聚合物的相互作用。分别对这些常见有机物进行了纳秒（6ns，1064nm）和皮秒（9ps，1053nm）激光损伤实验，获得了不同材料的损伤形貌、损伤阈值、烧蚀深度和烧蚀速率。实验结果表明聚四氟乙烯具有较高的抗损伤能力，对有机物材料的选择和使用范围有指导性作用。 二、有机污染物诱致光学元件的损伤。有机污染物与激光相互作用是系统中重要的污染物来源，其产生的污染物颗粒或者气溶胶会影响装置的洁净度，诱导光学元件损伤。本论文简述了光学元件损伤机理和测试方法以及颗粒污染物、膜状污染物诱导光学元件损伤模型。实验研究了皮秒激光（9ps，1053nm）辐照下，PETG 颗粒物污染物诱致光栅表面损伤特性。实验结果表明，当激光通量为 2.27 J/cm2时，测试的所有尺寸的 PETG 颗粒（>10μm）被激光辐照后，光栅表面均发生不同程度的损伤；颗粒尺寸越大，损伤区域越大。但随着激光通量的降低，诱致光栅损伤的 PETG 颗粒临界尺寸增加。获得了光学表面颗粒污染物允许极限尺寸与负载通量的关系曲线，有助于皮秒拍瓦装置的洁净控制。 三、光学元件表面分子态污染物沉积技术研究。本文简述了石英晶体微量天平和微纳光纤传感器的测试原理，根据视角因子等分模型对石英晶体微量天平和微纳光纤传感器样机进行了对比测试，比较了两种测试方法的特点。此外，还开展了分子态污染物在镀有 sol-gel 膜微纳光纤沉积特性的研究。实验结果表明，沉积在微纳光纤上的污染物质量与腔内污染物的浓度分布为非线性关系，有机污染物浓度越高越容易在微纳光纤表面沉积；镀 sol-gel 膜的微纳光纤比裸光纤更容易吸附污染物。Optical damage is the main issue that limits the high fluence operation of high power laser system. Organic contamination is one of the main pollution sources that reduce the optical damage threshold and induce damage of optics in high power laser facility. In order to reduce the organic contamination, it is practically significant to study the generation and detection of organic pollutants as well as contaminant-induced optical damage. It is crucial to improve the damage resistance of large optics and operating stability for high power laser system. The thesis consists of three parts reviewed as follows: 1. Laser damage of polymer materials. We review the physical features of common used polymer materials such as polytetrafluoroethylene, vacuum rubber, nitrile rubber and silicone rubber as well as the theoretical model of laser interaction with the polymer. For each of these materials, damage experiment by nanosecond pulsed laser(1064nm, 6ns) and picosecond pulsed laser(1053nm, 9ps) are carried out. The damage morphologies, damage thresholds, ablation rates and ablation depths of these materials are investigated. The experimental results show that polytetrafluoroethylene has the highest damage threshold, which gives guidance for the use of organic materials in high power laser system. 2. Laser interaction with polymers is the main pollutant source which can produce lots of particles or aerosols to contaminate the optics. We describe the damage mechanism and models of optics. We experimentally study the polymer particle induced damage of compression grating irradiated by picosecond pulsed laser at 1053nm. When laser fluence is larger than 2.27 J/cm2, the grating surface is always damaged regardless of the size of PET-G particles (larger than 10 μm). The larger the particle size, the greater the damage area. With the laser fluence decreases, the PETG particle size that induced damage of grantings is increased. Finally we get the relation between the laser fluence and the permitted size of particles on optical surface. 3. In order to monitor the volatilization and deposition of organic contaminants on optics surfaces, online detection of airborne molecular contaminant is investigated based on quartz crystal microbalance(QCM) and optical microfiber(OM). The detection characteristics of QCM and OM are experimentally compared in vacuum chamber based on the flow distribution model. The deposition characteristic of airborne molecular contaminant on sol-gel optical microfiber film is also studied. The experimental results show that the more the density of airborne molecular contamination is, the more it deposits on the optics. Sol-gel film is more efficient to absorb pollutants than the bare optical fiber
Contrast Measurement Techniques for Single-shot Ultra-short and Ultral-intense Laser Pulse
No full text超短超强激光装置的聚焦峰值功率已经达到1022 W/cm2，未来将可能发展到1025 W/cm2，为人类进行极端物理实验提供了可能，随着激光强度的提高，激光系统对信噪比的要求也随之增加，需达到1010甚至更高，这对信噪比测量（特别是单次脉冲信噪比测量）技术提出了极大的挑战。 针对目前超短超强激光单次脉冲信噪比测量的发展现状和技术需求，本文提出了谐振腔振荡式单次脉冲信噪比测量技术。该技术以三阶强度互相关理论为基础，利用谐振腔脉冲复制器，实现单个脉冲的复制，得到等比例强度衰减的基频被测脉冲序列和倍频扫描脉冲序列，在脉冲序列中不同脉冲对之间引入不同的时间延迟，从而实现对单次脉冲的扫描测量。同时谐振腔技术，将本不可被光电器件分辨的超短脉冲信号，延展为可分辨的信号，可以采用PMT直接进行测量。 本文对谐振腔振荡式单次脉冲信噪比测量技术进行了详细的理论分析，分析表明该技术可实现1010甚至更高信噪比的动态范围，获得50ps甚至更大的时间窗口，达到百飞秒的高时间分辨率，而且采用全口径光束测量，保证了测量信号的高保真度，谐振腔结构的稳定性也可以保证。另外，分析了色散、次脉冲等因素可能引入的误差，采取了相应措施消除影响。The focused peak power of the developed ultral-shot and ultral-intense laser system has reached 1022W/cm2, and will be 1025W/cm2 in the future, which provide humans a possibility for many extreme physical experiments. As the laser intensity being higher and higher, the contrast of the laser has to be more than 1010 or much higher. This a great challenge for the contrast measurement, especially for the single-shot contrast measurement. For the requirement of the contrast measurement of ultral-shot and ultral-intense laser system, a new cross-correlator structure based on resonator oscillation is proposed for contrast measurement of single-shot femtosecond pulse. Based on the theory of third order cross-correlation, using the replicator of resonator oscillation to replicate the single shot pulse, we get the sequences of fundamental frequency to be test and scanning second harmonic frequency attenuated at equal ratio. We set different time delay between every pair of pulses which include one fundamental frequency pulse and one second harmonic frequency pulse, and then the scanning measurement can be realized. Meanwhile, with the technique of resonator oscillation, the short signal which could not be distinguished will be extended to a sequence of signal which can be distinguished directly by the PMT. Analysis is taken on the resonator for its functions which indicates that high dynamic range >1010 (can be extended higher), time window as 50ps (can be extended larger) and high temporal resolution as about 100fs can be realized in this construction. Moreover, high fidelity is achieved since the pulse is measured with full aperture and the stability also can be guaranteed. Further, errors of the dispersion or minor pulses are also analyzed and avoided by some methods
Research of Lightweight Head-Mounted Display Optical System
No full text头戴显示器是将像源和光学系统组合后安装于眼镜或头盔上的小型目视系统，正逐渐从军事装备向民用设备发展，在工业、医疗以及教育等领域有着广泛的应用前景，有关的报道与成果层出不穷。尤其是近年来随着高分辨率微显示器以及光学和电子行业制造工业的长足发展，头戴显示器越发受到人们关注，各种新型的结构与设备逐渐涌现，其实用化与商业化的脚步也逐步加快。光学系统是头戴显示器的核心，其光学特性的优劣决定了头戴显示器的性能的好坏。为了使用户体验更加舒适，头戴显示器的重量和体积应尽可能地小，并和人眼的各项特性相符合。本文针对这些目标，开展了轻量化头戴显示器光学系统的研究工作。文章归纳总结了光学系统的各项设计需求，提出了一种新型的光学系统初始结构计算方法，设计了一种双透镜头戴显示器光学系统，加工了对应的原理样机。主要研究内容以及成果可以分为两个部分： (1) 提出了一种基于数值优化的光学系统初始结构求解方法。此方法能够根据初级像差平衡的原则自动分配系统的光焦度，可应用于多个场合。采用此方法可以对光学系统像差自动平衡的工作加以指导。此方法得出的结果是实际透镜，不需要在后期优化时添加透镜的厚度，因此不存在改变透镜厚度时，所引起的各项像差平衡的状态被破坏的问题。采用此方法设计出了头戴显示器光学系统初始结构。 (2) 设计了一种用于轻量化虚拟现实头戴显示器的光学系统，对其设计过程进行了详细阐述，加工了对应的原理样机。系统入瞳距为15mm，入瞳大小8mm，视场角为80°，系统总长约为61.5mm，像高30.5mm。边缘视场点列图平均半径约为50?m，中心视场点列图半径在30?m左右，人眼通过此系统观察显示器时，边缘视场的图像不会模糊，同时中心视场的像质也足够好。系统的垂轴色差得到了校正，最大畸变绝对值约为15%，全视场相对照度大于0.75。原理样机长宽高分别为140mm?100mm?80mm，质量约为220g。系统各项参数和像质满足设计需求，达到了轻量化的目标。Head-Mounted Display (HMD) refers to a miniaturized visual optical system with the combination of the image source and the optical system mounted on a helmet or glasses. It is gradually developed from military equipment to civilian equipment, and it is widely used in areas such as industry, medical treatment and education. There are a lot of reports relented to HMD around the world. With the rapid developments of high-resolution micro-display and manufacturing industry, more and more attention has been paid to the head-mounted display. A variety of new structures and equipment have been gradually emerged, and the pace of commercialization and application has been accelerated. The optical system is the core of the HMD, the optical characteristics of which determine the HMD performance quality. In order to improve the user’s experience, the weight and volume of the HMD should be as small as possible, and HMD should conform to the characteristics of the human eye. Aiming at these problems, this paper carried out a study of lightweight HMD optical system. This paper summarized the design requirements of the optical system, proposed a new method of calculating the initial structure of the optical system, designed a kind of double lens HMD optical system, designed and manufactured the corresponding prototype. The main research contents and the results can be divided into two parts. (1) A new method of calculating the initial structure of the optical system is proposed. The optical power of the lens can be automated distributed by the algorithm and it can be applied to more than one occasion. With this method it can be a guide to the work of optical aberration automatic balancing. This method results in several lens which have a thickness, so there is no problem of destroying the aberration balance by increase the thickness of lens. A set of head-mounted display optical initial structure is designed by using this method. (2) A lightweight head-mounted display optical system used in the area of virtual reality is proposed, and its design process is analyzed in detail, and the corresponding prototype is also designed and manufactured. The optical system’s pupil distance is 15mm, the entrance pupil diameter is 8mm, field of view is 80 °, the system total length is about 61.5mm, and the image height is 30.5mm. From these basic parameters it can be seen that the system has the excellent characteristics of lightweight and high immersion. The spot diagram average radius of the edge field is about 50?m, and the center field is about 30?m. When view the image display through this system, the image in the edges will not blur, while the center field image quality is also good enough. Lateral color aberration has been corrected. The absolute value of the maximum distortion is about 15%. Full field illustration contrast is greater than 0.75. The volume of the prototype is 140?100?80mm and the mass is about 220g. It can be seen that all of the optical parameters and the image quality do meet the design requirements, which achieved the goal of lightweight and high immersion
Study on the femtosecond laser guided corona discharge
No full text近年来，飞秒强激光脉冲在空气中形成的激光等离子体通道现象一直是当前科研领域的热点，飞秒光丝在很多领域都具有极其广泛的应用前景。由于飞秒激光具有很高的峰值功率，它在透明介质中传输时会产生光束的自聚焦，当由非线性克尔效应引起的自聚焦与等离子体散焦达到一种动态平衡时，就会形成很长的等离子体通道，称之为“飞秒成丝”现象。飞秒激光在传输介质中成丝之后，高强度激光脉冲可以实现远距离传输而不受瑞利距离限制，光丝通道内部含有大量的带电粒子。在成丝过程中伴随着诸多有趣的非线性效应，如：光强钳制、分子排列、高次谐波、自相位调制、锥角辐射等。正是由于这些有趣的物理现象，飞秒光丝被广泛的应用到超短脉冲压缩、白光激光雷达、遥感探测、THz辐射、人工降雨降雪、激光引雷等。因此，飞秒光丝的基本特性，尤其是光丝通道内等离子体密度空间分布对于飞秒光丝的应用具有重要意义。 本论文简要分析了飞秒激光在空气中成丝的物理过程以及光丝基本特性，并介绍了飞秒光丝在激光引雷中的应用以及飞秒激光等离子体通道的诊断方法。实验上开展了飞秒激光等离子体通道诱导电晕放电特性的初步研究，探究了几何构型对于飞秒光丝诱导电晕放电的影响，有利于理解光丝诱导高压的物理机制。此外，本文利用激光诱导电晕放电对低密度等离子体进行探测，并表征了飞秒光丝内等离子体密度的空间分布，同时对飞秒光丝的长度进行了精确测量，提出了一种探测等离子体密度空间分布的新方法。In recent years，laser filamentation produced by high peak power femtosecond pulse in air is one of the frontiers of high field laser physics. Femtosecond laser filament has shown broad applications in different fields. While propagating in optically transparent medium, high peak-power femtosecond pulses will collapse into a long plasma channel referred to as a“filament”due to the dynamic equilibrium between self-focusing due to Kerr effect and defocusing from self-generated plasma. Due to the filamentation, femtosecond pulse with high intensity can propagate for a long distance beyond the Rayleigh distance and the electron density inside the plasma channel is very high. Accompanying the generation of this long thin and relatively uniform ionization channel, some complex and fascinated nonlinear effects are observed, for example, intensity clamping, molecular alignment, harmonic generation, self-phase modulation, conical emission and so on. These nonlinear effects lead to many applications of filament in different fields, such as ultrashort pulse compression, LIDAR, remote sensing, terahertz emission, atmospheric water condensation and precipitation, lightning control. As a result, fully understanding the physical mechanism of the filamentation and filament characteristics are significantly important in filament applications. In this thesis, the physical mechanism of the filamentation and characteristics of the plasma channel were briefly introduced. The application of laser filament in lighting control and the methods to diagnose the plasma of filament were also presented. Some experiments on the filament guided corona discharge were carried out which illustrated the impact of geometric configuration. The results are useful to understand the physical mechanism of the filament guiding of high voltage. Besides, a new method based on laser induced corona discharges to probe weakly ionized plasma was demonstrated for the first time. The spatial distribution of plasma density inside the filament was characterized and the length of filament was more precisely measured by this technique. The results offer a new way to probe plasma density spatial distribution
Abnormal polarization in nitrogen ion laser fields induced by strong field ionization followed by seed amplification
No full textWe experimentally investigate elliptically polarized nitrogen ion laser fields by changing the polarization directions of the pump and probe pulses, which is interpreted as a result of strong birefringence of the gain medium near the wavelengths of the laser
Bidirectional grating compressors
No full textA bidirectional grating compressor for chirped pulse amplifiers is presented. It compresses a laser beam simultaneously in two opposite directions. The pulse compressor is shown to promote chirped pulse amplifiers' output energy without grating damages. To verify the practicability, an experiment is carried out. In addition, a crosscorrelation instrument is designed and set up to test the time synchronization between these two femtosecond pulses. (C) 2016 Elsevier B.V. All rights reserved
Frontiers in ultrafast optics and ultra-intense laser technology
No full text超快光学与超强激光技术是光学与激光领域的重大前沿,可望推动一批基础学科与高技术应用领域的发展.本文简要介绍了超快光学和超强激光技术领域的发展现状和趋势,包括超快激光和超强激光在粒子加速、阿秒科学、超快非线性光学、微纳制备、宽带光学频率梳等方向的发展与应用前景,以及发展更高性能的超强超短激光的前景和核心关键科学技术问题.国家自然科学基金Ultrafast optics and ultra-intense laser technology, which are important frontiers in the fields of optics and lasers, are expected to promote fundamental science discoveries and high-technology developments. In this paper, the current status and future trends of ultrafast optics and ultra-intense laser technology are introduced briefly, including the applications of ultrafast and ultra-intense lasers in particle acceleration, attosecond science, ultrafast nonlinear optics, micro/nano-fabrication, broadband optical frequency combs, etc. The future prospects and key techniques for generating ultra-intense and ultra-short laser pulses with higher performance are also described
Generation of high beam quality, high-energy and broadband tunable mid-infrared pulse from a KTA optical parametric amplifier
No full textNational Basic Research Program of China [2011CB808101]; National Natural Science Foundation of China [61078037, 11127901, 11134010, 61205208, 11204328]We have demonstrated efficient generation of high beam quality, high-energy and broadband tunable femtosecond mid-infrared pulses using a three-stage collinear optical parametric amplifier (OPA). The white-light continuum (WLC) seeded OPA setup, based on KTA crystal in three stages and pumped by a femtosecond laser pulse at 800 nm, is capable of producing idler wavelength ranging from 2.4 mu m to 4.0 mu m with energy up to 82 mu J at 3.27 mu m, which corresponds to signal energy of 350 mu J at 1060 nm. The output pulse has excellent intensity distribution with measured beam quality factor M-2 similar to 1.1 for signal and M-2 similar to 1.7 for idler. To our knowledge, this is the best beam quality reported in 3-5 Lim femtosecond OPA until now. The achieved mid-infrared pulse also has a good energy stability with a fluctuation of 1.01% rms over half an hour. (C) 2016 Elsevier B.V. All rights reserved
Generation of Nonlinear Vortex Precursors
No full textNational Natural Science Foundation of China (NNSF) [11374318, 11374315]; NSF of Shanghai [15ZR1430600]; 100-Talents Project of the Chinese Academy of SciencesWe numerically study the propagation of a few-cycle pulse carrying orbital angular momentum (OAM) through a dense atomic system. Nonlinear precursors consisting of high-order vortex harmonics are generated in the transmitted field due to carrier effects associated with ultrafast Bloch oscillation. The nonlinear precursors survive to propagation effects and are well separated with the main pulse, which provides a straightforward way to measure precursors. By virtue of carrying high-order OAM, the obtained vortex precursors as information carriers have potential applications in optical information and communication fields where controllable loss, large information-carrying capacity, and high speed communication are required