| 题名 | 高功率激光近场光束质量控制关键技术研究 |
| 作者 | 黄大杰 |
| 学位类别 | 博士 |
| 答辩日期 | 2013 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 林尊琪 |
| 关键词 | 光束质量控制 二元光学面板 可编程 数字微镜器件 液晶光阀 |
| 其他题名 | Key Techniques of Near-Field Beam Quality Control in High Power Laser |
| 中文摘要 | 在ICF高功率激光驱动器装置中，近场光束质量控制是提升系统运行通量水平、降低系统造价的有效手段之一。为提高装置的运行效率，控制技术逐渐由被动型向主动型发展，使光学系统的适应性、灵活性更加满足系统的需求。LLNL有五大技术获得2012R&D奖，其中之一的光寻址液晶光阀技术即为主动型光束质量控制技术的典型代表。该技术已被NIF装置的48路成功使用，用于对终端光学组件上的损伤点实施预屏蔽。这一措施大大延长了大口径光学元件的使用寿命，为NIF装置每年节省经费500万美金，对降低系统运行造价有重要意义。 本论文以高功率激光近场光束质量控制技术为主线，主要内容为： (1) 从理论上系统地分析了二元振幅技术的特点：定量分析了单元像素大小、滤波小孔尺寸对整形精度的影响，并分析了滤波小孔对不同频率成分强度分布的作用。计算结果表明：1，单元像素大小可以根据整形尺度、整形精度的要求来设计，当滤波小孔取10倍衍射极限，整形尺度 ，若要求整形误差小于10%，单元像素小于30微米即可，整形尺度增大，设计的像素大小可以同比例的增大。2，当滤波小孔大小取5倍衍射极限时，整形误差比较小，而且此时像素大小变化对整形误差的影响最小。3，为使某一频率的强度分布通过该滤波系统后仍可被分辨，滤波小孔可取的最小口径与该频率存在线性关系。 (2) 基于数字微镜器件的光学结构，利用各像素微镜可灵活实现二元振幅0和1的特点，实现了一种主动型的二元振幅技术。利用该技术对脉宽5ns、波长1053nm的脉冲光进行了空间强度整形，使光束的填充因子由33%提高为65%，光场调制度由52%降为28%，而且整形后的光斑大小与预期相符。 (3) 从原理上分析了液晶光阀技术的特点，重点从寻址方式、调制模式两个层面对液晶光阀进行了分析：1，从寻址方式角度对比了电寻址、光寻址液晶光阀的特点，重点分析了“黑栅效应”对后续光场的影响，结果表明：透射式电寻址器件中的“黑栅效应”不仅影响能量利用率，而且影响透射光的光束质量，“黑栅”结构使光束发生衍射，不透明电极的宽度决定这种衍射的强度；透射式光寻址器件，由于整个通光区域全部透光，能量利用率、透射均匀性要比电寻址器件优越。2，从调制模式角度对比了位相型、振幅型液晶光阀的特点，利用Jones矩阵方法对两种光阀的光调制特点进行理论分析，结果表明：对于向列型液晶层，可用于实现纯位相调制，不伴随附加的强度调制；对于扭曲向列液晶层，用于振幅调制时，会伴随有附加位相畸变，而且，附加位相值与振幅透过率存在对应关系。 (4) 分析了振幅型液晶光阀用于空间强度整形时，引入的附加位相对后续光场的影响。对于平顶超高斯强度分布的光斑，考虑附加位相后，自由传播1m时，中心区域调制度由5%提高为10%；自由传播2m时，中心区域调制度达到20%。 (5) 在国内首次研制成功国产化的光寻址液晶光阀的实验样机，本文中主要包括以下几方面的内容： I. 从原理上分析了器件结构、材料选择原则、器件光调制特性等，介绍了其中的写入光系统、透过率测试系统等。 II. 建立了此液晶光阀的等效电路模型，分别从理论、实验上分析了驱动电压对器件光调制特性的影响，结果表明：当驱动电压为27V@300Hz时，不仅光阀的透过率可以达到最大值，而且此时对光阀透过率的控制精度最高。 III. 对实验样机的关键参数进行测试，测试结果为：透过率为80%，开关比为40:1，直径7mm区域波前畸变小于0.3wavelength，这表明该实验样机达到的技术指标与目前国外同类型器件的指标接近（美国国家点火装置NIF报道的空间光调制器的技术指标为：透过率为85%~90%，开关比为50:1，波前畸变小于0.5wavelength）。 IV. 实验演示了该器件用于实现可调软边光阑、对终端光学组件上“损伤点”预屏蔽、对1053nm相干光空间强度整形等功能。 V. 对器件的动态波前畸变进行了测试与分析，测试结果表明：动态畸变小于0.3wavelength。 |
| 英文摘要 | Near-field beam quality control is one of the effective means to increase laser fluence and reduce the cost in high power laser systems. In order to improve the operating efficiency of the system, control technology has developed from the passive style to the active style. This development makes the optical system more flexible. The optically addressed liquid crystal light valve, which is a typical representative of the active technology in beam quality control, was one of the five technologies in LLNL which won 2012R&D Awards. This device has been used to shadow some small, isolated flaws on down stream optical components. This technology extends the service life of large aperture optical elements greatly and saves annually $500 million for NIF. The work mainly focuses on near-field beam quality control. The research results are summarized as follows: (1) The characteristics of binary amplitude technique are studied systematically. The impact of sizes of unit pixel and spatial filter pinhole on shaping accuracy is analyzed. How the filter aperture affects the intensity distribution of different frequency is explained. We can get the following results:1,pixel size could be chosen according to shaping scale and requirement of shaping accuracy, when the size of filter aperture is 10 times diffraction limit and shaping scale is , pixel size must be less than 30 microns to ensure the shaping error is less than 10%, pixel size could be proportional to shaping scale;2,When the filter size is five times diffraction limit, not only the shaping error is small, but the pixel size matters very little to the shaping error;3,The desirable filter size has a linear relationship with the frequency to make sure intensity distribution of this frequency pass. (2) Based on the optical structure of Digital Micromirror Device, an active binary amplitude technology is realized by real-time controlling each pixel micromirror. On this basis, the 1053nm laser pulse, which is 5ns wide, is spatially shaped. After shaping, Filling Factor increased from 33% to 65% and Field Modulation decreased from 52% to 28% which means near-field beam quality has been improved. Moreover, the size of the shaped beam was the same as we expected. (3) The characteristics of the liquid crystal light valve technology are studied from the principle. The addressing mode and the modulation mode are analyzed respectively:1,For different addressing modes, ‘black matrix effect’ is analyzed. The results show that ‘black matrix effect’ in transmission-type electronic addressed device affects not only energy utilization, but beam quality of transmitted light. ‘Black matrix’ diffracts the incident light, the diffraction intensity depends on the electrode width. This effect is absent in transmission-type optically addressed device. Both the energy utilization and light-admitting quality are better than those of electronic addressed device. 2,For different modulation modes: optical modulation of amplitude-type and phase-type liquid crystal light valve are analyzed using the Jones matrix method. We can get the following results: the nematic mode can be used for phase modulation without additional intensity distortion; when the twisted nematic mode is used for intensity modulation, additional phase distortion must be considered. This phase distoriton has a corresponding relationship with the intensity transmittance. (4) The phase distortion is analyzed when the amplitude-type liquid crystal light valve is used for laser beam shaping. When the spot with flat-top intensity distribution propagates 1m in free space, intensity modulation increases from 5% to 10%. If the distance of propagation is 2m, this modulation will increase to 20%. (5) The prototype of an optically addressed liquid crystal light valve has been developed for the first time in China. This chapter include the following contents: I. The device structure, material selection principles and characteristics of the device are analyzed from the principle. Both the addressing system and the transmittance-testing system are introduced. II. The equivalent circuit model of the device is built. The impact of the drive voltage on the device’s modulation is studied both in theory and experiment. The results show that when the operating voltage is 27V@300Hz, not only the transmittance is high, but it has very high precision of control. III. Key parameters of the device are tested. The results are shown as follows: Transmission is 80%, contrast is 40:1, wavefront distortion is less than 0.3wavelength. This means its performance is approaching international standards(Technical parameters of the device used in NIF are shown as follows: Transmission is 85%~90%, contrast is 50:1, wavefront distortion is less than 0.5wavelength). IV. The device has been used to implement the function of the soft edge aperture, shadowing the isolated flaws and spatially shaping 1053nm coherent light. V. The dynamic wavefront distortion of this light valve are tested and analyzed. The results show that the distortion is less than 0.3wavelength. |
| 语种 | 中文 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15749]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 黄大杰. 高功率激光近场光束质量控制关键技术研究[D]. 中国科学院上海光学精密机械研究所. 2013. |
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