涡旋光钛宝石激光器和径向偏振光形成级联纵向电场特性研究
文献类型：学位论文
| 作者 | 谭诗文 |
| 文献子类 | 硕士 |
| 导师 | 李建郎 |
| 关键词 | 圆柱矢量光束 cylindrical vector beam 涡旋光束 vortex beam 级联纵向电场 cascaded longitudinal electric field 电子加速 electron acceleration 钛宝石激光器 Ti:Al2O3 laser 点缺陷空间滤波 spot-defect spatial filter |
| 其他题名 | Vortex Ti:Al2O3 Laser and Cascaded Longitudinal Electric Fields Formed by Radially Polarized Beam |
| 英文摘要 | 偏振和相位是光的两个重要特征。本文从这两个特征出发，分别研究了圆柱矢量光束和涡旋光束。 圆柱矢量光束是一种空间非均匀偏振光，其光波横截面内的各个位置的电场（偏振）方向呈现空间圆对称分布。按偏振方向平行或者垂直于光束横截面的半径方向， 柱矢量光束可分为径向偏振光束或者切向偏振光束。该光束的中心为奇点，因而光强呈环状分布。在利用高数值孔径透镜聚焦时，径向偏振光束可以形成比均匀偏振（线偏振、圆偏振等）光更小的聚焦光斑，并在焦点处产生一个很强的纵向电场。这样的性质使得其在超分辨显微、高精度金属材料加工、粒子捕获和电子加速等领域具有宝贵的应用价值。 拉盖尔-高斯光就是一种典型的涡旋光束。它的电场振幅表达式中含有一个相位因子项exp(-il?)（l为角向阶数），这说明涡旋光沿传播方向形成螺旋状的相位波前，因而具有轨道角动量，另外，该光束横截面的光强也呈环形分布，中心位置为相位奇点。这样的特性使得涡旋光在量子信息处理、超分辨显微、光学捕获和操纵、激光切割与加工、引力波探测等领域具有重要的应用价值。 本硕士论文的第一个目的是开展径向偏振光束的特性研究。当高速运动的电子以一定夹角入射并进入到激光光束所形成的光场时，如果电子在运动过程中和光场始终保持固定的相位关系，即满足切伦科夫条件，那么基于反切伦科夫效应，电子将被光场加速到更高能量。值得关注的是，利用高数值孔径透镜（或锥形镜），径向偏振光在聚焦（或会聚）区域可以形成很强的纵向电场。它所形成纵向电场将为电子提供更强的电场力，从而使电子获得几百MeV甚至GeV数量级的能量。为了不再受限于电子与光场的单次作用长度，我们提出一种实现级联纵向电场的方法，利用锥面反射镜+筒形反射镜的复合结构，使得加速电场能够周期性排布，从而实现对电子的N次级联加速，这是本论文的主要任务之一。 本硕士论文的第二个目的是搭建涡旋光钛宝石(Ti:Al2O3)激光器。钛宝石激光器是一种可调谐激光器，其受激辐射发出的光为波长覆盖650至1100 nm的红光和近红外光。由于其增益带宽宽、可调谐范围长，因而具有产生飞秒或阿秒脉冲的能力，峰值功率可以达到太瓦甚至拍瓦量级，钛宝石激光系统在等离子体物理、离子加速等领域的应用非常有价值，它几乎已经成为实验室必备的激光系统。基于这种背景，如果能将让钛宝石激光器以涡旋光模式运转，意味着对原本激光器的重要升级，并且对于诸如强场激光物理等领域的各种应用将意义重大。而关于涡旋光束的产生方法是很丰富的，近来，在涡旋光Nd: YAG激光器和He-Ne激光器中，采用点缺陷镜作为腔内的空间滤波器进行模式选择得到了涡旋光束，该法是通过增大基模衍射损耗来抑制基模振荡，进而使得高阶模振荡输出的。鉴于点缺陷空间滤波的方法成本低廉，元件制作简单，本论文的另外一个主要任务就是将点缺陷空间滤波法与一般的商用钛宝石激光器结合起来，搭建一台涡旋光钛宝石激光器。 本论文的主要内容如下： 第一章，简要介绍了圆柱矢量光束和涡旋光束，分别分析了它们的特性。接着基于这些特性介绍了它们的应用价值。再者，我们总结了产生圆柱矢量光束和涡旋光束的方法。最后归纳了本论文的主要研究工作。 第二章，径向偏振光形成级联纵向电场的特性研究。首先依据矢量衍射理论，计算得到了径向偏振光经高数值孔径消球差透镜聚焦后形成的电场分布，然后类似地，采用基尔霍夫衍射理论，又结合径向偏振光偏振特性，得到其经锥面镜反射会聚后横截面呈零阶贝塞尔函数分布的纵向电场。我们分析了会聚区域光场的相干长度和横向宽度与入射光斑尺寸、锥面镜的锥顶角以及锥面镜出射端半径的关系。在此基础上，提出采用锥面镜+筒形反射镜的复合结构，通过设计合适的锥面镜和筒形镜参数，可以实现这种纵向电场的级联且周期性的分布。我们还分析了筒形反射镜的参数对所形成的级联纵向电场的横向宽度、周期、占空比等的影响。初步的分析结果表明，当所采用的锥面镜的顶角为60°，出射端半径和筒形反射镜的内宽半径均为999.682λ，可以实现周期为1154λ、占空比为1的纵向电场的级联，当用于电子加速时，加速区长度可达到米级。径向偏振光在经过锥面镜会聚时因相干可以形成很强的纵向电场，而这种级联纵向电场的设计将使得电子的加速区长度得到显著增加，为将电子加速到更高能量提供了可能。 第三章，涡旋光钛宝石(Ti:Al2O3)激光器实验研究。通过将腔内点缺陷滤波法与钛宝石激光器结合起来，成功实现了800 nm的一阶拉盖尔-高斯涡旋光输出。在研究过程中，我们调研了钛宝石激光器的特点，通过比较拉盖尔-高斯光束与基模高斯光束光斑的光强分布特点，理解了点缺陷空间滤波的物理原理；我们制作了不同直径的点缺陷，分别插入腔内进行了比较实验，证明了点缺陷法用于钛宝石激光器产生涡旋光输出的可行性，分析了点缺陷的尺寸对于激光输出功率和光斑质量的影响。在实验中，当点缺陷直径为140 μm时，我们获得了功率为135 mW的800 nm的一阶拉盖尔-高斯涡旋光输出，斜率效率为17.7%。 第四章，对本论文的研究结果进行了总结，指出研究过程中的不足之处，并提出了可能的解决方案。; Polarization is an important property of a laser beam. Different to conventional homogeneous polarization, the cylindrical vector beam (CVB) is characterized by the inhomogeneous polarization with cylindrical symmetry in both the amplitude and direction of electrical field vector. As the results, the transverse intensity pattern of CVB is doughnut-shaped with centeral singularity. The CVBs with radial and azimuthal polarizations can be tightly focused by a high numerical-aperture lens and form the strong electric or magnetic field in the longitudinal direction. Owing to these features, CVBs have been applied in many fields like high-resolution microscopy, high precision metal processing, particle manipulation and acceleration, etc. Phase is another important parameter of a laser beam. Except for the widely-explored fundamental Gaussian mode, high-order Laguerre-Gaussian (LG) modes are known as doughnut-like transverse field distributions and helical phase wavefronts. The exponential term exp(-il?) (where l refers to the azimuthal index) in its amplitude expression grants an orbit angular momentum (OAM) of l? to each photon of the laser beam, and therefore the high-order LG mode with l?0 can be called as vortex beam. These features make this vortex beam valuable for numerous applications such as optical trapping and manipulation, Bose-Einstein condensate, super-resolution microscopy, quantum communication and etc. In the first step, we foucsed on the feature of radially polarized beam (RPB). When an electron beam with high initial velocity passes through an electromagnetic wave, it is necessary for the electron to remain in an electricity field of constant phase, which means the component of electron velocity along the direction of wave propagation has to equal the velocity of light. Such condition is called Cherenkov condition and the interaction is associated with inverse Cherenkov effect. It is noteworthy that strong longitudinal electricity fields can be formed when RPBs are focused (or converged) by high numberical-aperture lens (or conical mirrors). The longitudinal electricity fields would provide electrons with energy increments of several hundred MeV even GeV. By the way, the energy increments require high laser powers. Based on these facts, we proposed a way to realize cascaded longitidinal electricity field to break the limitation of single interaction length. We designed a hybrid structure consisting of a conical mirror and a cylindrical reflection mirror (CRM) to form a periodic cascaded electricity field so as to accelerate electrons N times. This is one of the main tasks of our thesis. In the second step, we concentrated on the establishment of a Ti:Al2O3 laser that emits vortex light. Ti:Al2O3 lasers are tunable lasers that emit light with wavelength ranging from 650 to 1100 nm. Because of their wide gain bandwidth and long tuning range, they are able to generate femtosecond or attosecond pulses with high peak powers which can reach up to terawatt even petawatt level. A Ti:Al2O3 laser is an important equipment for the plasma physics, ion acceleration and other fields. What’s more, it is almost a standard desktop laser in many laboratories. Based on this background, if the Ti:Al2O3 laser can be operated in vortex mode, it means a significant upgrade to the original laser and will be more valuable for a variety of applications such as strong field laser physics and etc. Then we came to the problem of vortex’s generation. We noticed that a spot-defect mirror was used as the spatial filter in a Nd:YAG laser and a He-Ne laser to output vortex beam. The spot defect is used to suppress the oscillation of a Gaussian beam and result in the oscillation of a high order LG beam. In view of the low cost and the simple fabrication of spot-defect spatial filter (SDSF), we decided to modify an ever-built or commercial fundamental-mode laser into a vortex laser with the intervention of a SDSF. This is the other task of our whole research. The main contents of this thesis are as follows: In the Chapter I, the backgrounds on CVBs and vortex beams were briefly introduced, including their respective definations, the applications, as well as the methods to generate these beams. In the Chapter II, in view of the feature of tight focusing and subsequent formation of a strong longitudinal electric field of a RPB, we calculated the longitudinal component of electric field in the paraxial coherent region when a RPB was reflected by a conical mirror by using Kirchhoff diffraction theory, and it was concluded that the longitudinal component of electric field in the coherent region follows the form of zero order Bessel function. The effects of beam size, apex angle and tip size of conical mirror on the coherent length and coherent width were also discussed. With these results, we proposed a hybrid structure consisting of a conical mirror and a cylindrical reflection mirror (CRM) to form cascaded and periodic distribution of such longitudinal electric field. In the Chapter III, we demonstrated an vortex and LG01-mode Ti:Al2O3 laser by utilizing an intracavity spot defect for spatial filtering. At the 140-μm diameter of spot defect, the power of vortex laser output reached 135 mW, and the slope efficiency of laser was 17.7%. The effects of the sizes of spot defects on the transverse laser modes and laser powers were also discussed. In the Chapter IV, the results of the thesis were summarized, and the possible solutions for future research were put forward. |
| 学科主题 | 光学工程 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/30947]  |
| 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 | 谭诗文. 涡旋光钛宝石激光器和径向偏振光形成级联纵向电场特性研究[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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