磁悬浮旋转盘片激光器及被动调Q、矢量激光器研究
文献类型：学位论文
| 作者 | 韩侠辉 |
| 学位类别 | 博士 |
| 答辩日期 | 2016 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 李建郎 |
| 关键词 | 被动调Q激光器 瞬态温度 矢量偏振 磁悬浮 转盘激光器 |
| 其他题名 | Maglev Rotating Disk Laser and Passively Q-switched Vector Laser |
| 中文摘要 | 近年来，随着人们对光的偏振特性研究的深入，一些具有空间非均匀偏振的光束越来越吸引科研人员的关注。在空间非均匀偏振光束的大家族中，最引人注目的是空间各点的偏振态呈圆对称分布的圆柱矢量偏振光束。圆柱矢量偏振光束是矢量亥姆霍兹方程一组本征解，因而也是可以在激光谐振腔中稳定存在的本征模式，可以通过一定的选模方法由激光器直接输出。最近几年，可以直接输出矢量偏振光束的激光器是一个研究热点。径向偏振光束就是一种常见的圆柱矢量偏振光束，其振幅和偏振方向都具有轴对称性而且任意位置处的偏振方向都沿着半径方向。径向偏振光束具有紧聚焦的特性，经过高数值孔径的透镜聚焦后具有更小的焦斑尺寸而且焦点附近会形成一个很强的纵向电场，可以应用于高分辨成像，粒子捕获和电子加速等方面。此外，相较于传统的线偏振光和圆偏振光，有些金属材料对径向偏振光具有更大的吸收系数，因而其在工业加工领域也具有广阔的应用前景。上面提到的很多应用中需要用到高峰值功率、短脉宽的径向偏振激光脉冲，而被动调Q激光技术是一种简单而常用的获得高峰值功率、短脉宽的激光脉冲的方法。本研究利用光子晶体光栅镜作为选模元件，实现了高峰值功率、短脉宽的被动调Q径向偏振激光脉冲输出。 另外，随着激光二极管泵浦源的发展和激光材料制备、加工技术的不断进步，近年来半导体泵浦的全固态激光器发展迅速。虽然激光器能够输出的最高功率一直都在快速增长，但是在空间通信、激光武器、激光雷达和精细加工等领域，对同时具有高亮度的高功率固体激光器的需求也越来越强烈。然而，由于固体激光器的增益介质为固态，在工作的过程中不能流动，增益介质内沉积的热量只能通过介质表面带走，导致高平均功率运转时增益介质内部的温度梯度非常严重。固体激光增益介质内大的温度梯度会使得增益介质内部出现很强的应力，导致热透镜、热致双折射等效应，这就限制了固体激光器输出功率和亮度的进一步提高。如何有效的降低热光畸变，保证高功率运转过程中输出激光的亮度是固体激光器设计过程中的一个关键问题。为了解决这个问题，Basu和Bye提出了转盘激光器的概念。转盘激光器通过电机带着固体激光增益介质盘片转动，可以有效的减少热效应对激光器输出特性的危害，提高激光器输出激光的平均功率和亮度。本研究基于转盘激光器的概念，研究了输出皮秒脉冲的被动调Q的转盘激光器和磁悬浮的光驱动旋转盘片激光器。 本论文主要包括以下内容： 1) 以获得短宽度（纳秒或亚纳秒）、高峰值功率（>15kW量级）的矢量偏振脉冲和皮秒量级的转盘激光器脉冲为目标，进行了被动调Q的Nd:YAG激光器的优化设计。我们首先介绍了被动调Q的速率方程理论和基于速率方程的激光器参数优化设计的步骤。然后，基于这套理论对我们后面要实施的实验方案中相关参数进行了优化设计。 2) 理论研究了连续运转的被动调Q微片激光器增益介质中瞬态温度分布情况，并首次给出了LD端面、连续泵浦的被动调Q微片激光器增益介质中的瞬态温度分布的解析表达式。然后，利用得到的解析表达式分析了增益介质中准稳态温度分布的建立过程，并以常见的Nd:YAG/Cr4+:YAG和Yb:YAG/Cr4+:YAG被动调Q微片激光器系统为例分别讨论了激光四能级系统和激光三能级系统中瞬态温度分布情况。通过分析我们发现，在连续泵浦的被动调Q微片激光器的增益介质中，虽然温度分布具有瞬态特性，但是当达到准稳态以后，在激光脉冲发射阶段，增益介质中的温度是不随时间变化的，我们将这个温度定义为平衡温度，并给出了平衡温度的解析表达式。 3) 实验研究了具有<111>切向的YAG/Nd:YAG/Cr4+:YAG键合晶体的径向偏振被动调Q微片激光器。针对以前的研究中出现的矢量偏振激光光束的光斑不均匀问题，通过分析被动调Q激光器中激光晶体（Nd:YAG）的切向和可饱和吸收晶体（Cr4+:YAG）的切向对输出光斑均匀性的影响，首次在实验中采用<111>切向的Nd:YAG晶体和Cr4+:YAG晶体，获得了空间均匀对称的径向偏振激光脉冲输出。实验中输出激光脉冲的峰值功率达到了15.7 kW，脉冲能量达到了50.1 μJ ，脉冲宽度为3.2 ns，重复频率为8.1 kHz。最后，我们还提出了一种新的基于单轴晶体双反射效应的矢量偏振激光器实施方案，并给出了可以实现径向偏振和切向偏振激光输出的具体设计方案。 4) 首次研究并实验研制了输出皮秒脉冲的连续泵浦、被动调Q旋转盘片激光器。我们将转盘激光技术和被动调Q微片激光技术结合在一起，搭建了一台基于Nd:YAG/Cr4+:YAG键合晶体被动调Q的转盘激光器。首先保持键合晶体盘片不转，当入射的泵浦功率为5.25 W时，激光器输出平均功率为204 mW，脉冲宽度为585 ps，重复频率为7.9 kHz，峰值功率为44.1 kW。随后我们研究了对键合晶体盘片施加转动时，激光器输出脉冲特性的变化。实验发现当电机带动键合晶体盘片以0.5 Hz的频率缓慢转动时，可以消除不转时激光器输出主脉冲后面的尾脉冲，同时激光器输出脉冲的强度波动也会变小，稳定性也有明显的改善。然后继续增大电机转速，在电机转速调节范围内（0~10 Hz），激光器输出脉冲的峰值功率随着转速的加快，一直在升高。当转速达到10 Hz时，激光器输出脉冲的峰值功率比不转时提高了5倍左右。 5) 首次提出并实验实现了磁悬浮的旋转盘片激光器。我们首先介绍了热解石墨片的磁悬浮效应及其在光驱动下的转动特性，然后结合转盘激光器的概念提出了一种磁悬浮的转盘激光器方案。随后以Nd:YAG晶体薄片作为激光增益介质，在实验上初步验证了此方案的可行性。实验中利用LD端面泵浦的结构，在4 Hz转动速率下，实现了17.7 mW的TEM00单模激光输出，并且通过对比不转的情况，验证了晶体转动对激光器输出光束亮度的提高作用。 |
| 英文摘要 | As one of typical representative of light beams with spatially inhomogeneous polarization, cylindrical vector beams (CVBs) show cylindrical symmetry in both polarization and intensity and are known as the solutions of the full vector electromagnetic wave equation. Among various CVBs, radially polarized light is characterized by axis-symmetrical polarization with the electric field along its radial direction. By applying the high-numerical-aperture lens, radially polarized beam can be focused into a spot beyond the diffraction limit with a strong longitudinal electrical field. Such unique properties enable it to be useful for many areas, such as super resolution, optical trapping, electron acceleration, etc. Moreover, the metal material shows higher absorption efficiency to radially polarized beam than that of others, thus radially polarized beam is beneficial to efficient and high-precision material processing. As CVBs can stably exist as the cavity modes of a resonator, thus they can be directly produced from a laser cavity. Furthermore, many applications require radially polarized laser beam pulse that has high peak power and short duration. As we know, passive Q-switching is a common and simple method for producing short and energetic pulses. One topic of this dissertation focused on the generation of short-duration and energetic radially polarized laser pulse from a passively Q-switched solid-state laser. The second topic of this dissertation concentrated on diode-pumped solid-state lasers (DPSSL) in the rotary mechanism. As known, DPSSL have advantages in compactness and efficiency over other types, and can reach very high powers while maintaining a relatively good beam quality. Until now it has been applied in the various fields including direct-energy weapons, optical lidar and metal processing, etc. However, with the increase of pump power, heat deposition inside the gain medium of DPSSL would degrade the beam brightness and became a main bottleneck for further power scaling. One way to mitigate this heat deposition is to adopt the rotary disk laser geometry, as proposed by Basu and Bye, in which the rotation of the disk quickly removes the deposited heat in the region of laser excitation. Based on this background, we combined the rotating disk laser concept and the passively Q-switched microchip laser technology, and finally obtained picosecond laser pulse with increased pulse stability and peak power. And also we combined the optical control of maglev diamagnetic material and rotary disk laser, and proposed a new laser concept - maglev and rotating disk laser. The main work of this dissertation is as follows: 1.Theoretical model and optimized design to a passively Q-switched Nd: YAG laser were given to obtain nanosecond or subnanosecond vector pulse with >15kW peak power. Firstly, the theoretical model of passively Q-switched Nd:YAG laser was built by applying the rate equations and the optimal parameters of laser resonator was derived numerically. The obtained laser parameters (including length of laser cavity, optimum reflectivity of the output mirror and optimum initial transmission of the saturable absorber) was used as the guidance of subsequent laser experiments. 2.The analytical solutions for the transient temperature distribution in the gain media of continuous wave (CW) end-pumped passively Q-switched microchip lasers were derived for the first time. These analytical solutions of the transient temperature distribution in the gain media were used to evaluate the buildup of quasi-steady-state temperature distribution in the gain media. Then, this theoretical model was applied to analyze the transient temperature of four-level and three-level laser systems (i.e. Nd:YAG∕Cr4+:YAG and Yb:YAG∕Cr4+:YAG lasers), respectively. We found that although transient temperature in gain medium remained oscillatory behavior, the transient temperature at the pulsing stage of passively Q-switched microchip laser was a constant, and then we defined this constant as the equilibrium temperature and derived its analytical expression. 3.A radially polarized laser pulse from a passively Q-switched microchip laser with composite structure of <111> cut YAG/Nd:YAG/Cr4+:YAG crystal was demonstrated. We analyzed the reason of inhomogeneous distribution of light intensity in the existing passively Q-switched vector lasers, and found that both gain Nd:YAG crystal and saturable absorber Cr4+:YAG crystal along the cut direction could affect the uniformity of the output vector beam. In our experiment, by using <111> cut Nd:YAG and Cr4+:YAG crystals we obtained homogeneously radially polarized laser pulse output. At 6.5 W absorbed pump power, we obtained a radially polarized laser pulse with maximum peak power of 15.7 kW, minimum pulse duration of 3.2 ns, and repetition rate of 8.1 kHz. In the end, we proposed a new vector laser implementing scheme which was based on the bireflectance effect of uniaxial crystal and discussed the concrete design schemes for radially polarized beam and azimuthally polarized beam output, respectively. 4.By combining the rotating disk laser concept and the passively Q-switched microchip laser technology, we experimentally demonstrated, for the first time, a picosecond CW end-pumped passively Q-switched Nd:YAG/Cr4+:YAG rotary disk laser. In the case of no rotation and 5.25 W incident pump power, the average output power was 204 mW with maximum peak power of 44.1 kW, pulse duration of 585 ps, and repetition rate of 7.9 kHz. Then, we studied the influence of rotating on the characteristics of the output laser pulse, and we found that when compound disk rotaed at a frequency of 0.5 Hz, the tail pulse which existed in stationary mode disappeared, the intensity fluctuation of the output pulse decreased, and the stability was improved obviously. In the motor speed adjustment range (0~10 Hz), the peak power of the output pulse always increased with the speed of the rotating. For example, when the rotating frequency is 10 Hz, the peak power was increased five times higher than that in stationary state. 5.A maglev and rotary disk laser was proposed and demonstrated. By revisiting the levitation of diamagnetic material (pyrolytic graphite, PG) and its light-driven rotation, we combined its optical control and rotary disk laser, and proposed the concept of maglev and rotating disk laser. In the proof-of-concept experiment, a thin disk of Nd:YAG crystal was used as gain medium and attached to a PG plate levitated above a permanent magnets. With this arrangement, we realized a maglev and rotary Nd:YAG laser, and achieved a single-mode output of 17.7 mW at 447 mW of absorbed pumped power and a rotation frequency of ~ 4 Hz. As the detrimental thermal effects were alleviated, the brightness of the output beam was improved. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15993]  |
| 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 | 韩侠辉. 磁悬浮旋转盘片激光器及被动调Q、矢量激光器研究[D]. 中国科学院上海光学精密机械研究所. 2016. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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