级联锥形折射环形泵浦的模式可调的涡旋光激光器
文献类型：学位论文
| 作者 | 吴勇晓 |
| 文献子类 | 硕士 |
| 导师 | 李建郎 |
| 关键词 | 涡旋光束 Vortex beam 环形光泵浦 annular pumping Nd:YAG激光器 Nd:YAG laser 级联锥形折射 cascade conical refraction |
| 其他题名 | Switchable Excitation of LG01and LG03 Vortex Modes in Nd:YAG laser Pumped by Cascade Conical Refraction |
| 英文摘要 | 近来，以拉盖尔-高斯光束（Laguerre-Gaussian，LG）为代表的涡旋光束的特性引起了人们的广泛兴趣。LG光束是柱坐标系下标量波动方程的本征解，是激光器谐振腔的本征模式，其复振幅表达式包含一个螺旋相位因子exp(ilθ) （l表示角向节点数，也叫拓扑电荷数），沿光轴传播方向具有螺旋波前，因而其中的光子携带有轨道角动量，被称为涡旋光束。其次，该光束的中心存在相位奇点，其光强呈甜甜圈状的环形分布。这些独特的物理性质使得其在超分辨显微、光镊、量子纠缠等领域具有很高的应用价值。值得一提的是，高阶LG光束，特别是LG33模，包含多重环形强度分布，能够减少干涉仪反射镜中的热噪声，在引力波探测领域有很大的应用潜力。 目前，产生涡旋光的方法可大致分为两类：被动法、主动法。被动法是指通过模式转换元件，如柱透镜对、计算全息图、空间光调制器、螺旋相位板等，在谐振腔外将入射光转换为涡旋光。被动法产生的涡旋光的局限在于其光束质量取决于转换元件的加工精度，一般较差并且受限于转换元件的低损伤阈值难以获得高功率输出。主动法是指利用腔内选模元件、热透镜效应、环形光泵浦等进行模式选择，直接通过谐振腔振荡输出涡旋光束。环形光泵浦技术旨在将泵浦光束调制成环形强度分布，在激光器增益介质内部实现泵浦光的强度分布和激发光横向模式的有效的空间匹配，从而获得涡旋光束直接输出。显而易见，该方案在产生涡旋光束时具有高效率、高功率、高光束质量的优势。然而，目前已有的获取环形泵浦光的方法，如利用小孔光阑衍射、空心光纤、多模光纤离焦耦合、环形达曼光栅等，由于存在额外的插入损耗，转换效率都不高，通常在60-80％的范围内。 在本论文中，为了提高环形光的转换效率，我们提出了双轴晶体的锥形折射用于环形光泵浦当中。具体而言，锥形折射是指当圆偏振或非偏振光沿其中的一条光轴入射到双轴晶体内部，发生二维空间走离，从而在晶体后面的聚焦成像平面处演化为环形强度分布的现象。通常，双轴晶体对于泵浦光是不吸收的，而且利用镀膜的方法可以消除晶体前后表面上的菲涅耳反射。因此，利用双轴晶体的锥形折射可以将泵浦光整形为环形，而且有望获得百分之百的转换效率。在我们早期的研究中，已经基于单块双轴晶体的锥形折射获得单环的泵浦光，并利用其泵浦Nd:YAG激光器，获得了高效率的LG01模涡旋激光输出。 我们进一步注意到，当入射光沿光轴方向连续通过N个双轴晶体时，发生级联锥形折射，在聚焦成像平面上，可以产生同心多环的强度分布。对于N个双轴晶体的级联，聚焦成像平面上的环形模式是由2N-1个同心亮环组成，每个亮环半径是光束单独通过每个双轴晶体产生的锥形折射单环半径的加减组合；每个亮环的相对强度由两两相邻的两块双轴晶体的光轴平面夹角确定。因此，可利用双轴晶体级联锥形折射产生多个圆环用来泵浦固体激光介质，且同心圆环的强度分布与高阶拉盖尔-高斯光束的横向模式在空间上匹配，从而输出高阶拉盖尔-高斯光束的固体激光器。 在本研究中，为了验证这些设想，我们利用两块双轴晶体的级联锥形折射获得双环的泵浦光，将其用于泵浦Nd:YAG激光器获得高质量的涡旋激光输出，并且通过旋转其中一块双轴晶体可以方便的实现LG01模与LG03模之间的模式切换： LG光束及螺旋相位的检测仿真。基于低阶和高阶涡旋模式检测的需要，理论模拟了各阶 LG光束分别与平面波、球面波的干涉情况，干涉图样会在相位奇点附近出现“分叉”或“螺旋”条纹，根据分叉或是螺旋的数目和方向即可判断涡旋光束包含的拓扑电荷情况。 锥形折射理论。首先介绍了锥形折射背景，然后从几何光学和衍射理论角度描述锥形折射环的形成、环上能量分布及偏振特性，尤其是在聚焦成像平面；分析了入射光的偏振和模式对环上能量和偏振分布的影响。最后介绍了N级级联锥形折射的理论和实验，特别是两级锥形折射。 基于级联锥形折射环形光泵浦的模式可调的涡旋光激光器。利用改进的两级级联锥形折射装置，实现了实心泵浦光到同心双环泵浦光的转换，而且双环的半径及相对强度连续可调。实验测量级联锥形折射产生的同心双环形光的转换效率为96%，内外环的半径分别为121 μm 和 388 μm。然后分别将内、外环用于泵浦Nd:YAG激光器，利用内环泵浦时，激光器输出LG01模，阈值吸收泵浦功率为439 mW，斜坡效率为52.5%；在最大吸收泵浦功率为1.27 W时，激光输出功率为436 mW；利用外环泵浦时，激光器输出LG03模，阈值吸收泵浦功率为891 mW，斜坡效率为41.3%，在最大吸收泵浦功率为1.27 W时，激光输出功率为160 mW；通过绕光轴旋转第二块双轴晶体，可轻松实现模式转换。; Laguerre–Gaussian (LG) modes are a set of eigen solutions of paraxial scalar wave equation in the cylindrical coordinate system, the intrinsic doughnut-shaped intensity distribution and helical phase fronts of them endow the beam photon with orbital angular momentum, and thus they are referred to optical vortex beams. Up to dates, LG beams become attractive for emerging applications in the fields of optical trapping and manipulation, quantum entanglement, super-resolution microscopy and so on. Furthermore, the higher-order LG vortex modes with multi-ring power distributions, especially LG33 mode, are able to decrease the thermal noise in reflection mirror of an optics interferometer, therefore have great potential in gravitational wave detection and other high precision measurements. The vortex LG beams can be generated in both passive and active ways. The former refers to spatial transform of incoming Gaussian mode into the desired LG mode through external mode converters, such as cylindrical lenses pair, computer-generated hologram, spatial light modulator, spiral phase plate and etc. But the beam quality depends on machining precision of mode converters, is poor and the low damage threshold of converters also limit the output power enormously. The active method focuses on direct oscillation of LG mode inside laser cavity by using intra-cavity mode selector, thermal lens effect, annular pumping technique and so on. The annular pumping technique aims to reshape the pump light into annular intensity distribution for spatial overlapping with the oscillating mode inside the gain medium of laser, and thus it is beneficial for high mode quality and power scaling of the laser. However, ever-reported pump shaping routes (for example, utilizing capillary fiber, misaligned multimode fiber or circle Dammann grating) are of low shaping efficiency (typically in the range of 60–80%), due to the presence of additional insertion loss. In order to enhance the efficiency of annular pumping, the conical refraction of biaxial crystals (BC) is proposed for annular pump shaping. Generally, the BC is non-absorptive to pump light, and also Fresnel reflection on both facets of it can be avoided by using anti-reflection coating. So this method is expect to provide nearly unity shaping efficiency. When a focused circularly polarized or unpolarized light beam enters a BC along one of its optic axes, CR happens and the light beam propagates as a cone inside the crystal, emerging from the crystal as a cylinder, evolves into sharp ring intensity distribution at the focal image plane (FIP). In our early research, CR ring pumped solid-state laser has showed good performances in the generation of LG01-mode vortex beam with high laser efficiency. As known, the incident beam propagates along the optic axes of cascade biaxial crystals, known as cascade CR, multiple concentric rings can be generated. For an N-crystal cascade, where N is the number of applied BCs, the light pattern in the FIP consists of 2N-1 concentric rings, whose ring radii are combinations of those from individual crystals in the cascade and relative intensity of rings is determined by the azimuth angle between the orientations of the crystals. Therefore, the multiple rings pumping based on cascade BCs exhibits the potential for the generation of high-order LG vortex mode in end-pumped solid-state laser. In this study, to verify our assumption, we used the annular pump light originated from the cascade CR of two BCs to pump a Nd:YAG laser, and obtained the switchable emission of low- and high-order vortex modes. More details are given as follows: 1) Theoretical simulation of interference between LG beams and reference waves. Interference patterns between a LG beam and a reference plane beam or a spherical beam were simulated. A vortex beam interferes with a plane or spherical beam, resulting in the formation of fork or spiral fringes around the singularity. Topological charge can be obtained by the number of forks or spiral petals. 2) Conical refraction theory. Firstly, the background of conical refraction was introduced; then the formation of conical refraction rings was described from geometrical optics. The intensity and polarization distribution of rings, especially in the FIP, were depicted from diffraction theory. The effects of the polarization and intensity profile of the incident light on conical refraction were analyzed. Finally, the theory and experiment of N-cascade conical refraction were introduced, especially two-cascade conical refraction. 3) Cascade conical refraction for annular pumped LG01- and LG03-mode Nd:YAG laser. The annular pump light was originated from the modified two-cascade CR arrangement proposed by V. Peet. The measured shaping efficiency was 96% and the radii of the the inner and outer rings were 121 μm and 388 μm, respectively. We used the annular pump light to pump a Nd:YAG laser for selective lasing of LG01 and LG03 vortex modes. When only inner ring existed, the Nd:YAG laser started to emit at absorbed pump power (Pabs) exceeded 436 mW, then laser power increased linearly with Pabs at 52.5% slope efficiency, finally reached 439 mW at Pabs=1.27 W for LG01 mode. When only outer ring existed, the Nd:YAG laser started to emit when Pabs exceeded 891 mW, then laser power increased linearly with Pabs at a slope efficiency of 41.3%, finally laser power reached 160 mW at maximum pump power available where Pabs =1.27 W for LG03 mode. The switching for LG01 and LG03 excitation was realized by simply rotating the second biaxial crystal. |
| 学科主题 | 光学工程 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/31039]  |
| 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 | 吴勇晓. 级联锥形折射环形泵浦的模式可调的涡旋光激光器[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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