环形泵浦的涡旋光Nd:YAG激光器及其腔内倍频实验研究
文献类型：学位论文
| 作者 | 徐云 |
| 学位类别 | 硕士 |
| 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 | 涡旋光束 环形光泵浦 Nd:YAG激光器 圆环达曼光栅 倍频 |
| 其他题名 | Efficient Vortex Nd:YAG Lasers and Its Intracavity Frequency Doubling Under Annular Pumping |
| 中文摘要 | 拉盖尔-高斯(Laguerre-Gaussian, LG)光束是一种典型的涡旋光束。在LG光束的振幅表达式中含有一个相位因子项exp(ilθ)（l为拓扑电荷），这说明该光束沿传播方向形成螺旋形的相位波前，因而具有轨道角动量；另外，由于LG光束的中心位置处是一个相位奇点，因而中心的光强为零，使得一般情形下它的光强具有“面包圈”状的环形分布。涡旋光束的这些独特特性使得它在光子捕获和操控、超分辨显微镜、量子通信、引力波探测等众多领域有重要的应用价值。 目前，产生涡旋光的方法主要有被动法和主动法。前者是通过一些转换元件，如柱透镜、螺旋相位板等，在腔外将高斯模式转换为LG模式。但通过被动法得到的涡旋光一般光束质量较差，且转换效率较低、并且不能用于高功率情况。为了获得高功率、高光束质量的涡旋光，一般采用主动法，即直接通过激光谐振腔振荡输出涡旋光束，如：在腔内插入诸如螺旋相位板的光学选模元件；使用特殊的腔镜；利用热透镜效应进行模式选择等。在主动法中，因为具有螺旋相位分布的LG01*模式具有环形的强度分布，所以可使用环形泵浦光端面泵浦激光器，实现激发光与泵浦光模式的空间匹配，从而让激光器直接高效地输出涡旋光束。基于这一选模机理，国内外提出了利用离焦泵浦光、小孔光阑衍射、中空光纤、多模光纤离焦耦合等方式产生环形泵浦光的，并实现了相应的环形光泵浦的激光器。这种基于环形光泵的激光器可减少选模元件的插入，并可以满足高效产生高功率、高光束质量的涡旋光的需求。鉴于环形泵浦激光器具备以上这些特点和优势，本论文的主要任务之一是深入研究基于多模光纤和圆环达曼光栅(Circular Dammann Grating, CDG)泵浦整形的环形泵浦的涡旋光激光器。 与此同时，环形泵浦的激光器所输出的涡旋光的波长受限于增益介质的发射光谱。随着涡旋光束应用领域的不断拓展，如何将其产生的涡旋光的波段拓展至可见光和紫外波段逐渐成为一大研究热点。将经环形泵浦的涡旋光激光器输出的激光波长扩展至短波长的最有效的方法是利用非线性晶体的光学频率转换。迄今为止，已有一些课题组开展了涡旋光束倍频的相关研究，但他们的研究主要集中在腔外倍频，对利用激光器腔内倍频产生短波长涡旋光的报道相对较少。 本研究工作主要立足于以上两点，深入研究了基于多模光纤和圆环达曼光栅泵浦整形的环形泵浦的LG01模的涡旋光激光器，分别实验探索了该涡旋光激光器的腔外倍频和腔内倍频。本论文主要完成了以下几项工作： 1）螺旋相位的干涉检测仿真。分别对不同阶LG光束与参考平面波和参考球面波的干涉图样进行仿真。当涡旋光束与平面波干涉时会出现叉形条纹，与球面波干涉时会出现螺旋形条纹，涡旋光束中不同的拓扑电荷对应不同的分叉数或是螺旋数，且拓扑电荷的正负对应不同的分叉方向或是螺旋旋向，为后续实验中输出激光的相位检测提供基础。 2）圆环达曼光栅整形的环形泵浦的涡旋Nd:YAG激光器。将CDG引入泵浦转换系统，将泵浦光转换为环形泵浦光（转换效率为79.8%，衍射角39.9 mrad）。利用该环形泵浦光端面泵浦Nd:YAG激光器，获得了LG01涡旋激光输出。当吸收泵浦功率为6.38 W时，最大输出功率为1.86 W，斜率效率为34.5%。该激光器具有结构简单、紧凑、成本低、效率高等优点。 为了分析不同透镜组合对环形泵浦光以及激光器输出特性的影响，采用三种透镜组合进行实验，结果表明不同的透镜组合得到的环形泵浦光的光斑直径和宽度不同，而压缩环形泵浦光的光斑直径和环宽有利于降低激光器的阈值泵浦功率，提高激光输出效率，同时改善LG01模的纯度。 3）圆环达曼光栅整形的环形泵浦的涡旋Nd:YAG声光调Q激光器。在基于圆环达曼光栅整形的环形光泵浦的Nd:YAG激光器中，插入声光调Q开关，获得了高光束质量、线偏振、且具有螺旋相位的主动调Q脉冲激光输出。当吸收泵浦功率为5.6 W、声光调制频率为5 kHz时，激光脉冲的平均功率为470 mW，峰值功率为588 W，脉冲宽度为160 ns。 4）多模光纤整形的环形泵浦、线偏振涡旋Nd:YAG激光器。将多模光纤作为模式转换元件，通过离焦耦合将激光二极管的输出光束转换为环形泵浦光，在泵浦耦合系统像方焦平面处获得的环形泵浦光环宽为0.4 mm，光斑半径为250 μm，并且随着泵浦功率的增加一直保持对称、均匀的环形强度分布，具有很好的稳定性。 将该环形泵浦光端面泵浦Nd:YAG激光器，在激光谐振腔中插入Brewster板，获得1064 nm线偏振LG01模输出。当吸收泵浦功率为3.96 W时，最大激光输出功率为548 mW，斜率效率为22.3%，偏振纯度为330:1，输出激光中具有拓扑电荷为1的螺旋相位。 5）多模光纤整形的环形泵浦涡旋Nd:YAG激光器的腔外倍频实验研究。将利用多模光纤离焦耦合产生环形泵浦光的Nd:YAG激光器输出的线偏振涡旋光通过焦距为150 mm的透镜聚焦至a切LBO晶体，温度控制在149°C，实现532 nm倍频涡旋光的输出。当基频光功率为548 mW时，最大倍频光输出功率为0.187 mW，基频光-倍频光的转换效率为0.077%。 6）多模光纤整形的环形泵浦涡旋Nd:YAG激光器的腔内倍频实验研究。为了提高倍频转换效率，采用腔内倍频；为了防止倍频光进入泵浦源，采用折叠腔结构。首先分析了Nd:YAG晶体的热透镜焦距，根据ABCD矩阵进行了腔结构设计，得到相应的腔结构参数。 再在利用多模光纤离焦耦合产生环形泵浦光的Nd:YAG激光器的基础上实现了两种折叠腔结构的腔内倍频。一种是折叠镜与反射镜均为平面镜，非线性晶体采用a切LBO晶体，温度控制在149°C，获得532 nm涡旋光输出。当吸收泵浦功率在4.10~6.69 W的范围内，输出绿光在远场和近场均能保持较对称的环形分布，吸收泵浦功率-倍频光的转换效率为0.4%，x和y方向的M2因子分别为4.02和3.94，实验验证了腔内倍频方案的可行性。第二种是将后腔镜换成曲率半径为200 mm的平凹镜，并在激光晶体与折叠镜之间插入一个焦距为100 mm的透镜，调整LBO晶体的相对位置，实现了绿光的输出，吸收泵浦功率-倍频光的转换效率为0.1%。 |
| 英文摘要 | Laguerre-Gaussian (LG) beams are known as one set of stable solutions of paraxial scalar wave equation in the system of polar coordinates, and its transverse field distribution characterizes by an azimuthal angular dependence of the form exp(ilθ), where l is the topological charge (also the azimuthal index). Such beam has a phase singularity in the center and is called as optical vortices owing to the possessing of an orbital angular momentum. The LG modes, especially the first-order LG (LG01) mode, are becoming attractive for these unique properties, and have found numerous applications in optical tweezers, super-resolution microscopy, quantum cryptography, gravitational waves detection and etc. LG01 mode can be generated by using passive or active methods. The former refers to extra-cavity mode conversion of Gaussian beams through the various optical elements like cylindrical lenses, spiral phase plates, computer-generated holograms and so on. The latter points to the direct oscillation of LG01 mode inside a laser resonator, and usually it can be realized by inserting a mode-selective component into the resonator like spot-defect mirror, diffractive optical elements and etc. Among the various vortex lasers, the annular pumping technique shows unique potential in achieving high power and high beam brightness. This way avoids the adoption of an intra-cavity mode-selective component, and also assures the good spatial overlap between the pumping field and LG01 mode volume in the gain medium. Further, the lasing wavelength of vortex laser described above is limited by the emission spectrum of gain medium. With the increasing demands for vortex beams, it becomes significant to extend the vortex laser sources to visible and ultra-visible (UV) wavelengths. The convenient way to reach this aim is to apply the frequency conversion in these laser sources. Until now, some efforts have been devoted to the second harmonic generation (SHG) of vortex beams. However, most researches focused on extra-cavity SHG of vortex beams, and there have been rather few investigations on intra-cavity frequency doubling of the vortex laser. The dissertation included two parts. In Part I, we focused on Nd:YAG vortex lasers with annular pumping shaped by multimode fiber and circular Dammann grating (CDG), respectively. In Part II, we concentrated on extra-cavity and intra-cavity frequency doubling of vortex laser. More details are as follows: 1）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）Vortex and LG01-mode Nd:YAG laser involving a circular Dammann grating. By introducing a CDG into the pump unit, we demonstrated an end-pumped Nd:YAG laser that emitted vortex and LG01 mode with high laser efficiency and high power. In the scheme, CDG was used to reshape the pumping light into annular profile with diffraction efficiency of 79.8% and diffraction angle of 39.9 mrad, and the adaptation of it was realized easily by inserting it into the pump unit of a conventional end-pumped solid-state laser; the laser cavity was simple, compact and consisted of only a laser crystal and an output coupler. The beam power of this laser reached 1.86 W at an absorbed pump power of 6.38 W with a slope efficiency of 34.5%. 3）Acousto-optically Q-switched and vortex Nd:YAG laser by using circular Dammann grating for annular pumping. By adopting annular-shaped pumping based on CDG, we demonstrated an acousto-optically Q-switched Nd:YAG laser that emitted linearly-polarized LG01-mode vortex output. The averaged power of laser pulse was 470 mW, and the laser pulse showed 588-W peak power, 160-ns duration at 5.6-W absorbed pump power and 5-kHz repetition frequency. 4）Linearly polarized vortex Nd:YAG laser under annular pumping shaped by multimode fiber. By applying annular pumping and intracavity Brewster plate, we realized a linearly polarized Nd:YAG laser. The annular pumping profile was obtained by de-focal coupling the pump radiation into a conventional multimode fiber. The beam power of this laser reached 548 mW at an absorbed pump power of 3.96 W with a slope efficiency of 22.3% and a polarization extinction ratio of 330:1. The measurement showed that the laser beam had helical phase with a topological charge of 1. 5）Extra-cavity frequency doubling of annular-pumped vortex Nd:YAG laser. The vortex Nd:YAG laser described above was linearly polarized. Its 1064-nm output was focused into a temperature-controlled LiB3O5 (LBO) crystal. By adjusting the working temperature of LBO to 149℃, we obtained frequency- doubled laser output at 532 nm. The maximum power of SHG was 0.187 mW when the power of the fundamental laser is 548 mW. 6）Intra-cavity frequency doubling of annular-pumped vortex Nd:YAG laser. Firstly, to realize a vortex Nd:YAG laser that had a folded cavity geometry for the intracavity frequency doubling, we analyzed the thermal effect of Nd:YAG laser crystal and designed the folded cavity by using ABCD matrix method. Secondly, we investigated intra-cavity frequency doubling of a folded Nd:YAG laser that consisted of three plane-plane cavity mirrors. A LBO crystal was used as the intra-cavity frequency-doubling component under non-critical phase matching. We obtained linearly-polarized vortex light at 532 nm. The output maintained annular intensity distributions both in the far- and near- fields when the absorbed pump power was from 4.10 W to 6.69 W with the conversion efficiency of 0.4%. The M2 factors in x and y directions were 4.02 and 3.94, respectively. Finally, we investigated intra-cavity frequency doubling of a folded Nd:YAG laser that consisted of three cavity mirrors (two plane-plane mirrors and a plane-concave mirror with 200-mm radius of curvature). Inside the laser cavity, a lens with 100 mm focal length was placed between the laser crystal and the folding mirror, and also a LBO crystal was used as the intra-cavity frequency-doubling component under non-critical phase matching. By adjusting the LBO’s position, we realized the 532-nm laser output. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/16964]  |
| 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 | 徐云. 环形泵浦的涡旋光Nd:YAG激光器及其腔内倍频实验研究[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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