| 题名 | 光纤中的模式转换及环形光泵浦的矢量偏振Nd:YAG激光器 |
| 作者 | 方志强 |
| 学位类别 | 博士 |
| 答辩日期 | 2014 |
| 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 | 矢量偏振光 光纤 多模光纤 固体激光器 环形光泵浦 热致双折射 |
| 其他题名 | Mode conversion inside optical fiber and vector polarized Nd:YAG laser with annular-shaped pumping |
| 中文摘要 | 近年来，具有空间不均匀偏振态的矢量偏振光束，特别是径向偏振光，因其在众多领域中的潜在应用价值吸引了越来越多的关注。径向偏振光经高数值孔径物镜聚焦后可在焦点区域获得极强的纵向电场和远小于传统偏振光的焦斑尺寸，使得其在高分辨率光学显微镜、粒子捕获、电子加速、等离子体激发等领域有着广阔的应用前景。而金属材料对径向偏振光优异的吸收特性，使得径向偏振光在用于金属加工时可获得两倍于线偏振光或圆偏振光的效率提升，这也推动着径向偏振光在激光加工领域中的应用。随着径向偏振光应用领域的拓展，对低成本、高亮度、高效率和高功率的径向偏振光源的需求也越来越迫切。 本研究工作立足于这一需求，从主动法和被动法两个方向探索低成本、简易且高效的矢量偏振光的产生方法。本论文主要完成了如下的工作： 1) 少模光纤中的模式转换及矢量偏振光的产生。利用少模光纤作为模式转换器件，通过控制线偏振光的入射条件（偏移和倾斜），在四个不同的离轴入射条件下，分别实验实现了TM01模-径向偏振光，TE01模-角向偏振光，HE21模-混合偏振光和线偏振的LP11模的选择性激发。随后，针对选定的四种离轴入射条件，实验观察了入射光场偏振态的变化对光纤模式激发的影响。在此基础上通过干涉实验观察了光纤输出光束的波前相位与入射光场偏振态之间的关系。 2) 粗芯径、大数值孔径的多模光纤中空心光束的产生。利用粗芯径、大数值孔径的多模光纤作为模式转换器件，首次提出了通过离焦耦合调节将激光二极管输出光束转换为空心光的机制。实验中通过调节入射光的焦平面相对光纤端面的距离，在离焦距离为90 um时获得了环形光输出，转化效率为75%，光束M2因子为380。 3) 利用多模光纤转换得到的空心光泵浦Nd:YAG激光器，在没有任何腔内偏振选择元件的条件下，实现了径向偏振激光输出。实验中，利用多模光纤转换得到的808 nm环形光作为泵浦光，在平平腔结构的Nd:YAG激光器中获得了1.2 W的LG01模激光输出。激光器的斜率效率为28.3%，输出的径向偏振光的偏振度最高达83.3%。该激光器具有结构简单紧凑、效率高等优点。 在实验基础上，对环形光泵浦的Nd:YAG激光器输出径向偏振光的机理进行分析。理论计算表明，激光晶体中存在的热致双折射会导致径向偏振光和角向偏振光与环形泵浦光的空间重叠效率存在差异。 4) 利用多模光纤转换得到的环形光泵浦Nd:YAG激光器，通过在腔内插入可饱和吸收体Cr4+:YAG，实现了径向偏振的脉冲激光输出。该激光器输出平均功率最高达650 mW，斜率效率为32%。输出激光为径向偏振光且M2因子和偏振度分别为2.2和90%。在泵浦功率为3.8 W时，输出激光脉冲宽度为39 ns，峰值功率为3.3 kW，重复频率为2.5 kHz。并且在较高的泵浦功率下，通过轻微倾斜腔镜，输出光束偏振态可切换为角向偏振，同时在低泵浦功率下仍为径向偏振光。 5) 利用多模光纤转换得到的空心光泵浦Nd:YAG激光器，通过在腔内插入声光调Q开关，实现了主动调Q脉冲激光输出，输出激光为线偏振的LG01模。当重复频率为10 kHz时，最高获得了810 mW的激光输出，斜率效率为30%。在1 kHz的重复频率和4.4-W的泵浦功率下，输出激光脉冲的宽度为125 ns，峰值功率为1.6 kW。输出的LG01模的M2因子和偏振方向均不随泵浦功率和重复频率发生变化。 |
| 英文摘要 | In recent years, as one member of the families of vector polarized laser beams, radially polarized beam (RPB) which exhibits cylindrical symmetry polarization with the electric field along its radial direction and polarization singularity along the beam center, had attracted increasingly interest due to its several unique features. By applying the high-numerical-aperture (high NA) lens, RPB can be focused into a spot beyond the diffraction limit with a strong longitudinal electrical field. Such properties enable it to be useful for many areas, such as super resolution, optical trapping, electron acceleration, plasma excitation, and etc. Moreover, the metal material shows about higher absorption efficiency to RPB than that of linearly or circularly polarized light, and thus RPB is beneficial to efficient and high-precision material processing. For increasing demand on RPB, the dissertation focused on the low cost, simple and efficient methods of RPB generation. First, we realized vector polarized beam excitation inside a few-mode fiber by controlling the off-axis incidence of light. Following the viewpoint of mode conversion in few-mode fiber, we proposed a scheme of off-focus coupling to reshape the pump light from laser diode to annular shape. Then, continuous wave and pulsed RPB Nd:YAG laser were demonstrated by adopting the annular pump light. More detailed are listed as below: 1) The excitation of vector modes in a few-mode fiber by varying the polarization states of incident Gaussian beam at different off-axis conditions was investigated. As a result, the linearly polarized incidence orientated at four different off-axis conditions excited radially polarized TM01 mode, azimuthally polarized TE01 mode, hybrid polarized HE21 mode, or linearly polarized LP11 mode separately. Further, the left/right circularly polarized incidence excited linearly polarized LP11 mode and the excited mode was interpreted as the combination of HE21 and TM01 (/TE01) modes with equal amplitude. The left/right elliptically polarized incidence excited the two-lobe mode with the crescent conglutination between each pair of lobes and the excited mode was interpreted as the combination of HE21 mode and TM01 (or TE01) mode with unequal amplitude. 2) Generation of annular shaped pumping by multimode (MM) fiber with large core diameter and high NA. For the first time, we proposed a scheme to convert the light from laser diode (LD) into annular shape by controlling the off-focus condition of MM fiber. In experiment, at 90 um defocus distance, which was defined as the distance from the focal plane of incident beam and rear surface of fiber, annular shaped light could be formed in the fiber. The conversion efficient and M2 factor of annular shaped light was 75% and 380, respectively. 3) By applying annular-shaped pumping light, we demonstrated a radially polarized Nd:YAG laser without adopting any intracavity polarization selective component. The annular profile of the pumping light originated from the mode conversion inside a conventional multimode fiber under off-focus coupling condition. This laser had a simple plano-plano cavity and gave a maximum output power of 1.2 W at 1064 nm with a solpe efficiency of 28.3%. The polarization dgree of LG01 laser mode was 83.3%. The laser was compact and high efficiency。 Furthermore, a radial polarization selective mechanism of the annular pumped Nd:YAG laser was proposed. In theoretical, the thermal induced birefringence in laser crystal can induced the difference in overlap efficiencies for the radially and azimuthal polarized components with annular pumping. 4) We demonstrated, for the first time, a radially polarized and passively Q-switched Nd:YAG laser with annular-shaped pumping. The annular intensity profile of the pump light was originated from the mode conversion in a conventional multi-mode fiber under off-focus coupling. Radially polarized pulse from this laser had 3.3 kW peak power, 39 ns duration at 2.5-kHz repetition rate with ~90% degree of polarization. When the laser crystal was slightly tilted, we observed the pulsed laser output was switched to azimuthal polarization at high pump power and while remained radial polarization at lower pump power. 5) By adopting annular-shaped pumping, we demonstrated an acousto-optic Q-switched Nd:YAG laser with the linearly polarized LG01 mode output. The laser achieved a maximum average power of 810 mW with a slope efficiency of 30% at a 10 kHz repetition rate. With 4.4-W pump power and 1-kHz repetition rate, the laser produced pulse of 200 uJ with 125 ns pulse width. The M2 factor and polarization of the LG01 mode did not influenced by the pump power and repetition rate. |
| 语种 | 中文 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15863]  |
| 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 | 方志强. 光纤中的模式转换及环形光泵浦的矢量偏振Nd:YAG激光器[D]. 中国科学院上海光学精密机械研究所. 2014. |
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