1.Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100094, China 2.School of Optoelectronics, University of Chinese Academy of Sciences, Beijing 100049, China 3.Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 4.School of Physics Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
Fund Project:Project supported by the National Key R&D Program of China (Grant Nos. 2018YFB0407400, 2018YFB0407402), and the National Natural Science Foundation of China (Grant No. 6187030118)
Received Date:07 February 2021
Accepted Date:13 April 2021
Available Online:07 June 2021
Published Online:20 August 2021
Abstract:The high-average-power diode-pumped solid-state laser is one of the main research directions in the field of international laser technology, and has major applications in the fields like space exploration, precise detection, fusion research, etc. Under high-power pumping conditions, the conventional rods or slabs are not conducive to effectively removing waste heat. And the thermal effect causes the quality of the laser beam to deteriorate, which limits the further increase of the output power. In this article, the polygonal Nd: YAG thin disk is taken as a gain medium for the laser, and experiments verify that the side pumping method can obtain a higher output power of the all-solid-state pulsed laser while ensuring high beam quality. The gain medium is a regular pentagonal Nd: YAG thin disk with a side-cut-angle of 45°, the crystal thickness is 1.5 mm, and the diameter of the inscribed circle on the front face is 16 mm. Five laser diode arrays are placed symmetrically around the disk, and the pump surfaces are parallel to the sides of the disk. The pump laser propagates along the zigzag path between the upper and lower surface of the disk, thus improving the absorptive efficiency and pump uniformity. Through the optimization study of its gain characteristics and optical characteristics, the high-efficiency high-uniform pumping is achieved. Along the pump light coupling transmission path, the fast-axis collimator is used to control the beams in the fast-axis direction to be nearly parallel, and the large-area pump light is compressed through the coupling structure of cylindrical lens and light guide to match the size of the thin disk, and the pump coupling efficiency measured experimentally is 97%. When the Nd3+ doping concentration in the crystal is 0.3 at.%, the gain medium absorptive efficiency is 87%, and the root mea squared pump absorptive distribution uniformity in the gain medium is 3.21%. The fluorescence distribution of the gain medium is in good agreement with the simulated data. When the pump energy is 2.2 J, a laser output with an energy of 0.85 J is obtained, and optical-to-optical efficiency and slope efficiency are 38.8% and 40.1%, respectively. The single pulse energy stability is 2.7%(RMS) at 1 Hz frequency. In the stable cavity, the beam quality β - factor is measured to be about 10. Keywords:thin disk laser/ side pump/ pump uniformity
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2.1.激光器系统结构
我们设计采用的LDA侧面折返泵浦多边形薄片激光器的结构如图1所示, 增益介质为正五边形Nd:YAG薄片晶体. 为方便观察, 下方主图部分仅给出一个方向的泵浦耦合结构, 图1(a)为五边旋转对称泵浦结构俯视图. 多边形Nd:YAG晶体的侧面为泵浦光入射面, 呈梯形, 与端面成45°. 泵浦光正入射晶体侧面, 如图1(b)所示. 晶体前端面为激光输出面, 镀激光波长的增透膜. 晶体后端面与散热结构相连, 镀激光波长高反膜. 图 1 侧面泵浦多边形薄片激光器结构示意图 (a)结构俯视图; (b)局部放大图 Figure1. Schematic diagram of side-pumped polygonal thin-disk laser structure: (a) Top view of the structure; (b) partial enlarged view
单个LDA包含6个巴条, 单个巴条上有23个发光点, 单条激光二极管快轴发散角大, 需要采用快轴准直器(fast axis collimator, FAC)将发散角压缩至近平行, 准直后的快轴发散角 ≤ 0.5°. 泵浦源发光面积较大约为10 mm × 10 mm, 需要通过缩束耦合结构将其压缩至与多边形薄片侧面(入射面)尺寸匹配. 通过计算追迹光线, 泵浦光经耦合结构到达泵浦面时的光斑尺寸约为12.6 mm × 1.6 mm, 传输效率约为97.5%, 实际测量得到的传输耦合效率为97%. 22.2.增益介质内泵浦光吸收分布特性 -->
五边形Nd:YAG薄片增益介质的内切圆直径为16 mm, 厚度1.5 mm, 掺杂浓度0.3%(原子分数), 基于Zemax光学设计软件建模计算得出泵浦光在增益介质内分布, 如图2所示. 有效泵浦效率${\eta _{{\rm{a \text- eff}}}} = 65.1 \%$, 均匀性RMS为3.12%, 这说明泵浦光强度是均匀分布的. 图 2 晶体内被吸收的泵浦光强的二维和三维分布 Figure2. 2-D and 3-D distributions of the absorbed pump laser in the crystal.
22.3.增益介质荧光分布 -->
2.3.增益介质荧光分布
五边形Nd:YAG薄片增益介质的荧光分布如图3所示, 可见多边形增益介质内泵浦光分布均匀, 与仿真结果吻合. 图 3 增益介质内荧光分布 Figure3. Fluorescence distribution in disk.