偏振无关合束光栅的热畸变研究
文献类型：学位论文
| 作者 | 李林欣 |
| 文献子类 | 硕士 |
| 导师 | 晋云霞 |
| 关键词 | 光谱合成 spectral beam combining 偏振无关宽带光栅 polarization-independent broadband grating 光栅热效应 thermal effects 光束质量 beam propagation characteristics |
| 其他题名 | Study on thermal deformation of polarization-independent gratings used in spectrally combined laser system |
| 英文摘要 | 从上世纪七十年代开始，掺杂光纤激光器的研制在世界范围内引起广泛的关注。高功率光纤激光器的高速发展，使其在科研、医疗、工业以及国家安全等领域中都有着广泛的应用。目前，单束光纤激光器输出功率已达到数十千瓦量级。但是，由于更高功率密度引起的纤芯及端面光学损伤、激光器热效应和非线性效应严重限制了光纤激光器的输出功率与光束质量，因此将多束激光合成来提高合成功率输出的技术应运而生，简称合束。目前，国内外高亮度高功率的合束技术主要有两种―相干合成与光谱合成。相干合成通过控制多模块结构中单束激光器的输出相位，实现光束的远场相干合成。由于实现精确的相位控制难度较大，且经相干合成的光斑旁瓣能量难以抑制，因此相干合成技术近年来并未取得明显的应用进展。而光谱合成通过使用以光栅为代表的衍射元件，使不同波长的激光阵列实现远场强度合成，为实现高功率、高光束质量的合成输出提供了技术路径。千瓦以上的掺鐿光纤激光器高功率输出的偏振特性难以保障，因此应用于光谱合成系统中的光栅（简称合束光栅）的最佳选择是需要具有偏振无关特性，即在TE和TM偏振态下同时具有高衍射效率。为了提高光谱合成的激光阵列数从而提高输出功率，合束光栅同时也需具有宽带宽特性，即在宽波段具有高衍射效率。因此，具有偏振无关与宽带特性的合束光栅是光谱合成技术中的关键元件。然而，随着合束功率的提升由激光热沉积引起的光栅热畸变效应变得更为突出。光栅热畸变效应轻则对合成效率及合成光束的传播特性造成影响，重则损坏光栅。为了进一步提升光谱合成的输出功率与光束质量，研究合束光栅热畸变的作用机理显得尤为重要。 本文首先采用简易模式法与严格耦合波法讨论了光栅占宽比、光栅脊高度对偏振无关特性的影响，并设计制备了一款新型具有偏振无关宽带特性的合束光栅，然后将其应用于光谱合成系统，实现了高功率合成输出。 基于这款合束光栅，本文着重研究了光栅热畸变在光谱合成系统中的作用机理。光栅热畸变主要由连续激光辐照引起热积累效应引起，光栅的温升导致内部应力分布变化，从而引起光栅表面面形形变，衍射光的近场相位分布也随之改变，近场相位的调制导致远场强度分布的改变，从而导致光谱合成系统中光束传播特性的改变。因此，光栅热畸变主要体现为光栅表面面形畸变。根据热传导、热应力的基本原理，采用有限元分析方法，应用COMSOL模拟不同基底厚度、不同固定方式等情况下光栅的热形变分布，得出形变函数。再根据形变函数，结合严格耦合波理论，建立光栅热畸变物理模型，计算热畸变引起的近场相位调制。然后，根据光谱合成基本理论和标量衍射理论，计算光栅热畸变对合成光束传输特性的影响，模拟不同形变高度下合成光斑远场强度分布与光束质量M2。 在光栅热畸变模型建立的基础上，搭建光栅热畸变实时监测平台，监测不同功率密度激光辐照下的光栅表面面形、衍射光斑形貌、衍射光束的M2等，验证热畸变理论模型。采用载波傅立叶变换和泽尼克多项式拟合，可从观测干涉条纹图样中分离光栅面形信息。实验结果表明，光栅热畸变会导致远场光斑畸变，衰减光束质量M2，并且其变化趋势与光栅热畸变模型相吻合。; The rigid demand of high average laser power in the region of material processing, medical therapy, and national security constantly urge scientists to develop high power continuous laser. Although fiber lasers have been developed rapidly in the past years, the output power of single fiber is limited by the laser damage threshold, thermal effects, and nonlinear optical effects. At present, the main two kinds of laser combined technology are coherent beam combining (CBC) and spectral beam combining (SBC). Based on multi-module structure, CBC combines laser beams coherently using phase-lock technology. But it is quite difficult to control output phase precisely. SBC is a promising method to achieve high average power output without influencing the beam quality. The SBC is an incoherent combining technique which combines the output beams of many laser arrays into a single beam by an external cavity with the laser arrays having different wavelengths. A polarization-independent broadband grating plays a key role in the external cavity. With output power of combined beam increasing, it is inevitable that grating thermal deformation happens. Grating thermal deformation may have an influence on the beam propagation characteristics and combined efficiencies in SBC system. Furthermore, grating damage may happen with combined power increasing. It is quite important to study the influence of grating thermal deformation. In this paper, a new kind of polarization-independent broadband grating is designed by simple model method and rigorous coupled wave analysis. The grating is fabricated and implemented in SBC system. Based on the designed parameters of the grating, this paper studied how the grating thermal deformation influences the beam propagation characteristics in SBC system. Due to thermal effects, grating surface deformation happens and has an influence on near-field phase distribution. Based on basic principles of thermal conduction and thermal stress, grating surface deformation can be numerically calculated by business software COMSOL. On the other way, grating surface deformation can be observed by a Michelson interferometer. After handled by Fourier-transform method of fringe-pattern and Zernike polynomials interpretation, grating surface information can be separated from the fringe-pattern. According to the calculated or observed grating surface, the near-field phase distribution can be analyzed by rigorous coupled wave analysis. And then, based on the near-field phase distribution which is modulated by grating surface deformation, far-field intensity distribution can be attained by the scalar diffraction principle. The influence of grating thermal deformation on the combined beam propagation characteristics in SBC system can be analyzed by this method. |
| 学科主题 | 材料学 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/30984]  |
| 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 | 李林欣. 偏振无关合束光栅的热畸变研究[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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