LD端面泵浦高效紧凑型低噪声固体激光器研究
文献类型：学位论文
| 作者 | 林强 |
| 学位类别 | 博士 |
| 答辩日期 | 2008 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 朱健强 |
| 关键词 | 模重叠 晶体热效应 谐振腔敏感性 激光装调工艺 倍频噪声 |
| 其他题名 | LD End-pumped High Efficient Compact Low-noise Solid-state Lasers |
| 中文摘要 | 激光二极管（LD）泵浦固体激光器是近年来国际上发展较快、应用较广的新型激光器，由于LD高功率密度、高效率、长寿命、性能稳定、体积小等优势技术使紧凑型固体激光器的应用越来越广。端面泵浦方式是小型固体激光器中最常用的泵浦方式，特别是将输入腔模镀制在激光介质入射面上，使固体激光器结构更紧凑，这种紧凑型端面泵浦固体激光器中的腔模与增益分布在空间上关系密切，这种关系直接影响着激光输出的功率、效率以及稳定性等激光属性，因此，LD端面泵浦固体激光器的模特性问题就显得尤为重要。 本文首先分析了LD端面泵浦光束和激光腔模的空间重叠情况，将增益场和光子场的空间分布引入到耦合腔的速率方程中，计算了一些特定增益场激光输入输出关系，得到了高斯泵浦光束和高斯腔模分布情况输出功率和输入功率关系的线性近似解。分析得出，在泵浦光束为中心对称分布时，不同的增益强度分布，激光输入输出的差别不大，说明端面泵浦的激光器对于泵浦光束的分布形状要求并不是很严格。而泵浦光束和腔模尺寸比的不同对激光输出影响的很大。端面泵浦光束和腔模之间的耦合系数随泵浦光束和腔模尺寸比增大而减小，为了提高激光效率，需要适当降低泵浦光束尺寸，使泵浦光束尺寸小于腔模尺寸。 考虑到从泵浦源激光二极管发射的光束具有一定发散性和非对称性，分析了不同椭圆度情况下，激光阈值、斜率效率、输出功率与泵浦光束和模尺寸比的关系，优化了腔模设计。对于给定的泵浦光束尺寸，存在模尺寸的优化值，该优化值是泵浦功率的递增函数，这个结论可用于指导计算模半径和设计激光谐振腔，特别是对于光束形状为非对称的泵浦源LD。提出了端面泵浦TEM00模工作的固体激光器模优化设计方法，对非对称端面泵浦的固体激光器模关系作最优化分析，说明了泵浦光束越接近圆形，按照优化的腔模尺寸，激光效率越高，实例说明紧凑型端面泵浦固体激光的光光转换效率可达48％。 影响端面泵浦紧凑固体激光器功率稳定性的因素主要是激光器本身的热、结构失调引起的腔参数变化以及环境的温度变化。由于在端面泵浦紧凑型固体激光器中泵浦光束只是在激光介质中的局部产生作用，这使得激光介质具有很显著局部热效应。对于紧凑型固体激光器而言，由于其输入腔在晶体上的特性，热效应对光束波前的影响更大。针对这个问题，本文实测了平凹腔端面泵浦紧凑型固体激光器输出功率稳定性，用有限元法得到了这种激光器的热形变，分析腔模对热形变的敏感性。定义了腔失调敏感因子，得到了谐振腔对腔镜位置微小变化的敏感性。阐明了端面泵浦紧凑型固体激光器稳定谐振腔的优化设计过程，优化后激光输出光功率波动从8％提高到了优于1.5％。用设计实例验证了这种固体激光器谐振腔稳定性优化设计的可行性，基本解决了端面泵浦紧凑型固体激光器功率稳定性问题。阐述了两种通过反馈控制方式使激光器受外界环境温度的影响较小方法，以控制激光器的稳定性，并指出了两种反馈控制方式的反馈条件以及激光器稳定工作的保护措施。 固体激光器的红外光经倍频后产生短波的红、绿、蓝光在激光数据存储、激光印刷技术和激光医学诊断等方面的应用较为广泛。绿色激光和红色激光较容易获得，而腔内倍频的蓝色激光却较难获得，虽然LD端面泵浦Nd：YAG晶体BBO、LBO和PPKTP倍频可获得473nm蓝色激光输出，但由于这类激光器处于准三能级工作，从物理机理而言，激光工作运行效率不高，调整也很困难，因此腔内倍频蓝色激光器的工艺问题显得尤为重要。为了适应蓝色激光器生产需要，我们从Nd：YAG晶体946nm固体激光器模型出发，设计了腔内倍频473nm固体激光器装置。从工艺技术角度，设计了泵浦光耦合光学系统、激光器光学元件装调结构、激光器温度控制系统，详细描述了腔内倍频473nm固体激光器的装调过程，以满足对腔内倍频低噪声固体激光器的产业化要求。另外，针对这类腔内倍频激光器存在的“绿色噪声”问题，我们从振荡机理上分析这种蓝色激光噪声产生的机制，以描述腔内倍频固体激光的振荡特性，并对473nm蓝色激光器噪声作了初步分析，说明除非激光器处于单纵模工作状态，否则激光器中总是存在倍频噪声。而对于多纵模工作的激光器，随着泵浦功率的提高，激光腔内模的尖峰效应越来越明显，输出倍频激光噪声越来越大。初步实验结果表明，为了把噪声控制在实际需要的30％以内，需要适当控制泵浦功率，而且泵浦功率一般不能超出阈值的1.5倍。 |
| 英文摘要 | LD end-pumped solid-state lasers is a new type lasers, it is developed quickly and used popularly recently. With the advantage of high power intensity, high efficiency, long life, high stability and small volume of LD, the small solid-state lasers are used more widely. The end-pumped confuguration is the most commonly used in small solid-state lasers and especially the input cavity are filmed on the incident surface of laser medium which make the structure more compact. The cavity mode is tight connected with the gain distribution and the relationship should affect the output power, the slope efficiency and the stability of lasers directly. Therefore, the study of mode properties of LD end-pumped solid-state lasers is especially important . Firstly, the overlap of the gain distribution and laser cavity mode is analyzed. The gain distribution and photon field are included in the rate equations of coupled cavity so that the linear approximate solution of output power and input power is obtained by calculating when they are Gaussian pump and Gaussian cavity mode distribution. The result shows that when the pump beam is centrally symmetrical, output of laser changes little with different gain distribution so that it is not strictly require of pump beam profile for end-pump laser. But the output of laser is influenced strongly by the different ratio of the pump to mode radii. The coupling efficiency of end-pumped beam and cavity mode decreases with the increase of the ratio of the pump to mode radii. For the improvement of laser efficient, it is necessary to reduce the size of pump beam appropriately and make it smaller than that of cavity mode. As far as the beam divergence and asymmetry of LD are concerned, we analyze the threshold, slope efficiency and output power as functions of the ratio of the pump to mode radii for different ellipticity. For the given pump beam size, there is an optimized value of mode size and the optimized value is the increasing function of the pump power. The result should be guideline to the mode size and design the laser cavity, especially for asymmetry of LD. The design and optimization criteria for the mode size of end-pumped solid-state lasers are proposed, assuming TEM00 mode in the cavity. The optimum ratio between mode and pump beam size for elliptical pump beam is obtained. It is shown that the conversion efficiency of the laser is increasing with the ellipticity of pump beam. The experimental result for LD end-pumped solid-state lasers shows a fairly good agreement with the prediction of the model and the conversion efficiency of 48% is obtained. The main factor influencing output power stability of end-pumped compact solid-state laser is the change of resonator parameter caused by the heat, the misalignment and vibration of structure and the ambient temperature. Pump beam doesn’t acts on the complete laser crystal in LD end-pumped lasers, so there is large local heat deposition on the pumped part of crystal and local thermal effects. For the character with the input cavity on the crystal, the heat effect brings more impacts on the beam wave-front. Therefore, we have detected the output stability of end-pumped compact solid-state lasers with plan-concave cavities. The thermal distortion is obtained with finite elemental analysis (FEA) and the thermal stability of resonator is analyzed. Two misalignment sensitive factors are defined to analyze the mechanical sensitivity of cavity in terms of slight misalignment. The optimized design procedure of stable resonator is proposed for end-pumped compact solid-state lasers and the result of laser output fluctuation of 1.5% is achieved from 8%. The experimental proves the feasibility of optimized design procedure and solve the problem of stability of end-pumped compact solid-state lasers. Two methods to reduce the effect of ambient temperature are proposed to control the stability of lasers. The feedback condition of control method and the lasers protecting means are figured out. The red, green and blue lasers those are generated from intracavity frequency doubled infrared lasers are widely used in laser printing technology and laser medical diagnosis. The green and red light are easy to get but not for intracavity frequency doubled blue laser. Although the 473nm blue laser can be obtained from Nd:YAG crystal end-pumped by LD with BBO, LBO or PPKTP inserted into cavity, the efficiency of laser is low because of the quasi-three-level lasers and it is hard to assemble. Therefore, the alignment technology is important for intracavity frequency doubled blue laser. For the need of manufacture of blue lasers，the intracavity frequency doubled 473nm lasers is designed starting from the model of Nd:YAG 946nm lasers. The coupling optical system of pump beam, the alignment structure of optical element and temperature control system of lasers are designed. In addition, there is “green problem” which causes large amplitude fluctuation seriously. The producing procedure of intracavity frequency doubled lasers is demonstrated to describe the oscillation behavior of laser. The noise of 473nm blue laser is primarily analyzed which shows that unless operating with single longitudinal mode, there always exists noise in the resonator. And for the multi-longitudinal mode, the laser mode spiking behavior is more clear and the noise more large with increasing pump power. The primarily experimental result has shown that it is necessary to control the pump power less than 1.5 times of threshold so that noise level could be within 30% of the pratical requirement. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15602]  |
| 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 | 林强. LD端面泵浦高效紧凑型低噪声固体激光器研究[D]. 中国科学院上海光学精密机械研究所. 2008. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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