【作者】 王明哲；

【导师】 谭吉春； 景峰；

【作者基本信息】 国防科学技术大学 ， 物理学， 2011， 博士

【摘要】 热效应严重限制了高功率、高光束质量固体激光器的进一步发展。发展适用于重频、大能量激光器的新型热管理技术显得十分必要。本论文首先改进了已有的光热耦合的热效应分析理论,然后在此基础上,采用低温技术、相变冷却技术和径向偏振光技术,对Nd:glass、Yb:YAG固体激光器热管理进行了研究。主要内容包括以下四个方面:1.光热耦合的热效应分析理论(1)对传统热效应分析模型的改进和补充采用光线追迹方法,以二极管的微激射元为基本单位,设计高功率激光二极管阵列的端面泵浦耦合系统,并将光线追迹的结果作为介质热分析的生热源。建立了三维、瞬态的有限元模型求解介质的温度场、热应力分布,并提出了一种基于实测泵浦光斑求解热致波前畸变的方法,讨论了泵浦不均匀性的影响。实验上测量泵浦光斑,验证了泵浦耦合系统设计模型;使用热像仪测量钕玻璃介质的温度分布,校验了有限元方法的热传输计算模型。相比于传统模型,微激射元作为基本单位、实测泵浦光斑作为载荷等方法使模型更接近实际情况。(2)基于统计学的热致断裂概率分析模型及介质泵浦极限研究研究了热致断裂应力的评判标准和临界值概念,建立了一种统计学的热致断裂概率分析模型,分析了介质尺寸、应力、断裂概率三者的对应关系。分别研究了钕玻璃、Yb:YAG两种介质的断裂极限功率和吸收极限功率,指出不同重复频率下的泵浦极限机制不同。对17 kW,1 Hz泵浦的钕玻璃介质进行了断裂实验。(3)评估介质表面对流换热系数的方法提出了一种基于快响应热电偶测量不锈钢“替代片”温度变化,结合有限元计算模型反推表面对流换热系数的方法。通过寻求实测温度值和不同模拟参数下温度值的最小方差,得出某Yb:YAG放大器冷却构型表面对流换热系数值为3500 W/(m2 K)。该方法解决了对流换热系数计算模型不统一、实验中难以测量的问题。2.液氮制冷的低温Yb:YAG激光器低温下激光介质的热特性大幅改善,受激发射截面大幅提高。本文首先从材料特性的角度分析了温度对Yb:YAG介质的影响,研究了~100K低温激光器的热效应;然后研究了“电透镜”效应,并就Yb:YAG介质低温下的“电透镜”效应进行了理论分析,讨论了ASE效应、温度等因素的影响;针对低温激光器面临的泵浦传输难度增加的问题,提出了“楔形窗口”“实心棱镜”等解决方案;最后实验研究了12 kW端面泵浦的V型腔液氮冷却Yb:YAG激光器,对液氮制冷技术、系统效率以及最佳工作温度问题进行了讨论。3.利用相变潜热的固体激光介质散热技术(1)环路热管冷却的Yb:YAG放大器实验研究根据热阻理论以及多种新型热管技术的不同原理,设计了环路热管端面冷却Yb:YAG激光放大器和平板热管冷却的低温激光器,应用有限元软件ICEPAK建立了系统级热效应分析模型,实验上测量了6 kW泵浦的环路热管端面冷却Yb:YAG激光放大器性能,与水流冷却的对比表明,~1 Hz低重频下热管冷却完全可以代替水流冷却。(2)基于新型热管的非均匀散热理论与技术研究基于重频大能量激光器介质温度分布呈现的时、空不均匀性,提出了非均匀生热和非均匀散热两种热管理思路,即对原本温度高的地方减少生热、增强散热。与传统冷却方式对比的模拟结果表明:时空非均匀散热可以不同程度的降低温度梯度。例如:空间非均匀散热可将最高温度由362.3 K下降为347.6 K,最低温度从288.1 K升为292.6 K,从两方面共同减小了介质温度梯度;时间非均匀散热可将最高温度从362.3 K下降为336.1 K,最低温度不变。分别研究了利用嵌入式微热管阵列实现空间非均匀散热、利用脉动热管的振荡特性实现时间非均匀散热的技术,分析了基于热管技术实现非均匀散热的可行性。4.基于径向偏振光的光弹效应规避技术论述了径向偏振光在热管理中的优势。从规避光弹效应的角度出发,对径向偏振光的热致双焦点效应和热退偏效应进行了理论分析。重点分析了利用各向同性介质的热致双焦点产生径向偏振光的技术,该技术在利用热效应的同时,可规避部分热效应。研究了径向偏振光应用于固体激光热管理中的两个关键单元技术,即高度对称的泵浦场设计和补偿元件的设计,建立了环形激光二极管泵浦棒状放大器的设计理论,综合考虑了介质增益、系统效率、泵浦均匀性等因素。本论文对于光热效应分析模型的改进和对系统级热管理新技术的研究,可对新一代高功率固体激光器的设计和运行提供参考。更多还原

【Abstract】 Solid-state lasers are developing rapidly with the trends of high power, high beam quality, high efficiency and miniaturization, however, the thermal effects limit the further development severely, the innovative solutions are very necessary. In this dissertation, an optical-thermal coupled theoretical model is presented, from the points of uniform heat generation, high efficient heat removing and effective avoidance, the cryogenic laser technology, phase change cooling technology and the radially polarized light technology are studied which lead to the system-level thermal management. The major works are shown the following:1. Optical-thermal coupled thermal effects analysisUsing ray tracing method, based on the Laser Diode(LD) bar’s structure, the coupling system of high power LD array end-pumped laser is designed, and the results of ray tracing are taken as heat generation of thermal analysis. A three-dimensional, transient finite element model is established to solve the temperature and thermal stress distribution in laser medium. Thermal wavefront distortion is also solved with the measured pump profile, after that, the impact of pump uniformity is discussed. Measured pump profile verify the design model of the pump coupling system, and by thermal imaging of Nd:glass, the finite element heat transfer model is also verified. Compared to the conventional model, it is closer to the actual situation when using micro LD element as the basic unit of ray tracing and taking measured pump profile as load of thermal analysis.The evaluation criteria and critical value of thermal stress inducing fracture are reviewed. A Weibull statistical model is established to predict the possibility of fracture taking account of medium size and stress value. The fracture limit and absorption limit of Nd:glass and Yb:YAG are analyzed respectively. The mechanism of pump limit varies under different repetition rate. Fracture experiment of 17 kW, 1 Hz pumped Nd:glass is presented and the rationality of model is discussed.The surface heat transfer coefficient is estimated by using a fast thermocouple to measure the temperature evolution of false crystal, while compared with finite element analysis. Searching the lowest least square difference between measurement and simulation, a Yb:YAG amplifier surface heat transfer coefficient value of 3500 W/(m2K) is obtained. This method is simple and costless, while other calculation models are inconsistent and the direct experimental measurement is quite difficult.2. Liquid nitrogen cooled cryogenic Yb:YAG laserFirst, dependence of the Yb:YAG properties on temperature is reviewed and the thermal effects of laser medium with ~100K cooling are calculated, and then the electronic lensing effects for Yb:YAG medium are analyzed theoretically, the influence of cryogenic temperature is studied for the first time so far as we know. Comparing to the thermally induced refractive-index changes, the importance of electronic lensing is described. As liquid nitrogen(LN) cooling facing the problem of pump laser transmission, "wedge-shaped window," and "kaleidoscope homogenizer" are proposed as solutions. Finally, experimental study of a 12 kW LD pumped LN cooled V-shaped Yb:YAG laser is reported, LN refrigeration technique, system efficiency and other problems encountered in the experiment are discussed.3. Heat removing of medium with two-phase cooling technologyAccording to the thermal resistance theory and the various principles of novel heat pipe, loop heat pipe end cooling Yb:YAG laser amplifier and flat heat pipe cooled cryogenic laser are designed. Using finite element software ICEPAK, a system-level thermal analysis model is established. By experimental measurements on the 6 kW LD end-pumped loop heat pipe cooling Yb:YAG laser amplifier performance versus water cooling, the results show that in low repetition rate(~1Hz) water cooling can be replaced by heat pipe cooling.Based on the temporal and spatial asymmetry of medium temperature in rep-rated high energy regime, two novel ideas of non-uniform heat generation and non-uniform heat removing are presented. For example, the spatial non-uniform heat removing concept is increasing heat dissipation where the temperature is high. Compared with conventional cooling methods, the simulation results show that temporal and spatial non-uniform heat removing can reduce temperature gradient to a certain extent. By using embedded micro heat pipe arrays and pulsating heat pipe, the spatial and temporal non-uniform cooling can be achieved respectively. The idea of non-uniform cooling is proposed for the first time as we know.4. Application of radially polarized beam to avoid photoelastic effectsTo avoid the photoelastic effects, the thermal bifocusing mechanism and depolarization on radially polarized beam are theoretically studied, its superiority in thermal management is pointed out. In particular, the generating method of making use of thermal bifocusing in isotropic medium is investigated and its predominance is emphasized.Two key issues of applying the radially polarized beam in thermal management of high power solid-state lasers are analyzed, which are the designs of highly symmetric pump profile and aberration compensation components. The complete design theory of LD ring side-pumped rod amplifier is proposed, factors such as gain coefficient, system efficiency and pump uniformity are all taking into account.In conclusion, this paper improves the model of the optical-thermal effect analysis and proposes system-level thermal management with three new technologies, the results will be helpful in the design and operation of high power solid-state lasers.更多还原