Research On New Cooling Technology For Thermal Management Of High Power Solid-state Lasers

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.