Heat transfer for diode side-pumped YAG slabs

Thermal heating is a major limiting factor in scaling the average power of a solid-state laser. The heat transfer coefficient is affected by the coolant flow rate, the physical properties of the laser slab and the coolant, and the pumping cavity geometry. The relationship between the heating effects and the heat transfer coefficient has been studied by considering the variations of thermal conductivity and expansion coefficient of the laser slab with temperature. It is concluded that different heat transfer coefficients should be adopted according to different heat intensities inside the laser slabs in order to obtain better pumping input as well as to optimize the cooling effects.     

Thermal and optical properties of diode side-pumped solid state laser rods

Thermal heating is a major limiting factor in scaling the average power of a solid-state laser. The relationship between the heat intensity in a laser slab and the heat transfer coefficient has been studied by considering the variations of thermal conductivity and expansion coefficient of the laser slab with temperature. In this paper, the laser rod has been studied by including its thermal, stress and optical aberration effects. It is pointed out that different heat transfer coefficients should be adopted according to different heat intensities inside the laser rod in order to obtain better pumping input as well as to optimize the cooling effects.     

Thermal lensing of diode side-pumped solid-state lasers

The pump energy distribution in a diode side-pumped solid-state laser, is an overlap of propagating Gaussian beams. A simple model has been developed to calculate the thermal focal length of a diode side-pumped solid-state laser, which is based on a thermal model with a Gaussian heat density in any cross section of a laser rod. It can be seen that as the waists of pump beams increase, the energy distribution tends to be uniform and the thermal focal length tends to be long, which means a smaller thermal focusing.     

Diffraction losses of Nd:YAG and Yb:YAG laser crystals

Based on space-dependent rate equations, the lowest threshold input power for a diode end-pumped solid-state laser is obtained for the pump spot size w_p→0. However, as the pump beam waist is decreased, the thermally induced effects in the laser rod would be very high. Diffraction losses caused by radial and tangential variations of refractive index have been analyzed and compared for the Nd : YAG and the Yb : YAG at room temperature (300 K) and liquid-nitrogen temperature (77 K).     

Influence of pump beam size on laser diode end-pumped solid state lasers

Based on space-dependent rate equation, the lowest threshold input power for diode end-pumped solid state lasers is obtained for pump spot size w_p → 0. However, when the pump beam waist is decreased, the far-field half-angle increases due to the same beam quality, which will influence the match between the pump volume and mode volume. Most importantly, when the pump beam waist is decreased, the temperature and temperature gradient in the laser rod would be very high due to the resulting heat, which would cause material distortion, variation of refractive index, instability of laser resonator etc. We analyze the temperature distributions with different pump beam sizes and conclude that the condition w_p ≈ w₀ (the radius of the laser beam at the laser rod) is often taken as a rough guide to the optimum case.     

Thermal lensing and spherical aberration in high-power transversally pumped laser rods

A precise knowledge of the thermal effects in laser crystals is very important for high-power laser design, but some relevant parameters are most often neglected in their calculation. In this paper, with the example of transversally pumped, high-power Nd:YAG lasers, we show the importance of the thermal dependence of the gain medium’s physical properties such as the thermal conductivity, the dn/dT and the expansion coefficient on the thermal aberration and thermal lensing values; we also prove that the choice of the resonator can influence the amount of spherical aberration. Finally, analytically calculated values are confronted to experimental values, and show a very good agreement.     

评估固体激光器对流换热系数的方法

为提高固体激光器的热管理效果,提出了一种使用316L不锈钢片代替激光晶体评估固体激光器表面对流换热系数的方法。在低冷却水温的条件下,使用快响应热电偶对替代片和出口水流的动态温度进行测量,应用有限元方法计算不同对流换热系数下替代片的动态温度,讨论了对流换热系数对重频大能量激光器热效应的影响。通过寻找测量值和计算值的最小方差,得到该冷却结构下的激光器表面对流换热系数为3 500 W·m-2·K-1。     

评估固体激光器对流换热系数的方法

 为提高固体激光器的热管理效果,提出了一种使用316L不锈钢片代替激光晶体评估固体激光器表面对流换热系数的方法。在低冷却水温的条件下,使用快响应热电偶对替代片和出口水流的动态温度进行测量,应用有限元方法计算不同对流换热系数下替代片的动态温度,讨论了对流换热系数对重频大能量激光器热效应的影响。通过寻找测量值和计算值的最小方差,得到该冷却结构下的激光器表面对流换热系数为3 500 W·m^-2·K^-1。     

激光相干合成中单元的相位和振幅波动对合成光束远场分布的影响

运用傅里叶光学分析法推导出系统抖动造成单个光源的相位和振幅发生波动时的远场光强表达式,以1维阵列为例分析了系统抖动对远场的影响。研究表明:随着参与合束的发光单元数目的增加,尖峰变锐,能量更集中;系统抖动引起了远场峰值光强的减少,出现了本底现象,破坏了远场的对称性和光束质量;激光阵列单个光源的相位随机抖动应该控制在3/8范围内;相干合束发光单元数目越多,系统抖动对远场的影响越小。     

射流式水冷镜对流换热系数的优化

介绍了射流水冷技术并将其应用到高能激光器反射镜的冷却中,设计了射流式水冷镜。采用计算流体力学的方法,利用通用有限元软件ANSYS的流体分析模块FLOTRAN对单孔内射流进行了数值模拟,详细讨论了各孔参数对换热性能的影响,实现了孔参数的优化。结果表明:在水冷镜中采用射流技术可以获得很好的对流换热效果,换热系数可达100 kW/(m2·K)以上,且具有结构简单、可控性强等特点。     

Efficient diode-pumped intracavity frequency-doubled CW Nd : YLF laser emitting in the red

We demonstrate an efficient continuous-wave diode-pumped Nd : YLF laser emitting in the red (660.5 and 657 nm) by intracavity frequency doubling with a LBO crystal. We obtained more than 1 W of average power (in two output beams) for 12 W of pump power with a TEM₀₀ mode. This power is to our knowledge the highest obtained with a frequency-doubled Nd : YLF laser emitting in the red. Comparison between “a-cut” and “c-cut” crystals has been carried out in terms of performance and wavelength emission.     

纳米流体对流换热系数增大机理

纳米流体流动换热能力优于传统流体介质.研究了纳米流体热物性的提升和热散射对其对流换热系数的影响.结果表明,纳米颗粒的加入,优化了介质的热物性,增大了导热系数,强化了纳米流体内颗粒、流体以及流道管壁碰撞和相互作用,同时加强了流体的混合脉动和湍流,从而增大了对流换热系数. 关键词： 纳米流体 换热系数 热散射     

An improved transmission line laser model for multimode laser diodes incorporating thermal effects

An improved transmission line model to study the thermal effects in semiconductor laser diodes is reported in this paper. The temperature effects in the laser characteristics are obtained by incorporating temperature dependent gain and carrier density equations for the laser cavity. These primary factors are introduced in the regular transmission line laser model to estimate the static and dynamic characteristics of an 1.3μm InGaAsP double heterostructure laser diode. The results show good agreement with the experimental observation and solution of rate equations referred in the literature. The key feature of this model is that it provides the laser spectra at various temperatures. Based on the model, time dependent evolution of optical spectrum, temperature dependent optical output and frequency chirp are evaluated. Further the distribution of photon and electron density within the cavity is also determined.     

High conversion efficiency solar laser pumping by a light-guide/2D-CPC cavity

A simple and efficient light-guide/2D-CPC solar pumping approach is proposed. A fused silica light-guide assembly is used to transmit 6 kW concentrated solar power from the focal spot of a large parabolic mirror to the entrance aperture of a 2D-CPC pump cavity, where a long and thin Nd:YAG rod is efficiently pumped. Numerical calculations are made for different light-guides, 2D-CPC cavities and laser rods. The laser output power is investigated through finite element analysis. With 4 mm diameter rod, the maximum calculated laser power of 75.8 W is obtained, corresponding to the conversion efficiency of more than 11 W/m². The tracking error dependent laser power losses are lower than 4%. A small scale prototype was constructed and tested, reaching 8.1 W/m² conversion efficiency.     

Properties and laser oscillation of the (Nd,Y) pentaphosphate system

We report on fluorescence and other properties of the system (Nd, Y)P₅O₁₄. With a single crystal of 1.32 mm length containing approximately one atomic percent yttrium we obtained pulsed laser oscillation at 1.05 microns wavelength with peak powers exceeding 200 Watts.     

组合曲线回归法计算螺旋扁管管内对流传热系数

针对螺旋扁管壁面温度难于准确测试的特点,提出采用线性曲线回归与非线性曲线回归相结合的方法进行管内传热系数计算.进行了光管与螺旋扁管管内对流传热实验,并用该方法计算管内对流传热系数.光管传热系数计算值与经典公式的大偏差小于6%.螺旋扁管管内传热系数的计算值略大于通过测量壁温获得的传热系数值,分析了二者存在差异的原因.组合...     

弥补激光棒热透镜的最佳端面研磨半径理论计算

从激光晶体的热效应理论出发，把棒状激光晶体等效为梯度折射率棒．利用梯度折射率光学理论，由梯度折射率棒的光焦度公式，推导了更为精确的激光棒端面研磨半径公式．并把它与通常使用的端面研磨半径公式进行了比较．结果发现，它们的曲线在激光棒耗散功率比较大时，吻合的比较好（≥450w）；在小耗散功率时，两曲线差别很大（〈450W）；并且在耗散功率为某一定值时（420W左右），有一个无穷点．最后理论分析了它们差异的原因．     

Experimental study of high-power pulse side-pumped Nd:YAG laser

The paper reports on the characterization of a compact and simple side-pumped 0.538 J×100 Hz pulse Nd:YAG laser. A side-pumping configuration with 100 laser diode bars is used in the laser head. We also experimentally studied the laser performance of the diode-pumped Nd:YAG laser head in the free running and Q-switched operation under different repetition rates.     

CO2 laser welding of copper slabs

For the first time it has been possible to laser weld copper sheets up to 3 mm thick with a 2 kW cw CO₂ laser. The main questions arising from the process are indicated.     

基于侧面热对流的LD泵浦Yb:KGd(WO4)2热透镜效应研究

基于对LD端面泵浦Yb∶KGW激光晶体工作特点的分析,建立了符合条件的激光晶体热模型.考虑晶体侧面的热对流,运用MATLAB和FORTRAN软件,通过有限差分法求解各向异性介质热传导方程,得到了该晶体的温度场分布、端面热形变分布和不同泵浦功率下的热焦距.结果表明:在泵浦功率为14 W时,泵浦面最高温度达272.8℃、最...