Theoretical prediction and experimental validation of the angle-of-arrival probability density of a laser beam in a strong plane-flame turbulence

The propagation of a laser beam through a plane turbulent flame is studied using geometrical optics approximation. The random fluctuations of the refractive index caused by a strong thermal turbulence in the flame create random perturbations of the laser beam direction. From the Markovian process model applied along the whole random path of the beam, the theoretical probability density of the laser beam angle-of-arrival is derived from the analytical solution of the Einstein-Fokker-Kolmogorov equation, which we have determined in terms of a series expansion of spherical harmonics. An experimental setup and a method for measuring this probability density are described. The experimental results obtained are shown to agree with the theoretical predictions.     

Heat Transfer Analysis on Laser-tissue Thermal Interaction Using Heterogeneous Model

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Role of epidermis in the optics and thermal physics of human skin

Some engineering approaches to the problems of optics and thermophysics of biological tissues have been developed. The light fields both within and beyond biological medium have been studied. The role of the epidermis in the formation of these fields is revealed. It is shown that in the general case epidermis cannot be considered as a spectral filter that only absorbs and does not scatter light. A convenient approximation of the fluence rate in a two-layer (epidermis and dermis) tissue is proposed, which is the sum of two exponential functions of depth in the epidermis. This approximation made it possible to obtain an analytical solution to the problem of heating biological tissues by an external narrow light beam. It is found that epidermis only slightly affects the temperature fields under blue light irradiation. However, when an external source of red light is used, the epidermis works as a heater and can increase several times the dermis temperature in comparison with a single-layer tissue. The reasons for these heating features are discussed. Examples of corresponding calculations are given.     

激光二极管端面泵浦Yb:YAG圆棒热效应研究

基于激光二极管端面泵浦Yb∶YAG棒工作特点的分析,提出了端面绝热、周边恒温的激光晶体热分析模型,采用了一种新的热传导方程求解方法,得到了超高斯光束端面泵浦Yb∶YAG棒温度场的一般解析表达式。同时分析了不同阶次、不同光斑半径、不同功率超高斯光束以及晶体参数改变时对于Yb∶YAG棒温度场分布的影响。研究结果表明,若准直聚焦到Yb∶YAG棒泵浦面42.5W的光束具有4阶超高斯强度分布时,掺Yb3+质量分数为10.0at.%、长度为2.5mm、半径为2mm的Yb∶YAG棒的泵浦面获得74.20℃的最高温升。新的热传导方程求解方法在研究激光棒温度场分布方面具有计算量小、精度高等特点。研究结果对减小激光晶体的热效应,提高全固态Yb∶YAG激光器性能提供了理论依据。     

纳秒级脉冲激光诱导Zn掺杂过程中GaN/Al2O3材料的温度分布及热形变解析分析

分析了纳秒级脉冲激光作用下GaN的激光诱导Zn的掺杂过程.利用简化的一维模型，给出一种比较直观的脉冲激光辐照下GaN/Al₂O₃材料温度分布的解析形式，得到了GaN材料表面温度与激光辐照时间的关系以及材料形变与深度的关系.在激光脉冲作用时，GaN材料表面的温度与辐照时间的平方根成正比.脉冲过后，材料温度分布梯度和热形变分布随深度发生变化，接近表面的温度梯度最大，热形变量也最大.而在连续脉冲作用时表面的温度呈锯齿状不断升高. 关键词： 激光诱导 2O₃')" href="#">GaN/Al₂O₃ 温度分布 热形变     

基于液晶空间光调制器的太赫兹波频谱调制

频谱可调制的太赫兹波具有广泛的应用价值。利用一台纯相位式的液晶空间光调制器对飞秒激光脉冲进行空间整形,通过改变飞秒激光脉冲的横向空间分布,实现太赫兹波频谱的调制。在实验中,利用光泵浦整流方式产生太赫兹波,并利用太赫兹时域光谱系统对太赫兹信号进行探测。通过GS算法在液晶空间光调制器上加载不同的相位图,获得了不同的飞秒激光脉冲横向空间分布。通过改变探测距离和飞秒脉冲的空间分布参数,实现了太赫兹波频谱的调制。还利用菲涅尔衍射算法对这一过程进行了理论模拟,理论模拟结果与实验结果吻合的较好,这充分说明了基于飞秒脉冲空间整形的太赫兹光谱调制技术的可行性。     

采用解析法对Nd:YAG单晶光纤的温度场研究

采用解析法对Nd:YAG单晶光纤激光器热效应相关的光纤温度场分布进行研究。建立了Nd:YAG单晶光纤热模型,在单晶光纤所满足的边界条件下通过解析求解热传导方程,得到在高功率808 nm泵浦光抽运下产生946 nm激光的单晶光纤温度场分布,并与传统Nd∶YAG激光晶体的温度场进行比较,然后分别与同泵浦条件下的有限元数值方法的分析结果进行研究对比,最后分析泵浦光参数、单晶光纤参数等对温度场的影响。结果表明,功率为86 W的泵浦光入射至单晶光纤端面的最高温升仅为30.98℃,明显优于同泵浦条件下传统Nd∶YAG晶体的端面温升结果94.37℃,与有限元数值法得到的Nd:YAG单晶光纤19℃温升结果相比,该解析法结果更接近于实验的测量值31℃,能够更精确描述晶体光纤温度场。本文可对单晶光纤激光器热效应的精确研究提供一定参考,进而有利于提高单晶光纤激光器的性能。     

Heat Transfer Analysis on Laser-tissue Thermal Interaction Using Heterogeneous Model

Considering the heterogeneous thermal properties of living tissues,a multi-layer heat transfer model for laser-tissue thermal interaction is presented in this paper.An analytical solution to the temperature transients within two tissue layers is obtained using the Laplace integral transform method.Heat transfer behaviors inside the tissues before evaporation are investigated.The critical time required to vaporize the surface tissue and the thermal damage depth are numerically predicted.It was shown that there is evident difference in temperature predictions between the uniform model and the heterogeneous one.This study is useful to laser surgery,such as ophthalmology,dermatology and neurosurgery which request high accuracy on temperature prediction and heat deposition.     

Laser surface melting: an analytical approach to calculate heat and mass flows in liquid phase

Summary The paper describes a three-dimensional analytical model to calculate the thermal and velocity fields induced in a melted pool during high-power laser irradiation. The dependence of the thermophysical properties of matter on temperature is accounted for. The thermal field in solid phase is also discussed. The beam power accounting for the melting process is calculated by means of an energy balance involving the total energy in liquid phase plus the energy flowing through the liquid-solid interface.     

Modeling the interactions between light and crystallizing polymer during fast cooling

In this work, an experimental set-up able to quench thin polymer films whilst recording the sample thermal history as well as the overall and depolarized light intensities of a laser beam emerging from the sample is described. The interactions between the light beam and the crystallizing material have been modeled accounting for absorption and scattering phenomena. The proposed model was found to be able to reproduce the experimentally observed behavior of light intensities and it was validated by comparison with conventional DSC analysis. On the basis of this model, a method to obtain crystallinity evolution is proposed and applied to some fast cooling runs. The method was applied to quenching runs of an iPP carried out under cooling conditions more than one order of magnitude faster than those allowed for the DSC technique. The crystallization temperature dependence upon cooling rate was obtained from these runs in a cooling rate range never before explored. PACS 81.05.Lg; 81.70.Fy     

Diffusion-deformational instability and subboundary formation in the recrystallization of semiconductor-on-insulator films

A novel (defect-deformational) mechanism for subboundary formation in laser beam recrystallization of semiconductor films is proposed. The theory is developed yielding explicit analytical expressions for the distance between subboundaries as a function of film thickness and the scanning velocity of the laser beam. The theoretical results obtained agree well with the experimental results.     

激光辐照引起的材料温度场和热应力场的瞬态分布

 光电探测器吸收激光后的温升以及因温升造成的各种现象,致使探测器遭受到不同程度的损伤。利用热弹性理论对CO2激光器辐照K9玻璃材料进行研究,建立激光辐照材料温升及热应力分布二维平面模型,通过解析计算得到由激光辐照半导体材料引起的温度场和应力场的瞬态分布。研究表明, K9玻璃材料的激光辐照损伤阀值与辐照时间和光斑半径相关。在同一条件下,造成的热应力损伤阀值较熔融损伤的低,故K9玻璃材料的破坏形态为热应力破坏。      

强激光辐照下窗口材料靶传热及热应力破裂的解析分析

用有限Ｈａｎｋｅｌ积分变换法对轴对称强激光束辐照下平面薄靶（或薄膜）传热及热应力分布进行了解析推导，得到了随时间演变的温度场及相应的热应力场的解析解。对比有关实验，对强激光束辐照窗口材料（例如ＭｇＦ２晶体、Ｋ９玻璃）靶引起其脆性热应力破裂进行了分析，详细讨论了激光强度、激光－靶能量耦合系数、辐照时间及光斑尺寸等因素对靶的破坏的影响。     

Experimental and numerical investigation on structural effects of laser pulses for modal parameter measurement

In this paper it has been described part of the research devoted to the development of a complete non-intrusive experimental modal analysis procedure based on laser techniques both for excitation and for measurement. In particular, attention has been focused on the thermal effects generated by laser pulses on the excited structure. An analytical model of the energy exchange between the light pulse and the target surface is proposed together with a finite element model of thermal and mechanical behaviour of the structure under excitation. Both the models (analytical and numerical) have been experimentally validated by measuring the thermal and the vibration responses induced by the laser pulses. The experimental part of the study has been performed on a cantilever beam excited with laser pulses from an Nd : YAG source (532 nm, 100 mJ/pulse) using an high-speed infrared camera and a scanning laser Doppler vibrometer. Results from this work can be used to improve understanding concerning the features of laser excitation and to establish a mechanical equivalent system of forces and moments, useful in order to increase the accuracy in the measurements of modal parameters when laser pulses are used as excitation sources.     

A 3D dynamic numerical approach for temperature and thermal stress distributions in multilayer laser solid freeform fabrication process

This paper presents a 3D transient numerical approach for modeling the multilayer laser solid freeform fabrication (LSFF) process. Using this modeling approach, the geometry of the deposited material as well as temperature and thermal stress fields across the process domain can be predicted in a dynamic fashion. In the proposed method, coupled thermal and stress domains are numerically obtained assuming a decoupled interaction between the laser beam and powder stream. To predict the time-dependent geometry of the deposited material, once the melt pool boundary is obtained, the process domain is discretized in a cross-sectional fashion based on the powder feed rate, elapsed time, and intersection of the melt pool and powder stream area on the workpiece. Layers of additive material are then added onto the non-planar domain. Main process parameters affected by a multilayer deposition due to the formation of non-planar surfaces, such as powder catchment, are incorporated into the modeling approach to enhance the accuracy of the results. To demonstrate the proposed algorithm, fabrication of a four-layer thin wall of AISI 304 L stainless steel on a workpiece with the same material is modeled. The geometry of the wall, temperature, and stress fields across the modeling domain are dynamically predicted throughout the process. The model is used to investigate the effect of preheating and clamping the workpiece to the positioning table. Results show that preheating improves the process by reducing the thermal stresses as well as the settling time for the formation of a steady-state melt pool in the first layer. In addition, clamping the workpiece can also decrease thermal stresses at its critical locations (i.e. deposition region). In terms of geometrical aspects, the results show that the temperature and the thickness of the deposited layers increase at the end-points of layers 2–4. The reliability and the accuracy of the model are experimentally verified.     

Analysis of temperature distributions in diode-pumped alkali vapor lasers

Assuming the pump light to be a Gaussian beam and considering the medium absorption, we establish the heat conduction model to describe the temperature distribution in diode-pumped alkali vapor lasers (DPALs) with two different pumping ways. Combining with the experimental parameters in cesium laser, the spatial temperature distribution picture, the longitudinal and the radial temperature distributions of two cesium lasers are obtained by numerical solving the model. Influences of the pump power, the beam waist and the absorption coefficient on the axial and the radial temperature distribution are calculated and compared for the two Cs lasers. The result suggests that Cs laser pumped by two laser diode arrays can improve the spatial temperature distribution condition and reduce the longitudinal temperature gradient, which is good for reducing the thermal effects and improving the output characteristics of DPALs.     

数值模拟抽运分布对端泵激光器晶体热透镜球差的影响

为了研究端面抽运情况下,激光晶体在不同分布的抽运光抽运时热透镜球差的变化,通过对稳态热传导方程和Zernike多项式的求解,建立了热透镜球差与抽运光强度分布的模型,对模型进行了理论分析和仿真研究,并对仿真结果做了进一步理论和仿真分析.结果表明:在相同的抽运功率下,二阶超高斯分布抽运光抽运时球差最大,且随着抽运分布系数k的增大(除高斯分布外)球差逐渐减小;随着抽运功率的增加,抽运分布系数k对球差的影响逐渐加重,且不同分布系数k所产生的球差差距逐渐增大;并对二阶超高斯分布抽运光抽运得到最强激光功率的照射范围进行了理论分析和仿真分析,得知在相同抽运功率下,二阶超高斯分布抽运光得到最强激光功率的范围最宽为0.30—0.63倍高斯半径.     

Numerical and experimental investigation of seam welding with a pulsed laser

The present work develops two numerical models to compute thermal phenomena during pulsed laser welding. The first one which is based on finite difference model calculates the welding width and its penetration by utilizing heat transfer equations. Parametric design capabilities of the finite element code ANSYS were also employed for the simulation of the second model. The transient temperature profiles, the fusion dimensions and the heat affected zones (HAZ) have been calculated here. The thermo-physical properties are dependent on temperature and so a nonlinear solution is employed. It is found that the temperature profile and penetration depth are strongly dependent on the pulse parameters of laser beam. Finally, the results of the two models and the experimental outcomes are compared.     

BBO晶体倍频中的温度场与光场耦合模拟

首先建立了无限大晶体截面近似下的时-空4维光场和温度场耦合的倍频模型. 并在瓦级功率激光抽运下,利用解析公式和迭代方法数值模拟了BBO晶体内的倍频过程.得到了基频光和倍频光的振幅和转换效率曲线以及温度场分布,展示了温度场及光场的时间演化过程. 其次分析了抽运功率和晶体长度对转换效率和温度场的影响. 结果给出,温度分布对转换效率和光束质量都有较大影响;对热效应引起的相位失配进行补偿能够很好地改善转换效率的下降,因此必须在实际应用中加以考虑. 关键词： 倍频 温度场 BBO晶体     

Exact solution for temerature field due to non-equilibrium heating of solid substrate

Non-equilibrium energy transfer between electron and lattice sub-systems due to short-pulse heating is formulated and the closed form solution for electron and lattice site temperatures is presented. The electron kinetic theory approach is incorporated to formulate non-equilibrium energy transfer in the electron and lattice sub-systems. The method of Lie point symmetries is used in the exact solution of governing energy equation. In the analysis, the volumetric heat source, representing the laser heating pulse, and surface heat source, corresponding to short thermal contact of the surface, are incorporated and the analytical solutions for each heating source are presented. Electron temperature distribution obtained from the closed form solution is compared with its counterpart predicted from the numerical simulation. It is found that the results obtained from the closed form agree well with electron temperature predictions obtained from numerical simulation.