Hyperbolic heat equations in laser generated ultrasound models

The generation of ultrasound by pulsed lasers is a thermoelastic process, and in the literature on general thermoelasticity there are presently three different forms of the heat equation in popular use, the classical, Lord-Schulman (LS), and Green-Lindsay (GL) heat equations. The question may thus arise as to which heat equation should be used to model laser generated ultrasound. The purpose of this work is to summarize the current rationale for using these different heat equations, in order to provide a basis for choosing one of the forms. A review of the classical, LS and GL theories is given, and the potential advantages of hyperbolic heat flow theories in laser ultrasonics are discussed. A numerical example is given that clearly shows the small time differences these theories predict, and also points out potential problems with using hyperbolic heat equations on small time scales.     

用于激光测距的1 572 nm窄带滤光膜

介绍了用于激光测距机的1 572 nm窄带滤光膜的用途、特性和制备方法。利用计算机优化软件对多腔结构进行计算后, 得到了损耗小、制备方便、重复性好的膜系设计。讨论了薄膜的光谱、温漂及均匀性等主要技术问题后,给出了特性测试和环境实验结果。目前相关的带宽12 nm滤光膜应用在新型激光测距机中分别达到了精度±5 m和重复频率(1±0.1)Hz下11 km的使用效果。     

窄带干涉滤光片抗击不同输出方式激光的损伤特性研究

利用Nd∶YAG调Q单脉冲激光和自由脉冲激光对硬膜窄带干涉滤光片进行激光损伤阈值的测试,并且采用表面热透镜技术测量了滤光片的吸收率。实验发现：窄带干涉滤光片的吸收率和激光损伤阈值强烈依赖于辐照激光波长与窄带干涉滤光片通带的相对位置;在调Q单脉冲激光作用下,不同中心波长的滤光片损伤形貌存在明显的差别,而在自由脉冲激光作用下,各滤光片的损伤形貌则趋于相同,均表现为典型的热熔烧蚀破坏。根据实验结果,结合损伤形貌,通过驻波场理论对激光作用下滤光片内电场分布的分析与模拟,探讨了两种激光模式作用下滤光片的损伤特征和损伤机理的不同特点。     

Hyperbolic heat conduction effects caused by temporally modulated laser pulses

In non-destructive testing (NDT), the intensity of laser pulses may be temporally modulated to improve the signal-to-noise ratio of ultrasonic signals. The purpose of this work is to determine the time scale on which hyperbolic heat conduction effects are significant in ultrasonic displacements and stresses for temporally modulated laser pulses, under the assumption that the heat wave speed equals the longitudinal wave speed. A one-dimensional model with a finite train of heat flux pulses is investigated. Using the Green and Lindsay model, it is found that the critical modulation frequency in steel, above which hyperbolic heat conduction effects will become signficant, is 2.26 GHz. A dimensionless factor is given to calculate critical modulation frequencies in other isotropic materials. The signal enhancement potential of temporal intensity modulation is apparent in the numerical results.     

Heat generation by impulse ultrasound

An original method allowing to get a system of nonlinear evolution equations for the interacting modes applies to a problem of the heat generation by non-periodic ultrasound, including impulse one. The basic idea and final equations for the thermoviscous plane flow are presented. The limit of periodic source is traced. The numerical calculations were based on the pulse solution of the Burgers equation as an ultrasound source. Some illustrations on temporal behavior of the medium expansion caused by the pulse ultrasound are presented.     

Laser Accelerated, High Quality Ion Beams

Intense beams of protons and heavy ions have been observed in ultra-intense laser-solid interaction experiments. Thereby, a considerable fraction of the laser energy is transferred to collimated beams of energetic ions (e.g. up to 50 MeV protons; 100 MeV fluorine), which makes these beams highly interesting for various applications. Experimental results indicate very short pulse duration and an excellent beam quality, leading to beam intensities in the TW range. To characterize the beam quality and its dependence on laser parameters and target conditions, we performed experiments at several high-power laser systems. We found a strong dependence on the target rear surface conditions allowing to tailor the ion beam by an appropriate target design. We also succeeded in the generation of heavy ion beams by suppressing the proton amount at the target surface. We will present recent experimental results demonstrating a transverse beam emittance far superior to accelerator-based ion beams. Finally, we will discuss the prospect of laser-accelerated ion beams as new diagnostics in laser-solid interaction experiements. Special fields of interest are proton radiography, electric field imaging, and relativistic electron transport inside the target.     

相控阵高强度聚焦超声的研究进展

相控阵高强度聚焦超声（high Intensity focused ultrasound,HIFU）技术可以通过电子调相自由控制聚焦区域中焦点的形状、位置、个数等,实现高精度、高效率的治疗。文章主要介绍了相控聚焦超声的原理、阵型设计、声场优化、控制算法、电路设计以及换能器材料等几个方面。     

周期结构GaAs晶体ps脉冲差频产生窄带THz辐射的研究

计算模拟了在ps脉冲抽运周期结构GaAs晶体差频产生窄带THz波过程中,对于不同波长、不同脉宽的抽运光,周期结构GaAs晶体的走离长度和最佳周期长度的变化,并对计算结果进行了理论分析.研究了周期结构GaAs晶体的畴数对THz波光谱的影响.根据GaAs晶体温度色散公式,计算并分析了晶体最佳周期长度的温度调谐特性,研究了温度变化对THz波光谱的影响,提出了通过温度调谐实现在周期结构GaAs晶体中产生宽调谐、窄带宽THz波的新方法,该计算结果为下一步的实验研究提供了理论基础. 关键词： ps脉冲 差频产生窄带THz波 周期结构GaAs晶体 温度调谐     

高强度聚焦超声的临床应用

高强度聚焦超声是近几年发展起来的一种新的非介入性肿瘤治疗技术,临床应用越来越广,文章就其在不同肿瘤如前列腺癌、肝癌、胰腺癌、子宫肌瘤等治疗中的应用及进展进行评述。     

三种不同端面光纤列阵和半导体激光列阵耦合的(数值)模拟

半导体激光列阵的输出光束有较大的发散角和较强的不对称性，极大地限制了其在各领域中的应用。为了改善半导体激光列阵输出光束的质量，必须采用特殊的光学系统进行光束整形。一种简单有效的方法是利用光纤列阵实现光束由线性排列到圆形排列的转换。对三种特殊制备的光纤列阵和半导体激光列阵的耦合特性进行了数值模拟研究。结果显示，相对于球形端面光纤列阵和锥形端面光纤列阵，球顶锥形端面光纤列阵和半导体激光列阵有着更高的耦合效率(90％以上)。此外，利用球顶锥形端面光纤列阵还可以进一步压缩输出光束的发散角，从而获得更高质量的输出光束。     

高强度聚焦超声的临床应用

高强度聚焦超声是近几年发展起来的一种新的非介入性肿瘤治疗技术,临床应用越来越广,文章就其在不同肿瘤如前列腺癌、肝癌、胰腺癌、子宫肌瘤等治疗中的应用及进展进行评述。     

大功率垂直腔面发射激光器的相干性测量与分析

通过有机化学气象沉积(MOCVD)技术在n型GaAs衬底上生长制作了发射波长为850nm的VCSELs4×4列阵器件,介绍了VCSELs的制作工艺流程.对器件进行了相干性测量,计算了干涉条纹可见度,分析了影响干涉条纹可见度的因素.     

33 W quasi-continuous-wave narrow-band sodium D_(2a) laser by sum-frequency generation in LBO

We demonstrate an all-solid quasi-continuous-wave （QCW） narrow-band source tunable to sodium D2a line at 589.159 nm. The source is based on sum-frequency mixing between lasers at 1064 nm and 1319 nm in a LBO crystal. The 1064 nm and 1319 nm lasers are produced from two diode side-pumped Nd：YAG master oscillator power amplifier （MOPA） laser systems, respectively. A 33 W output of 589 nm laser is obtained with beam quality factor M^2 = 1.25, frequency stability better than ±0.2 GHz and linewidth less than 0.44 GHz. A prototype 589 nm laser system is assembled, and a sodium laser guided star has been successfully observed in the field test.     

Laser generation of convergent acoustic waves for materials inspection

A laser technique for generating convergent acoustic waves is described. The optically probed Rayleigh wave in the centre of convergence shows an amplification factor of the order of 20 with respect to a collimated surface wave. Applications to the ultrasonic characterization of layered materials and to crack detection are described.     

基于磁光子晶体的低损耗窄带THz滤波器

本文提出一种采用石榴石型铁氧体磁性材料的太赫兹滤波器,利用波导线缺陷和腔内点缺陷的耦合特性,通过改变腔内介质柱半径及分布,实现对某个波长的耦合,达到了高效率滤波的功能;改变外磁场的大小,影响铁氧体材料的磁导率变化,使谐振频率发生改变,从而对THz波进行滤波.应用平面波展开法(PWM)和时域差分有限法(FDTD)进行仿真分析,研究结果表明,该滤波器其插入损耗为0.0997 d B,3 d B带宽为8.22 GHz,实现了低损耗窄带滤波.     

多光束激光远场干涉实验研究

设计了多组多孔径光阑,将激光扩束后通过自制的多孔径光阑,获得多光束相干激光光源.基于多光束干涉理论,通过改变不同的光阑,得到多组点状阵列分布的远场干涉图样,结果表明远场中心主瓣具有随着占空比减小能量降低、旁瓣增多的分布规律.实验结果表明,相干源呈正三角形分布时能量集中度较好.对高功率激光相干合成具有一定的参考意义.     

新型垂直腔面发射半导体激光器阵列

设计出四次质子注入工艺制备垂直腔面发射激光器(VCSEL)阵列的方法,实现了对阵列中单元器件间的隔离以及对单元器件注入电流限制的分别作用。一方面通过对VCSEL外延片上分布布拉格反射镜(DBR)两次较浅的质子注入形成高电阻区域实现对阵列中单元器件间的隔离,另一方面通过再次的两次较深度的可以达到有源区上表面的质子注入形成高电阻区域实现对单元器件注入电流的限制。由瞬态热传导方程对阵列中单元器件间的热相互作用进行了理论分析。采用四次质子注入工艺实现了2×2、3×3简单的二维GaAs/AlGaAs量子阱VCSEL阵列,并对器件的激射近场、光谱特性及功率等进行了测量。     

激光引信中阵列激光二极管光束参数的测量方法

根据激光引信中激光器的脉冲功率高、发散角大的特殊测量要求,比较了ISO 11146标准文件对激光束的3种测量方法,以二阶矩法为基础,运用机械扫描法测量光束截面的功率密度,进而再现出光斑图形。采用光能量降为最大强度的1/e2来定义光束束宽,通过等强线求出了宽度和远场发散角。     

角反射器耦合的InGaAs应变层量子阱锁相列阵激光器的模式特性

采用反应离子刻蚀技术，设计、制备了一种角反射器耦合的１０单元ＩｎＧａＡｓ应变层单量子阱镇相列阵激光器。在高达４×Ｉｔｈ的工作电流范围内，获得了主单瓣远场输出，单瓣束宽最低达０．６４°，接近衍射极限。考虑了角反射器引入的纵模与侧模之间的耦合，及载流子注入引起的反折射率导引，用微扰理论作了模拟计算，表明角反射器耦合是锁相列阵的一种新的耦合机制；主瓣对应于同相锁定，支瓣是由周期性微扰引入的高阶本征模。     

有界噪声和谐和激励联合作用下一类非线性系统的混沌研究

研究一类有界噪声和谐和激励联合作用下的非线性系统，首先用多尺度方法将该系统约化，针对约化后的平均系统，利用随机Melnikov过程方法结合均方值准则导出随机系统可能产生混沌运动的临界条件，结果表明在一定的参数范围内，随着Weiner过程强度参数值的增大，混沌的临界激励幅值先递减继而递增. 同时，用两类数值方法即最大Lyapunov指数法和Poincare截面法验证了解析结果. 关键词： 有界噪声 多尺度 随机Melnikov过程 混沌