激光诱导等离子体开关控制脉宽实验研究

 利用激光诱导等离子体开关技术,在1 064 nm的Nd：YAG固体激光器上获得了脉宽4.4~6.4 ns的短脉冲激光输出。激光电离空气产生的等离子体开关控制脉冲宽度时,聚焦透镜焦距越短,压缩后的脉宽越窄,但激光能量损耗越大。压缩后的激光脉宽与激光能量近似成双曲线关系。在控制脉宽光路的焦点处放置带孔的Cu薄片可抑制等离子体的扩散,得到了脉宽最短可达4.4 ns的激光输出。     

Vibration testing based on impulse response excited by laser ablation

This paper proposes an innovative vibration testing method based on impulse response excited by laser ablation. In conventional vibration testing using an impulse hammer, high-frequency elements of over tens of kilohertz are barely present in the excitation force. A pulsed high-power YAG laser is used in this study for producing an ideal impulse force on a structural surface. Illuminating a point on a metal with the well-focused YAG laser, laser ablation is caused by generation of plasma on the metal. As a result, an ideal impulse excitation force generated by laser ablation is applied to the point on the structure. Therefore, it is possible to measure high-frequency FRFs due to the laser excitation. A water droplet overlay on the metal is used to adjust the force magnitude of laser excitation. An aluminum block that has nine natural frequencies below 40 kHz is employed as a test piece. The validity of the proposed method is verified by comparing the FRFs of the block obtained by the laser excitation, impulse hammer, and finite element analysis. Furthermore, the relationship between accuracy of FRF measurements and sensitivity of sensors is investigated.     

Nanoparticles by Laser Ablation

This review concerns nanoparticles collected in the form of nanopowder or a colloidal solution by laser ablating a solid target that lies in a gaseous or a liquid environment. The paper discusses the advantages of the method as compared with other methods for nanoparticle synthesis, outlines the factors on which the properties of the produced nanoparticles depend, explains the mechanisms and models involved in the generation of nanoparticles by laser ablation, clarifies the differences between nanoparticle generation in gaseous and liquid environments, presents some experimental desigins and equipment used by the several groups for nanoparticle generation by laser ablation, describes the techniques used for “tuning” the width of the nanoparticles size distribution, and finally presents a few interesting examples of nanoparticles generated by laser ablation.     

激光等离子体闪光高速摄影法实验研究

 通过对激光等离子体进行诊断,可以获取激光等离子体重要的参数和演变规律。使用高速CCD对激光等离子体闪光进行拍摄,研究了不同激光能量下激光诱导空气击穿产生的等离子体的形状、大小、颜色以及激光等离子的发展变化特性。实验表明：等离子体在空间上呈发光液滴状,且逆着激光束的方向膨胀。随着激光能量的增大,等离子体在水平方向和竖直方向的尺寸逐渐增大,增大趋势逐步变小。     

高重频CO_2激光损伤HgCdTe晶体的数值分析

针对CO2激光作用下HgCdTe晶体的损伤问题进行了数值分析。首先,建立了高重频CO2激光损伤Hg0.784Cd0.216Te晶体的三维热传导物理模型;然后,利用有限元方法计算了单脉冲和高重频CO2激光作用下,Hg0.784Cd0.216Te晶体的损伤阈值;最后,分析了激光重频以及辐照时间对晶体损伤阈值的影响。研究结果表明:单脉冲激光辐照下,晶体的损伤阈值为64.5 J/cm2;高重频(f>1 kHz)激光辐照下,激光重频的改变对晶体损伤阈值的影响较小,损伤阈值应由平均功率密度表征,且与辐照时间密切相关;辐照时间的增加,可以有效地减小晶体的损伤阈值,当激光辐照功率密度<1.95 kW/cm2时,不会发生晶体损伤。研究结果对高重频CO2激光在激光加工以及激光防护的应用方面具有指导意义。     

Laser diode beam shaping by optical interference

We recently proposed a novel beam shaping technique that employs Lloyd’s mirror interference. In this study, we apply this technique to three commercial laser diodes: laser diodes used for optical pumping of solid-state lasers, for laser beam printers, and for laser displays. The elliptical output beams from these laser diodes could be transformed into nearly circular beams by inserting a mirror-polished GaAs substrate below the active layer of each laser diode and adjusting its height. The experimentally observed far-field patterns were predicted fairly well by numerical calculations based on Huygens’ integral. We confirmed that our beam shaping technique is applicable to laser diodes with various wavelengths and vertical beam divergence angles. We also describe the monolithic configuration of the beam shaping system, which can be fabricated by dry etching.     

激光轴向偏焦法平整化CVD金刚石膜

利用Nd:YAG型金刚石精密激光切割机,采用激光轴向偏焦法对化学气相沉积（CVD）法制备的金刚石膜表面进行扫描式平整化处理,利用扫描电子显微镜（SEM）、粗糙度仪和金相显微镜对平整化后的金刚石表面进行表征,研究了激光充电电压和焦点位置对扫描凹槽宽度和深度的影响,以及扫描间距对平整化效果的影响。研究结果表明:扫描凹槽宽度随激光充电电压的升高而增大;凹槽深度随激光充电电压的升高而增大,随偏焦量的增大而增大。激光轴向偏焦法对CVD金刚石膜进行平整化处理后,其粗糙度显著减小,利用氢等离子体对其表面进行刻蚀处理,能够有效去除表层石墨,从而达到理想的平整化效果。     

Analytical model for ion acceleration by high-intensity laser pulses

We present a general expression for the maximum ion energy observed in experiments with thin foils irradiated by high-intensity laser pulses. The analytical model is based on a radially confined surface charge set up by laser accelerated electrons on the target rear side. The only input parameters are the properties of the laser pulse and the target thickness. The predicted maximum ion energy and the optimal laser pulse duration are supported by dedicated experiments for a broad range of different ions.     

Absorption modification by laser irradiation in DKDP crystals

Laser-induced modification at 355 nm of deuterated potassium dihydrogen phosphate(DKDP) crystals following exposure to nanosecond(ns) and sub-ns laser irradiation is investigated in order to probe the absorption mechanism in damage initiation. Laser damage resistance is greatly improved by sub-ns laser conditioning,whereas only a little improvement occurred after ns laser conditioning at the same laser fluence. Moreover, scattering and transmittance variations after the two types of laser conditioning indicate similar reduction of linear absorption. However, by contrast, large differences on nonlinear absorption modification are discovered using Z-scan measurement. This characteristic absorption modification by laser irradiation provides evidence that a nonlinear absorption mechanism plays a key role in damage initiation at 355 nm.     

Removal mechanisms of micro-scale particles by surface wave in laser cleaning

This paper is to investigate the mechanisms of micro-scale particle removal by surface wave, which was induced by a short pulse laser in a cleaning process. The authors analyzed the adhesive forces of particles on substrate surface and the clearance force produced by surface wave in laser cleaning. The physical model of particle removal by laser-induced surface wave was established to predict the removal area and the processing conditions of laser cleaning. In this research, a KrF excimer laser was applied to irradiate 304 stainless steel specimen distributed with copper particles to generate surface wave for copper particle removal. Considering that a time-varying and uniformly distributed heat source irradiates on material surface with thermao-elastic behavior, the displacement and acceleration of substrate induced by a pulsed laser were solved by an uncoupled thermal–mechanical analysis based on the finite element method. The processing parameters such as laser energy, laser spot size are discussed, respectively. A series of laser cleaning experiments were designed to compare with computation results. The results show that the removal area by surface wave beyond the laser spot increases with the laser energy and that, the surface acceleration decreases with the increase of the laser spot size.     

芳纶纤维复合材料对激光的吸收特性研究

 利用双积分球光电管系统实验研究了不同厚度的芳纶纤维复合材料在不同强度的1.06μm激光辐照下（低于烧蚀阈值的激光强度）的反射和透射特性，在材料厚度一定时，反射率随着激光功率密度的增加而增加，透射率则呈现指数衰减；激光功率一定时，反射率随着材料厚度的增加而增加，透射率则呈现衰减。通过反射率和透过率可以确定出表面吸收和体吸收的激光能量份额。     

Laser capabilities of CuBr mixture excited by RF discharge

Our investigations demonstrated that utilizing copper bromide (CuBr) mixture as a source of Cu atoms in a RF-excited discharge can be a promising alternative to the Cu sputtered system, when the development of Cu ion gas laser is considered. Both spectroscopic and laser investigations showed that the threshold input power for lasing was reduced about 5 times using the CuBr-based system instead of the Cu-sputtered system. Pulsed and CW laser oscillation on Cu⁺ transitions in the near IR spectral region was obtained in RF-excited He-CuBr discharge operated at 13.56 MHz and 27.12 MHz. At input RF power of 800 W, a laser output power of 10 mW at the 780.8 nm Cu ion laser line was achieved. An increase of laser output power by a factor of two, as well as better Cu vapour axial distribution and better discharge stability, was attained when DC discharge was superimposed on the RF discharge. Laser gain on 11 UV Cu ion lines was observed in RF-excited Ne-CuBr discharge. basing on the obtained results, we consider the CuBr laser system excited by RF discharge capable of generating UV laser radiation at relatively low input power. Received 4 January 1999     

长脉冲高能激光对金属靶材烧蚀实验研究

为研究长脉冲高能激光与金属靶材相互作用的机理,使用2种激光器对铝、钢、铜和钛靶材进行烧蚀试验。记录靶材穿透时的激光轰击次数,测量穿过通孔的激光能量和孔的面积,得到不同材料在不同能量密度下的烧蚀率。实验表明：能量密度低的激光束,即使对靶材可能有烧蚀作用,它的烧蚀率要比能量密度高的激光束小很多。能量密度达到造成液态质量迁移条件的长脉冲激光,可获得较大的烧蚀率和截面积较大的通孔。     

Perforation of biomodels and biotissues by powerful laser pulses

In vitro perforation of gelatin-based biomodels and myocardium tissue by powerful CO₂ and YAG:Er laser pulses is considered. Features of the thermal and shock-wave effects on the walls of the laser channel are investigated. Patterns of laser channeling in gelatin by a single YAG:Er laser pulse were studied.     

Spatial beam shaping by quartz crystal piano-convex lens

Based on the optical activity of quartz crystal, we proposed a scheme for shaping the spatial intensity distribution of a linearly polarized laser beam by utilizing a quartz crystal piano-convex lens in combination with a polarizer. The intensity profile of the shaped laser beam can be easily switched from one profile to another by controlling the polarization direction of the incident laser beam.     

激光烧蚀CH薄膜特性

报道了神光Ⅱ装置上激光膜相互作用的实验,实验利用能量计和光电管测量激光与CH薄膜作用前后的能量和时间波形的变化。结果表明:激光对CH薄膜透过率由作用产生的激光等离子体电子密度决定;功率密度为1.01014 W/cm2的激光与CH薄膜作用下,入射激光能量脉冲前沿与薄膜作用产生激光等离子体;在200 ps时,激光等离子体电子密度稀疏到临界密度以下,激光开始透过等离子体;在700 ps后,等离子体密度远低于临界密度,激光完全透过激光等离子体。     

Periodic nanostructures self-formed on silicon and silicon carbide by femtosecond laser irradiation

Laser-induced periodic surface structures (LIPSS) were formed on Si and SiC surfaces by irradiations with femtosecond laser pulses in air. Different kinds of self-organized structures appeared on Si and SiC at laser fluences slightly higher than the damage threshold, which was measured by confocal laser scanning microscope. The characteristic spatial periodicity of every observed structure was estimated reading the peak values of the 2D Fourier transform power spectra obtained from SEM images. The evolution of the spatial periodicity was finally studied with respect to both the laser fluence and the number of laser pulses. As already observed for metals, the behavior of the spatial periodicity on laser fluence can be related to the parametric decay of laser light into surface plasma waves. Our results suggest a wide applicability of the parametric decay model on different materials, making the model a useful tool in view of different applications of LIPSS.     

Preparation of solder pads by selective laser scanning

We propose a new laser preparation technique to solder Sn-Ag3.5-Cu0.7 on a copper clad laminate （CCL）. The experiment is conducted by selective laser heating and melting the thin solder layer and then preprinting it on CCL in order to form the matrix with solder pads. Through the analysis of macro morphology of the matrix with solder pads and microstructure of single pads, this technique is proved to be suitable for preparing solder pads and that the solder pads are of good mechanical properties. The results also reveal that high frequency laser pulse is beneficial to the formation of better solder pad, and that the 12-W fiber laser with a beam diameter of 0.030 mm can solder Sn-Ag3.5-Cu0.7 successfully on CCL at 500-kHz pulse frequency. The optimized parameters of laser soldering on CCL are as follows： the laser power is 12 W, the scanning speed is 1.0 mm/s, the beam diameter is 0.030 ram, the lead-free solder is Sn-Ag3.5-Cu0.7, and the laser pulse frequency is 500 kHz.     

高功率半导体激光端面泵浦方形掺Nd3+离子激光晶体热形变研究

为了解决高功率半导体激光器端面泵浦激光晶体引起的热效应问题,激光晶体泵浦端面的热形变必须进行准确的计算.通过对于全固态激光器中激光晶体的工作特点分析,建立了矩形截面激光晶体热分析模型.基于热传导方程,提出了泊松方程的一种新解,并获得了矩形截面激光晶体端面热形变分布的一般解析表达式.同时讨论了半导体激光器偏心泵浦激光晶体给端面热形变带来的影响.与有限元分析方法以及其他数值分析方法相比,解析分析方法不会给计算引入任何的误差.热形变的解析分析为解决激光晶体的热效应问题以及提高激光器的性能提供了理论的依据.     

激光冲击对E690高强钢激光熔覆修复微观组织的影响

为研究激光冲击对E690高强钢激光熔覆修复层微观组织的影响,选用专用金属粉末对E690高强钢试样预制凹坑进行激光熔覆修复,并使用脉冲激光对激光熔覆层进行冲击强化处理,同时采用扫描电镜、透射电镜和X射线应力分析仪分别对激光冲击前后激光熔覆层的微观组织和表面残余应力进行检测。结果表明:激光熔覆修复后,激光熔覆层组织为等轴晶,熔覆层与E690高强钢基体之间冶金结合良好,其表面残余应力为均匀分布的压应力。经激光冲击后,激光熔覆层截面晶粒得到细化,并观察到大量的形变孪晶,互相平行的孪晶界分割熔覆层粗大晶粒,在激光熔覆层的晶粒细化过程中发挥着重要作用;试样表层位错在{110}滑移面上发生交滑移,在晶界周围形成了位错缠结。经激光冲击后,激光熔覆层冲击区域表面残余压应力数值相较于冲击前提升了1.1倍。