Investigation of high power laser coatings

1Introduction Opticalthinfilmcoatingisoneofthewea kestopticsinICFlasersystems.Totally,there aretwoaspectswhichinfluencethedevelopment ofthelasersystems.Thefirstaspectislaser damageresistanceofthecoatings,whichpuzzled coatingengineersformanyyears[1].Although muchgreatprogresshasbeenmadeintheoretical analysisofdamagemechanismsandoptimization ofdepositionprocedures,therewerestillmany sealedproblemsrestrictingthefurtherimprove mentofthelaserinduceddamagethreshold(LIDT)[27].Thelaser induceddama…     

基于轴棱锥用主被动方式产生短 脉冲高功率近似无衍射光

利用轴棱锥构成的Bessel-Gauss谐振腔,首次由灯泵Nd:YAG调Q激光器主动式直接输出纳秒高功率近似无衍射Bessel-Gauss光脉冲。应用衍射理论给出的Bessel-Gauss场分布模型对结果进行分析,数值模拟和实验结果基本吻合。同时利用带抗共振环(ARR)的Nd:YAG调Q激光器输出的高稳定纳秒高斯脉冲,通过Axicon的光束整形,由被动方式获得近似理想的纳秒无衍射零阶Bessel光。利用胶片扫描法记录了光强分布的精细结构,测定得出无衍射光的中心光斑半径约为90μm,峰值功率密度高达2.3×10⁹ W/cm²。分析比较了主动和被动方式产生纳秒无衍射光的特性和优缺点。     

阶段离子束辅助法制备基频减反膜

在研究阶段离子束辅助制备方式对薄膜性质影响的基础上,采用电子枪蒸发及离子束辅助沉积制备了氧化铪及氧化硅单层膜,采用阶段离子束辅助沉积及全程非离子束辅助沉积制备了基频减反膜。测量了所有样品的弱吸收、残余应力和激光损伤阈值。结果发现,相对电子枪热蒸发制备的样品,离子束辅助沉积的单层膜具有大的弱吸收、低的激光损伤阈值,且张应力减小,压应力增加;阶段离子束辅助沉积制备的减反膜剩余应力变小,弱吸收稍微增加,激光损伤阈值从10.91 J/cm²增加到18 J/cm²。分析表明,离子束辅助沉积在引入提高样品激光损伤阈值有利因素的同时,也引入 了不利因素,阶段离子束辅助沉积在引入有利因素的同时,有效减少了不利因素的引入,从而提高了样品的激光损伤阈值。     

透射式光热透镜技术的原理和应用

透射式光热透镜技术是光热偏转技术的新发展。介绍了透射式光热透镜技术测量光学玻璃、光学薄膜弱吸收的基本原理和装置。用Matlab对透射式光热透射技术的原理进行了模拟,对K9玻璃以及在K9玻璃上镀制的薄膜样品的弱吸收进行了测试。结果表明,透射式光热透镜技术是一种高灵敏度的热波检测技术,其灵敏度最高可以达到0.1ppm,而且可以对膜层中的缺陷进行检测。     

ON THE SIMULTANEOUS EFFECT OF BEAM DEFLECTION AND THERMAL LENSING PHENOMENA IN PHOTOTHERMAL SPECTROMETRY

Abstract

A general model that accounts for a combined effect of thermal lensing and photothermal beam deflection was derived. The model enables the estimation of contributions to the measured photothermal signal from each of the two phenomena. Experimental parameters such as modulation frequency, the pump beam diameter, and the position of detector can be optimized using this model in order to minimize the unwanted effects of misalignment or laser pointing instability, and to improve the accuracy of thermal lens and photothermal beam deflection spectrometry.     

ZnO薄膜非线性光学特性的实验研究

利用金属有机化学气相沉积(MOCVD)技术在蓝宝石衬底上生长一层高质量的ZnO薄膜。为了考察沉积温度对样品的非线性特性的影响,在200~500 ℃生长了一系列ZnO薄膜。用X射线衍射谱(XRD)及扫描电镜(SEM)对样品结构进行了评价。以Nd:YAG激光器输出的1.06 μm的激光为基频光,对ZnO薄膜样品的二阶及三阶非线性光学特性进行了实验研究。实验发现,对于250 ℃沉积温度的样品有较强的非线性效应,实验测得的二阶非线性极化张量 χ ⁽²⁾_ZZZ=9.2 pm/V, 三阶有效非线性系数χ⁽³⁾=5.28×10^－20 m²/V²。     

宽带掺钛蓝宝石激光器的理论及实验的初步研究

本文从理论上阐述了掺钛蓝宝石的激光特性并提出了谐振腔设计的优化方案。在室温条件下,实现了调Q倍频YAG激光泵浦下直腔式掺钛蓝宝石激光器的脉冲激光运转。波段范围为820nm～920nm。     

Femtosecond near-field scanning optical microscopy

We have developed an instrument for optically measuring carrier dynamics in thin-film materials with ≈150 nm lateral resolution, ≈250 fs temporal resolution and high sensitivity. This is accomplished by combining an ultrafast pump–probe laser spectroscopic technique with a near-field scanning optical microscope. A diffraction-limited pump and near-field probe configuration is used, with a novel detection system that allows for either two-colour or degenerate pump and probe photon energies, permitting greater measurement flexibility than that reported in earlier published work. The capabilities of this instrument are proven through near-field degenerate pump–probe studies of carrier dynamics in GaAs/AlGaAs single quantum well samples locally patterned by focused ion beam (FIB) implantation. We find that lateral carrier diffusion across the nanometre-scale FIB pattern plays a significant role in the decay of the excited carriers within ≈1 μm of the implanted stripes, an effect which could not have been resolved with a far-field system.     

极坐标激光直写系统光轴和转轴对准的光栅反射对准法

提出一种光栅反射对准法:在极坐标激光直写系统中利用光栅反射聚焦光斑来校准光轴和工件台的转动轴;被光栅反射的聚焦光斑成像在CCD上。当聚焦透镜的光轴和工件台的转动轴存在对准误差时,如果转动光栅,激光光斑将在光栅上扫描出一个圆形轨迹。通过观测这个轨迹,能估算出光轴和转轴的对准误差并相应地做出调整减小对准误差。采用光栅反射法,现将光轴和转轴的对准误差减少到小于0.5μm。     

Nanometer edge profile measurement of diamond cutting tools by atomic force microscope with optical alignment sensor

This paper describes an atomic force microscope (AFM) based instrument for nanometer edge profile measurements of diamond cutting tools. The instrument is combined with an AFM unit and an optical sensor for alignment of the AFM probe tip with the top of the diamond cutting tool edge in the submicrometer range. In the optical sensor, a laser beam from a laser diode is focused to generate a small beam spot with a diameter of approximately 10 μm at the beam waist, and then received by a photodiode. The tool edge top and the AFM probe tip are brought to the center of the beam waist, respectively, through monitoring the variation of the photodiode output. To reduce the influence of the electronic noise on the photodiode output so that the positioning resolution can be improved, a modulation technique is employed that modulates the photodiode output to an AC signal by driving the laser diode with a sinusoidal current. Alignment experiments and edge profile measurements are carried out.     

Femtosecond pump/supercontinuum-probe spectroscopy: optimized setup and signal analysis for single-shot spectral referencing

A setup for pump/supercontinuum-probe spectroscopy is described which (i) is optimized to cancel fluctuations of the probe light by single-shot referencing, and (ii) extends the probe range into the near-uv (1000-270 nm). Reflective optics allow 50 μm spot size in the sample and upon entry into two separate spectrographs. The correlation γ(same) between sample and reference readings of probe light level at every pixel exceeds 0.99, compared to γ(consec)<0.92 reported for consecutive referencing. Statistical analysis provides the confidence interval of the induced optical density, ΔOD. For demonstration we first examine a dye (Hoechst 33258) bound in the minor groove of double-stranded DNA. A weak 1.1 ps spectral oscillation in the fluorescence region, assigned to DNA breathing, is shown to be significant. A second example concerns the weak vibrational structure around t=0 which reflects stimulated Raman processes. With 1% fluctuations of probe power, baseline noise for a transient absorption spectrum becomes 25 μOD rms in 1 s at 1 kHz, allowing to record resonance Raman spectra of flavine adenine dinucleotide in the S(0) and S(1) state.     

X射线椭圆弯晶谱仪实验研究

利用椭圆从一个焦点发射出的光线经椭圆面反射必汇聚于另一焦点的性质,设计了基于620～6200eV的X射线椭圆弯晶谱仪.文中分析了系统原理,完成了谱仪光学色散系统、探测系统及仪器的研制.采用KAP、LiF、PET、MiCa作为色散元件,其2d值在0.4～2.6nm、Bragg角为30°～67.5°,晶体尺寸为120×8×0.2mm,偏心率为0.9586,焦距1350mm,光程长1456mm,分析器基底材料用数控铣床加工.实验在上海"神光Ⅱ"号装置上实施,激光能量为150J,波长为0.35mm,真空度为3×10^-3Pa.为达到光学对准,采用了小点光源以及精密望远镜对中.实验结果显示出金靶在0.63～0.79nm范围,其分辨力(△λ/λ)达到了1/486.谱的分辨力还与晶体特征和激光靶源尺寸有关.谱仪性能良好、使用方便、简单,具有高的收集效率和分辨能力,能够有效获取激光惯性约束聚变中激光等离子体发射光谱的丰富信息.     

激光光盘伺服槽及预制格式刻划机控制系统研究与设计

本文描述了激光光盘伺服槽及预制格式刻划机的主轴回转。直线微进给、自动调焦、激光控制等精密控制系统,对光盘预格式化进行了理论研究。主轴回转稳速精度达到5×10^-6,刻槽螺距精度达到1.6μm±0.03μm。     

Femtosecond near-field scanning optical microscopy

We have developed an instrument for optically measuring carrier dynamics in thin-film materials with approximately 150 nm lateral resolution, approximately 250 fs temporal resolution and high sensitivity. This is accomplished by combining an ultrafast pump-probe laser spectroscopic technique with a near-field scanning optical microscope. A diffraction-limited pump and near-field probe configuration is used, with a novel detection system that allows for either two-colour or degenerate pump and probe photon energies, permitting greater measurement flexibility than that reported in earlier published work. The capabilities of this instrument are proven through near-field degenerate pump-probe studies of carrier dynamics in GaAs/AIGaAs single quantum well samples locally patterned by focused ion beam (FIB) implantation. We find that lateral carrier diffusion across the nanometre-scale FIB pattern plays a significant role in the decay of the excited carriers within approximately 1 microm of the implanted stripes, an effect which could not have been resolved with a far-field system.     

A Novel Atomic Force Microscope with High Stability and Scan Speed

Abstract

A novel atomic force microscope (AFM) has been developed. Unlike conventional AFM systems, its cantilever and tip were set up in the X direction with respect to the sample. It can practically eliminate the crawling effect of the probe itself. Meanwhile, using a beam splitter, we devised a unique path optical beam deflection method for the measurement of the cantilever's displacement with its minimized structure and an optimal light‐amplifying ratio; the position of the sensitive detector (PSD) is just in front of the visual sight of operator. This makes observation and operation easier. Furthermore, the PSD attached to the translation stage can be adjusted and, thus, adapts the setpoint of imaging force to different samples. In this way, our new AFM provides high stability and scan speed. The highest scan rate is about 40 lines/s or 10s for a 400×400‐pixel, 3 µm×3 µm image.     

Near-field and confocal surface-enhanced resonance Raman spectroscopy at cryogenic temperatures

For laser spectroscopy at variable temperatures with high spatial resolution a combined scanning near‐field optical and confocal microscope was developed. Rhodamine 6G (R6G) dye molecules dispersed on silver nano‐particles or nano‐clusters were investigated. For optical excitation of the molecules, either an aperture probe or a focused laser spot in confocal arrangement were employed. Raman spectra in the wavenumber range between 300 cm^?1 and 3000 cm^?1 at room temperatures down to 8.5 K were recorded. Many of the observed Raman lines can be associated with the structure of the adsorbed molecule. Intensity fluctuations in spectral sequences were observed down to 77 K and are indicative of single molecule sensitivity.     

Effects on mechanical properties in electron beam welding of TC4 alloy by laser shock processing

The surface of TC4 titanium alloy welding line by electron beam welding (EBW) was processed by high power Q-switched and repetition-rate Nd: glass laser. Effects of laser power and spot diameter on residual stress and microhardness of the TC4 alloy welding line by laser shock processing (LSP) have been analyzed. Results show that residual stresses almost do not change as laser power is 45.9 J, spot diameter is ϕ9 mm; While laser power is 45.9 J, spot diameter less than ϕ3 mm, the distribution of residual stress in welding line occurs obvious variation, which residual stress increase obviously with spot diameter decrease. When power density is bigger than 1.8 × 10¹⁰ W/cm², residual stresses of electron beam welding line occur change by LSP, which improve obviously residual stress distribution; while laser power is bigger than 1.2 × 10¹⁰ W/cm², the surface micro-hardness of electron beam welding line occurs change by LSP, which improve obviously micro-hardness distribution. Mechanical properties of TC4 titanium alloy welding line will be improved by LSP, which provides experimental foundation for further controlling the distributions of residual stress and micro-hardness during laser shock processing. __________ Translated from Journal of Jiangsu University (Natural Science), 2006, 27(3): 207–210 [译自: 江苏大学学报 (自然科学版)]     

Optical characteristics of thin film coating and measurement of its thickness

Theoretical analysis of the optical characteristics of thin film coatings was carried out. The relations of refractive index, phase change and film thickness were studied. As a result, a new optical method of measuring thickness of thin metallic coatings was developed. Further, the technique can also be used to find the refractive index of the thin film and the phase change of the light beam owing to being reflected from the coating. It is shown that the new method is very accurate for measuring the thin coating thickness. A specimen coating was tested with a polarised laser beam and the measured phase change falls between the reasonable range of 0 and π.     

基于旁瓣光束衍射反演的强激光远场焦斑测量方法

针对大型激光装置使用纹影法无法实现旁瓣光束弱信号区域光强分布精确测量的问题,提出了基于旁瓣光束衍射反演的纹影法强激光远场焦斑测量方法.采用逆向推演间接测量的研究方法,沿光路传播逆方向推导,以旁瓣光束衍射光强图像和相位图像作为输入,通过计算获得未遮挡前旁瓣光束远场焦斑分布.相比传统基于纹影的远场焦斑测量方法,本文的主要改...     

LD端面泵浦铯蒸气激光器的模式匹配

以光纤耦合半导体激光器作为泵浦源,5mm长的铯蒸气池作为激光增益介质,开展了端面泵浦铯蒸气激光器的模式匹配实验研究。分析了泵浦光聚焦光斑半径和聚焦位置对铯激光输出性能的影响。以激光器的工作斜效率和光光效率为指标对各模式匹配参数进行了优化,同时对激光器的阈值泵浦功率进行了研究。结果表明:在一定的激光振荡模束腰下,存在最佳的泵浦光聚焦光斑半径使斜效率最大。此外,聚焦位置在蒸气池中央时有利于提高斜效率和光光效率。对阈值泵浦功率的研究显示,阈值泵浦功率随泵浦光聚焦光斑半径的减小而减小,而且当泵浦光聚焦于蒸气池前端时有利于降低阈值泵浦功率。基于以上研究,获得了一组最佳模式匹配参数,即泵浦光聚焦光斑半径为333μm,激光振荡模束腰为167μm,泵浦光聚焦位置位于蒸气池中央。