激光等离子体效应对薄膜损伤特性的影响

高能激光的发展对光学元件的抗损伤能力要求越来越高,其中光学薄膜是最薄弱的环节之一。实验研究了激光的聚焦位置对石英基片上HfO2/SiO2减反射薄膜损伤形貌的影响,研究发现:激光等离子体的高压冲击波对薄膜产生强烈的冲击剥离效应,其压强随膨胀半径的增加而迅速减小。激光等离子体光谱的辐射波长小于入射激光波长,这会增强薄膜对辐射光能量的吸收;位于深紫外波段、能量大于HfO2薄膜带隙的光子能量,将被薄膜直接吸收,从而加剧薄膜的电离破坏。激光等离子体的辐射效应和冲击波效应的共同作用决定了薄膜的损伤形貌。当激光聚焦到薄膜表面时,冲击波压强极大会使薄膜发生大面积的电离去除,同时基底发生击穿;当两者距离大到一定距离时,冲击波只会使得中心处小面积薄膜发生剥离,基底未出现断裂。     

退火对EBE,IBS和ALD沉积HfO2薄膜的抗激光损伤性能影响

结合自身实验条件采用电子束蒸发(EBE)、离子束溅射(IBS)和原子层沉积(ALD)三种工艺制备了HfO2薄膜,对其进行退火实验,采用1064 nm Nd:YAG激光测定了即时沉积和退火后各HfO2薄膜的抗激光损伤能力。研究发现,ALD HfO2薄膜的激光损伤阈值最高,EBE HfO2薄膜次之,IBS HfO2薄膜的损伤阈值最低;300℃退火对各工艺薄膜抗激光损伤能力的影响均为负面,500℃退火则会显著降低ALD HfO2薄膜的抗激光损伤能力。     

原子层沉积HfO_2薄膜的激光损伤特性分析

以目前激光惯性约束聚变中应用最广泛的高折射率材料二氧化铪(HfO2)为研究对象,在熔石英基底上分别采用TEMAH和HfCl4前驱体制备了HfO2薄膜,沉积温度分别为100,200和300℃。采用椭偏仪和激光量热计对薄膜的光学性能进行了测量分析,采用X射线衍射仪(XRD)对薄膜的微结构进行了测量。最后在小口径激光阈值测量平台上按照1-on-1测量模式对薄膜的损伤阈值进行了测试,并采用扫描电子显微镜(SEM)对损伤形貌进行了分析。研究表明,用同一种前驱体源时,随着沉积温度升高,薄膜折射率增加,吸收增多,损伤阈值降低。在相同温度下,采用有机源制备的薄膜更容易在薄膜内部形成有机残留,导致薄膜吸收增加,损伤阈值降低。采用HfCl4作为前驱体源在100℃制备氧化铪薄膜时,损伤阈值能够达到31.8J/cm2(1064nm,3ns)。     

HfO_2单层膜的吸收和激光损伤阈值测试

薄膜吸收是降低膜层激光损伤阈值的重要原因,为了研究薄膜吸收对激光损伤阈值的影响,对HfO2单层膜在1 064 nm处的吸收及其在不同波长激光辐照下的损伤阈值进行了测试和分析。研究结果表明:薄膜的激光损伤阈值由薄膜吸收平均值(决定于薄膜中缺陷的种类和数量)和吸收均匀性(决定于薄膜中缺陷的分布)共同决定;根据HfO2单层膜在1064 nm波长处的吸收值,不但可以定性判断薄膜在1 064 nm波长,而且还可以判断在其它波长激光辐照下的抗激光损伤能力。     

PECVD技术制备单层光学薄膜抗激光损伤特性研究

采用PECVD技术在BK7玻璃基底上沉积了不同厚度的单层SiO2（折射率为1.46）和SiNx（折射率为1.84）光学薄膜,并对这2种膜层进行抗激光损伤阈值（LIDT）测试,分析讨论了PECVD技术制备的单层光学薄膜与抗激光损伤特性之间的关系。实验结果表明:PECVD技术制备的单层SiO2薄膜有较高的LIDT,薄膜光学厚度在o/4~o/2之间时,在光学厚度为350 nm时,LIDT有最小值21.7 J/cm2,光学厚度为433 nm时,LIDT有最大值27.9 J/cm2。SiNx薄膜的LIDT随着光学厚度增加而减小,在光学厚度为o/4时,LIDT有最大值29.3 J/cm2,光学厚度为o/2时,LIDT有最小值4.9 J/cm2。     

激光诱导击穿光谱在薄膜损伤分析中的应用

提出了应用激光诱导击穿光谱技术对薄膜损伤特性进行表征的方法,研究了纳秒脉冲激光作用下薄膜损伤时的等离子体光谱特征,并应用该技术对薄膜的抗激光损伤特性进行了测量.实验测得,HfO2单层膜在78mJ的激光能量的辐照下,薄膜损伤时的等离子体温度为2 807.4K,且电子密度为7.4×1017cm-3.利用识别薄膜损伤时的等离子体光谱的特征,准确地判断了薄膜是否损伤,避免了薄膜损伤的误判现象.结果表明,激光诱导击穿光谱技术适用于薄膜的激光损伤测试中,并且是一种非常有效的测试分析方法.     

缺陷对HfO2薄膜的激光弱吸收与损伤阈值的影响

激光弱吸收是导致光学薄膜损伤的重要原因。在(5~43)mPa的氧分压下制备并测试了一组HfO2薄膜。实验发现,当氧分压小于20mPa时,薄膜弱吸收越大,损伤阈值越低;当氧分压大于20mPa时,薄膜的损伤阈值与弱吸收并不一一对应,具有较高弱吸收的薄膜可能同时具有较高的损伤阈值。建立了缺陷模型,采用有限元法模拟了缺陷对弱吸收测量和损伤阈值测量的影响,分析了缺陷尺寸、密度、吸收系数对弱吸收和损伤阈值的影响。研究结果显示,吸收系数高于薄膜1 000倍的缺陷可以降低薄膜的损伤阈值1 000倍,却并不影响薄膜的弱吸收。缺陷对HfO2薄膜的激光弱吸收与损伤阈值测试有完全不同的影响,是导致某些薄膜弱吸收与损伤阈值背离的原因。     

激光诱导多层光学薄膜损伤分析与仿真

提出一种全面分析光学薄膜损伤特性的方法,根据热传导理论与电子增殖理论建立激光辐照下多层介质膜的损伤理论模型。以HfO₂/SiO₂多层高反膜为例,计算红外纳秒脉冲激光作用下膜系内部的温度场、应力场以及自由电子数密度分布,对其热学特性与电子增殖特性进行综合评估后,得到不同输入条件下膜系的损伤阈值。结果表明,薄膜材料的损伤特性会受到驻波场的影响,在1064 nm波长的激光辐照下HfO₂/SiO₂ 多层介质薄膜的热致应力损伤效应先于热熔融效应先于场效应发生,且薄膜中SiO₂层发生热损伤,而薄膜并未发生场损伤,此外薄膜的损伤阈值随着激光脉宽的增大而增大。     

1064 nm激光对中性密度滤光片的损伤机理研究

中性密度滤光片的典型结构是在K9玻璃上镀金属膜,来实现对激光的有效吸收.由于损伤阈值较低,严重限制了其在高能激光系统中的应用.实验研究了较高激光能量密度下滤光片的损伤形貌和损伤机理.损伤形貌的变化特征是:随着激光能量密度的增加,滤光片先出现损伤点,后以损伤点为中心产生裂纹,且裂纹长度逐渐变长,最终连接成线状和块状,导致大面积的薄膜脱落.建立了缺陷吸收激光能量升温致中性密度滤光片表面薄膜损伤的模型,计算了薄膜表面的温度和应力分布,讨论了薄膜表面不均匀温升造成的径向、环向和轴向热应力分布.理论分析显示:环向应力是造成薄膜沿径向产生裂纹的主要原因.当激光能量密度大于约2.2 J/cm²,杂质粒子半径大于140 nm且相邻杂质粒子之间的距离小于10 μ m时,裂纹才能大量连接起来引起薄膜的大面积脱落.     

单脉冲和多脉冲飞秒激光对单层光学薄膜的损伤

 利用掺钛的蓝宝石飞秒激光系统输出的单脉冲和多脉冲飞秒激光(中心波长800 nm,脉宽50 fs,靶面聚焦直径Ф 40 μm),分别对BK7玻璃基底上厚约500 nm的单层HfO₂和单层ZrO₂薄膜进行辐照,得到了这两种薄膜在1-on-1和1 000-on-1测试方法下的激光损伤阈值。实验发现,两种方法下HfO₂单层膜的阈值均比ZrO₂单层膜的阈值高。从简化的Keldysh多光子离化理论出发,认为HfO₂薄膜材料的带比ZrO₂的宽是导致上述结果的主要原因。同时,同一种薄膜的多脉冲下的阈值比单脉冲下的低,原因是多脉冲下,飞秒激光对光学薄膜的损伤存在累积效应。     

Guided-Wave Two-Dimensional Acousto-Optic Scanner Using Proton-Exchanged Lithium Niobate Waveguide

Two-dimensional scanning of a 0.6328 mum guided-light beam has been realized using noncolinear acousto-optic (AO) coplanar Bragg diffraction together with colinear AO guided-mode to substrate radiation-mode conversion in a Z-cut Xpropagation LiNbO3 proton-exchanged (PE) waveguide. The two surface acoustic (SAW) waves utilized are at the center frequencies of 500 and 200 MHz, propagating in the Y and X axes, respectively. Two-dimensional scanning of approximately 720 resolvable light beam spots, namely, 18 40 (horizontal vertical) scanning, has been demonstrated using a light beam of 1.0 mm aperture. The total number of resolvable beam spots can be greatly increased from 720 by simply utilizing SAW transducers of larger bandwidth and a light beam of greater aperture. It should also be possible to significantly increase the diffraction efficiency from 3 % by optimizing the parameters of the PE waveguide and the SAWs.     

Effect ofγ-radiation on the spectral luminescence characteristics of impure cadmium tungstate crystals

Results of a study of the effect of γ-radiation on the spectral luminescence properties of cadmium tungstate crystals doped with silver, bismuth, and molybdenum cations are presented. Spectral characteristics of the nondnnnoped crystals are briefly described. Absorption and photo and X-ray luminescence spectra of the crystals taken before and after exposure to γ-radiation (5.5·10⁴ Gy) are compared. It is found that the spectral characteristics of the crystals doped with silver, bismuth, and molybdenum cations do not change markedly after the exposure. The relation between the type of impurity-induced defects, individual characteristics of the impurity cations, and the character of the effect of γ-radiation on the spectral luminescence properties of impure crystals is analyzed (preliminarily). Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 55–60, January–February, 1997.     

Femtosecond Transform-Limited Pulse Generation by Compensating for the Linear Chirp of SPM Spectra in Dispersion Shifted Fibers

A simple technique of pulse compression, based on the linear chirp compensation of self-phase modulation (SPM) spectra in dispersion shifted fibers, is demonstrated. The optimization procedure is carried out, for a short span of a single-mode fiber, using a parabolic law, which describes the behavior of the squared output pulse width versus the pump peak power in the case of Gaussian pulses. The experimental results give a minimum pulse duration of 233 fs, which is in good agreement with the model. Shorter and coherent pulses, down to 90 fs, have been obtained by inserting an interference filter at the optical output.     

Sorting of bending magnets for the SSRF booster

The Shanghai Synchrotron Radiation Facility(SSRF)booster ring,a full energy injector for the storage ring,is deigned to accelerate the electron beam energy from 150MeV to 3.5GeV that demands high extraction efficiency at the extraction energy with low beam loss rate when electrons are ramping.Closed orbit distortion(COD)caused by bending magnet field uniformity errors which affects the machine performance harmfully could be effectively reduced by bending magnet location sorting.Considering the affections of random errors in measurement,both ideal sorting and realistic sorting are studied based on measured bending magnet field uniformity errors and one reasonable combination of bending magnets which can reduce the horizontal COD by a factor of 5is given as the final installation sequence of the booster bending magnets in this paper.     

Measurement of nonlinear coefficient of optical fiber based on small chirped soliton transmission

We measure the waveform and phase curves of short optical pulses before and after transmission over different lengths of fibers by use of the pulse analyzer with the frequency-resolved optical gating (FROG),and numerically simulate pulse evolution under the experimental conditions.The nonlinear coefficient of the fiber is given by comparing the experimental results with the numerical ones.Difference between the experiment and numerical simulation is analyzed.     

Resonance scattering of canonical elastic shells in absorbing fluid medium

Resonance scattering of elastic spherical shell and cylindrical shell while the surrounding fluid medium has absorption is studied. The normal mode solution derived using exact elastic theory and the separation of variables is still applicable. However, the scattering form function has to be modified for the absorbing medium, otherwise the unreasonable result would be obtained. The backscattering form function in the absorbing medium is redefined, and the form function of elastic spherical and cylindrical shell with vacuum or solid matter filled is calculated in various absorption conditions. The results show that the absorption of surrounding fluid leads to notable attenuation of the coincidence resonances in the mid-frequency, but it has a little influence on the low-frequency resonance scattering induced by the filler inside the shell.     

Single-beam self-referenced phase-sensitive surface plasmon resonance sensor with high detection resolution

A versatile and low-cost single-beam self-referenced phase-sensitive surface plasmon resonance(SPR)sensing system with ultra-high resolution performance is presented.The system exhibits a root-mean-square phase fluctuation of ±0.0028.over a period of 45 min.i.e.a resolution of±5.2×10-9 refractive index units.The enhanced performance has been achieved through the incorporation of three design elements:a true single-beam configuration enabling complete self-referencing so that only the phase change associated with SPR gets detected,a differential measurement scheme to eliminate spurious signals not related to the sensor response,and the elimination of retardation drifts by incorporating temperature stabilization in the liquid crystal phase modulato .Our design should bring the detection sensitivity of non-labeling SPR biosensing closer to that achievable by conventional fluorescence-based techniques.     

Influences of SiO2 protective layers and annealing on the laser-induced damage threshold of Ta2O5 films

Ta2O5 films are prepared on BK7 substrates with conventional electron beam evaporation deposition.The effects of SiO2 protective layers and annealing on the laser-induced damage threshold (LIDT) of the films are investigated.The results show that SiO2 protective layers exert little influence on the electric field intensity(EFI)distribution,microstructure and microdefect density but increase the absorption slightly.Annealing iS effective on decreasing the microdefect density and the absorption of the films.Both SiO2 protective layers and annealing are beneficial to the damage resistance of the films and the latter is more effective to improve the LIDT.Moreover,the maximal LIDT of Ta2O5 films is achieved by the combination of SiO2 protective layers and annealing.     

A stereo matching algorithm using multi-peak candidate matches and geometric constraints

Gray cross correlation matching technique is adopted to extract candidate matches with gray cross correla- tion coefficients less than some certain range of maximal correlation coefficient called multi-peak candidate matches. Multi-peak candidates are extracted corresponding to three closest feature points at first. The corresponding multi-peak candidate matches are used to construct the model polygon. Correspondence is determined based on the local geometric relations between the three feature points and the multi-peak candidates. The disparity test and the global consistency checkout are applied to eliminate the remaining ambiguous matches that are not removed by the local geometric relational test. Experimental results show that the proposed algorithm is feasible and accurate.