中红外组合激光场调控宽带超连续谱的产生

本文对原子在波长为2000nm、脉宽为12.5fs的中红外驱动脉冲和脉宽为1.3fs的紫外控制脉冲叠加形成的组合场中产生的高次谐波进行了研究.通过组合场驱动氦原子得到了谱宽为230eV的超连续谱.利用组合场产生的谐波比单独利用中红外脉冲产生的谐波的强度高了3个量级.对超连续谱进行滤波并调节组合场中两束激光的延迟时间,可以直接产生100as的单个脉冲.我们发现超连续谱的宽度和位置几乎不会因为驱动场和控制场强度的变化而改变,这种性质有利于从实验上获得单个宽谱阿秒脉冲.     

利用两色组合激光场驱动氦原子产生单个阿秒脉冲

对由波长为1600 nm和800 nm两色激光形成组合场驱动氦原子产生的高次谐波进行了研究,通过改变组合场中主要发射高次谐波期间电场的上升沿和下降沿随时间不同的变化率,使电子长路经和短路径发射谐波的特性发生变化,将由此产生的高次谐波进行叠加时可以得到更短的阿秒脉冲,再调节两束激光场的延迟时间,能够有效地实现抑制电子短路径的贡献,从而得到脉宽为33.7 as的单个阿秒脉冲.     

利用多周期空间非均匀啁啾场产生单个短阿秒脉冲

提出了一种直接生成单个短阿秒脉冲的有效方案.采用数值求解一维含时薛定谔方程,理论研究了多周期空间非均匀啁啾场驱动预激发氦离子的高次谐波辐射和阿秒脉冲的产生.结果表明：在空间非均匀啁啾场作用下,谐波截止可扩展至1050阶次,并且高于300阶次以上的谐波是连续的.由于连续谱来自单一量子路径的贡献,滤出连续谱上880-1120阶次的谐波,直接产生了一个脉宽为11.4 as的单个脉冲.通过增加驱动脉冲的持续时间,可获得一个脉宽短至10 as的单个脉冲.     

利用低频场控制轨道直接产生低于50阿秒的单个脉冲

提出一种直接得到脉宽小于50as的单个脉冲的新方法.利用波长为800nm脉宽为5fs的超短脉冲叠加上波长为10μm,强度为超短脉冲强度的10％且脉宽足够长的低频场脉冲驱动氦原子,谐波谱可以拓展到I_p+8.2U_p,同时,高于4.2U_p的谐波几乎是连续的.如果继续调节超短脉冲的载波包络相位,超连续谱可以继续拓宽,从而可以得到低于50as的单个脉冲. 关键词： 阿秒脉冲 超连续谱     

利用正交偏振双色场产生孤立阿秒脉冲

利用强场近似理论开展了正交偏振双色场与氦原子相互作用产生高次谐波和阿秒脉冲的理论研究。正交偏振双色场由少周期的4 fs钛宝石驱动脉冲激光和与它偏振方向垂直的8 fs倍频控制脉冲构成。研究发现,通过合理选择两束脉冲之间的相对相位,能够控制高次谐波发射过程中长、短电子轨道的选择。当相对相位调整为1.2π时,平台谐波主要来自短轨道电子的贡献,由于其运动时间短、波包扩散少,且没有与长轨道电子谐波产生干涉,沿驱动脉冲电场方向的高次谐波谱具有较高强度和较小调制幅度的超连续平台区,通过对第120次到第180次超连续谐波进行傅里叶变换,可产生脉宽为54 as的高强度孤立阿秒脉冲。所提方案对组合脉冲相对相位的选取要求并不严苛,在0.3π的变化范围内皆可获得脉宽较短的孤立阿秒脉冲,同时控制脉冲电场强度的变化对上述数值模拟结果的影响也很小。     

非平行偏振双色场驱动产生脉宽稳定的单个宽谱阿秒脉冲

提出一种直接得到脉宽稳定的单个宽谱阿秒脉冲的新方法.利用波长为800 nm脉宽为5 fs的超短脉冲叠加上偏振方向与主脉冲成π/3,脉宽同样为5 fs的二次谐波脉冲驱动氦原子,可以得到宽度达到50 eV的超连续谱.当二次谐波的强度大于10¹⁴ W/cm²时,超连续谱的位置以及谱宽几乎不会随着二次谐波脉冲的强度的改变而改变.对85—125 eV的超连续谱进行滤波可以直接得到100 as左右的单个阿秒脉冲,这个性质对于实验上获得单个宽谱阿秒脉冲而言是非常有利的. 关键词： 阿秒脉冲 超连续谱     

传播效应对宽带极紫外连续谱的影响

宽带极紫外连续谱是获得阿秒脉冲的重要途径,在此过程中传播效应是影响宽带极紫外连续谱产生的重要因素,本文针对传播效应对宽带极紫外连续谱的影响,采用慢变波近似的一阶传播方程,研究了传播过程对双色场周期量级激光脉冲产生宽带连续谱及其对应的单个阿秒脉冲的影响.通过分析不同聚焦位置和介质长度的传播过程,发现介质位于焦点之后有利于产生连续谱,同时随着介质长度的变长,虽然单个阿秒脉冲峰值强度有所提高,但是产生的阿秒脉冲宽度也会增加.进一步的分析表明,在特定的双色场延迟优化下,传播效应不仅使得阿秒脉冲强度增强,还可以获得与单原子模型下具有同样脉宽的单个阿秒脉冲,而不会因为传播效应导致阿秒脉冲宽度变宽.     

光脉冲宽度达到阿秒

世界上最短的光脉冲已从飞秒 (10 -15ｓ)进入阿秒量级 (10 -18ｓ) .10名科学家组成的国际性小组宣布 ,他们已经产生并测量到单个的 6 5 0ａｓ脉宽的软Ｘ射线脉冲 .这些科学家来自奥地利的维也纳技术大学、加拿大渥太华的国家研究中心和德国的伯来菲尔德大学 .他们发展的实验工具和技术可以进行原子和分子中束缚电子动力学的阿秒分辨率光谱研究 .该科学家小组用 75 0ｎｍ波长、7ｆｓ脉宽、中心强度为 9× 10 14 Ｗ ｃｍ2 的激光照射氖气 ,产生一系列极紫外和软Ｘ射线区的高次谐波 (波长可达 14ｎｍ) .使软Ｘ射线与可见光共线并通过锆带通滤波…     

利用两色红外组合场驱动氦原子产生单个阿秒脉冲

利用数值求解原子在强激光场中的含时薛定谔方程,对一维氦原子处于两色红外组合场中产生的高次谐波进行了研究,研究发现,在组合场驱动下,谐波谱的截止位置可以拓展到 ,从而得到脉宽较短的多个阿秒脉冲,通过改变两束激光的相位延迟,最终得到了33.8as的单个脉冲。进一步利用小波变换分析了阿秒脉冲的发射特征,发现单个阿秒脉冲实现是对电子长路径抑制的结果.     

利用两色红外组合场驱动氦原子产生单个阿秒脉冲

利用数值求解原子在强激光场中的含时薛定谔方程,对一维氦原子处于两色红外组合场中产生的高次谐波进行了研究,研究发现,在组合场驱动下,谐波谱的截止位置可以拓展到Ip+5.7Up,从而得到脉宽较短的多个阿秒脉冲,通过改变两束激光的相位延迟,最终得到了33.8as的单个脉冲.进一步利用小波变换分析了阿秒脉冲的发射特征,发现单个阿秒脉冲实现是对电子长路径抑制的结果.     

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.     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

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.