超强脉肿激光在低密度等离子体中的相对论自导引效应

分析了相对论条件下激光超短脉冲在等离子体中的传输特性，在傍轴近似和慢变振幅近似条件下，推导了折射率、电子密度、静电场以及电子空腔尺度的表达式。当激光功率超过产生自导引阈值功率时，激光束斑沿着传输光轴方向振荡。在有质动力产生的压力非常强时，聚焦光束中央部分的电子被全部排开形成电子空腔。给出了电子空腔的尺寸以及在出现空腔时的处理方法。在超过形成电子空腔的阈值功率（Pc≈2.5TW)时，空腔的尺度几科与激光功率无关，这意味着电子空腔阻止了激光脉冲的进一步聚焦。     

飞秒激光在透明介质中诱导光学微腔的实验研究

鉴于飞秒激光脉冲持续时间极短且峰值功率极高，将其紧聚焦到透明介质体内部时，易引发双光子效应、碰撞电离、雪崩击穿等一系列非线性过程，在焦点处产生微爆，从而形成微腔结构。提出采用25fs的激光脉冲在透明介质内部诱导形成微腔结构。分析了微腔的能量阈值。结合三维精密位移台，制备了三维微腔点阵。探讨了超短激光脉冲在透明介质内部形成微腔结构的方法与基本实验参数。试验发现：采用更短脉宽的飞秒脉冲时可以降低微腔形成的能量阈值；通过调整飞秒激光功率、脉冲作用次数和光束聚焦情况等因素，可以有效改变微腔的纵深比；在数值孔径较低时因无法实现紧聚焦，故不能形成微腔。     

高温超导体超导态的光场发射研究

场致发射在低于场致发射电场阈值的场强作用下,发射体受脉冲激光照射时将会产生光场发射本文对高温超导体临界温度以下光场发射做一定的研究,并与高温超导体一般的场致发射情况做了一定的比较,两者有不同的发射规律．１引言 光场发射是发射体处于电场强度不及但接近场致发射阈值场强的条件下,输人激光脉冲激发电子发射的现象,其发射电子流受光脉冲控制．在低于场致发射电场阈值的场强作用下,高温超导体受脉冲激光照射也可以产生光场发射．当温度降到临界温度以下时高温超导体材料由正常态变成超导态,体内的电子形成库柏对,能隙会引…     

紧聚焦的超短超强激光脉冲在真空中加速斜入射的相对论电子

研究了紧聚焦的线偏振飞秒超强高斯激光脉冲俘获并剧烈加速斜入射低能相对论电子的效应 ，发现被俘获的电子在激光脉冲纵向有质动力的强大加速作用下，可以获得GeV量级的能量 ，并详细研究了入射电子的初能量、斜入射角、电子与激光脉冲的相对延迟时间和激光脉冲 宽度等条件对电子能量增益的影响，发现当激光脉宽超过10λ时，脉宽对电子能量增益影响 不大. 关键词： 电子加速 有质动力 能量增益 束腰     

强脉冲CO_2激光对红外材料的破坏现象

实验表明强激光对红外材料的破坏分为两种类型:烧蚀和冲击破坏。在空气中,光功率密度低于大气的光学击穿阈值时,以烧蚀为主。超过击穿阈值时,冲击破坏是主要的。在我们的实验条件下,用强激光辐照红外脆性材料的靶,靶极易产生微裂纹或粉碎性破坏,其破坏效果与激光的脉冲能量、功率密度、脉冲宽度,以及着靶面积有关。为了直观,文中给出了不少辐照损伤的照片。     

Compton散射下激光等离子体界面附近电子的运动

研究了多光子非线性Compton散射下等离子体中两个不同密度区的界面附近耦合激光场中电子的运动, 导出了电子的横向动量与纵向动量相互关系的一般方程.研究发现：介质的非均匀性导致电子运动特性的重要变化,在一定的条件下,这种非均匀性有利于注入电子获得加速;散射可有效地降低弹推阈值.当耦合脉冲强度低于弹推阈值时,电子最终被脉冲超过,但无论对初始时刻静止的电子还是运动的电子,能量增益均不为0,且随注入能量的提高而迅速地增加.当耦合脉冲强度高于弹推阈值时,电子最终超过脉冲而获得很高的能量增益,其值远远高于均匀等离子体的情况.在入射相同能量的情况下,耦合光会使电子获得更高的能量增益.     

基于Nd∶GdVO4晶体的双波长被动调Q激光器

提出了一种基于Nd∶GdVO₄晶体的双波长正交偏振被动调Q激光器。建立了对应的速率方程模型,研究了激光器输出双波长脉冲和不同输出镜反射率条件下泵浦功率对激光输出时域特性的影响。理论研究结果表明,通过调节输出镜反射率改变双波长阈值反转粒子数密度,当π偏振阈值反转粒子数密度小于σ偏振阈值反转粒子数密度且差值较小时,激光器可以输出双波长被动调Q脉冲激光,通过增大泵浦功率可以依次产生π偏振单一波长脉冲、双波长多对一脉冲、双波长一对一脉冲、双波长一对多脉冲以及σ偏振单一波长脉冲。搭建实验装置,设置π偏振输出镜反射率为0.60,σ偏振输出镜反射率为0.95,对泵浦功率和激光输出时域特性之间的关系进行验证。随着泵浦功率的增大,激光器依次输出具有上述时域特性的脉冲激光,与数值仿真结果一致。当泵浦功率为5.51 W时,激光器输出正交偏振双波长一对一脉冲激光,其中π偏振和σ偏振的波长分别为1 063.23 nm和1 065.52 nm,平均功率分别为323 mW和462 mW,脉冲峰值功率分别为11.62 W和20.35 W,脉冲宽度分别为185 ns和168 ns,脉冲重复频率为1...     

强脉冲CO2激光对红外材料的破坏现象

实验表明强激光对红外材料的破坏分为两种类型:烧蚀和冲击破坏。在空气中,光功率密度低于大气的光学击穿阈值时,以烧蚀为主。超过击穿阈值时,冲击破坏是主要的。在我们的实验条件下,用强激光辐照红外脆性材料的靶,靶极易产生微裂纹或粉碎性破坏,其破坏效果与激光的脉冲能量、功率密度、脉冲宽度,以及着靶面积有关。为了直观,文中给出了不少辐照损伤的照片。 关键词：     

空间约束对激光诱导等离子体演化和光谱的影响

应用纳秒脉冲激光烧蚀铝的动力学模型分析了激光诱导等离子体的演化过程。通过设置有无空腔的不同情况，研究和讨论了空间约束对等离子体和光谱信号的影响，并得到了等离子体演化过程中的电子温度和电子数密度。基于局域热平衡的假设，计算了铝在396.15 nm波长处的谱线强度。与无空腔条件下产生的等离子体相比，有空腔时等离子体的电子温度和电子数密度都明显提高。随着空腔宽度的增加，增强效果减弱，光谱信号强度先升后降，在空腔宽度为1.4 cm处获得最大值。建立了实验装置，实验结果与仿真结果吻合得较好，在同一宽度下得到了最大的信号强度值。模拟和实验结果提供了膨胀过程中等离子体空间和时间分布的数值信息，并解释了空腔存在对等离子体演化产生影响的机制。     

中空等离子体通道中强激光脉冲对电子的加速

分析了中空等离子体通道内传播的强激光脉冲电磁场中注入电子的运动,发现当激光脉冲强度超过某个阈值时,电子能获得有效的纵向加速并自动与脉冲分离.所得结果为实验提供了参考和依据.     

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.