国外点滴

<正> 英国Oriel科学有限公司开发了一种供无线电通讯系统使用的纤维光学微波透光延迟系统。该系统能够延迟千兆赫微波或没有畸变的10~(-12)s(最大可达10~(-8)s)脉冲信号。系统由三部分组成:千兆赫/10~(-12)秒电-光转换器、单模延迟纤维、千兆赫/10~(-12)秒光-电转换器。这     

阿秒光谱技术的探讨(综述)

<正> 1965年美国贝尔实验室首先用红宝石激光器获得ps超短光脉冲。从此高速光谱技术进入超高速领域,即ps光谱技术。不久又出现了ps扫描摄影机,使ps技术逐步应用到物理学、化学、生物学以及光电子学等各个方面,研究各种超快速过程。在1965年以前的100多年间,高速光谱技术只提高了两个数量级,即从10~(-7)s发展到10~(-9)s。然而,从六十年代初出现激光器算起,到现在才20多年,高速光谱技术却从10~(-9)s发展到了10~(-14)s,提高了五个数量级,接近了10~(-15)s,即fs的范畴。贝尔实验室在这个领域一直占领先地位。它在1981年得到90fs的     

世界最短的激光脉冲

<正> 美国麻省理工学院最近成功获得持续时间只有15×10~(15)s的超短激光脉冲。这是到目前为止世界上最短的光脉冲。早在1982年夏季美国贝尔实验室就获得了30×10~(15)s的白光脉冲。这次麻省理工学院的新成果是在改进贝尔实验室早期成果之后获得的。光脉冲是由一些基波相干叠加组成的。这些基波彼此干涉后形成了脉冲。脉冲波的宽度与基波的频率范围成正比。也就是说,频带越     

碱金属钾蒸气混合双光子跃迁的研究

利用脉冲染料可调谐激光激发原子密度为10~(14)～10~(18)cm~(-3)的稠密碱金属钾蒸气,在5780(?)～5920(?)波段内测量了激光共振电离光谱.结果表明,除了钾原子4s—nd(n=10～16)、4s—ns(n=12～18)双光子跃迁以外,还出现了钾4P—7s、4P—5d跃迁.这种跃迁的电离光谱信号是由于在钾蒸气中钾分子-原子混合双光子跃迁而产生的.本文对钾蒸气混合双光子跃迁进行了讨论.     

消除激光接收中的荧光干扰

在固体激光器中,都是用闪光灯抽运法激励工作物质。一股,闪光灯的闪光时间在10~(-4)～10~(-3)数量级,在此期间,激光工作物质发出荧光,它在谐振腔来回反射,被激光工作物质放大,由前反射镜输出所需要的激光脉冲,激光脉冲的持续时间在10~(-8)数量级。激光测距机包括有发射光学系统和接收光学脉冲系统,一般这两个系统靠得很近,激光脉冲通过发     

基于傅里叶变换的光纤陀螺测试环境自评估技术

提出了一种基于傅里叶变换的光纤陀螺(FOG)测试环境自评估技术。测试结果表明,FOG零偏稳定性由环境3中的0.0015 (°)/h(100 s, 1σ)(数据100 s平滑后的标准差)降低到环境4中的0.0019 (°)/h (100 s, 1σ);随机游走系数由环境3中的2.1565×10~(-4)(°)/h~(1/2)降低到环境4中的2.8876×10~(-4)(°)/h~(1/2)。对另一只脉冲输出的陀螺进行了不同环境下的测试,零偏稳定性由环境3中的0.0013 (°)/h (100 s, 1σ)降低到环境4中的0.0021 (°)/h (100 s, 1σ)。通过两只陀螺的实验,验证了所提自评估技术的有效性,为高精度FOG的精度测试提供了指导。     

飞秒激光脉冲产生研究

本文通过对被动锁模染料激光器锁模动力学特性分析,讨论了对撞脉冲锁模(CPM)环型染料激光器产生持续期为10~(-12)秒到10~(-15)秒量级的高稳定度光脉冲的锁模机理。选择有大吸收,发射截面和很短恢复时间的R6G增益介质和有更大吸收截面的DODCI可饱和吸收体,能够获得极短的光脉冲。为获得几十飞秒光脉冲的稳定运     

染料激光器输出特性的实验观测

一、前言自60年代染料激光器问世以来,由于它不仅具有激光器的一般特性,而且由于有机染料受激幅射产生的激光的波长已复盖了由紫外32lnm到红外1.3μm范围,同时染料激光器产生的超短脉冲的时间宽度已经可以压缩至15×10~(-15)s,其光带宽可以窄到δλ=6×10~(-5)nm。这些特性便使染料激光器在近年来得到迅速的发展。高单色性的可     

盐水溶液跨混合纤维素膜扩散机制的定量可视化实验研究

介绍了一种改进的高时空分辨率相移干涉仪,测量了不同浓度的NaCl溶液跨过5 μm孔径醋酸-硝酸混合纤维素膜进入去离子水的跨膜扩散过程。实验观察到NaCl溶液在膜-液界面附近的质量输运行为受膜结构的影响而表现出的特征:相对分散的浓度高值分布、均流传质现象、对流的快速纵向进行和横向扩散的缓慢发展等。实验计算得10.0 mg/mL、7.5 mg/mL、5.0mg/mL、2.5 mg/mL 和 1.0 mg/mL 的 NaCl 溶液跨膜传质系数分别为 3.0×10~(-6) m~2/s、8.5×10-7 m~2/s、7.8×10~(-7)m~2/s、2.9× 10~(-7)m~2/s和6.0×10~(-8)m~2/s。本实验结果可为跨膜扩散机理的研究提供参考。     

双波长输出He—Ne激光器测量氦等离子体中电子密度时空分布

用3.39μm和0.63μm同时振荡的双波长He-Ne激光器,首次测量了脉冲氦等离子体中电子密度(10~(14)～10~(15)cm~(-3)的时间和径向空间分布.     

Measuring Light Field of Light Source with High Directional Resolution Using Mirrored Ball and Pinhole Camera

This paper presents a method of measuring a light field of a light source with high directional resolution using a mirrored ball and a pinhole camera. The light field describes a spatial and directional distribution of radiances from the light source. The directional distribution is expanded by a reflection on the mirrored ball, and the radiances are measured by a charge-coupled device (CCD) camera with a pinhole lens. The light source is laterally moved by a robot arm to measure the directionally expanded light field, and each pixel on a CCD can obtain the radiances from the light source through the pinhole lens with high directional resolution. The light field is estimated from the pixel value and the position of each pixel using a ray tracing technique. The light field of a krypton lamp was experimentally measured by the proposed method, and the accuracy of the measurement was evaluated against the irradiances measured by a spectro-radiometer at sample points.     

Influences of decoherence on the generation of a macroscopic superposition state using weak cross-Kerr effect

Considering a system in which a single photon and a coherent field propagate through a Kerr medium, when the weak cross-Kerr interaction between the coherent state and the single photon under decoherence is involved, this paper derives analytically a macroscopic superposition state by the superoperator method and investigates the influences of decoherence on the coherence properties of the obtained state. It finds that the macroscopic superposition state will experience evolution from a pure superposition state to a mixed state in a dissipative environment and the Kerr effect makes the field display a periodic revival from decoherence for a short time.     

The Klein-Gordon and the Dirac Oscillators in a Noncommutative Space

We study the Dirac and the Klein-Gordon oscillators in a noncommutative space. It is shown that the Klein-Gordon oscillator in a noncommutative space has a similar behaviour to the dynamics of a particle in a commutative space and in a constant magnetic field. The Dirac oscillator in a noncommutative space has a similar equation to the equation of motion for a relativistic fermion in a commutative space and in a magnetic field, however a new exotic term appears, which implies that a charged fermion in a noncommutative space has an electric dipole moment.     

Reconstruction of the acoustic impedance profile in a plate using an inverse spectral procedure

An inverse spectral procedure was applied to reconstruct the acoustic impedance profile along the thickness direction of a plate using its thickness resonance frequencies, density and thickness. For a successful reconstruction, the material-property profile must be symmetric about the mid-plane of the plate. Several cases of numerical simulations, including plates with a few layers and with a high number of layers are described. The calculated resonance frequencies were used to reconstruct the acoustic impedance profile, a process that was successful for all cases. We assume that a plate with a high number of layers, each with a different but constant acoustic impedance, simulates a plate with a smoothly varying acoustic impedance profile. It can be concluded that such a plate, which generates small, virtually undetectable, internally reflected waves, can also be reconstructed. In the special case of a plate of unknown thickness and unknown but constant density, the method is still useful, because a relative variation of the material property can be reconstructed using only the resonance frequencies. An experiment using a resonance-mode electromagnetic acoustic transducer (resonance-mode EMAT) is also described. EMAT is a non-contact ultrasonic method that can measure thickness resonance frequencies, making it appropriate for this method. Some examples of applications are measurement of the temperature profile inside a rolled metal sheet, measurement of a clad metal plate, and monitoring of a metal casting.     

Brownian motion on a manifold

The question of the existence and correct form of equations describing Brownian motion on a manifold cannot be answered by mathematics alone, but requires a study of the underlying physics. As in classical mechanics, manifolds enter through the transformation of variables needed to account for the presence of constraints. The constraints are either due to a physical agency that forces the motion to remain on a manifold, or they represent conserved quantities of the equation of motion themselves. Also the Brownian motion is described either by a Smoluchowski diffusion equation or by a Kramers equation. The four cases lead to the following conclusions, (i) Smoluchowski diffusion with a conserved quantity reduces to a diffusion equation on the manifold; (ii) The same is true for diffusion with a physical constraint in three dimensions, but in more dimensions it may happen thatno autonomous equation on the manifold results; (iii) A Kramers equation with a conserved quantity reduces to an equation on the manifold, but in general not of the form of a Kramers equation; (iv) The Kramers equation with a physical constraint reduces to an autonomous Kramers equation on the manifold only for a special shape of that constraint. Throughout, only a certain type of physical constraints has been envisaged, and global questions are ignored. Finally, the customary heuristic construction of a Fokker-Planck equation for a mechanical system on a manifold is demonstrated for the case of Brownian rotation of a rigid body, and its shortcomings are emphasized.     

具有态守恒赝隙的光子晶体中两能级原子自发辐射的增强与抑制

论证了在赝带隙光子晶体中存在一个全频率域态总数守恒规则，在完全带隙光子晶体中还存在一个局域态总数守恒规则.态总数守恒规则指出，如果一个光子晶体的态密度在某些频率范围存在相对于等效介质态密度的谷，则一定由其他频率范围内相对于等效介质态密度的峰来补偿.使用符合态总数守恒规则的态密度模型，解释了态密度调制导致的自发辐射谱增强、抑制、变窄、红移、蓝移以及谱分裂等光子晶体中的量子光学现象.该理论比较适合研究在具有赝带隙的光子晶体中大量随机分布的发光原子或分子的自发辐射行为. 关键词： 光子晶体 自发辐射 态密度 光子赝带隙     

Boundary crisis and transient in a dissipative relativistic standard map

Some dynamical properties for a problem concerning the acceleration of particles in a wave packet are studied. The model is described in terms of a two-dimensional nonlinear map obtained from a Hamiltonian which describes the motion of a relativistic standard map. The phase space is mixed in the sense that there are regular and chaotic regions coexisting. When dissipation is introduced, the property of area preservation is broken and attractors emerge. We have shown that a tiny increase of the dissipation causes a change in the phase space. A chaotic attractor as well as its basin of attraction are destroyed thereby leading the system to experience a boundary crisis. We have characterized such a boundary crisis via a collision of the chaotic attractor with the stable manifold of a saddle fixed point. Once the chaotic attractor is destroyed, a chaotic transient described by a power law with exponent −1 is observed.     

NUCLEATE POOL BOILING HEAT TRANSFER OF BINARY MIXTURES WITH DIFFERENT BOILING RANGES

The nucleate pool boiling heat transfer data on a smooth flat surface were measured for three binary mixtures of HC600a/HFC134a, HC600a/HC290, and HC600a/HFC23. Much effort was made to investigate the influence of the boiling range on the pool-boiling heat transfer performance. From the experimental results, the HC600a/HFC23 mixture with a wide boiling range showed lower heat transfer coefficients (HTCs) than the mixture with a narrow boiling range such as HC600a/HFC134a and HC600a/HC290 systems. The measured data were also compared with the results predicted by five well-known correlations. It can be found that the average deviation is less than 25% for mixtures with narrow boiling ranges, but a larger deviation for mixtures with wide boiling ranges.     

Direct measurement of group dispersion of optical components using white-light spectral interferometry

We present a simple white-light spectral interferometric technique employing a low-resolution spectrometer for a direct measurement of the group dispersion of optical components over a wide wavelength range. The technique utilizes an unbalanced Mach-Zehnder interferometer with a component under test inserted in one arm and the other arm with adjustable path length. We record a series of spectral interferograms to measure the equalization wavelength as a function of the path length difference. We measure the absolute group refractive index as a function of wavelength for a quartz crystal of known thickness and the relative one for optical fiber. In the latter case we use a microscope objective in front and a lens behind the fiber and subtract their group dispersion, which is measured by a technique of tandem interferometry including also a Michelson interferometer.     

Reprint of : Scattering approach to scanning gate microscopy

We present a perturbative approach to the conductance change caused by a weakly invasive scattering potential in a two-dimensional electron gas. The resulting expressions are used to investigate the relationship between the conductance change measured in scanning gate microscopy as a function of the position of a scattering tip and local electronic quantities like the current density. We use a semiclassical approach to treat the case of a strong hard-wall scatterer in a half-plane facing a reflectionless channel. The resulting conductance change is consistent with the numerically calculated quantum conductance.