转换波勘探中的低速带效应

主要讨论了3个问题:①低速带表层的纵横波速度比对多分量记录矢量性的影响;②多分量记录上折射波的类型;③形成转换折射波的条件。理论分析表明,在转换波勘探中,当低速带表层的纵横波速度比较大时多分量记录表现为标量波场,当纵横波速度比不是很大时则表现为矢量波场;模型分析表明,垂直方向记录上的折射纵波主要是下行和上行纵波,水平方向记录的转换折射横波主要是下行纵波和上行横波。文章还分析了低速带底层介质和下伏基岩不同纵横波速度对转换折射横波振幅的影响,认为形成转换折射横波的条件是低速带底层横波速度远远小于下伏介质的横波速度。     

Features of laser diagnostics of laminar spherical and annular inhomogeneities

The direct problem of refraction by a laser plane in a medium with nonmonotone radial profile of the index of refraction is solved. The possibility of solving the inverse problem on the basis of the Abel integral is analyzed. A computer simulation of the direct problem for models of hot and cold layers in a liquid in the presence, or absence, of an inhomogeneity core, is conducted. A technique for diagnosis of transparent spherical and cylindrical inhomogeneities based on a combined laser-computer refraction method is proposed. __________ Translated from Izmeritel’naya Tekhnika, No. 1, pp. 34–37, January, 2007.     

Modeling the influence of edge effects on refraction of a laser beam by a temperature inhomogeneity

The influence of edge effects on refraction of a laser beam in flat-layered optical inhomogeities of finite dimension is modeled. An algorithm for numerical calculation of the trajectory of a geometrical optical beam in an arbitrary two-dimensional inhomogeneous field of indices of refractions is developed. A comparison is performed with the results of a calculation of the beam for the case of a temperature inhomogeneity, taking into account edge effects and with edge effects ignored. __________ Translated from Izmeritel’naya Tekhnika, No. 7, pp. 28–31, July, 2008.     

Negative refraction in a rotational plasma metamaterial

The features of negative refraction are investigated by using a rotational plasma metamaterial. It is assumed that the plasma metamaterial is composed of plasma and dielectric material periodically. The analytical results show that the plasma density, plasma filling factor, and rotation angle significantly changed the properties of negative refraction. Interestingly, the plasma metamaterial without exhibiting negative refraction effect will show negative refraction when rotating the optical axis of the metamaterial, while the range of incident angle for negative refraction will be reduced or even disappear for the plasma metamaterial exhibiting negative refraction effect when rotating the optical axis. Moreover, the full-angle negative refraction can be obtained by specific rotation angle and plasma density. The effects of plasma density, plasma filling factor and the rotation angle on the properties of THE negative refraction effect are also explored and discussed in detail.     

折射波法在地矿勘探中的应用研究

地震折射波法是一种较为成熟的地球物理勘探方法,广泛应用于地矿、隧道、水电及核工业等各个行业,但由于受地形限制较大,故近些年使用较少。本文重在阐述在处理方法得当、地形较平坦时,折射波法对测定覆盖层的厚度等还是具有较好的效果。     

Power attenuation and focus shift of solid state laser beam caused by laser-induced plume

Currently, remote laser welding using solid-sate lasers is widespread in industry. Meanwhile, it is well known that the laser-induced plume blown up from the processing point affects penetration in laser welding, through the attenuation and the refraction of the laser beam. These phenomena in carbon dioxide laser welding have been investigated well and it is widely recognized that using the shielding gas flow to blow away the laser-induced plume is very important. However, in remote laser welding it is not easy to maintain the shielding gas flow to the processing point. By the way, these phenomena depend on the wavelength of the laser. So, quantitative knowledge of the attenuation and refraction of the solid state laser beam are necessary in achieving stable penetration in remote laser welding with this laser. This study was made to determine the attenuation coefficient and the amount of the effective focus shift caused by refraction of the laser beam in the plume, through melt run experiments with a YAG laser. The attenuation coefficient of the laser beam was estimated to be 0.00090 mm^?1 from the dependence of the cross-sectional area of weld metal on the laser power and the plume length. This value is about one twentieth of the attenuation coefficient of a carbon dioxide laser beam at welding found in the literature. The amount of focus shift was estimated to be 0.67 mm per 100 mm plume length, from the dependency of penetration depth on the defocusing distance and the plume length. Comparing the 3 mm of plume length, this value is centesimal of the estimated value by Beck et al. [The effect of plasma formation on beam focusing in deep penetration welding with CO₂ lasers. J. Phys. D: Appl. Phys. 1995;28:2430–2442] in CO₂ laser welding. Therefore, a solid-state laser such as a YAG laser is considered to be a suitable laser source for remote laser welding.     

Analysis of plasmonic Bloch waves and band structures of 1D plasmonic photonic crystals

We consider such passbands that appear due to excitation of plasmons inside the one-dimensional photonic crystal with elementary cells containing layers of positive and negative permittivities. The corresponding Bloch waves can be regarded as surface plasmon chains that are subsequently excited at the layers’ boundaries. The band structure depends on the elementary cell composition that also determines the plasmon type. The excited plasmons can be similar to surface plasmons that propagate along a dielectric slab “sandwiched” between two semi-spaces with negative permittivity, or to surface plasmons that propagate along a negative permittivity slab sandwiched between two dielectric semi-spaces. It is shown that the existence of negative refraction does not depend on the frequency dispersion of permittivity. The latter fact permits us to distinguish six elementary cell compositions with qualitatively different band structures. The frequency evolution of the band structure of a plasmonic PC with a given frequency dispersion of the permittivity can be considered as a transition between various kinds of band structures.     

大功率半导体激光加工光致等离子体折射效应

为了研究大功率半导体激光加工过程中,光致等离子体对激光光束显著的屏蔽作用,以波长为976nm、光斑尺寸为0.5mm1mm、最大功率为4kW的半导体激光加工系统为实验基础,采用了与实际相符的光致等离子体电子密度数学模型和几何光学ABCD矩阵算法,从吸收和折射两方面对光致等离子体的屏蔽作用进行了理论分析和实验研究,得到了在光致等离子体电子密度ne1.01018/cm3的条件下,光致等离子体的折射效应才是引起半导体激光光束屏蔽的主要原因这一结果。结果表明,光致等离子体改变了聚焦光束的形态,使其焦点下移、光斑变大、能量密度变小,其效果类似于一个非线性梯度折射率的负透镜。     

不同激光脉冲作用下ZnS晶体非线性光学特性

用Z-scan技术在532 nm的皮秒激光脉冲和800 nm飞秒激光脉冲作用下分别研究了ZnS晶体的非线性吸收及非线性折射特性。实验结果表明,ZnS晶体在532 nm的皮秒激光脉冲作用下非线性吸收为双光子吸收,其非线性吸收系数为5.310-11 m/W,在800 nm的飞秒激光脉冲作用下非线性吸收为三光子吸收,非线性吸收系数为0.5910-21 m2/W2;在532 nm皮秒激光脉冲作用下,ZnS晶体的非线性折射率符号为负,自由载流子产生的非线性折射率的改变占主导,而800 nm飞秒激光脉冲作用下,ZnS晶体的非线性折射率符号为正,束缚电子产生的非线性折射率的改变为主要因素。     

近红外透射光谱技术快速测定烟用香精理化指标

应用近红外透射光谱分析技术,建立了烟用香精相对密度、折光指数、酸值的近红外预测模型,并对光谱预处理方法进行讨论。将模型的预测结果与标准方法测定结果进行比较,在显著性水平为5%的条件下,两种方法不存在显著性差异。该方法操作简便,可快速、准确地用于烟用香精理化指标的检测。