Effect of the mode composition of laser radiation on the intracavity formation of periodic surface structures

We investigated the effect of the mode composition (longitudinal and transverse modes) of laser radiation on the basic parameters of the stroke pattern formed on a treated surface placed inside a laser cavity. We found that the contrast of the stroke pattern formed by a laser beam with a complex set of longitudinal modes depends, in the first place, on the width of the laser radiation spectrum and the laser cavity length and is virtually independent of the angle of rotation of the beam. In the region of small rotation angles the broadening of strokes caused by the effect of transverse modes becomes predominant. B. I. Stepanov Institute of Physics, Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikaladnoi Spektroskopii, Vol. 64, No. 1, pp. 50–54, January–February, 1997.     

Measurement of nonlinear refractive index of Ag and Au nano-particles using moiré technique

In this paper we have measured second order nonlinear refractive index of Ag and Au nano-particles using the moiré deflectometry with a CW He-Ne laser at wavelength of 632.8 nm. The main advantages of this technique are its extreme experimental simplicity and low sensitivity to external distribution in comparison with other interfrometric methods that need very stable experimental set-up. Because of self-focusing (or self-defocusing) that is due to intensity dependent refractive index, presence of sample in the moiré deflectometry set-up lead to rotation of moiré fringes. By moving the sample in set-up we have measured these rotation angles in different displacements from focus point of lens used in this set-up and then we have calculated the nonlinear refractive index of Ag and Au nano-particles using their values. Also we have obtained the sign of n₂ regard to the direction of rotations. Consequently by using the Mathematica software we have exhibited the suitable fitness of theoretical curve with experimental results.     

Detailed mechanism of trans–cis photoisomerization of azobenzene studied by semiclassical dynamics simulation

A realistic dynamics simulation study is reported for the trans–cis photoisomerization of azobenzene. The simulation follows both nπ* and ππ* excitations and each excitation is induced by a 50 fs (FWHM) laser pulse. The simulation results show that, for both excitations, the reaction path is predominated by the rotation coordinate of the NN bond. The simulation finds that the CNN inversion angles expand as soon as the rotation starts. The expansion of the CNN bond angles permits the molecule to rotate efficiently. It is therefore suggested that the photoisomerization of trans-azobenzene follows an inversion-assisted rotation path. These simulation results are significant for understanding the mechanism of this important process.     

Holographic prism as a new optical element: II. Method of measurement of reproducible angles

To determine the precise values of angles stored and reproduced by a new multivalued measure of plane angles, i.e., a holographic prism, it is expedient to use a procedure for collating two circular scales, one of which is determined by the prism upon its rotation on a rotary table, while the other is the device’s own scale. The two scales are considered to be equivalent. The collation procedure is discussed taking into account target (deviations of sought values of angles of both scales from their nominal values) and a number of nuisance parameters of the procedure. The scheme of the collation device is considered. The geometric model that determines sources of errors caused by design features of signal reading and recording devices of the holographic prism is analyzed.     

Rotation of laser beam polarization in acousto-optic devices

When a linear, polarized laser beam is passed through an acousto-optic device, the polarization direction of the outgoing beam is often different from that of the entering beam, and thus a rotation of the polarization angle takes place. This paper shows how the polarization rotation and the intensity can be calculated for different polarization angles. Experimental results obtained with a single crystal germanium frequency shifter in the 10.6 μm region agreed well with theory.     

Three dimensional alignment of molecules using elliptically polarized laser fields

We demonstrate, theoretically and experimentally, that an intense, elliptically polarized, nonresonant laser field can simultaneously force all three axes of a molecule to align along given axes fixed in space, thus inhibiting the free rotation in all three Euler angles. Theoretically, the effect is illustrated through time dependent quantum mechanical calculations. Experimentally, 3, 4-dibromothiophene molecules are aligned with a nanosecond laser pulse. The alignment is probed by 2D ion imaging of the fragments from a 20 fs laser pulse induced Coulomb explosion.     

Spontaneous Faraday rotation due to strong laser radiation in a plasma

Theoretical studies on intense laser radiation interacting with plasma reveal an induced nonlinear birefringence which turns out to be spontaneous Faraday rotation. Here, this rotation is called nonlinear Faraday rotation (NFR). The expressions for the nonlinear refractive indices of the laser fields are calculated both in relativistic and non-relativistic limits for a two-component plasma. The NFR angles due to nonlinear induced birefringence are derived in the absence of externally imposed magnetic fields. It is shown that, in the relativistic limit, the electron motion plays an important role in producing NFR and so in the generation of induced magnetic fields.     

道威棱镜的偏振特性及偏振补偿研究

为了避免机载光电吊舱中共口径光学系统内部由于道威棱镜旋转引起的激光照射脉冲偏振态的变化,利用琼斯矩阵对道威棱镜的偏振特性与四分之一波片、半波片补偿道威棱镜旋转引起的激光脉冲偏振态变化进行了理论分析和实验验证。结果表明:线偏振的激光脉冲通过旋转一定角度的道威棱镜时,激光脉冲偏振态变为椭圆偏振,偏振态发生变化;而激光脉冲首先通过旋转一定角度的四分之一波片与半波片时,可使通过道威棱镜系统的激光脉冲偏振态保持不变,且两波片旋转角度与道威棱镜旋转角度之间存在一种非线性关系。采用偏振补偿方法可有效避免机载共口径光学系统中道威棱镜引起的激光脉冲偏振态变化,提高激光脉冲能量利用率,降低激光脉冲后向散射抑制难度。     

Optical activity of a cadmium diphosphide crystal

We have experimentally studied the characteristic features of a laser beam, polarized at angles of ± 45° to the horizontal (vertical), passing through plates of varying thickness cut from a CdP₂ crystal and placed between crossed polarizers, as a function of their angle of rotation relative to the vertical axis. We show that a periodic variation in the intensity is connected with the variation in polarization of the laser beam. We have established that the laser beam remains linearly polarized not only when it passes through the sample along the optic axis, but also when it travels at up to a 17.5° angle to the optic axis. In this case, the specific rotation of the plane of polarization along the optic axis is 68.2 degrees/mm for a thicker plate and 83.3 degrees/mm for a thin plate. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 5, pp. 690–693, September–October, 2006.     

基于子结构技术的光机装置反射镜组件的稳定性分析

反射镜组件是光机装置的重要组成部分,反射镜片的转角稳定性对光路的传输有着直接影响。为了分析获得在地脉动载荷作用下反射镜片的转角响应,分别基于整体结构建模技术和子结构技术建立了某光机装置简化结构的有限元模型,并分析获得了装置中两个反射镜片的转角响应。两种方法得到的反射镜片的转角响应差别分别为3.99%,11.84%,说明基于子结构技术开展光机装置反射镜架组件的稳定性分析是可行的。     

Model-based algorithm for rotation invariant target recognition using laser radar range imagery

The out-of-plane rotation invariance is demonstrated for a recognition system for laser radar (ladar) range imagery. The key of our method is to transform an original ladar range image of scene data into a height-range image. We demonstrate experimentally that our method provides the out-of-plane rotation invariance property and preserves in-plane rotation invariant characteristics at the same time. We also show experimentally the relationship between the recognition rate and depression angles.     

Characterization of the laser cleaving on glass sheets with a line-shape laser beam

A CO₂ laser with a line-shape beam was used to cleave a soda-lime glass substrate at various beam-rotation angles to the cutting direction. The stress distribution on the glass substrate cleaved by the laser beam has been analyzed in this study. An uncoupled thermal-elastic analysis was achieved by the ABAQUS software based on the finite element method. The numerical results show that the stress field of the fracture is caused by a complex stress state and the cleavages are significantly affected by the heat diffusion and beam rotation angle. At the rotation angle of zero degree to the cleaving direction, the phenomena of the chip formation have been found due to a large temperature gradient at the cleaving depth of the glass substrate.     

Determination of the moment of inertia of methyl groups: Analysis of the millimeterwave spectra of cis-propanal and methylthioethyne

The ground state rotational spectra of methylthioethyne and cis-propanal have been investigated. An overall fit of the measurements using the Internal Axis Method has allowed us to accurately determine the internal rotation parameters. The moments of inertia of methyl groups obtained from an internal rotation analysis are reviewed. They are compared to the values calculated using two empirical correlations found by McKean: between the isolated stretching frequencies and the corresponding CH bond lengths or the HCH angles.     

Experimental investigation of stochastic pulsation and formation of light bullets with megagauss magnetic fields by an intense laser pulse propagating in a preionized plasma

The generation of extremely stable light bullets in a preformed plasma near critical density has been observed experimentally during the interaction of intense picosecond laser beam with a metallic target in air. Optical probing measurements indicate the formation of pulsating channels, typically of about 5 microm in diameter, directed towards a heating laser beam, as well as of disconnected massive plasma blocks moving also towards the laser beam. The velocities of the dense plasma blocks reach the values of 4.5x10(8) cm/s. The blocks are stable during their acceleration and propagation in air. Self-generated magnetic fields up to 4-7 MG were observed by means of the Faraday rotation of a probe laser beam.     

Manifestation of gyrotropy upon light scattering in paratellurite

Upon scattering of laser radiation at different wavelengths, three optical effects—rotation of the plane of polarization of linearly polarized light, dependence of the specific rotation on the radiation wavelength, and the dependence of the scattered light intensity on the angle between the scattered and incident beams—are simultaneously observed. The intensity distributions of the scattered laser light at 0.488, 0.531 nm, and 0.633 μm along the optical axis of a thick sample of a TeO₂ single crystal are visualized. Based on the patterns obtained, the values of the specific rotation are calculated, and the curve of the optical rotation for the visible range is obtained.     

The application of a rotating-wave-plate stokes polarimeter for measurement of the optical rotation angle

An optical scheme based on Stokes-Mueller formalism and rotating-wave-plate Stokes polarimeter is successfully developed to measure the optical rotation angle in a chiral medium. The average relative error in the measured rotation angle of a half-wave plate is determined to be 1.16%. The average relative error in the measured rotation angles of glucose solutions with concentrations ranging from 0 to 1.2 g/dl is determined to be 3.78%. The correlation coefficient between the measured rotation angle and the glucose concentration is found to be 0.99950, while the standard deviation is just 0.00376°. From the inspection of measured rotation angle in the sol-gel materials containing C₁₇H₁₇ClO₆ with concentrations ranging from 0 to 0.0665 g/ml, the average relative error in the measured rotation angles is determined to be 3.63%. Consequently, the derived algorithm for measuring the rotation angle of a chiral medium is feasible, and the developed system is evaluated with a precision of 5.4% approximately in rotation angle measurement.     

Effect of non-Markovian dynamics on the violation of tripartite Bell inequalities

The effect of the non-Markovian dynamics on the violation of Bell inequalities for a generic tripartite entangled state has been investigated. Our results imply that the dynamics of Bell violation depends not only on the coupling strengths between the system and the environment but also on the rotation angles. Under both the strong and weak couplings, the Bell nonlocality can be eliminated by the decoherence induced by the environment in finite time when the rotation angles take certain values.     

A new electro-optic modulated circular heterodyne interferometer for measuring the rotation angle in a chiral medium

A new optical polarimeter capable of measuring the rotation angle in a chiral medium is developed successfully, and is an extension of previously developed simultaneous or sequential measurement system of the principal axis and retardance in linearly birefringent materials. The polarimeter for measuring the rotation angle is based on an electro-optic modulated circular heterodyne interferometer and using phase-lock technique to measure the rotation angle directly and precisely. The validity of the proposed design is demonstrated by the measurement of rotation angle in a half-wave plate and the glucose sample. The average relative error in rotation angle level of 0.00284° has been obtained for a half-wave plate. A correlation coefficient value of 0.9999975 is determined; it indicates a highly linear relationship between the reference values and the measured rotation angles. Moreover, a standard deviation in rotation angle level of 0.005275° has been obtained for glucose solutions with concentrations ranging from 0 to 1.2 g/dl in 0.2 g/dl increments, with a correlation coefficient value of 0.99915 between the reference and the measured values. This setup is compact in configuration, and is easy in calibration. The linearity and resolution characteristics of this system are comparable to those previous studies adopting phase-sensitive techniques.     

Generation of sheath in magnetized plasma under the impact of slow rotation

In this study, we have tried to investigate the generation of sheath in magnetized plasma rotating with a uniform angular velocity about an axis making an angle with the direction of plasma-acoustic wave propagation. In a marked contrast to the earlier studies, here the simultaneous impact of slow rotation and external magnetic field has been taken into consideration. Previous studies have revealed that the Coriolis force generated from rotation has a tendency to produce an equivalent magnetic field effect as and when the ionized medium rotates. The variations of sheath potential with normalized distance for different values of angles of rotation as well as for different values of Mach number have also been investigated for typical plasma parameters.     

激光多普勒自主测速仪天线安装误差对测速精度的影响

为了减小激光多普勒自主测速仪天线安装误差引起的测速误差,以多普勒频移原理为基础,通过旋转矩阵的复合变换推导出测速误差与安装误差间的函数关系,分析了三个安装误差角对三维测速精度的影响,并对旋转矩阵采用Bursa模型线性化后在整体最小二乘准则下给出安装误差角的最优估计,从而实现测速误差的补偿。仿真结果表明:测速误差随速度的增大而增大,影响纵向、横向测速精度的主要因素是偏航角,影响垂向测速精度的主要因素是纵倾角,补偿后三维测速误差显著减小。