超声多波聚焦及声偏振方向控制方法*

在超声多波聚焦思想的基础上,通过数值模拟计算,分析了时间反转法的多波聚焦特性以及对声场偏振方向进行控制的可行性。结果表明,在待测目标的不同位置处,时间反转法都能够实现多波聚焦的效果,使具有不同传播速度、不同偏振特性的多种声波自适应聚焦。但是,在介质的近表面处,由于受到表面波的影响,多波聚焦声场仍然具有椭圆偏振特性,无法实现声场偏振方向精确控制的目的;而位于介质内部的多波聚焦点受到表面波影响很小,数值计算结果表明此时多波聚焦声场具有线偏振特性,通过改变声源前后两个脉冲的激发幅度和相位,可以控制声场的偏振方向,达到偏振方向扫描的目的。该文的研究为精确检测裂纹方向或界面性质提供一种可能的途径。     

Dielectric properties of short sisal/coir hybrid fibre reinforced natural rubber composites

The dielectric properties, such as dielectric constant, volume resistivity and dielectric loss factor, of sisal/coir hybrid fibre reinforced natural rubber composites have been studied as a function of fibre loading, fibre ratio, frequency, chemical modification of fibres and the presence of a bonding agent. The dielectric constant values have been found to be higher for fibre filled systems than pure natural rubber. This has been attributed to the polarization exerted by the incorporation of fibres into the matrix. Dielectric constant values were observed to be decreased with increase in frequency due to the decreased interfacial and orientation polarization at higher frequencies. Whereas dielectric constant increases with fibre loading because of the increment in number of polar groups after the addition of hydrophilic lignocellulosic fibres. The volume resistivity of the composites was found to be decreased with fibre loading and a percolation threshold has been obtained at 15.6% volume of fibres. Fibre treatment, such as alkali, acetylation, benzoylation, peroxide and permanganate, were carried out to improve the adhesion between fibres and matrix. The dielectric constant values were lower for systems consisting of fibres subjected to chemical treatments due to the increased hydrophobicity of fibres. The addition of a two-component dry bonding agent consisting of hexamethylene tetramine and resorcinol, used for the improvement of interfacial adhesion between the matrix and fibres, reduced the dielectric constant of the composites. When the weight percentage of sisal fibre was increased in the total fibre content of the hybrid composites, the dielectric constant was found to increase. The added fibres and different chemical treatments for them increased the dielectric dissipation factor. A dielectric relaxation has been observed at a frequency of 5 MHz.     

Light propagation in a strong external magnetic field with regard to quantum corrections

Linearized motion equations for electromagnetic wave field in the external magnetic field have been solved for the PVLAS experiment configuration within the low-energy approximation of quantum electrodynamics. It has been shown that the wave propagation velocity depends on the initial direction of the plane of wave polarization. Dispersion laws corresponding to the waves with mutually perpendicular directions of polarization have been established. The dependence of ellipticity of laser radiation field on initial polarization and the value of the external magnetic field has been obtained. The ellipticity parameter for the configuration of the system used in the PVLAS experiment has been found.     

Extremely short vector solitons under the conditions of conical refraction

Propagation of extremely short electromagnetic pulses in a biaxial crystal under the conditions of conical refraction has been considered. The system of wave equations taking into account the dispersion contribution of the crystal lattice ions to the polarization response of the medium and a nonlinearity of the polarization response of electrons has been derived. It has been shown that under certain conditions this system can be reduced to an equation which is integrable by means of the inverse scattering transformation method. The proper Lax pair has been found. Physical analysis of the steady-state pulse solution of the system of wave equations has been performed.     

Propagating modes of a lens-like medium

The wave equation for a weakly guiding medium having a cylindrical symmetry and parabolic index distribution has been solved exactly. Simple power series are obtained for the field distributions of the modes, which illustrate the type of fields that will be found in graded core fibres away from the core cladding discontinuity. These results are discussed and compared to the modes predicted for other types of fibres.     

Features of magneto-optical resonances in an elliptically polarized traveling light wave

The parameters of nonlinear absorption magneto-optical resonances in the Hanle configuration have been studied as functions of the ellipticity of a traveling light wave. It has been found that these parameters (amplitude, width, and amplitude-to-width ratio) depend strongly on the polarization of the light wave. In particular, the resonance amplitude can increase by more than an order of magnitude when the polarization changes from linear to optimal elliptic. It has been shown that this effect is associated with the Doppler frequency shift for atoms in a gas. The theoretical results have been corroborated in experiments in Rb vapor.     

Polarization properties of elliptical core non-hexagonal symmetry polymer photonic crystal fibre

In this paper, polarization properties and propagation characteristics of polymer photonic crystal fibres with elliptical core and non-hexagonal symmetry structure are investigated by using the full vectorial plane wave method. The results show that the birefringence of the fibre is induced by asymmetries of both the cladding and the core. Moreover, by adjusting the non-symmetrical ratio factor of cladding $\eta$ from 0.4 to 1 in step 0.1, we find the optimized design parameters of the fibre with high birefringence and limited polarization mode dispersion, operating in a single mode regime at an appropriate wavelength range. The range of wavelength approaches the visible and near-infrared which is consistent with the communication windows of polymer optical fibres.     

An Optical Model for Polarization Microscopy Analysis of Pulp Fibre-to-Fibre Bonds

Pulp fibre-to-fibre bonds were studied using polarization microscopy and microtome cuts. The experiments showed considerable discrepancies between these two experimental methods. While microtome cuts clearly show if a bond between two fibres has formed, polarization microscopy cannot unambiguously discern between crossed unbonded fibres and bonded fibres; also certain bonds cannot be detected with this method. To examine these shortcomings, a physical model of polarization microscopy of bonded and unbonded pulp fibers was built. Experimental validation of the model gave good agreement between calculations and reflectance measurements. Calculations based on this model clearly demonstrate that only bonded fibres resembling a plane parallel plate show as bonds. However, crossings of unbonded fibers also appear as bonds if the two fibres are flat and plane parallel to each other. The model provides a consistent interpretation for polarization microscopy imaging of pulp fibre bonds, an important topic in research of mechanical and optical properties of fibrous composites like paper.     

FDTD Simulation for the Focusing Property of Extended Hemispherical Lens as Receiving Antenna

The focusing property of extended hemispherical lens, on which a linearly polarized plane wave is incident normally, is simulated by FDTD method. The focusing process is presented pictorially at different moment. Focal length is obtained and compared with that computed by other method. Good agreement has been found. The coupling between the focused incident wave and a waveguide in the focal plane of the lens is also simulated. The lens is used as antenna at millimeter wavelengths.     

Light polarization modulation in single-mode optical fibres

A theoretical and experimental study of light polarization modulation in single-mode anisotropic optical fibres is presented. The problem of increasing polarization modulation efficiency in slightly and strongly anisotropic fibres is considered. The requirements to be met by the optical configuration for efficient polarization modulation with both slightly and strongly anisotropic fibres are discussed. The polarization modulation signal is found to be immune to external effects on the fibre under compensated birefringence. An experimental polarization modulation-based fibredyne video transmission data highway is presented.     

Polarization-reversal-induced damping of left-hand polarized wave on electron cyclotron resonance

The left-hand polarized wave, which has been believed not to be related to electron cyclotron resonance (ECR), is for the first time demonstrated to be damped near the ECR point in an inhomogeneously magnetized plasma as a result of the polarization reversal to the right-hand polarized wave. The polarization reversal is found to be caused by the existence of the boundary between a plasma and a vacuum region, the local point of which shifts depending on the plasma column radius. This phenomenon is quantitatively explained in terms of the dispersion relation including the effect of the radial boundary condition.     

Polarization of Tapered Semiconductor Travelling-Wave Amplifiers

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An examination of the effect of polarization on the radiation losses of bent optical fibres

It has long been recognized that the bending losses in weakly guiding optical fibres are independent of the polarization for large bend radius. Here, we show this fact using the volume equivalent current method. The procedure is then applied to a helically bent fibre and it is shown that the radiation from the helical fibre is also independent of the polarization as long as the fibre is weakly guiding.     

Thomson scattering induced by gravitational waves

We investigate the effects of a weak gravitational wave, modelled as a gaussian wavepacket, on the polarization state of an electromagnetic field enclosed in a cavity. Our approach is semiclassical, in that the electromagnetic field is described as a quantum field, while the gravitational perturbation is treated classically, as a slightly curved background spacetime. Assuming that before the interaction the electromagnetic field has been prepared in a given polarization state, we show that – due to the gravitational scattering with the wave – some photons having different polarization states are found in the cavity at late times. Such polarization scattering has some resemblance with Thomson scattering, well-known in Quantum Electrodynamics: hence the motivation for the title. We give a numerical estimate of the resulting photon polarization spreading in the case of a typical gravitational burst from a final supernova rebound. We also briefly comment about the possible influence of such gravitational scattering on the Cosmic Microwave Background (CMB) polarization.     

Stokes parameters of an electromagnetic light wave on scattering

Stokes parameters of an electromagnetic light wave on scattering have been discussed, which permit us to study the spectral degree of polarization of the far-zone scattered field. An example of scattering of a spatially coherent electromagnetic light wave from a quasi-homogeneous medium has been discussed to illustrate the changes of the Stokes parameters and the changes of the spectral degree of polarization of the far-zone scattered field.     

Goniopolarimetry: Space-borne radio astronomy with imaging capabilities

The principles of space-based low-frequency radio astronomy (below 50 MHz) are briefly introduced. As the wavelength range considered does not allow the use of focusing systems, goniopolarimetric (or direction-finding) techniques have been developed. These inversion techniques provide the direction of the wave vector, the polarization state and the flux of the observed electromagnetic wave. In case of spatially extended sources, we can also infer an order of magnitude of the apparent source size. These techniques are presented, and their limitations are discussed. An example from a recent study illustrates the techniques and capabilities.     

General demonstration of principal states of polarization and real-time monitoring of polarization mode dispersion in optical fibres

We investigated the general properties of polarization effects in optical fibres and demonstrated the existence of socalled principal states of polarization (PSP), which mean the fixed points in mathematics, in different polarization effects,such as birefringence and polarization mode dispersion, by using fixed point theory. Furthermore, a time evolution vector is defined to describe the time evolution of polarization state in optical fibres, which is used to investigate the time evolution of polarization mode dispersion vector (PDV), including differential group delay and PSP. The experimental results of real-time monitoring of PDV by using this method are reported. To our knowledge, this is the first report on monitoring PSP evolution in optical fibres.     

Focusing of singular beams

The process of focusing of an inhomogeneously polarized beam passed through a system consisting of a uniaxial crystal and a polarization filter is considered. It is shown that the action of this system is equivalent to the action of a phase transparency with a complex relief of the refractive index. In particular, if the angle between the axes of the quarter-wave plate and the polarizer is equal to 45°, this transparency has a helicoidal relief. Simultaneously, two singular beams with different curvatures of their wavefronts arise in the transparency. The action of the lens consists in formation of a combined singular beam having three focal waists. The boundary wave arising upon focusing is able not only to form a new spectrum of toroidal vortices, but also to straighten the focusing region, forming a fairly extended range with a weak divergence.