Sound wave scattering by a spherical scatterer located near an ice surface

An echo signal is simulated, which is reflected from a spherical scatterer located near an ice surface. The homogeneous water medium in which the scatterer is located is assumed semi-infinite. For the scattering coefficients of the sphere, asymptotic formulas are obtained by the saddle point method, which can be used for sufficiently large distances between the source emitting a spherical wave and the scatterer. For the occurring branch cut integrals using the steepest descent method, asymptotic expressions are also obtained. Numerical results are obtained for an acoustically rigid sphere and an ice sphere. The density of the ice medium and speed of longitudinal waves in it coincide with the analogous parameters of the ice cover. In a wide frequency range of 8–12 kHz, echo signals are compared that have been calculated for two models of media: a water half-space bordering an ice half-space and an ice-covered homogeneous waveguide with a fluid bottom under the assumption that the source placed in the water layer is directional. It is shown that in a large distance interval between the source and the spherical scatterer, the half-space model sufficiently accurately describes the echo signal while substantially reducing calculation time (by approximately a factor of 10 for the waveguide with a depth of 200 m and a sandy bottom considered in the paper).     

Analysis of a compliantly suspended acoustic velocity sensor

The dynamics of a compliantly suspended acoustic velocity sensor having a spherical geometry are analyzed using theory and experiment. The analysis starts with a review of the motion associated with an unconstrained solid sphere when subjected to an acoustic plane wave in an unbounded inviscid fluid medium. The theory is then modified to account for the inclusion of an inertial sensor and an external suspension system. Accordingly, the open-circuit receiving response of a geophone-based and accelerometer-based device is derived. Density variations associated with the sphere and the surrounding fluid medium are assessed along with the effects fluid viscosity. Wave effects in the sphere and the suspension system are also analyzed.     

Natural Electromagnetic Waves in a Circular Waveguide with a Chiral and a Dielectric Layer

Based on particular solutions of a system of wave equations which describe electromagnetic processes in an unbounded biisotropic medium and are represented in the form of generalized power series, a dispersion equation is obtained for a two-layer circular waveguide with a chiral and a dielectric layer. The solution of the dispersion equation is analyzed for some types of natural waves.     

Integrated optical cross strip polarizer concept

Passing across an abrupt junction from a thick vertically bimodal waveguide to a thinner single mode segment, guided light can undergo complete destructive interference, provided that the geometry and the phases of the modes in the initial segment are properly adjusted. We propose to employ this effect to realize a simple polarizer configuration, using a strip that is etched from a planar waveguide. A beam of light is made to pass the strip perpendicularly. The light enters from the single mode waveguide outside the strip into the strip segment, which is configured to support two modes. At the end of the strip, apart from reflections, the amount of power that is guided in the following lower segment depends on the local phases of the two modes. These phases are different for TE and TM light, hence we may expect a polarization dependent power transfer, resulting in polarizer performance for a properly selected geometry. The paper describes in detail the modeling of the device in terms of rigorous mode expansion. Design guidelines and tolerance requirements for geometric and material parameters are discussed. For typical Si₃N₄/SiO₂ materials, our calculations predict a peak performance of 34 dB polarization discrimination and 0.3 dB insertion loss for a device with a total length of about 12 m that selects TE polarization at a wavelength of 1.3 m.     

Grazing‐incidence small‐angle X‐ray scattering from Ge nanodots self‐organized on Si(001) examined with soft X‐rays

Grazing‐incidence small‐angle X‐ray scattering (GISAXS) measurements with soft X‐rays have been applied to Ge nanodots capped with a Si layer. Spatially anisotropic distribution of nanodots resulted in strongly asymmetric GISAXS patterns in the q_y direction in the soft X‐ray region, which have not been observed with conventional hard X‐rays. However, such apparent differences were explained by performing a GISAXS intensity calculation on the Ewald sphere, i.e. taking the curvature of Ewald sphere into account.     

Determination of structure of weak scatterers from holographic images

For a weak scatterer a three dimensional Fourier transform of the image formed by a generalized hologram is shown to yield directly the Fourier transform of the scattering potential evaluated on the Ewald sphere. The entire scattering potential can be approximately determined if the object is illuminated from all directions in a series of experiments and if the scattering potential is essentially bandlimited to within a sphere of radius 4π/λ.     

海洋波导中刚性球及旋转椭球前向散射场时频特征的畸变

利用波导中目标声散射理论的简正波方法,数值模拟了自由空间和浅海均匀波导中刚性球、旋转椭球的散射场,分析了波导中刚性体前向散射时频特征的畸变规律。仿真结果表明:波导中刚性体的前向散射时域波形与频谱特征受波导制约;目标散射场的固有特征受波导色散特征的调制,时频谱上呈现出由于色散的简正波间的耦合而导致各简正波能量条纹界限模糊的现象。时频特征的畸变程度与组成散射共振系统的波导、目标的尺度以及散射体布放深度有关,随波导底质透声能力和深度增加、散射体散射强度(信混比)增强而减弱。      

Electric currents on ideally conducting body of revolution during transient diffraction

The problem of scattering of an electromagnetic pulse by an ideally conducting body of revolution in a homogeneous isotropic medium is presented. Mutually independent systems of integrofunctional equations are derived for individual azimuthal current harmonics, taking into account axial symmetry of the body. Transient relations between currents on a sphere or on a finite circular cone are analyzed for the case of diffraction of a Gaussian pulse.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 27, No. 1, pp. 87–95, January, 1984.     

Diffraction of guided waves by the end face of a dielectric slab waveguide terminated with a finite conductive strip

The diffraction of guided waves by the end face of a dielectric slab waveguide short circuited with a finite conductive strip is analyzed. An integral equation technique is employed to formulate the corresponding boundary problem. The unknown term in this integral equations is the electric field E(x) on the terminal plane of the waveguide. The homogeneous term is determined from the incident guided wave. A method of moments technique is employed to compute approximately the electric field E(x) by using Laguerre functions as describing and testing functions. The reflection coefficients of the guided waves are computed by using the approximate expression of the E(x) field. Numerical results are given for several guide and conductor plate dimensions.     

近似声斗篷的多层介质覆盖水中弹性球壳的声散射

对水中弹性球壳的低频覆盖层消声效果和消声机理进行研究。声斗篷(Acoustic cloak)是各向异性的超常材料,根据有效介质近似理论(Effective MediumApproximationTheory),采用各向同性多层介质近似各向异性材料的声斗篷,推导出覆盖多层介质水中弹性球壳散射声场的解析表达式,计算了弹性球壳覆盖多层介质前后的散射形态函数、谐振模态以及声场分布,分析了覆盖前后球壳的声散射特性和声传播机理。研究表明,覆盖层内的声波在多层介质之间的声传播方向发生改变,声场形成弯曲变形,声能流绕过目标,这不仅极大的降低了低频散射强度,而且使到达弹性体表面的声场强度非常小,散射频响中除了0阶子波产生的第一个谐振峰外,无法激发弹性球壳的其它谐振模态,有效的抑制弹性球壳的谐振散射。      

A Riemann–Hilbert Problem for Propagation of Electromagnetic Waves in an Inhomogeneous,Dispersive Ω Waveguide

We consider the inverse scattering problem for a model of electromagnetic wave propagation in a rectangular waveguide filled with dispersive material. The waveguide is inhomogeneous in the longitudinal direction but homogeneous in the transverse directions. Dispersive properties of the material are described by a single-resonance Lorentz model. By reformulating the scattering problem in the frequency domain as a Riemann–Hilbert problem, we prove that the constitutive parameters of the inhomogeneous waveguide are reconstructed uniquely from the scattering data.     

On the problem of an inhomogeneous sphere placed in a homogeneous alternating magnetic field

The results of a numerical analysis of the equivalent magnetic moment of a two-layered sphere magnetized by a homogeneous alternating magnetic field are reported. The particular case of a hollow sphere is examined. It is found that for values of the parameter 60, the hollow sphere has the same magnetic moment as a solid sphere.     

A proposal for the generation of optical frequency comb in temperature insensitive microcavity

We numerically simulate the generation of an optical frequency comb(OFC) in a microring based on the traditional Si_3N_4 strip waveguide and a temperature compensated slot waveguide.The results show that OFCs are susceptible to temperature with strip waveguide while they can keep stable when temperature changes 10 Kin either low-Q(10~5) or highQ(10~6) microcavity with the well-designed slot waveguide,which has great superiority in practical applications where the temperature drift of the cavity due to the intense pump or surrounding change is unavoidable.     

光通信中的玻璃无源器件的制备与研究

本文报道了以高质量的BK7光学玻璃为衬底,通过稀释Ag+-Na+离子交换技术,制备出低损耗的表面条波导,并用近场法测量出条波导的模场分布曲线。给出光纤与波导耦合的失配损耗为0.54dB,条波导传输损耗为0.21dB/cm.以及在此工作基础上,成功地研制了适用于大容量相于光通信系统的1×2和1×4单模波导分束器。     

Efficient coupling of hybrid optical waveguide with a sharp bend structure for high integration

A hybrid optical waveguide with a \(90^{\circ }\) sharp bend comprising a dielectric straight waveguide, a tapered dielectric strip waveguide, and a microscale metal gap waveguide is proposed, modeled, fabricated, and characterized with the aim of improving the efficiency of light coupling between the dielectric and plasmonic waveguides. The simulation result using the full-vector finite-difference time domain shows a total transmissivity of about 63 % at a wavelength of 1,550 nm. A set of hybrid optical waveguide with a \(90^{\circ }\) bend is fabricated via the two-step photolithography and a metal lift-off process. From the measured result for the characteristics of the fabricated hybrid optical waveguide, the transmission loss was estimated to be about 17 dB, which is in stark contrast with the simulation value. Nevertheless, such a novel coupling scheme may be of potential use in high-density photonic integration applications.     

Fluctuation theorems for dielectrics with periodic boundary conditions

We derive fluctuation theorems for dielectrics with periodic boundary conditions defined by a Bravais lattice in which the configuration of a large number of permanent dipoles in the unit cell is repeated periodically. We use the electrostatic approximation and show that it is essential to consider bounded geometry. We consider in particular geometries adapted to the Ewald summation used in computer calculations, namely ellipsoids, with as special cases a slab and a sphere. The fluctuations depend strongly on the chosen geometry.     

有限波束作用下掩埋球体的散射声场计算

计算并分析了海底掩埋物体的三维散射声场。采用“等效垂直线列阵”方法来进行有限波束的建模,并将浅海波导中点声源散射声场的波数积分计算方法,推广到有限波束作用下海底掩埋物体的散射场计算,本文导出了物体位于沉积层中的散射声场计算公式。计算结果表明,在波导中垂直面内的散射声场,与沉积层中点源形成的声场非常相似;当距离较远时,散射声波呈柱面波衰减。文章还分析利用海面反射以提高散射能量的可能性,表明波束指向性及波束宽度对散射声场有较大影响。     

Photo-thermal-elastic interaction in an unbounded semiconducting medium with spherical cavity due to pulse heat flux

In this work, the photothermal waves in an unbounded semiconducting medium with spherical cavity are studied. This problem is solved using the theory of coupled plasma, thermal, and elastic wave. An unbounded material, elastic semiconductor containing a spherical cavity with isotropic and homogeneous thermal and elastic properties has been considered. The inner surface of the cavity is taken traction-free and subjected to an exponentially decaying pulse boundary heat flux. Laplace transform techniques and eigenvalue approach were used to obtain the analytical solutions. Numerical computations have been done for silicon-like semiconductor material, and the results are presented graphically to estimate the effect of time and the coupling between the plasma, thermal, and elastic waves.     

Spontaneous multipole radiation in a condensed medium

The probability of the spontaneous emission of electromagnetic radiation of an arbitrary multipolarity in an infinite homogeneous isotropic nonabsorbent condensed medium with constant permittivity and permeability is determined using the perturbation theory for quantum electrodynamics. The local field inside a sphere is calculated for fields of arbitrary configuration in the medium with the help of the real and virtual cavity models.