粒子模拟中波导激励源的设计与实现

 讨论了粒子模拟中用时域有限差分(FDTD)法设计波导激励源的常见问题,并根据波的传播特性,提出一种模拟波导激励源的新方法,给出通用的计算公式,讨论了几种特殊情况下的算法修正。给出了2维柱坐标系下磁绝缘线振荡器(MILO)计算实例,结果表明：该方法能模拟较为复杂的波导激励源,并有效消除了激励源边界处的虚假反射,与传统波导激励源模拟方法比较,验证了该方法的正确性与优越性。     

Efficiency of excitation of a quasioptical open resonator by a waveguide

We study theoretically and experimentally the efficiency of excitation of an open resonator by a waveguide. A two-dimensional model of a quasioptical two-mirror open resonator, which is excited through a coupling element in the form of an open end of a plane-parallel waveguide, is proposed. The field of the open resonator is represented as the superposition of Gaussian wave beams and its scattering on the coupling element is described by solving the problem of diffraction of waveguide waves and Gaussian wave beams on an open end of a flanged waveguide. Properties of the mirrors and the resonator loss are specified from physical considerations. The calculation results are compared with the experimental data, and a physical interpretation of the obtained results is offered. The factors which influence the efficiency of excitation of an open resonator are identified. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 52, No. 3, pp. 231–249, March 2009.     

Ion wave excitation by radiation pressure in a plasma waveguide

After turn-off of high-power pulsed microwaves, a large-amplitude ion wave is generated which has half of the wavelength of the guided wave. The electromagnetic radiation pressure is responsible for the excitation of the ion acoustic wave.     

Models of non-linear waveguide excitation by non-stationary light beam

The propagation of a non-stationary light beam through a linear/non-linear discontinuity in a planar waveguide is considered by using an approximate approach based on the Generalised Non-Linear Schrodinger Equation (GNLSE) and the rigorous Transmission Line Modelling (TLM) Method. The objective is to compare the different theoretical approaches that can be used for the design of guiding structures with pulse shaping and signal processing functionality. Reflection and transmission of the light beam propagating through the junction of linear and non-linear waveguides are studied with respect to a dimensionless parameter that defines the increase of the refractive index induced by high-intensity radiation. In conclusion, the routes to efficient modelling are discussed.     

On the influence of radiation wavelength on the efficiency of excitation of near-surface waveguides

We show the results of numerical study of the dependence of narrow radio-beam propagation on the wavelength in a near-surface waveguide formed above the calm sea. The waveguide excitation efficiency, which is understood as a trapped part of the current beam power, and the waveguide-power loss rate due to diffraction-de-excitation have been found for wavelengths in the range of 0.8–10 cm and vertical profile of refractive index close to those really observed in the atmosphere Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 7, pp. 882–890, July, 1996.The author thanks A. Yu. Derkach and N. P. Santalov for assistance in computations.     

Two-dimensional coupled mode equation for grating waveguide excitation by a focused beam

The problem of grating coupling of a focused incident beam under non-normal incidence into a slab waveguide is given a complete three-dimensional (3D) solution. The diffracted field is expressed as the Fourier integral of a regular part and of a singular part resulting from the existence of the coupled guided mode. A suitable expression of the field in the neighborhood of the pole and a rigorous definition of the modal field lead to a generalized coupled mode equation relating the incident field and the two-dimensional (2D) modal field propagating in the plane of the slab waveguide. The phenomenological parameters involved in the coupled wave equation: the propagation constant, the radiation coefficient as well as the modal field shape are derived from the exact treatment of plane wave diffraction in the same structure. The solution of the complete coupling problem is given in the particular case of a Gaussian incident beam, and of a high index step-index waveguide.     

Enhanced two-photon fluorescence excitation by resonant grating waveguide structures

Enhanced two-photon fluorescence (TPF) spectroscopy with novel high-finesse resonant polymeric grating waveguide structures (GWSs) is presented. Under resonant conditions the field enhancement at the surface of a GWS can be exploited for TPF spectroscopy without the need for highly focused laser excitation light. We compare the TPF obtained by placing a drop of tetramethylrhodamine (TMR) on top of a GWS with that obtained with TMR on top of a glass substrate. Our procedure and results indicate that the detection of TPF can be improved by a factor of 10 with resonant GWSs.     

Correlation structure of the sound field produced in a deep ocean by a near-surface sound source

Experimental data obtained by studying the sound field produced in the first and second convergence zones by an omnidirectional pseudonoise source operating in the kilohertz frequency range at a depth of ～10 m are presented. The measurements of the cross correlation and the time spectra are performed for the signals received from different directions in the vertical plane by one narrow-beam, 40-m array and by two such arrays with the array centers positioned at different depths (200 and 450 m). The results of the experiments show that, for the signals arriving over different ray paths, the cross-correlation coefficients and the fluctuations of the time spectra obtained by using the reception at one depth and at two different depths are practically identical.     

Electromagnetic-wave excitation and diffraction by metal cylinder inside flat waveguide

Electromagnetic-wave excitation by a vertical electric dipole located inside a flat waveguide containing a vertical circular metal cylinder is examined. An exact solution is obtained, and the diffracted field inside the waveguide is analyzed. Algorithms are provided for electromagnetic-field calculation in a Rayleigh approximation and in a short-wave approximation. Results of numerical calculations of fields and directivity characteristics are presented. The effect of the cylinder on the radiation power of the dipole in the waveguide is analyzed.Nizhny Novgorod State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 36, No. 11, pp. 965–976, November, 1993.     

High-order mode of polymeric waveguide selective excitation

Conditions for the selective excitation of multimode waveguide high-order modes by power trans-formation of fundamental modes of a tunnel connected waveguide are calculated. It is shown that the power conversion factor can reach 100% under phase-matching conditions, which can be satisfied by the selection of a second waveguide refractive index.     

楔形海域中三维效应对声源距离估计的影响分析*

楔形海域中声传播会出现显著的三维效应,导致水平方向上特定接收位置的声信号出现模态多次到达或者模态影区。利用射线模型,首先对楔形波导中的传播损失进行计算,分析三维效应的存在对于声波传播的影响, 结果表明,随着距离的增加,水平折射效应愈加明显,所得的三维结果与利用二维模型计算所得相差逐渐增大。然后对比二维声场和三维声场中的接收波形,并对其进行模态分离,证明在某些接收位置将会出现明显的模态多次到达或模态影区。最后通过匹配全波解以及分离出的各号简正波所对应的脉冲波形进行声源距离估计研究,并分析三维效应对于声源距离估计结果的影响。结果表明,由于三维效应的存在,使用三维模型能够得到准确估计声源距离,而利用二维模型进行估计得到的结果与实际距离相差较多,总体上偏大。     

Electromagnetic field generated by a modulated moving point source in a planarly layered waveguide

In the present work, we consider a modulated point source in an arbitrary motion in an isotropic planarly layered waveguide. The radiation field generated by this source is represented in the form of double oscillatory integrals in terms of the time and the frequency, depending on the large parameter λ. By means of the stationary phase method, we analyze, in the waveguide, the Doppler effect, the retarded time, and the Vavilov–Cherenkov radiation. Numerically, the problem of the moving source is approached by the method of spectral parameter power series.     

5.2-5.6-microm source tunable by frequency conversion in a GaAs-based waveguide

A tunable mid-IR source obtained by difference-frequency generation is demonstrated in a selectively oxidized GaAsAlAs multilayer waveguide. We designed the waveguide to present the required form birefringence for phase matching of the nonlinear interaction. We took special care to lower losses for the mid-IR radiation. IR tunability from 5.2 to 5.6 mum was achieved by variation of the waveguide temperature and one pump wavelength. IR output power as great as 0.12 muW was obtained with the product of two pump powers of 7 mW(2). Losses of ~50 cm(-1) were measured for the mid-IR radiation. These losses are attributed to surface scattering.     

Phase-modulation fluorometer using a phase-modulated excitation light source

We propose a phase-modulation fluorometer (PMF) with a light-emitting diode (LED) or a laser diode (LD) used as an excitation light source (ELS) that is driven in the phase-modulation (PM) mode. The PM-ELS generates many frequency sidebands that spread in the vicinity of carrier frequency f _c with the interval of modulation frequency f _m depending on the maximum phase deviation Δφ. The scheme enables us to derive fluorescence lifetime values of a multicomponent sample at one time. We show a typical numerical simulation result for explaining the principle of operation. To demonstrate the effectiveness of the proposed PMF, we have measured fluorescence lifetimes of three kinds of inorganic fluorescent glasses and that of a mixture solution of 1 × 10^?6M rhodamine 6G and 1 × 10^?6 M coumarin 152 in ethanol with a volume ratio of 1: 1.     

Laser excitation of a fracture source for elastic waves

We show that elastic waves can be excited at a fracture inside a transparent sample by focusing laser light directly onto this fracture. The associated displacement field, measured by a laser interferometer, has pronounced waves that are diffracted at the fracture tips. We confirm that these are tip diffractions from direct excitation of the fracture by comparing them with tip diffractions from scattered elastic waves excited on the exterior of the sample. Being able to investigate fractures-in this case in an optically transparent material-via direct excitation opens the door to more detailed studies of fracture properties in general.