Nonuniform Currents Research Articles

Nonuniform injection current induced unusual chirp behavior of a four-electrode bistable distributed Bragg reflector laser

A complete analysis of the chirp of a four-electrode bistable distributed Bragg reflector laser is given in theory and by experiment. Experimentally, the chirp investigation is realized by a Mach-Zehnder interferometer. In small-signal regime, the modulus and phase of 2/spl beta//m, frequency and amplitude modulation index ratio, is measured for different injection currents and shows an unusual behavior against the modulation frequency. A theoretical model including nonuniform injection current shows the same behavior of 2/spl beta//m against the modulation frequency and proves that the nonuniform carrier distribution inside the cavity is the origin of the specific behavior. The large-signal analysis provides a study of the instantaneous chirp behavior during optical switching. >

The currents on a cylinder illuminated by a general obliquely-incident plane wave

A relation is presented that determines the total current induced by a general plane wave obliquely incident or a perfectly electrically conducting cylinder of arbitrary cross section from the total current induced by a normally incident plane wave. Remarkably, this same relation ran also be used to determine the physical optics (PO) and nonuniform (NU) currents for oblique incidence directly from the PO and NU currents, respectively, for normal incidence.

Perturbed h-method without the Lagrange multiplier for three-dimensional nonlinear magnetostatic problems

A new formulation of the three-dimensional nonlinear magnetostatic problem is presented. This is a perturbed h-method without the Lagrange multiplier. To verify this method, the benchmark problem 13 defined in the TEAM Workshop for a nonlinear magnetostatic problem is solved and the results are compared with the experimental results measured by Nakata et al. (1990) of Okayama University. Also the practical problem of determining the magnetic field excited by a non-uniform sense current of an MR head is solved. >

Nonlocal resistivity in the vortex liquid regime of type-II superconductors.

We develop a phenomenological hydrodynamic description of the transport properties of a viscous vortex liquid in realistic geometries with nonuniform currents, including the effects of the boundaries of the sample. This approach should be useful for modeling multiterminal transport measurements in the vortex liquid regime of, for example, YBCO.

Wave spectrum filtration by current and wave height calculation

Theoretical models for the interaction between wind-generated wave spectra and currents are worked out on the basis of wave dispersion equations with nonuniform currents. The relationships obtained for critical velocity of surface adverse current are compared with experimental data from a laboratory wave channel. As a result of wave-current interaction, the filtration of the wave frequency spectrum by current causes the mean wave height to diminish. The results of the study of wave frequency spectrum filtration by an adverse current and by a following current when the waves approach the current at some angle, are compared with experimental field and laboratory results. The available field data (Hayes, 1980) verify the effect of partial spectrum filtration due to a caustic. The calculation of the wave spectrum frequency filtration by an adverse current correlates satisfactorily with the experimental investigations of the pneumatic barriers.

Double reflection of capillary/gravity waves by a non-uniform current: a boundary-layer theory

When a train of gravity waves encounters an opposing current, the wavelength is shortened and the waves may be reflected. If capillarity is included, the shortened waves may be reflected for a second time and experience further shortening. By this process the initially long gravity waves can be damped by viscosity quickly without breaking. In this paper a boundary-layer approximation is obtained close to the reflection points, and is matched to the ray approximations outside. This is then applied to the propagation of a wavepacket. Damping is accounted for in the ray solution and the result is compared to the undamped solution. The case where the two reflection points coalesce is also considered. It is found that as the separation between the reflection points decreases, the wavepacket appears to remain longer in the region of reflections relative to the width of this region.

Application of incremental length diffraction coefficients to calculate the pattern effects of the rim and surface cracks of a reflector antenna

Incremental length diffraction coefficients (ILDCs) for the half-plane are integrated around the rim of a paraboloid reflector antenna to obtain well-behaved far fields of the nonuniform current for all angles of observation. These far fields, when added to the physical optics far field, produce a more accurate total far field of the reflector. Excellent agreement with the far fields obtained from a method-of-moments solution to the electric field integral equation applied to a 20-wavelength-diameter reflector shows that the cross polarization, farther-out sidelobes, and fields near nulls of reflector antennas can be appreciably modified by the fields of the nonuniform currents. ILDCs are also used to investigate the effect of cracks on the surface of reflectors that can result from the imperfect fitting together of panels to form large reflectors. Three models of cracks are studied. Significant pattern effects are found, depending on the model and orientation of the cracks.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A uniform ray approximation of the scattering by polyhedral structures including higher order terms

A uniform ray representation of the far field scattered by flat plate structures is investigated by postulating an approximation of the surface current on each face of the object, which is subsequently integrated either in closed form or asymptotically in terms of the well-tabulated edge transition function. Specifically, the current on each plate is approximated, in addition to the usual physical optics (PO) component, by a primary nonuniform current, obtained from the canonical solution to the wedge problem and truncated at edges of the plate, as well as a secondary nonuniform current induced by doubly diffracted fields and expressed in terms of an equivalent edge source. The superimposed effect of the rays resulting from the primary and secondary nonuniform current integration improves the agreement of the calculated pattern as compared with method of moments computations.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Diffraction coefficients and field patterns of obtuse angle dielectric wedge illuminated by E-polarized plane wave

The diffraction problem is treated for the incidence of an E-polarized plane wave on both interfaces of an obtuse dielectric wedge. Based on the dual integral equation, the total field is obtained by the sum of the physical optics solution and the edge-diffracted correction term. Calculated diffraction coefficients and field patterns are plotted in figures for a wedge angle of 120 degrees , incident angles of 60 degrees and 70 degrees , and relative dielectric constants of 2 and 10. It is shown that the Neumann expansion to the nonuniform currents provides a more accurate correction to the physical optics currents than the multiple expansion as the angle of dielectric wedge increases.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Diffraction by an arbitrary-angled dielectric wedge. II. Correction to physical optics solution

For pt.I see ibid., vol.39, no.9, p.1272-81 (1991). The error of the physical optics solution for the E-polarized plane wave incidence in connection with diffraction by an arbitrary-angled dielectric wedge is corrected by calculating the nonuniform current distributed along the dielectric interfaces. Two kinds of series expansions to the nonuniform current are employed. One is an asymptotic expansion as the multipole line source located at the edge of the dielectric wedge, since the correction field seems to be a cylindrical wave emanating from the edge in the far-field region. The other is arbitrary electric and magnetic surface currents expanded by infinite series of the Bessel functions, i.e. the Neumann expansion, of which fractional order is chosen to satisfy the edge condition near the edge of the dielectric wedge in the static limit. Both of the two different expansion coefficients for a wedge angle of 45 degrees , relative dielectric constants 2, 10, and 100, and the E-polarized incident angle of 150 degrees are evaluated by solving the dual series equation numerically after finite truncation. >

The Integral Equation Method in the Problem of Electromagnetic Waves Diffraction by Complex Bodies

ABSTRACT A numerical approach based on the method of integral equations is used to solve a very large class of applied electrodynamic problems. First of all, many problems of diffraction and excitation for bodies of different nature and form were solved. Representation of unknown currents as a sum of “uniform” and “nonuniform” components permits the use of integral equations to solve such problems for semi-infinite structures. It gives the opportunity to determine the diffraction coefficients used in Keller‘s geometrical theory of diffraction, and nonuniform currents used in Ufimtsev‘s method of edge waves for cases where canonical problems have no analytical solutions. It extends the field of application of these asymptotic methods. In addition, the method of integral equations can be used to consider objects supporting surface waves.

The modulational theory of nonlinear gravity-wave in fluid with varying depth and nonuniform current

By means of WKB expansions, new fourth order evolution equations are derived for two-dimensional Stokes waves over the bottom with arbitrary depth. The effects of slowly varying depthh=h(e2x) and currentU=U(e2x,e2t,e4z) on the evolution of a packet of Stokes waves are considered as well. In addition, numerical simulation is performed for the evolution of single envelope by finite-difference method.

Evolution of a gravity wave spectrum through a current gradient

Transformation of a wind wave spectrum through a region of nonuniform current is investigated. It is shown how an evolution equation for the spectral action can be arrived at by the scaling arguments introduced by Phillips (1985). A numerical model based on this equation is applied to two different current systems (a head‐on current and a shear current). The model also contains an indicator of excess wave breaking activity. It is pointed out that the short wave part of the spectrum (less than 10 cm, say) is not expected to be well predicted by models like the one presented here. This is probably because excess wave breaking of the longer waves is expected to generate short waves or short‐scale surface roughness, an effect not accounted for in these models.

Nonuniform currents on a conducting wedge illuminated by a TM plane wave

Closed-form expressions for nonuniform currents induced on a perfectly conducting infinite wedge illuminated by a TM plane wave are presented. Results computed by using these expressions are in good agreement with ones of the eigenfunction solution of the wedge.

不等流沿岸流における流速分布の相似性について

不等流沿岸流における流速分布の相似性について

Closed-form expressions for uniform currents on a wedge illuminated by TM plane wave

Closed-form expressions for nonuniform currents on a perfectly conducting, infinite wedge illuminated by a transverse magnetic plane wave are presented. The expressions are derived by requiring that they agree with the current predicted by the eigenfunction solution close to the edge and J.B. Keller's geometrical theory of diffraction (1962) far from the edge. The angle of incidence is arbitrary and the expressions remain uniformly valid even for glancing angles of incidence when the geometrical optics boundaries are in the vicinity of the wedge faces. The formulas presented are simple, involving Fresnel functions with complex arguments. These functions can be expressed in terms of complimentary error functions which may be computed using standard subroutine packages. Exact expressions for nonuniform currents are available for the two special cases of half-planes and infinite planes. Closed-form expressions for the axial electric field, and hence all the field components in the vicinity of the wedge axes, are also obtained. Currents computed using expressions obtained are compared with currents computed from the eigenfunction solution of the wedge, with good agreement throughout.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

87:6077 Behavior of the surface gravity wave spectrum on a horizontally nonuniform current: Lavrenov, I.V., 1986. Izv. Atmos. Ocean Phys. (a translation of Fiz. Atmos. Okeana), 22(5):398–402

87:6077 Behavior of the surface gravity wave spectrum on a horizontally nonuniform current: Lavrenov, I.V., 1986. Izv. Atmos. Ocean Phys. (a translation of Fiz. Atmos. Okeana), 22(5):398–402

Nonuniform currents on a wedge illuminated by a TE plane wave

Closed-form expressions for nonuniform currents on a perfectly conducting, infinite wedge illuminated by transverse electric (TE) plane wave are presented. These expressions are derived by requiring that they coincide with the current predicted by the asymptotic diffraction method far from the edge and, further, that they agree with the current predicted by the eigenfunction solution at the edge. The angle of incidence is arbitrary and our expressions remain valid even for glancing angles of incidence when either one or both faces of the wedge are in the vicinity of a geometric optic (GO) boundary. Formulas presented here are simple involving the well-known modified Fresnel functions but are not uniform. Exact expressions for nonuniform currents are available for the two special cases of half-plane and infinite plane. For these special cases, our solution reduces to the exact solution. Currents computed using the expressions developed here are compared with currents computed from the eigenfunction solution of the wedge. Good agreement is obtained throughout.

Multifluxon dynamics in driven Josephson junctions

The dynamics of fluxons in a long Josephson junction driven by time-varying nonuniform bias currents are described by a generalization of the sine-Gordon equation. This equation has solitary wave solutions which correspond to current vortices or quantized packets of magnetic flux in the junction. As with the sine-Gordon equation, multifluxon solutions may be demonstrated for the long Josephson junction. Our numerical calculations show that several fluxons may be launched or annihilated at the end of a junction. We also show multiple steady state conditions which correspond to one or more flux quanta trapped in the junction.

Current and current shear effects in the parabolic approximation for underwater sound channels

The effect of currents on the acoustic pressure field in an underwater sound channel is investigated. Based on fundamental fluid equations, model equations are formulated for sound pressure while including nonuniform currents in the source–receiver plane. Application of parabolic-type approximations yields a collection of parabolic equations. Each of these is valid in a different domain determined by the magnitudes of current speed, current shear, and depth variation of sound speed. Under certain conditions, it is possible to interpret current effects in terms of an effective sound speed. Using this effective sound speed in an existing numerical code, we examine sound speed in a shallow water isospeed channel with a simple shear flow and a lossy bottom. It is found that even small currents can induce very substantial variations in relative intensity. The degree of variation depends upon current speed, source and receiver geometry, and acoustic frequency. Particular emphasis is placed on intensity-difference predictions in reciprocal sound transmissions in the presence of an ocean current.