Nonuniform Coupling Research Articles

High-power Čerenkov microwave oscillators utilizing High-Current nanosecond Electron beams

A short review is given of results obtained at the Institute of High-Current Electronics of the Siberian Branch of the Russian Academy of Sciences on generating high-power microwave radiation. Most of the research was devoted to a study of stimulated Cerenkov radiation from relativistic electron beams. It is shown that the efficiency of a relativistic 3-cm backward wave tube with a nonuniform coupling resistance can reach 35%. High-frequency radiation was discovered in the emission spectrum of the Cerenkov oscillators and it was shown that the nature of the radiation was associated with the stimulated scattering of low-frequency radiation by the relativistic electrons. Radiation with a power of 500 MW was obtained in the 8-mm wavelength range using a two-beam Cerenkov oscillator. High-current pulse-periodic nanosecond accelerators with a charging device utilizing a Tesla transformer were used in the experiments. The possibility was demonstrated of generating high-power microwave radiation with a pulse-repetition frequency of up to 100 Hz. An average power of ∼500 W was achieved from the relativistic oscillators. A relativistic backward wave tube with a high-current electron beam was used to make a prototype nanosecond radar device. Some of the results presented were obtained jointly with the Russian Academy of Sciences Institute of Applied Physics. Questions concerning multiwave Cerenkov interaction are not considered in this paper.

Variation of coupling coefficients by sampled gratings in complex coupled distributed-feedback lasers

We present an efficient method for an almost arbitrary variation of the complex coupling coefficient of distributed feedback lasers by using superstructure gratings, As shown by detailed transfer matrix model calculations the coupling coefficient can be reduced locally according to the duty cycle of a sampled grating. The additional supermodes caused by the superstructure can be strongly suppressed by choosing a small superperiod so that the supermodes are shifted apart from the gain curve. This novel approach for the realization of nonuniform coupling coefficients is applied to analyze experimentally the influence of the complex coupling strength on the performance of loss coupled InGaAs-InGaAlAs-InP-DFB lasers. >

The functional significance of the 10-Hz sympathetic rhythm: a hypothesis.

This paper reviews work from our laboratory on the 10-Hz rhythm in sympathetic nerve discharge of the cat and offers a hypothesis on its functional significance. The rhythm is ubiquitous to the discharges of sympathetic nerves with different cardiovascular targets and arises from a system of coupled nonlinear brain stem oscillators, each of which has a selective relationship with a different portion of the spinal sympathetic outflow. The 10-Hz rhythmic discharges of sets of sympathetic nerves are differentially related and the pattern of relationships in one experiment can be the reverse of that in the next. We hypothesize that nonuniform and dynamic coupling of the central circuits controlling different sympathetic nerves is the basis for the formulation of complex cardiovascular response patterns that include differential changes in regional blood flows.

New family of solvable 1D Heisenberg models

Starting from a Calogero--Sutherland model with hyperbolic interaction confined by an external field with Morse potential we construct a Heisenberg spin chain with exchange interaction $\propto 1/\sinh^2 x$ on a lattice given in terms of the zeroes of Laguerre polynomials. Varying the strength of the Morse potential the Haldane--Shastry and harmonic spin chains are reproduced. The spectrum of the models in this class is found to be that of a classical one-dimensional Ising chain with nonuniform nearest neighbour coupling in a nonuniform magnetic field which allows to study the thermodynamics in the limit of infinite chains.

Differential relationships among the 10-Hz rhythmic discharges of sympathetic nerves with different targets

Partial coherence analysis was used to remove the influences of the central circuits controlling a sympathetic nerve (as reflected by its discharges) on the coherence of the 10-Hz discharges of other sympathetic nerves in unanesthetized decerebrate or urethan-anesthetized cats. In many cases, partialization reduced but did not eliminate the sharp peak near 10 Hz in the coherence functions relating the discharges of sympathetic nerve pairs. This observation implies that the central sources of the 10-Hz rhythmic discharges of any nerve are not identical to those responsible for the rhythm recorded from any other nerve. Partial coherence analysis also revealed differential relationships among the 10-Hz rhythmic discharges of sympathetic nerves with different targets. Importantly, the pattern of differential relationships observed in one experiment could be the reverse of that in the next. Although the basis for the differential relationships is not yet clear, nonuniform coupling of multiple brain stem 10-Hz oscillators and/or nonuniform cross talk between spinal circuits controlling different sympathetic nerves may be involved.

Non-uniform resonant coupling effect on 2D electron transport in δ-doped double quantum well structures

Effects of resonant coupling on 2D electron transport, especially the wavefunction-dependent scattering of electrons, have been studied in novel double quantum well (DQW) field effect structures in which a sheet of ionized donors is inserted in one of the wells. When the two relevant wavefunctions arc modulated by a gate voltage V g from localized ones to those extending over both wells, the channel resistance R shows a resonant increase, resulting in a peak-to-valley ratio of 2.5 in R- V g characteristics at 1.5 K. Magnetic-field dependence of the channel resistivity has been investigated to elucidate features of resonant coupling at two different donor concentrations N δ. It has been found that the formation of coupled wavefunctions takes place uniformly only in the sample with low N δ, whereas the resonant coupling in the sample with high N δ (> 10 15 m −2) becomes substantially non-uniform or position dependent in the channel and leads to incomplete delocalization of wavefunctions.

Initiating reentry: the role of nonuniform anisotropy in small circuits.

Until recently only two types of media have been considered to provide the nonuniformities necessary to initiate cardiac reentry: (1) continuous isotropic media with intrinsic repolarization inhomogeneities; and (2) continuous isotropic media free of inhomogeneities in which repolarization nonuniformities are introduced transiently. The purpose of this article is to establish cellular coupling as a basis for arrhythmias by placing a new type of inhomogeneity, nonuniform anisotropy due to sparse side-to-side coupling between cells, in an overall perspective with the other nonuniformities that lead to reentry. Review of experimental and theoretical models of reentry leads to the following picture: with slowed conduction, reentrant circuits diminish in size and the nonuniformities necessary for reentry are provided by nonuniform anisotropy. Repolarization nonuniformities create functionally different pathways for reentrant circuits of relatively large size (> 30-50 mm2). Nonuniform anisotropic cellular coupling, which is associated with underlying microfibrosis, makes it possible for reentry to occur in small areas (< 10-15 mm2). A general property found in nonuniform anisotropic bundles is the presence of functionally different pathways in the absence of intrinsic repolarization inhomogeneities--one of fast longitudinal conduction with a longer refractory period, and another of "very slow" transverse conduction with a shorter refractory period. Since it is not known if nonuniform anisotropy exists in the AV node, the best known structure with small reentrant circuits, we performed microscopic extracellular measurements in the AV node of the rabbit. The transitional zone of the AV node was found to have markedly nonuniform anisotropic conduction properties. The analysis provides the view that functionally different pathways of small reentrant circuits, including those of the AV node, need to be reevaluated in terms of the role of nonuniform anisotropic cellular coupling.

DFB laser with nonuniform coupling coefficient realized by double-layer buried grating

A technique for the realization of a nonuniform grating coupling coefficient kappa , and its implementation in distributed-feedback (DFB) lasers is presented. This technique, which is an extension of the buried-grating concept, offers precise control of the coupling coefficient and enables a kappa -ratio greater than 10. DFB lasers with an asymmetric kappa -variation have been fabricated using this method. A front-to-rear facet output power ratio close to five was obtained, qualitatively in agreement with simulations. >

Characterization of Surface Plasmons on Metal-oxide-semiconductor Structures

Abstract Using the Otto (prism-air gap-sample) configuration p-polarized light of wavelength 632·8 nm has been coupled with greater than 80% efficiency to surface plasmons on the aluminium electrode of silicon-silicon dioxide-aluminium structures. The results show that if the average power per unit area dissipated on the metal film exceeds approximately 1 mW mm−2, then the coupling gap and thus the characteristics of the surface plasmon resonance are noticeably altered. In modelling the optical response of such systems the inclusion of both a non-uniform air coupling gap and a thin cermet layer at the aluminium surface may be necessary.

Classification of Sign Changes in Borehole TEM Decays

Sign changes at a particular receiver location are commonly observed in downhole TEM data as a function of delay time. There are many causes but generally they can be classified as being due to either a change in the sense of coupling between the downhole probe and the analagous secondary field, or a reversal of the analagous secondary field itself. The most common coupling-related reversals are due to analagous current migration in finite plate-like, overburden and host conductors. On the other hand, analagous secondary field reversals are more frequently seen in multiple conductor cases. In particular the impulse response exhibits a sign reversal for the case of a conductor directly under a transmitter placed on an overburden. In this case the step response does not. For the case of the same conductor located at some horizontal distance away from the transmitter, the step response may exhibit a sign reversal while the impulse response may exhibit two. The overall response is further complicated by the fact that the superimposed overburden response changes sign as well. Sign reversals may also arise from non-uniform primary field coupling, current channeling and induced polarization effects and these can add further uncertainty to the interpretation process.

Heteronuclear chemical-shift correlation in the presence of nonuniform one-bond coupling constants

Efficiency of heteronuclear chemical-shift correlation spectroscopy has been improved by adoption of bilinear pulses which selectively flip only distant protons thus eliminating most homonuclear couplings. In the presence of nonuniform one-bond coupling constants fixed delays cannot be adjusted accurately. Study of resulting spurious peaks quantitatively explained their intensities, and it also suggested development of a new pulse sequence for measurements of coupling constants 1 J CH.

Frequency Plateaus in a Chain of Weakly Coupled Oscillators, I.

A chain of $n + 1$ weakly coupled oscillators with a linear gradient in natural frequencies is shown to exhibit “frequency plateaus,” or sequences of oscillators having the same frequency, with a jump in frequency from one plateau to another. We first show that the equations for the coupled oscillators admit an invariant $(n + 1)$-torus on which the equations have a special form, one in which an n-dimensional subsystem is approximately invariant. We then show that when the linear gradient becomes too steep to allow phaselocking, there emerges a large-scale invariant circle in this n-dimensional system which corresponds to the existence of a pair of plateaus, and whose homotopy class within the n-torus corresponds to the position of the frequency jump. Also discussed are the effects of anisotropic and nonuniform coupling.

Coupling Characteristics of Nonradiative Dielectric Waveguides

An analytical method is presented for predicting coupling characteristics of dielectric strips in the nonradiative dielectric waveguide. Starting with an approximate but very accurate expression for the coupling coefficient between parallel dielectric strips, the scattering coefficients for nonuniform coupling structures are derived in simple closed-form expressions by taking the effect of the field deformation at the curved sections into account. The coupling coefficient of the nonradiative dielectric waveguide is found to be so large compared to those of other dielectric waveguides that complete power transfer can be attained with coupled polystyrene bends having a curvature radius as small as 20 mm at 50 GHz. The theory was verified experimentally for various coupling structures. As an application toward millimeter-wave integrated circuits, 0-dB couplers, quadrature hybrid couplers, and an in-phase power divider were constructed based on the present analysis. A comparison of theory and experimental data of these fabricated coupling circuits suggests that the effect of the nondegenerate modes in the straight and curved guides must be included in the analysis to further improve the theory.

Microwave filters with nonperiodic transmission characteristic

AbstractA microwave filter with nonuniform‐coupled transmission lines is suggested. In contrast to known transmission‐line microwave filters the transmission characteristic does not change periodically with frequency. The synthesis procedure shown reduces the design problem to the known synthesis of directional couplers with nonuniform coupling factor. A table of calculated coupling factor values of the microwave filter with an equi‐ripple passband of 0.05 dB up to 1 dB and an equi‐ripple stopband of 60 dB is given. A practical design example shows good agreement with theoretical results.

Second-order effects in acoustic surface-wave filters: design methods

Second-order effects, which may be defined as those effects which prevent acoustic surface-wave filters from behaving as ideal transversal filters, are described systematically. Explicit exact and approximate formulas are given for the effect of acoustic multireflections and external matching circuits on both the main signal response and triple transit echoes. The effect of non-uniform tap coupling across the acoustic beam is described, and weighting methods thatprovide a more uniform coupling are presented. Other secondary effects like electrostatic or quasistatic end effects, beam diffraction, mass loading and bulk wave excitation are briefly described. A method for the correction of positioning errors or delay errors by experimental means is described.

Erratum: Theory of optical-waveguide distributed lasers with nonuniform gain and coupling

Coupled‐mode theory is applied to distributed feedback lasers having step variations in the longitudinal distributions of both the gain and the coupling. Threshold solutions for the required laser gain and the longitudinal mode spectra are presented for the modified geometries. Numerical computations show that these modifications on the uniform structures considered heretofore allow for optimization of the laser characteristics such as lowering the threshold gains and providing controllable higher‐order‐mode rejection, as well as unidirectional operation.

Theory of optical-waveguide distributed lasers with nonuniform gain and coupling

Coupled-mode theory is applied to distributed feedback lasers having step variations in the longitudinal distributions of both the gain and the coupling. Threshold solutions for the required laser gain and the longitudinal mode spectra are presented for the modified geometries. Numerical computations show that these modifications on the uniform structures considered heretofore allow for optimization of the laser characteristics such as lowering the threshold gains and providing controllable higher-order-mode rejection, as well as unidirectional operation.

Effect of nonuniform coupling on junction paraconductivity

The effect of spatial variations of the junction coupling on the excess current-voltage characteristics is studied in the paraconducting regime. A nonlinear increase of the peak voltage V p(ϵ) with ϵ is predicted in agreement with experiments.

Magnetostatic Echoes

Coherent excitation of an ensemble of higher-order magnetostatic modes in a ferromagnet by a sequence of two or three microwave pulses can result in the reradiation of echo pulses1 due to phase coherent states of the many excited modes ocurring at times subsequent to pulse application. The pulse spectrum breadth Δω must be appreciably greater than the frequency breadth of the individual modes. A model for the magnetostatic mode echo can be formulated in a manner similar to that of the spin echo.2 The salient features of the magnetostatic model echo differing from that of the spin echo are a consequence of a restriction to small angles of magnetization opening in the ferromagnetic experiment, nonuniform coupling to the participating magnetostatic modes, and the possibility of instability effects abstracting excited modes from the ensemble contributing to the mode echo. Excitation by two pulses of time separation τ should result in the appearance of a train of echoes spaced at time intervals τ following the second pulse. The decay of the primary two pulse echo as a function of increasing τ yields a relaxation time T2 characterizing an average rate of decay of the transverse energy of the excited mode ensemble. The decay of a three-pulse echo, occurring at time T+τ for excitation pulses applied at times 0, τ, and T will be governed by longitudinal relaxation if T is varied with τ held constant. Magnetostatic mode echo phenomena have been observed in high-purity single-crystal YIG spheres, disks, and rods at microwave X-band frequencies. The samples are contained in low Q cavities or terminated waveguide. Pulse durations of 10−8 sec are used at peak powers ranging from 10−2 to 10 W. Measurements of the two-pulse echo decay constant as a function of temperature over the range 1.6° to 300°K yield results compatible with the temperature dependence of the total linewidth ΔH for this material using the relationship T2=2/γΔH. A magnetization relaxation time at 300°K of T2=0.75 μsec has been observed in a [100] YIG rod of diameter 0.1 in. and length 0.3 in. with Hdc parallel to the rod axis. The existence of magnetostatic mode echoes appears to be experimentally favored by large samples with geometry other than spherical. This is a consequence of a higher density of modes in certain frequency intervals for nonspherical geometries, thus allowing more modes to be accomodated within the pulse spectrum breadth. Furthermore it is easier to excite higher order modes in such samples as evidenced, for example, by the experiments of Dillon.3 The echo appears most strongly with increasing Hdc in the magnetostatic mode region. A decrease in transverse relaxation time is noted as the high-field limit for the appearance of the echo is approached. This latter result appears to be associated with the density of degenerate spin wave states in this region. Multiple two-pulse echoes are easily observed in the YIG rod at 300°K. The amplitude of the secondary two-pulse echoes is found to be a critical function of the excitation pulse level and may exceed that of the primary echo under some conditions of excitation. Measurements of T1 by means of the three-pulse echo technique, yield roughly the relationship T1∼T2/2. The observation is complicated by the existence of a large number of echoes arising from three closely spaced pulses.

Nonuniform Waveguide High-Pass Filters with Extremely Steep Cutoff

The design of a tapered waveguide high-pass filter with very steep cutoff characteristics near the cutoff frequency and very low reflections for frequencies beyond the cutoff is studied on the basis of nonuniform or inhomogeneous transmission line theory. The complex input reflection coefficient due to the presence of a section of nonuniform waveguide is obtained through a new approach by formulating the problem in terms of a pair of coupled differential equations of forward wave and reflected wave with varying propagation constants and nonuniform coupling coefficients. The solution of the reflection coefficient appears in the form of an infinite series of integrals and can be reduced, for the case of very gentle tapering to the simple form of fourier integral previously obtained by others. The general solution thus obtained is valid even if 1) the tapering along the waveguide is not gradual, and 2) the tapered section is terminated in an arbitrary impedance. It is shown that among many illustrated simple trial functions of impedance variation along the taper, the exponential function raised to cosine square yields reflection characteristics with the steepest rise near the cutoff and the lowest reflections for all frequencies beyond the cutoff. The steep rise near cutoff frequency is phenomenal, since, for example, at the nominal cutoff of 55 kMc the reflection reduces to about -50 db within 0.18 kMc, i.e., the transition region from the stop band to pass band at -50 db reflection is only about 0.33 per cent of cutoff. The same design procedure for the high-pass filter can be used for waveguide transitions of extremely wide band and very low reflections.