Leaky Structures Research Articles

Leaky wave antenna with amplitude controlled beam steering based on composite right/left-handed transmission lines

Abstract. An antenna comprising two different composite right/left-handed transmission line structures is proposed which enables easy beam steering at an operation frequency of 10 GHz. The composite right/left-handed transmission lines are based on planar, periodically arranged via free unit cells, implemented in microstrip technology. Both transmission lines exhibit the infinite wavelength phenomenon which occurs at 9.72 GHz and 9.89 GHz, respectively. Thus, operating the different leaky wave structures at 10 GHz, radiation with azimuth angles of ±8° and ±17° can be achieved depending on the selected input port. In order to obtain a tunable main beam direction, the radiation patterns of both structures are superimposed by feeding them simultaneously. The influence of each guiding structure, and hence the direction of the main beam, can be controlled via the feeding amplitude. As a result of this, the beam can be steered between ±17° with a gain of up to 10 dBi. The guiding structures are arranged in parallel with a clearance of a=12.2 mm which is less than half of the wavelength in free space. This allows in a further step the attachment of additional guiding structures in order to increase the tunable angle range or creating an antenna array with a small beamwidth in the elevation plane without the occurrence of grating lobes. An antenna prototype was fabricated and validated by measurements.

Matrix method for analysis of optical waveguides and quantum well structures

The one dimensional wave equation $$ \frac{{d^2 \Psi }} {{dx^2 }} + \Gamma ^2 (x)\Psi (x) = 0 $$ appears in quantum mechanics, optical waveguide theory, plasma physics and many other diverse areas. For example, in waveguide theory $$ \Gamma ^2 (x) = [k_0^2 n^2 (x) - \beta ^2 ] $$ where k 0 is the free space wave number, n(x) is the refractive index distribution and β is the propagation constant. In quantum mechanics $$ \Gamma ^2 (x) = \frac{{2\mu }} {{n^2 }}[E - V(x)] $$ where the symbols have their usual meaning. In this paper we will describe the matrix method for the solution of the wave equation and apply it to obtaining the eigenvalues. The method can also be used for the analysis of leaky structures.

Solving dielectric and plasmonic waveguide dispersion relations on a pocket calculator

We present a robust iterative technique for solving complex transcendental dispersion equations routinely encountered in integrated optics. Our method especially befits the multilayer dielectric and plasmonic waveguides forming the basis structures for a host of contemporary nanophotonic devices. The solution algorithm ports seamlessly from the real to the complex domain--i.e., no extra complexity results when dealing with leaky structures or those with material/metal loss. Unlike several existing numerical approaches, our algorithm exhibits markedly-reduced sensitivity to the initial guess and allows for straightforward implementation on a pocket calculator.

Realization of the compact static Fourier transform spectrometer LLIFTS in glass integrated optics

The realization and the characterization of the leaky loop integrated Fourier transform spectrometer (LLIFTS) is described. The principle of the LLIFTS lies on a two-beam interferometer in planar design using a leaky loop waveguide structure. The interference pattern is measured at the edge of the component. The LLIFTS has been realized using the silver/sodium ion exchange on glass substrate technology, which is low cost and requires only a single lithography step. A mask has been designed considering a numerical model recently developed. Interference patterns have been measured in the wavelength range from 1500 to 1630 nm. Wavelength resolutions of 14 and 11 nm have been measured, respectively, on the 350 and the 500 microm radii leaky loop structures on a compact optical device.

Inertia-induced coherent structures in a time-periodic viscous mixing flow

Inertia-induced changes in transport properties of an incompressible viscous time-periodic flow due to fluid inertia (nonzero Reynolds numbers Re) are studied in terms of the topological properties of volume-preserving maps. In the noninertial Stokes limit (vanishing Re), the flow relates to a so-called one-action map. However, the corresponding invariant surfaces are topologically equivalent to spheres rather than the common case of tori. This has fundamental ramifications for the response to small departures from the noninertial limit and leads to a new type of response scenario: resonance-induced merger of coherent structures. Thus several coexisting families of two-dimensional coherent structures are formed that make up two classes: fully closed structures and leaky structures. Fully closed structures restrict motion as in a one-action map; leaky structures have open boundaries that connect with a locally chaotic region through which exchange of material with other leaky structures occurs. For large departures from the noninertial limit the above structures vanish and the topology becomes determined by isolated periodic points and associated manifolds. This results in unrestricted chaotic motion.

Study of leaky waves: Application to hollow waveguides, to arrow waveguides and to the SiGe LEDs

In this paper a new approach is proposed for the determination of resonance characteristics of leaky guiding structures. We show that in the conventional transfer matrix approach some modifications are needed for the description of leaky structures. Our approach is based on resonance considerations and does not make recourse to complex propagation constants. It is applied to hollow and ARROW waveguides. We have used this approach for the optimization of the vertical LEDs based on SiGe.

Detailed Study of Quasi-bound Modes in Leaky Structures

Abstract The inconsistency of the progressive wave boundary conditions for the quasi-bound modes in open leaky structures is shown and a new boundary value formulation is developed. The results obtained are tested for leaky waveguides with delta function barriers for which a simple analytical characteristic equation is available. The method is applied for the determination of eigenvalues of prism-loaded multilayer waveguides. The influence of the prism coupler on the waveguide modes is investigated in detail for slab and four-layer waveguides. The method is compared with several approximate methods.

Regional congruence and divergence of glucose transporters (GLUT1) and capillaries in rat brains.

The association of glucose transporters (GLUT1) and brain capillaries was tested in different brain structures of rats by a direct comparison of the topologies of capillaries and GLUT1 in identical brain sections. Antibody staining of capillaries (fibronectin) and GLUT1 were made visible by fluorescence microscopy. The results showed differences between brain structures containing a tight and a leaky blood-brain barrier. All capillaries of brain structures with a tight blood-brain barrier showed congruent staining of GLUT1 and capillary morphology. The circumventricular organs that are known to have leaky barrier capillaries were stained by fibronectin antibodies but not by GLUT1 antibodies. Ependymal cells showed moderate staining by GLUT1 antibodies both in areas with tight and leaky barriers. The subcommissural organ appeared to be unique showing neither capillary nor GLUT1 stain. It is concluded that glucose transporters (GLUT1) exist in all brain capillaries of blood-brain barrier structures, whereas they are absent in leaky barrier structures. Moderate amounts of glucose transporter (GLUT1) can also be detected in ependymal cells.

Rigorous spectral analysis of leaky structures: application to the prism coupling problem

This work presents a rigorous spectral analysis of the leakage phenomenon in integrated optical structures. We first show that the “leaky mode” of a dielectric waveguide actually represents a group of radiation modes around a central component satisfying the transverse resonance of the structure. Then we give a rigorous integral formulation for the calculation of the coupled power in structures like the prism coupling structure. The approximations for the case of weak coupling are then analysed to show their limits of validity. With these approximations we get the same formula existing in the literature.

Subsurface Pressures and Leak-off Tests in the Netherlands and their Implications for Prospectivity, Safety, and Well Design

More than 4500 formation pressures and leak-off test (LOT) of Dutch wells have been compiled and validated. When plotted vs. depth, these data exhibit a remarkable separation, the formation pressures being consistently lower than the LOT measurements. The band that separates these two sets of data has been called the [open quotes]lower bound[close quotes] with respect to the LOTs. This lower bound corresponds to the maximum expected formation pressures, which seem to be related to the minimum formation strength of the rock. In the Dutch northern offshore, numerous valid dry structures have formation pressures close to or at the lower bound. At a depth of 2000 m, this corresponds to overpressure of only 75 bars (1100 psi). Where a regional aquifer can be established, the protected trap concept is applicable. These types of traps are located downdip of potentially leaky structures and have a greater chance of having retained their charge, and maximum hydrocarbon column lengths can also be established. Futhermore, overpressures can lead to porosity preservation and increase the volume of gas in place. Prospect should be ranked accordingly. The identification of overpressured zones and the prediction of the most likely pressures to be expected have a significant effectmore » on exploration programs and planning. Adequate rig selection and well design will reduce costs and increase drilling safety.« less

Chapter 29: Tracer uptake by circumventricular organs — a relative measure of blood supply to the brain

The permeability difference between “tight brain regions” and “leaky brain structures” is illustrated by example, in which the tritiated adenosine analog N -ethylcarboxamidoadenosine was injected intracarotidally. The radioactivity was in this case revealed by autoradiography. The tracer amount accumulated in a given experiment by the blood–brain barrier (BBB)-protected, tight brain regions frequently shows a rather pronounced fluctuation from one animal to another. One main reason for this is that the portion of the bolus injected into the common carotid artery that distributes to the brain via the internal carotid artery varies, because of the variable amounts of the bolus diverted largely to the external carotid and pterygopalatine arteries. To be able to disregard interindividual uptake differences in the tight brain regions caused by such a differing tracer supply, it has proved useful to relate the measured accumulation values to the corresponding values obtained for the circumventricular organs. Because of the leakiness of their capillaries, the tracer uptake by these regions indicates very well the actual tracer supply to the brain.

Bent planar waveguides and whispering gallery modes: a new method of analysis

A matrix method for analyzing bent planar optical waveguides is discussed. The method is a modification of an earlier method which yields bend loss directly, inasmuch as a nonuniform refractive index is approximated by a series of linear profiles rather than a series of uniform profiles. The method can be used with absorbing or leaky structures. The effect of whispering gallery modes has also been studied. It appears that a whispering gallery explanation given by H.J. Harris and P.F. Castle (1986) may not be adequate.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Alternative expression of the dispersion equation in multilayered structures

An alternative approach that directly obtains the dispersion equation for each mode of a multilayered integrated optic waveguide is presented. The form of the dispersion equation can be easily numerically implemented; the presence of a single solution greatly simplifies the roots finding. This new approach does not present any limitation even dealing with almost degenerate modes, with leaky or lossy structures.

Analytical investigations of leaky and absorbing planar structures

Analytical expressions for the loss coefficient and the perturbed propagation constant are presented for planar leaky structures which may have media of complex refractive indices. Numerical calculations are reported for the loss variation with parameters such as the buffer refractive index, buffer thickness and superstrate refractive index.

Dynamic aspect of inter-sertoli junctions in monkeys

The extensive tight junctions that link Sertoli cells were studied in testes of monkeys (Cercopithecus aetiops) in conventional thin sections, lathanum-perfused specimens, and free-fracture replicas. The results demonstrated that tight junctions linked Sertoli cells from the base, and in conventional sections they were identifiable as punctate points of occlusions. Colloidal lanthanum penetrated many of these points. Freeze-fracture replicas showed a large number of strands and displayed various degrees of complexity; this variability was associated with the exposed area. Areas with less organized and regularly spaced strands, breakdown of strands, and organizing and disorganizing particles were related to the dynamic aspect of these junctions, which continuously disassembled and reorganized to allow the transfer of germ cells from the basal to the adluminal compartment. This report hypothesizes that the blood—testis barrier may result from the summation of the resistances contributed by a large number of moderately leaky structures arranged in series. The coexistence of septate with tight junctions was never observed, since all strands were of the tight junction type. These were frequently associated with communicating junctions.

Intercellular Junctions of the Secretory Ameloblasts

In A study of the ultrastruncture of the secretory ameloblasts, fifty neonatal rats were perfused with Karnovsky's fixative or fixative containing tannic acid and glutaraldehyde. The distribution and structural features of cell junctions in the ameloblasts were studied by thin sectioning and freeze-fracture technique.The longitudinal sections of fractured ameloblasts were observed as long, narrow cells, which were divided by single or double ridges into 2-3 faces on the lateral surface. In thin sections and freeze-fracture replicas, gap junctions and desmosomes were found on the basal surfaces, and gap junctions of various sizes and outlines were found on the lateral surfaces of the ameloblasts. In replica images, the focal linear ridges of macular tight junctions were seen on the lateral surfaces, and closely associated with small clusters of the aggregated particles of gap junctions.In thin sections and replicas, the junctional complexes which consist of tight and gap junctions and desmosomes were observed at both proximal and distal ends of the secretory ameloblasts. The proximal junctional complex consisted of 1-4 tight junctional strands, and in completely sealed the paracellular spaces of the ameloblasts. In contrast, the well-developed meshwork of tight junctions occupied the distal end of the cell body. This distal junctional complex was more extensive than that of proximal, and completely occluded the paracellular spaces of the ameloblasts.However the discontinuous and free-ending strands of tight junctions were commonly observed in the junctional complex. Discontinuous strands are thought to indicate the incomplete fusion of particles in tight junctional strands. Leaky structures of these junctional complexes must he concerned with the penetration of calcium and the other molecules throughout the paracellular spaces in the ameloblast layer into the enamel.

Leaky-wave antennas I: Rectangular waveguides

A microwave network approach is employed for the description and analysis of leaky-wave antennas. This approach is based on a transverse resonance procedure which yields the complex propagation constants for the leaky waves. A perturbation technique is then applied to the resonance equation to obtain results in simple and practical form. These procedures are illustrated by application to a number of practical leaky rectangular waveguide structures. Very good agreement is obtained between the theoretical results and the measured values.