Waves In Opposite Directions Research Articles

A Bifilar Helices-Based Bidirectional Same-Sense Circularly Polarized Antenna With Wide Bandwidth and High Gain

This communication presents a novel bifilar helices based bidirectional same-sense circularly polarized (CP) antenna. It consists of two conical bifilar helices with a back-to-back configuration and a wide band feeding network. Since the helices are identical and each of them radiates unidirectional CP waves, the proposed antenna is able to produce same-sense CP waves in opposite directions. For each helix, a pair of conformal fan-shaped structures are designed for impedance matching, while, they act as reflecting surfaces for generating endfire radiation. A parallel strip line is adopted to connect the two helices and provide differential feeds for them. Thanks to the wide band feature of both the bifilar helices and the feeding network, the proposed antenna exhibits superior bandwidth performance over the existing studies. In addition, due to the full metal structure, dielectric loss can be avoided and thus the proposed antenna has the advantage of high gain. To validate the design idea, a prototype is fabricated by using metal 3-D printing technology. The experimental results show that the 3-dB axial ratio bandwidth (ARBW) is 40% from 1.9 to 2.86 GHz and the peak gain is 8 dBic.

Additively Manufactured Dual Circularly Polarized Antennas With Bidirectional Same-Sense Radiation and Wide Bandwidth Characteristics

This communication presents two additively manufactured bidirectional circularly polarized (CP) antennas. Each antenna radiates the same-sense CP waves in opposite directions and has the capability of altering the CP state to either right-hand CP (RHCP) or left-hand CP (LHCP) by switching the feeding ports. They adopt the same scheme of combining a linearly polarized (LP) bidirectional source and two polarizers of the same type. Different polarizers, a single-slab dielectric polarizer (Ant 1) and a linearly tapered slot polarizer (Ant 2), are proposed and applied for them, respectively. The LP source adopts a circular waveguide-based structure. By altering the orthogonal degenerate modes in the waveguide, the CP states can be switched correspondingly. To validate the design ideas, prototypes of the two antennas are fabricated and tested. Experimental results show that the axial ratio bandwidth (ARBW) of Ant 1 and Ant 2 is 30.3% (4.9–6.66 GHz) and 17.9% (5.26–6.3 GHz), respectively. Within the ARBW, their gain is 8.4± 0.8 and 7.2± 1 dBic, respectively. These results demonstrate that both antennas exhibit the advantages of wide band, high gain, and ease of fabrication over the conventional designs.

Алгоритмический метод расчета отражения и прохождения волны через многослойную структуру. Часть 1. Матричный алгоритм

The matrix algorithm for calculation of single-dimension wave through multi-layer structure is proposed. The case of simultaneous incidence on structure two waves in opposite directions is investigated. The structure is presented as successive uniform layers having different parameters. In the basis of method is established the analogy between propagation of wave in multi-layer wave-conducting structure and chain which consist of successive connected four-pole blocks of propagation and connection. The results received on the model of four-pole blocks are generalized to the case of propagation of one-dimension waves in multi-layer structure. It is found the resulting matrix which is formatted by successive product of propagation and connection matrix for structure in a whole. On the basis of calculation scheme receiving elements of last matrix from the elements of previous matrix is proposed the algorithm which allows by recurrent way to find the resulting matrix for arbitrary number of medium. On the basis of proposed algorithm, the amplitudes of leaving out structure waves through the amplitude of coming waves may be calculated. Also, the coefficients of reflection and passing by energy for the structure as a whole may be calculated. For the convenience of machine programming, it is proposed the block-scheme of program which realizes the algorithm. This program includes the under-program which calculates the elements of last matrix from the elements of previous matrix. The proposed the recurrent formulas which allows to generalize proposed algorithm to the propagation of electromagnetic waves. It is proposed the recommendations which allow to generalize proposed algorithm to the waves which do not have the harmonic character.

Total Surface Current Vector and Shear From a Sequence of Satellite Images: Effect of Waves in Opposite Directions

AbstractThe total surface current velocity (TSCV)—the horizontal vector quantity that advects seawater—is an essential climate variable, with few observations available today. The TSCV can be derived from the phase speed of surface gravity waves, and the estimates of the phase speeds of different wavelengths could give a measure of the vertical shear. Here, we combine 10‐m resolution Level‐1C of the Sentinel‐2 Multispectral Instrument, acquired with time lags up to 1 s, and numerical simulation of these images. Retrieving the near‐surface shear requires a specific attention to waves in opposing directions when estimating a single‐phase speed from the phase difference in an image pair. Opposing waves lead to errors in phase speeds that are most frequent for shorter wavelengths. We propose an alternative method using a least squares fit of the current speed and amplitudes of waves in opposing directions to the observed complex amplitudes of a sequence of three images. When applied to Sentinel‐2, this method generally provides more noisy estimate of the current. A byproduct of this analysis is the “opposition spectrum” which is a key quantity in the sources of microseisms and microbaroms. For future possible sensors, the retrieval of TSCV and shear can benefit from increased time lags, resolution, and exposure time of acquisition. These findings should allow new investigations of near‐surface ocean processes including regions of freshwater influence or internal waves, using existing satellite missions such as Sentinel‐2, and provide a basis for the design of future optical instruments.

Programming nonreciprocity and reversibility in multistable mechanical metamaterials

Nonreciprocity can be passively achieved by harnessing material nonlinearities. In particular, networks of nonlinear bistable elements with asymmetric energy landscapes have recently been shown to support unidirectional transition waves. However, in these systems energy can be transferred only when the elements switch from the higher to the lower energy well, allowing for a one-time signal transmission. Here, we show that in a mechanical metamaterial comprising a 1D array of bistable arches nonreciprocity and reversibility can be independently programmed and are not mutually exclusive. By connecting shallow arches with symmetric energy wells and decreasing energy barriers, we design a reversible mechanical diode that can sustain multiple signal transmissions. Further, by alternating arches with symmetric and asymmetric energy landscapes we realize a nonreciprocal chain that enables propagation of different transition waves in opposite directions.

Plane waves reversibility

This work simulates reversibility of plane waves in different ways. We start making theoretical classical reversing of a plane wave in two different ways exchanging t by –t as first step. In one case, we additionally flip the temporal orientation of the magnetic field. In the other case, we flip the electric field. Therefore, we can compare two classical approaches to time reversed electromagnetism on plane waves. On the other hand, we obtain two different mechanically reversed plane electromagnetic waves out of the frame of the electromagnetics reversibility theory. A theoretical experiment makes these effects, where, an infinite plane current generates two plane waves in opposite directions. After this, the waves are made to return by two different ways: (1) by retro reflecting and (2) by moving back the wave. Finally, the returning plane waves insides over a conductor plane in order to induce plane currents in the conductor. The goal is to complete reversibility cycles including the charges movement. The returning waves and the induced currents will be compared themselves in all the cases. Charges movements are also included in the discussion in order to have an additional felling of the waves reversibility and physical insight of time-reversed waves. It is used a plane waves theoretical experiment created by Feynman as a starting point [1]

Measurement of the Acoustic Non-Linearity Parameter of Materials by Exciting Reversed-Phase Rayleigh Waves in Opposite Directions.

The acoustic non-linearity parameter of Rayleigh waves can be used to detect various defects (such as dislocation and micro-cracks) on material surfaces of thick-plate structures; however, it is generally low and likely to be masked by noise. Moreover, conventional methods used with non-linear Rayleigh waves exhibit a low detection efficiency. To tackle these problems, a method of exciting reversed-phase Rayleigh waves in opposite directions is proposed to measure the acoustic non-linearity parameter of materials. For that, two angle beam wedge transducers were placed at the two ends of the upper surface of a specimen to excite two Rayleigh waves of opposite phases, while a normal transducer was installed in the middle of the upper surface to receive them. By taking specimens of 0Cr17Ni4Cu4Nb martensitic stainless steel subjected to fatigue damage as an example, a finite element simulation model was established to test the proposed method of measuring the acoustic non-linearity parameter. The simulation results show that the amplitude of fundamentals is significantly reduced due to offset, while that of second harmonics greatly increases due to superposition because of the opposite phases of the excited signals, and the acoustic non-linearity parameter thus increases. The experimental research on fatigue damage specimens was carried out using this method. The test result was consistent with the simulation result. Thus, the method of exciting reversed-phase Rayleigh waves in opposite directions can remarkably increase the acoustic non-linearity parameter. Additionally, synchronous excitation with double-angle beam wedge transducers can double the detection efficiency.

Numerical Study of the Hydrodynamics of Waves and Currents and Their Effects in Pier Scouring

The scour around cylindrical piles due to codirectional and opposite waves and currents is studied with Reynolds Averaged Navier–Stokes (RANS) equations via REEF3D numeric modeling. First, a calibration process was made through a comparison with the experimental data available in the literature. Subsequently, not only the hydrodynamics, but also the expected scour for a set of scenarios, which were defined by the relative velocity of the current ( U C W ), were studied numerically. The results obtained show that the hydrodynamics around the pile for codirectional or opposite waves and currents not have significant differences when analyzed in terms of their velocities, vorticities and mean shear stresses, since the currents proved to be more relevant compared to the net flow. The equilibrium scour, estimated by the extrapolation of the numerical data with the equation by Sheppard, enabled us to estimate values close to those described in the literature. From this extrapolation, it was verified that the dimensionless scour would be less when the waves and currents are from opposite directions. The U C W parameter is an indicator used to adequately measure the interactions between the currents and waves under conditions of codirectional flow. Nevertheless, it is recommended to modify this parameter for currents and waves in opposite directions, and an equation is proposed for this case.

High‐Efficiency and Tunable Circular‐Polarization Beam Splitting with a Liquid‐Filled All‐Metallic Catenary Meta‐Mirror

AbstractCircular‐polarization beam splitting is one functionality of interest in polarization control. However, current splitters suffer from limitations in either efficiency or tunability. It is theoretically and experimentally demonstrated that a meta‐mirror composed of catenary‐shaped metallic gratings can efficiently deflect left‐handed and right‐handed circularly polarized waves in opposite directions with power efficiency of over 90%. This ultrahigh efficiency is attributed to the nearly perfect polarization conversion and continuous phase profile of the metallic catenary structure. Furthermore, the meta‐mirror demonstrates high beam‐splitting tunability with ethanol injected into the catenary groove. By controlling ethanol depth, the working frequency for peak deflection can be continuously tuned from 10.5 to 8.5 GHz. It is believed that this approach can provide valuable guidance for efficient and dynamic control of electromagnetic waves for various applications.

Dispersion and non-reciprocal elastic wave propagation in a membrane coupled with a uniform flow

In this paper, we carry out an analytical study to investigate the dispersive and non-reciprocal properties of harmonic elastic wave propagation in a membrane on an elastic foundation. One side of the membrane is in contact with a uniform inviscid and incompressible flow. The analysis shows that the frequency spectrum and the dispersion curve are not symmetric, therefore breaking the principle of reciprocity. We show that the dynamics of the wave propagation of the system depends on the dimensionless phase velocity of the membrane and the dimensionless stiffness of the elastic foundation. The system possesses one region where the phase velocity of the propagating waves in opposite directions is different, and another where the waves travel only in one direction (directional band gap). There also exist regions in which only evanescent and spatially growing waves are excited.

Mirrorless optical parametric oscillators with stitching faults: backward downconversion efficiency and coherence gain versus stochastic pump bandwidth

Parametric downconversion in submicronic periodically poled (PP) quadratic materials allows for the generation of signal and idler waves in opposite directions. The distributed feedback mechanism enables a backward mirrorless optical parametric oscillator (BMOPO), which has been experimentally realized using a periodically poled KTiOPO (PPKTP) crystal with 800 nm periodicity. A remarkable spectral property of the BMOPO is that the bandwidth of the forward co-propagating wave is comparable to the pump bandwidth, whereas that of the backward wave is typically several orders of magnitude narrower. In the backward idler configuration realized in PPKTP the backward idler exhibits a coherence gain of two orders of magnitude. In this paper we will consider the backward signal configuration, where the effect of coherence enhancement of the backward signal wave is especially pronounced for exact group velocity matching of the co-propagating pump and idler waves. This quasi-phase-matched (QPM) scheme requires a 335 nm scale poling periodicity, which may be achieved in GaN waveguides, combining e-beam lithography and epitaxy. However, during the lithography step necessary to obtain the PP material, stitching faults may occur. The BMOPO device is then formed by a sequence of 200 micron long PP elements stuck together. We assume that the stitching faults do not exceed half of the poling period in order to prevent the parametric reverse process. The original three-wave QPM model with the explicit periodicity of the nonlinear parametric coupling is necessary for taking into account the periodicity faults in the junctions. Thanks to the high performance computing facility available at Nice University, we analyze the parametric downconversion efficiency and the coherence gain of the backward signal versus the stochastic pump bandwidth in a large pump-bandwidth range. We show that if the stitching faults do not exceed a quarter of the poling period, i.e., less than 80 nm, which is technically feasible, the efficiency is comparable to the BMOPO in the perfect PP material. In any case, for an incoherent pump of broad spectral bandwidth we obtain a coherence gain of almost three orders of magnitude.

Temperature Changes Induced by the Portevin-Le Châtelier (PLC) Effect during Tensile Test Based on the Example of CuZn37 Brass

This paper presents results of research on PLC effect in CuZn37 brass. A thermographic camera was applied in the tests for determining surface temperature distributions of the tested specimens during tension. Spatial-temporal diagrams were prepared on the basis of those distributions. The tests were performed at two strain rates. No significant difference in the tension curve course depending on the above rate was found. Significant difference was observedas regards the course of specimen temperature changes. Increased frequency of the permanent strain wave propagating along the specimen centerline was observed together with the increase of the wave propagation speed for the higher strain rate. Simultaneous initiation of two propagating waves in opposite directions was observed in case of the higher strain rate.

Directional Amplification with a Josephson Circuit

Non-reciprocal devices, which have different transmission coefficients for propagating waves in opposite directions, are crucial components in many low noise quantum measurements. In most schemes, magneto-optical effects provide the necessary non-reciprocity. In contrast, the proof-of-principle device presented here, consists of two on-chip coupled Josephson parametric converters (JPCs), which achieves directionality by exploiting the non-reciprocal phase response of the JPC in the trans-gain mode. The non-reciprocity of the device is controlled in-situ by varying the amplitude and phase difference of two independent microwave pump tones feeding the system. At the desired working point and for a signal frequency of 8.453 GHz, the device achieves a forward power gain of 15 dB within a dynamical bandwidth of 9 MHz, a reverse gain of -6 dB and suppression of the reflected signal by 8 dB. We also find that the amplifier adds a noise equivalent to less than one and a half photons at the signal frequency (referred to the input). It can process up to 3 photons at the signal frequency per inverse dynamical bandwidth. With a directional amplifier operating along the principles of this device, qubit and readout preamplifier could be integrated on the same chip.

Splitting the surface wave in metal/dielectric nanostructures

We investigate a modified surface wave splitter with a double-layer structure, which consists of symmetrical metallic grating and an asymmetrical dielectric, using the finite-difference time-domain (FDTD) simulation method. The metal/dielectric interface structure at this two-side aperture can support bound waves of different wavelengths, thus guiding waves in opposite directions. The covered dielectric films play an important role in the enhancement and confinement of the diffraction wave by the waveguide modes. The simulation result shows that the optical intensities of the guided surface wave at wavelengths of 760-nm and 1000-nm are about 100 times and 4∼5 times those of the weaker side, respectively, which means that the surface wave is split by the proposed device.

Surface-emitted terahertz-wave generation by ridged periodically poled lithium niobate and enhancement by mixing of two terahertz waves

Surface-emitted terahertz- (THz-) wave generation by difference-frequency mixing with ridge-shaped periodically poled lithium niobate (PPLN) was demonstrated. The PPLN had a ridge height of 300 microm, a thickness of 20 microm, and an interaction length of 35 mm. The ridge behaves as a slab waveguide for optical pump beams. The PPLN gives rise to THz waves in opposite directions, perpendicular to the pump-beam direction. Reflecting the THz wave on one side and overlapping it with the THz wave on the other side increased the total THz-wave intensity approximately 2.7 times compared with that without reflection and mixing.

Application of the finite difference time domain algorithm to quasi‐static field analysis

In this paper, the use of the standard finite difference time domain (FDTD) algorithm is extended to quasi‐static electromagnetic field problems. While straightforward application of the standard FDTD algorithm at very low frequencies leads to excessively long simulation times, we show that for linear structures this problem can be circumvented by using a ramp excitation function. The use of appropriate absorbing boundary conditions such as Berenger's perfectly matched layer is also shown to be necessary. By combining two plane waves in opposite directions, a uniform electric or magnetic field can be created so that the electric and magnetic field solutions are decoupled, as required in quasi‐static analysis. Calculations of the induced fields and currents in a human model exposed to power line frequency fields provide a realistic example of an application of this novel FDTD technique.

Confined emission by high-frequency capacitative discharges

The application of electrical fields of radio frequency by external electrodes to cells that contain mixtures of caesium with argon, krypton or xenon at low temperatures (<340 K) produces capacitive discharges in columns, that show discontinuities where the discharge is confined. In this work an explanation is given based on the propagation of two electrical waves in opposite directions that produce a quasi-potential.

3.2 - Acoustic wave amplitude modulation of a multimode ring laser

Synchronous intercavity amplitude modulation of a ring laser operating well above threshold causes the numerous axial modes in each oppositely directed (OD) wave to lock into fixed amplitudes and phases. The light in the cavity is composed of two oppositely directed pulses of roughly 30-cm spatial extent. Saturation of the active medium dictates the location of the modulator for symmetric treatment of the OD pulses. If the modulator is located symmetrically (natural crossing), the pulses cross at the modulator and have identical amplitudes. If the modulator is located asymmetrically, the OD pulses generally have different amplitudes, experience different net population inversions in the active medium, and do not cross at the modulator. A frequency offset between OD pulses also appears when the modulator is located asymmetrically. Other phase-locking phenomena including extinction of one of the OD waves are observed. When the modulator is located symmetrically, the rotation sensing is improved with multimode phase-locked operation over that of the same ring laser operating near threshold. Frequency locking still occurs, however, at about three times the earth rate, or approximately 200 Hz. The locking frequency is found to depend upon modulator depth and frequency. The frequency locking may result from internally reflected pulses which are necessarily coincident with the proper pulses when the modulator is located symmetrically, and a modulation scheme has been devised to reduce such effects.

SYMPTOMS ASSOCIATED WITH DUODENAL RETENTION AND REVERSE MOTILITY

In a previous communication, 1 it was pointed out that a mass of barium injected into the duodenum was partially divided and passed in opposite directions along that viscus. Similar movements were shown to occur in conditions that permitted the accumulation of barium at any point along the course of the duodenum following the taking of a barium meal. From these observations it was concluded that distention of the duodenum was sufficient stimulus to excite contractions of the musculature about the mass and the subsequent passage of contraction waves in opposite directions along the longitudinal axis of the duodenum. Roentgenographic examinations of the gastro-intestinal tract on a series of 398 consecutive patients over a period of three years corroborate the findings and conclusions previously derived. Early in the study of duodenal motility, it became apparent that definite symptoms were either directly associated with or immediately followed reverse movements of barium.