Cutoff Transition Research Articles

Sparse Antenna Array With Flat-Top and Sharp Cutoff Radiation Patterns

A sparse antenna array with flat-top and sharp cut-off radiation patterns is presented in this work. The array consists of sixteen antenna elements, which is realized by applying a symmetrical sparse configuration with uniform excitation phase. This configuration simplifies the excitations of the array while maintaining excellent radiation performance. The dual-band dual-polarized base station antenna based on the proposed sparse antenna arrays in the bands of 1.71 GHz ~ 2.17 GHz and 2.30 GHz ~ 2.69 GHz is fabricated and measured. The measured results are in very good agreement with the predicted ones, showing that the port reflection coefficient is less than -15.0 dB, and the isolation is greater than 28.5 dB. In addition, the array has a gain of 10.1 to 12.3 dB, a half-power beamwidth (HPBW) of 45° ± 5° in both the horizontal and vertical planes, a cut-off transition of less than 17.4° from -3 dB to -20 dB between the main beam and the sidelobe area, and a sidelobe level suppressed to below -21.1 dB. Apart from the good radiation performance, the proposed sparse antenna array has a compact size with very few antenna elements, which is very suitable for communication in a crowded scene.

A Novel Adaptive Wavelet Thresholding with Identical Correlation Shrinkage Function for ECG Noise Removal

On the basis of wavelet theory, a novel Adaptive wavelet thresholding method (AWT) is proposed for the ECG signal enhancement. The best base wavelet for ECG signal filtering can be automatically obtained through the cross correlation coefficient and the energy to entropy ratio. The variable universal threshold (VarUniversal) is applied to different decomposition level so as to suppress diverse noise. To achieve a smooth cut-off transition, an identical correlation shrinkage function (IcoShrinkage) is also adopted in the AWT according to its correlation coefficients with the hard thresholding and the soft thresholding. The performance of AWT is compared with four threshold approaches and six shrinkage functions, respectively, on the basis of 150 practical ECG signals of 30 subjects. The filtering results reveal that the AWT can adaptively choose an optimal base wavelet for a specific ECG signal. With the VarUniversal threshold and IcoShrinkage, the AWT obtains the better filtering results than the other compared methods.

Coating design of a broad band visible transmitting filter with sharp ultraviolet and infrared cut-on and cut-off transitions

Guided optimization of a good starting design can lead to a final design that can be reverse engineered. We apply this to a wide bandpass filter and indicate its deposition with optical monitoring.

Flow and geometry induced scattering of high frequency acoustic duct modes

Cut-on cut-off transition of acoustic modes in hard-walled ducts with irrotational mean flow is well understood for Helmholtz numbers of order unity. Previous finite-element simulations of this phenomenon, however, appear to indicate the possibility of energy scattering into neighbouring modes at moderately large Helmholtz numbers. In this paper, such scattering phenomena are explained and predicted in slowly varying aeroengine ducts using a multiple-scales approach. It is found that, for sufficiently high frequencies, two mechanisms exist whereby energy can be scattered into neighbouring modes by an incident propagating mode. One mechanism occurs only when there is a mean flow inside the duct and induces scattering at significantly lower frequencies than the other mechanism which remains present without mean flow. A coupled system of ordinary differential equations is derived and then solved numerically for a number of example cases to obtain the corresponding transmitted and reflected amplitudes of the scattered modes as well as the overall acoustic pressure field. The theory appears to demonstrate that some exchange of energy between the acoustic and mean flow fields occurs during scattering.

Enhanced optical transmission at the cutoff transition

The phenomenon of extraordinary transmission in the optical regime for circular hole arrays in optically thick metal films is studied as a function of hole size and depth. In the limit of small holes compared to the depth, the transmission properties follow a waveguide type behavior. By describing the transmission process as resulting from the interference between a resonant and a non-resonant contribution, a transition is clearly revealed through the specific spectral variations of the resonance at a given hole depth. This transition is associated to a change in the attenuation through the hole as its size increases, and corresponds to the optimal condition for surface plasmon excitation.

Theoretical investigation of factors controlling the operating characteristics of quantum dot lasers: a review

Carrier-density-dependent internal optical loss sets an upper limit for operating temperatures and considerably reduces the characteristic temperature of a quantum dot (QD) laser. Such a loss also constrains the shallowest potential well depth and the smallest tolerable size of a QD at which the lasing can be attained. In a laser with a single layer of QDs, this loss can strongly limit the output power and cause a rollover of the light-current curve; it also imposes limitations on the conventional method of determining the internal quantum efficiency. Excited-state-mediated capture of carriers from the waveguide into the QD ground-state places a fundamental limitation on ground-state lasing - the output power saturates at high injection currents. The saturation power is controlled by the transition time between the excited- and ground-state in a QD. The longest, cut-off transition time exists, beyond which no ground-state lasing is possible. The maximum number of longitudinal modes that can oscillate in a QD laser increases with increasing surface density of QDs and remains limited (first increases and then decreases) with increasing scatter in the QD-size. In addition to the maximum tolerable scatter, there also exists the minimum scatter in the QD-size for each higher-order mode to start lasing.

Excited-State-Mediated Capture of Carriers Into the Ground State and the Saturation of Optical Power in Quantum-Dot Lasers

Excited-state-mediated capture of carriers from the waveguide into the lasing ground-state in quantum dots (QDs) is studied. Such a two-step capture places a fundamental limitation on ground-state lasing-the output power saturates at high injection currents. The saturation power is controlled by the transition time between the excited- and ground-state in a QD. The longest, cut-off transition time exists, beyond which no ground-state lasing is possible

A new large N phase transition in YM 2

Inspired by the interpretation of two-dimensional Yang–Mills theory on a cylinder as a random walk on the gauge group, we point out the existence of a large N transition which is the gauge theory analogue of the cutoff transition in random walks. The transition occurs in the strong coupling region, with the 't Hooft coupling scaling as α log N , at a critical value of α ( α = 4 on the sphere). The two phases below and above the transition are studied in detail. The effective number of degrees of freedom and the free energy are found to be proportional to N 2 − α 2 below the transition and to vanish altogether above it. The expectation value of a Wilson loop is calculated to the leading order and found to coincide in both phases with the strong coupling value.

Large- N transitions for generalized Yang–Mills theories in [formula omitted] dimensions

We describe the entire phase structure of a large number of colour generalized Yang–Mills theories in 1 + 1 dimensions. This is illustrated by the explicit computation for a quartic plus quadratic model. We show that the Douglas–Kazakov and cut-off transitions are naturally present for generalized Yang–Mills theories separating the phase space into three regions: a dilute one a strongly interacting one and a degenerate one. Each region is separated into sub-phases. For the first two regions the transitions between sub-phases are described by the Jurkiewicz–Zalewski analysis. The cut-off transition and degenerated phase arise only for a finite number of colours. We present second-order phase transitions between sub-phases of the degenerate phase.

High-frequency Green's function for a semi-infinite array of electric dipoles on an infinite grounded stratified dielectric slab: part III $phase-matched wave interactions and numerical results

The particular effects of phase matching in interactions between different wave species and interaction mechanisms pertaining to the asymptotic Green's function for a semi-infinite phased array of parallel dipoles on an infinite grounded stratified dielectric slab are addressed. The two previous parts have lead to a grouping of certain asymptotic terms, which provides physically appealing interpretations of interacting wave processes that involve slab-modulated propagating (radiating) Floquet waves (FW), slab-guided leaky waves (LW), evanescent (nonradiating) FWs and surface waves (SW), together with their associated conical edge-coupled wave constituents. Special attention is now given to the role, in these wave interaction functions,of the difference between the propagating wave numbers (i.e., the degree of phase matching) that characterizes any pair of propagating FWs and LWs, and evanescent FWs and SWs. Cutoff transitions of FWs from the evanescent to the propagating regime are described also in terms of a second wave interaction function, structured similarly to the first. In connection with applications, the interaction between SWs and evanescent FWs, as well as FW transitions from evanescent to propagating, are discussed from the perspective of scan blindness in actual arrays. Various numerical studies highlight the role of phase matching in different problem scenarios.

A uniformly valid multiple scales solution for cut-on cut-off transition of sound in flow ducts

Starting from a multiple-scales approach to model sound transmission through a slowly varying duct of arbitrary cross section, an explicit analytical solution is derived for a mode undergoing cut-on cut-off transition. The solution is a composite one, removing the singularity that appears in the WKB approximation by encompassing the inner boundary-layer solution with the behaviour far upstream and downstream. The solution should not only prove to be a useful benchmark for computational aeroacoustic models, but also enable a designer to continue to use multiple-scales theory to examine sound transmission even when cut-on cut-off transitional modes are present.

Azimuthal Sound Mode Propagation in Axisymmetric Flow Ducts

Assuming an axisymmetric mean flow and acoustic boundary conditions, a set of three-dimensional axisymmetric equations suitable for each azimuthal sound mode of fluctuation is derived based on the three-dimensional linearized Euler equations. The independent two-dimensional model for each azimuthal sound mode is deduced by a Fourier series decomposition. A computational aeroacoustics approach is then applied to solve each two-dimensional equation system. The dispersion-relation-preserving finite difference scheme is implemented for spatial discretization, whereas the 2N storage low-dissipation and low-dispersion Runge-Kutta scheme is applied for time integration. Appropriate boundary conditions are prescribed at the various boundary regions. The numerical procedure is first validated by cases of a straight circular pipe and an annular duct subjected to a subsonic uniform mean flow, the numerical results of which show very good agreement with the analytical solutions. A further numerical example is presented for an axisymmetric inlet duct with an aeroenginelike geometry including an internal spinner. The aeroacoustic computation is based on an inviscid irrotational mean flow calculated by a second-order computational fluid dynamics solver. The acoustic solutions agree well with the existing finite element and multiple-scales solutions in the literature. Finally, the cuton, cutoff transition phenomena are investigated based on the proposed numerical approach. The transition behavior is found to be highly dependent on the mean flowfield in addition to the known geometric effect. These results demonstrate the feasibility of the proposed model and solution procedure.

Two-Dimensional Gauge Theories of the Symmetric Group S n in the Large-n Limit

We study the two-dimensional gauge theory of the symmetric group S n describing the statistics of branched n-coverings of Riemann surfaces. We consider the theory defined on the disc and on the sphere in the large-n limit. A non trivial phase structure emerges, with various phases corresponding to different connectivity properties of the covering surface. We show that any gauge theory on a two-dimensional surface of genus zero is equivalent to a random walk on the gauge group manifold: in the case of S n , one of the phase transitions we find can be interpreted as a cutoff phenomenon in the corresponding random walk. A connection with the theory of phase transitions in random graphs is also pointed out. Finally we discuss how our results may be related to the known phase transitions in Yang-Mills theory. We discover that a cutoff transition occurs also in two dimensional Yang-Mills theory on a sphere, in a large N limit where the coupling constant is scaled with N with an extra log N compared to the standard ‘t Hooft scaling.

Sound propagation in slowly varying lined flow ducts of arbitrary cross-section

Sound transmission through ducts of constant cross-section with a uniform inviscid mean flow and a constant acoustic lining (impedance wall) is classically described by a modal expansion, where the modes are eigenfunctions of the corresponding Laplace eigenvalue problem along a duct cross-section. A natural extension for ducts with cross-section and wall impedance that are varying slowly (compared to a typical acoustic wavelength and a typical duct radius) in the axial direction is a multiple-scales solution. This has been done for the simpler problem of circular ducts with homentropic irrotational flow. In the present paper, this solution is generalized to the problem of ducts of arbitrary cross-section. It is shown that the multiple-scales problem allows an exact solution, given the cross-sectional Laplace eigensolutions. The formulation includes both hollow and annular geometries. In addition, the turning point analysis is given for a single hard-wall cut-on, cut-off transition. This appears to yield the same reflection and transmission coefficients as in the circular duct problem.

Frequency-domain Green's function for a planar periodic semi-infinite phased array. II. Diffracted wave phenomenology

For pt.I see ibid., vol.48, no.1, p.67-74, 2000. This second part of a two-paper sequence deals with the physical interpretation of the rigorously derived high-frequency asymptotic wave-field solution in part I, pertaining to a semi-infinite phased array of parallel dipole radiators. The asymptotic solution contains two parts that represent contributions due to truncated Floquet waves (FWs) and to the corresponding edge diffractions, respectively. The phenomenology of the FW-excited diffracted fields is discussed in detail. All possible combinations of propagating (radiating) and evanescent (nonradiating) FW and diffracted contributions are considered. The format is a generalization of the conventional geometrical theory of diffraction (GTD) for smooth truncated aperture distributions to the truncated periodicity-induced FW distributions with their corresponding FW-modulated edge diffractions. Ray paths for propagating diffracted waves are defined according to a generalized Fermat principle, which is also valid by analytic continuation for evanescent diffracted ray fields. The mechanism of uniform compensation for the FW-field discontinuities (across their truncation shadow boundaries) by the diffracted waves is explored for propagating ad evanescent FWs, including the cutoff transition from the propagating to the evanescent regime for both the FW and diffracted constituents. Illustrative examples demonstrate: (1) the accuracy and efficiency of the high-frequency algorithm under conditions that involve the various wave processes outlined above and (2) the cogent interpretation of the results in terms of the uniform FW-modulated GTD.

Spatiotemporal Shaping of Terahertz Pulses

We report temporal shaping of few-cycle terahertz pulses, using a slit in a conductive screen as a high-pass filter. The filter’s cutoff frequency was tuned by changing the width of the slit; the slope of the cutoff transition was altered by changing the thickness of the screen. We measured the transmission function of the filters, using large-aperture photoconducting antennas to create and detect the incident and transmitted electric field. Our experimental results were in excellent agreement with the performed finite-difference time-domain simulations of the propagation of the pulse through the slit. When the screen thickness was greater than the slit width, the filter was well modeled by a short, planar waveguide. Using a simple transfer function, we accurately describe the sharp cutoff and dispersion of such a filter. © 1997 Optical Society of America Recently terahertz (THz) pulses have proved useful for a wide variety of applications, including far-infrared 1 / time-domain spectroscopy, 2 study and control of Rydberg atoms, 3 T-ray imaging of optically opaque materials, 4 and impulse-ranging studies. 5 Applications exploit the features of THz pulses that distinguish them from optical pulses, namely their broad bandwidth and long center wavelength. The broad bandwidth allows for the exciting possibility for sculpting the pulses to meet a desired goal by modifying their spectra. Although shaped THz pulses have been made by modifying the optical pump intensity profile 6 or by manufacturing more complex emitters, 7 there are

Spatiotemporal shaping of terahertz pulses

We report temporal shaping of few-cycle terahertz pulses, using a slit in a conductive screen as a high-pass filter. The filter's cutoff frequency was tuned by changing the width of the slit; the slope of the cutoff transition was altered by changing the thickness of the screen. We measured the transmission function of the filters, using large-aperture photoconducting antennas to create and detect the incident and transmitted electric field. Our experimental results were in excellent agreement with the performed finite-difference time-domain simulations of the propagation of the pulse through the slit. When the screen thickness was greater than the slit width, the filter was well modeled by a short, planar waveguide. Using a simple transfer function, we accurately describe the sharp cutoff and dispersion of such a filter.

Design of zero-phase recursive 2-D variable filters with quadrantal symmetries

The digital filters with adjustable frequency-domain characteristics are called variable filters. Variable filters are useful in the applications where the filter characteristics are needed to be changeable during the course of signal processing. In such cases, if the existing traditional constant filter design techniques are applied to the design of new filters to satisfy the new desired characteristics when necessary, it will take a huge amount of design time. So it is desirable to have an efficient method which can fast obtain the new desired frequency-domain characteristics. Generally speaking, the frequency-domain characteristics of variable filters are determined by a set of spectral parameters such as cutoff frequency, transition bandwidth and passband width. Therefore, the characteristics of variable filters are the multi-dimensional (M-D) functions of such spectral parameters. This paper proposes an efficient technique which simplifies the difficult problem of designing a 2-D variable filter with quadrantally symmetric magnitude characteristics as the simple one that only needs the normal one-dimensional (1-D) constant digital filter designs and 1-D polynomial approximations. In applying such 2-D variable filters, only varying the part of 1-D polynomials can easily obtain new desired frequency-domain characteristics.

Technique for an interrupt-immune two-mode fiber-optic strain sensor

The concept for an interrupt-immune two-mode sensor based on the absolute variation of the two-mode modulation depth in the single-mode cutoff transition region as a function of strain is developed and demonstrated. While the minimum resolution for absolute measurement greatly depends on the detection electronics, with the experimental apparatus used for the present research a resolution of 400 microstrain, which corresponds to a shift in the cutoff wavelength by 83 pm or a 1.2-dB reduction in the modulation depth, was achieved. The minimum resolution may be improved by optimizing the cutoff transition curve of the fiber.

A simple method of producing far-infrared high-pass filters

A method of making high-pass waveguide filters for the far infrared is described. The filters are formed from arrays of tightly packed narrow-bore metal tubes. Such filters feature sharp cut-off transitions in the region between 20 cm-1 and 40 cm-1. Transmission above cut-off is comparable to that in similar filters produced by drilling holes in thin metal sheets. Typically the transmissions just above cut-off are about 5% while that just below was measured as less than 0.1%.