Cutoff Frequencies Of Modes Research Articles

Elliptical fibre dielectric waveguides: a transverse transmission line analysis

The authors present a new method for determining the optical properties of arbitrary refractive index elliptical core fibres. The method applies the elliptic cylindrical coordinate system to Maxwell's equations and appropriately, using Fourier Transforms, operated on an elliptical core optical fibre waveguide, derives a flexible algorithm, which can be used to determine the optical properties of elliptical fibre waveguides. By dividing the fibre into a series of uniform consecutive thin dielectric layers transversely across the elliptical cross section with suitable transforming functions representing voltage and current themselves based on the electromagnetic field components, leads to transverse transmission line equations. The resonance frequencies of the cascaded matrices determine the mode propagation constants. The analysis leads to a numerical algorithm for calculating the exact modes of propagation constants. The method is analytically exact and computationally accurate, leading to algorithmic implementation, without making use of Mathieu functions which are solutions for step index waveguide. The advantage of this work is also the fact that it can be applied to arbitrary refractive index profiles. They present results of modal b ¯ − V diagrams for various fibre ellipticities and also apply this method to determine the birefringence of graded index profile fibres and mode cut-off frequencies.

Concurrence of two identical atoms in a rectangular waveguide: linear approximation with single excitation

We study two two-level systems (TLSs) interacting with a reservoir of guided modes confined in a rectangular waveguide. For the energy separation of the identical TLSs far away from the cutoff frequencies of transverse modes, the delay-differential equations are obtained with single excitation initial in the TLSs. The effects of the inter-TLS distance on the time evolution of the concurrence of the TLSs are examined.

Studying the Phase of the Ambient Noise Cross-Correlation Function of an Oceanic Waveguide Field

Advantages and limitations of a new way of estimating the cutoff frequencies of hydroacoustic modes using data on the phase of the cross-correlation function of a shallow sea noise field are considered. The results from numerical modeling are compared to experimental data recorded in the Barents Sea.

Quantitative Analysis of Photon Density of States for One-Dimensional Photonic Crystals in a Rectangular Waveguide

Light propagation in one-dimensional (1D) photonic crystals (PCs) enclosed in a rectangular waveguide is investigated in order to achieve a complete photonic band gap (PBG) while avoiding the difficulty in fabricating 3D PCs. This work complements our two previous articles (Phys. Rev. E) that quantitatively analyzed omnidirectional light propagation in 1D and 2D PCs, respectively, both showing that a complete PBG cannot exist if an evanescent wave propagation is involved. Here, we present a quantitative analysis of the transmission functions, the band structures, and the photon density of states (PDOS) for both the transverse electric (TE) and transverse magnetic (TM) polarization modes of the periodic multilayer heterostructure confined in a rectangular waveguide. The PDOS of the quasi-1D photonic crystal for both the TE and TM modes are obtained, respectively. It is demonstrated that a “complete PBG” can be obtained for some frequency ranges and categorized into three types: (1) below the cutoff frequency of the fundamental TE mode, (2) within the PBG of the fundamental TE mode but below the cutoff frequency of the next higher order mode, and (3) within an overlap of the PBGs of either TE modes, TM modes, or both. These results are of general importance and relevance to the dipole radiation or spontaneous emission by an atom in quasi-1D periodic structures and may have applications in future photonic quantum technologies.

Band-pass filters based on periodic structures in SIW technology

In this work, the study of two different topologies of S-band band-pass filters based on periodic structures in substrate integrated waveguide (SIW) technology is theoretical and experimentally addressed. The first topology consists of periodic rectangular perforations of the SIW substrate. In this filter topology, the lower cutoff frequency is determined by the cutoff frequency of the first Floquet mode of the periodic structure, while the upper cutoff frequency and the rejection band are conditioned by its first band-gap. The second topology consists of a periodic array of square complementary split-ring resonators (CSRRs) etched on the waveguide surface. In this case, the use of subwavelength resonators with evanescent-wave transmission leads to a much smaller filter, given that the pass-band is below the cutoff frequency of the waveguide. The effect of the different dimensions of the unit cell in the filters characteristics (lower cutoff frequency, pass-band, and rejection-band associated to the first band-gap of the periodic structure) is analyzed through the dispersion diagram of the periodic cell. Finally, two band-pass filters based on each topology have been designed and built, showing in both cases good matching and low insertion loss in the pass-band, and a deep rejection band.

Evidence for low density holes in Jupiter\u2019s ionosphere

Intense electromagnetic impulses induced by Jupiter’s lightning have been recognised to produce both low-frequency dispersed whistler emissions and non-dispersed radio pulses. Here we report the discovery of electromagnetic pulses associated with Jovian lightning. Detected by the Juno Waves instrument during its polar perijove passes, the dispersed millisecond pulses called Jupiter dispersed pulses (JDPs) provide evidence of low density holes in Jupiter’s ionosphere. 445 of these JDP emissions have been observed in snapshots of electric field waveforms. Assuming that the maximum delay occurs in the vicinity of the free space ordinary mode cutoff frequency, we estimate the characteristic plasma densities (5.1 to 250 cm−3) and lengths (0.6 km to 1.3 × 105 km) of plasma irregularities along the line of propagation from lightning to Juno. These irregularities show a direct link to low plasma density holes with ≤250 cm−3 in the nightside ionosphere.

Wideband Epsilon‐Near‐Zero Supercoupling Control through Substrate‐Integrated Impedance Surface

AbstractThe supercoupling phenomenon of electromagnetic wave through arbitrary shaped epsilon‐near‐zero (ENZ) material channels has attracted considerable attention recently, leading to numerous intriguing applications. It has been demonstrated that around the cutoff frequency of fundamental TE10 mode, waveguide behaves as the 2D homogenous ENZ material. However, for a given waveguide geometry, the ENZ operating bandwidth is inherently narrow and the center frequency is fixed. Here, a reconfigurable ENZ medium is proposed and experimentally demonstrated with a wideband‐tunable supercoupling effect by means of inserting a metasurface inside the waveguide, named as substrate‐integrated impedance surface (SIIS). With different insertion depth (which can be mechanically tuned) of SIIS, the ENZ supercoupling frequency can be precisely controlled. The proposed technique not only achieves wideband tuning of the ENZ supercoupling, especially to control the enhanced group delay of the electromagnetic waves, thereby offering a promising way toward for tunable slow‐light devices, such as light storage and processing components.

1.1 THz tenth harmonic mixer based on planar GaAs Schottky diode

Here, the tenth harmonic mixing technology based on planar GaAs Schottky diode is first proposed for the 1.1 terahertz (THz) frequency conversion module. The first higher order mode cut-off frequency of the suspended microstrip line has been investigated to determine the appropriate cross-sectional dimension of transmission line shield cavity in the radio frequency (RF) circuit portion. The height of shield cavity of transmission line is designed to be discontinuous in order to facilitate the assembly of the diode. A modelling approach combining field and circuit is used to achieve joint simulation of linear passive structure and diode non-linear characteristics. The simulation results show that the frequency conversion loss of this mixer is <55 dB in the RF frequency range of 1.03–1.154 THz, and the best frequency conversion loss is 50 dB at RF of 1.098 THz.

Cutoff Frequencies of a Dielectric-Loaded Rectangular Waveguide With Arbitrary Anisotropic Surface Impedance

The coupled transcendental equations describing cutoff frequencies of TE and TM modes are deduced for a rectangular waveguide with arbitrary wall impedance. In the case of TE modes, their generalization is made to a rectangular guiding structure, which possesses both distributed wall impedance and inhomogeneous dielectric loading. Effective impedance is obtained for imperfectly conducting surface incorporating periodic rectangular corrugations along the direction of mode propagation. Circular waveguide bounded by such corrugated surface and rectangular impedance waveguide are considered as numerical examples. For these waveguides, the computations performed to determine the cutoff frequencies of TE guiding modes are validated against the results of the perturbation theory and the finite-element method.

A Simple Approximate Formula for the Location of the TM and TE Mode

This paper, try to formulate and solve the TM and TE modal equations of the guided surface waves supported by a multilayered structure of uniaxially anisotropic dielectrics. Leads to a concise form of these we proposed a new method for determining the TM and TE characteristic equations, expressed in terms of a 4 × 4 matrix multiplication, which is easily implemented. To validate the general expression an example of a double-layered structure is used. We presented numerical results for the cutoff frequencies of the surface wave modes of a five-layer isotropic structure. Finally, a simple approximate formula for the location of the TM0 mode is also given.

Cutoff frequencies of accelerating modes propagation inside a cylindrical waveguide filled with radially inhomogeneous magnetized plasma

The cutoff frequencies of accelerating hybrid EH0l mode (l = 1, 2, 3, and 4) propagation are studied in a cylindrical waveguide filled with radially inhomogeneous plasma in the presence of an external static magnetic field applied along the waveguide axis. To drive the analytical cutoff equations, a model based on the Bessel-Fourier expansion, which considers strong spatial dispersion and a finite external magnetic field, is utilized. In order to examine the integrity of the cutoff equations, the well-known cutoff expression related to the hybrid EH wave in a homogeneous magnetized plasma waveguide is recalculated with the help of a new model. The cutoff equations are solved numerically, and the effects of the external magnetic field and plasma density are investigated on the cutoff frequencies of the EH0l modes (l = 1, 2, 3, and 4) in the particular waveguide with a radius of 2.1 cm. By studying the effect of the waveguide radius on the cutoff frequencies, it is revealed that the waveguide radius plays a decisive role in the behavior of the cutoff frequencies with the external static magnetic field and plasma density. It is found that a critical radius can be realized for the waveguide so that the cutoff frequency treatment with the external magnetic field and plasma density is reversed in it.

Beam Diffraction Effects in the Backward Wave Regions of Viscoelastic Leaky Lamb Modes for Plate Transmission at Normal Incidence.

Plane-wave theory for fluid-embedded isotropic plates is often used in ultrasonic guided-wave applications, and to estimate wall thickness, corrosion, or sound velocities in plates and pipes. In such structures, measured ultrasonic transmission through the solid material is affected by acoustic beam diffraction effects, and the results may deviate from plane-wave descriptions, which are insufficient to describe the complex effects that occur. When exciting a fluid-embedded steel plate with a pulsed ultrasonic beam at normal incidence, resonance frequency downshift, axial sound pressure level increase, and beam narrowing have been observed, for measured resonance peaks in the frequency regions of certain leaky Lamb mode branches of the plate. In the ranges of other leaky Lamb mode branches, the observed effects are different. Measurements, finite element, and angular spectrum modeling are used to indicate a close connection between these beam diffraction phenomena and the backward wave characteristics of certain leaky Lamb mode pairs, in the frequency and Poisson's ratio regions around coincidence of two Lamb mode cutoff frequencies of similar symmetry. In particular, such observations made for the steel plate's fundamental thickness-extensional (TE) mode appear to be caused by acoustic beam excitation of the backward wave regions of the S-2vl and S2vl leaky Lamb modes.

Electron density variability of nighttime D region ionosphere in Vietnamese and Japanese sectors

AbstractRecording tweek atmospherics on geomagnetically quiet days in 2014 at Tay Nguyen University (TNU) (12.65°N, 108.02°E), Vietnam, and at Kagoshima (KAG) (31.48°N, 130.72°E), Japan, we investigated the nighttime electron density variability of the D region ionosphere between the equatorial‐low‐latitude Vietnamese and the low‐ to middle‐latitude Japanese sectors. We estimated the reflection height and electron density of the D region ionosphere using the first‐order mode cutoff frequency of tweek atmospherics. The results observed at both stations show that the mean electron density at the reflection height during winter season was higher than that during summer and equinox seasons. The electron density observed at TNU gradually decreased from 20:15 to 4:15 LT from winter to equinox and to summer. The electron density observed at KAG increased from 20:20 to 4:20 LT during summer and winter seasons. The mean electron density during 2014 observed at TNU (25.0 cm−3) was higher by 2.1 cm−3 than that observed at KAG (22.9 cm−3). During 2014, the nighttime electron density variations show a moderate positive correlation with the sunspot number but show weak to no correlation with the galactic cosmic rays. We suggest that the seasonal variations in the nighttime electron density could be significantly caused by the enhancement of geocoronal hydrogen Lyman α intensity and seasonal variation of nitric oxide density in the lower ionosphere.

THE BLOCKAGE EFFECT IN WAVEGUIDE BENDS IN THE VICINITY OF THE CUTOFF FREQUENCY OF THE SECOND-ORDER MODE

In the paper, a rigorous mathematical model of electromagnetic wave diffraction in the bend of a plane-parallel waveguide is developed. Numerical simulation of such a waveguide bend has revealed the effect of its complete blockage at a frequency close to the cutoff frequency of the second-order mode. The resonant nature of this effect has been established to be consisting in excitation of a blocking oscillation in such a waveguide irregularity. It is concluded that a waveguide bend can be considered as an open waveguide resonator. Importance of the solved problem is determined by wide application of waveguide bends in the microwave technology.

Low cross-talk, deep subwavelength plasmonic metal/insulator/metal waveguide intersections with broadband tunability

We suggest a low cross-talk plasmonic cross-connector based on a metal/insulator/metal cavity and waveguides. We separately investigate the isolated cavity mode, the waveguide mode, and the combination of cavity and waveguide modes using a finite-different time-domain method. Due to resonant tunneling and the cutoff frequency of the odd waveguide mode, our proposed structure achieves a high throughput transmission ratio and eliminates cross-talk. Furthermore, the proposed structure has a broadband tunability of 587 nm, which can be achieved by modulating the cavity air gap thickness. This structure enables the miniaturization of photonic integrated circuits and sensing applications.

Analysis of Plasma-Loaded Noncorrugated and Triangular Corrugated Coaxial Cavity

A field analysis of plasma-loaded noncorrugated coaxial cavity and plasma-loaded triangular corrugated cavities for megawatt-class gyrotrons has been undertaken. The presence of magnetic field in plasma changes its characteristics to an anisotropic media. In plasma waveguide, three families of modes can exist: plasma modes (space charge modes), cyclotron modes, and waveguide EH and HE modes. The modes present in the plasma are hybrid modes with all six nonzero field components. The presence of the low-density plasma in a gyrotron cavity leads to the coupling of these families of modes and causes a shift in the eigenvalue of the operating (desired) mode. In this paper, the dispersion relation for a plasma-loaded noncorrugated and triangular corrugated coaxial cavity has been derived. For the gyrotron case, where the cyclotron frequency of the plasma is approximately equal to the operating frequency and the operating frequency is near mode cutoff frequency in the cavity, the desired HE mode couples with the cyclotron mode. This coupling leads to shift in the eigenvalue of the HE mode.

Simple but accurate method to compute LP11mode cutoff frequency of nonlinear optical fibers by Chebyshev technique

We propose a simple method to compute the first higher order mode cutoff frequency of a single-mode fiber in the presence of optical nonlinearity. We consider optical Kerr-type nonlinearity for these fibers having dispersion-shifted and dispersion-flattened profiles. Our analysis involves the Chebyshev method but introduces a linearization of cubic terms of fields due to Kerr-type nonlinearity. Our results are shown to match excellently with the available exact numerical values. Thus, this formulation and stepwise algorithm should be considered as a simple but accurate alternative to deeply involved numerical technique and used as a guideline in calculating and minimizing the modal noise in single-mode nonlinear fibers.

Electromagnetic analysis of the main and higher modes on the onion-like carbon tubes

ABSTRACTThe electric and magnetic field distributions as well as the dispersion characteristics of open cylindrical tube (hollow-core) waveguides are analysed in this work. The analysed waveguides are made of an onion-like carbon (OLC) material. The solution of the boundary problem was fulfilled by the partial area method. It was discovered the very complicated dependencies of the phase and attenuation constants on the waveguide radii. Such dependencies arise because the OLC material is the highly dispersive and absorbing one. The fundamental and four higher modes can propagate in our investigated waveguides. We have analysed the high-frequency cut-off frequency of modes that is dependent on the tube waveguide external radius. The certain electromagnetic (EM) mode can propagate only on the frequencies lower than the mentioned cut-off frequency. For this reason, this cut-off frequency has an important use in practical applications. We presented here the EM field distributions of modes. We discovered that it is possible to reach the one-mode regime of OLC tube waveguide at the certain external radius. The fundamental and first higher modes’ EM energy is concentrated in the OLC material between the internal and external surfaces of the waveguide.

Sound transmission acrosss a plenum window with an active noise cancellation system

A series of experiments was conducted in the present study in an attempt to understand how the implementation of active noise control would affect the sound transmission across a plenum window. A simple squared pressure control scheme with three error sensors equi-spaced on the vertical centerline of the indoor window opening was adopted. The secondary control loudspeakers were fixed close to the outdoor window opening. The effects of secondary control loudspeaker orientation relative to the plenum cavity and their locations on the performance of the active control were investigated. Results show that the active control system with two loudspeakers located symmetrically about the plenum cavity horizontal centerline facing directly the incoming noise gives the best performance. However, the effective frequency range is restricted to that below the first asymmetric mode cut-off frequency in the window gap direction. The active control performance is improved when the reverberation strength of the receiver chamber is reduced in general.

Exact Expressions for Vector Mode Cutoff in Three-Layer Step-Index Fibers

Two-layer step-index fibers have well-known cutoff solutions, while cutoff solutions for three-layer step-index fiber exist for only one geometry (ring fiber). We derive exact expressions for cutoff frequencies of both vector and scalar modes for all geometries of three-layer step-index fibers. While vector modes have been solved for these fibers, to the best of our knowledge, this is the first time cutoff expressions are reported. Expressions for cutoff help predict the number of supported modes for a given fiber profile. Using these cutoff expressions, we deduce characteristics of fiber modal content as a function of fiber profile.