Lossy Waveguides Research Articles

ATTENUATION CONSTANTS OF RADIO WAVES IN LOSSY-WALLED RECTANGULAR WAVEGUIDES

At the ultra-high frequencies (UHF) common to portable radios, the mine tunnel acts as a dielectric waveguide, directing and absorbing energy as a radio signal propagates. Understanding radio propagation behavior in a dielectric waveguide is critical for designing reliable, optimized communication systems in an underground mine. One of the major parameters used to predict the power attenuation in lossy waveguides is the attenuation constant. In this paper, we theoretically and experimentally investigate the attenuation constants for a rectangular waveguide with dielectric walls. We provide a new derivation of the attenuation constant based on the classic Fresnel re∞ection coe-cients. The new derivation takes advantage of ray representation of plane waves and provides more insight into understanding radio attenuation in tunnels. We also investigate the impact of difierent parameters on the attenuation constant, including the tunnel transverse dimensions, permittivity, conductivity, frequency, and polarization, with an aim to flnd their theoretical optimal values that result in the minimum power loss. Additionally, measurements of the attenuation constants of the dominant mode at difierent frequencies (455, 915, 2450, and 5800MHz) for a straight concrete tunnel are presented and compared to theoretical predictions. It is shown that the analytical results match the measured results very well at all four frequencies.

Nonlinear distortion of optical pulses by self-produced free carriers in short or highly lossy silicon-based waveguides

An explicit analytical solution for the asymmetric attenuation of optical pulses by self-produced free carriers in silicon waveguides is derived. It allows us to quantify the pulse distortion and to calculate explicitly the free-carrier density and the nonlinear phase shifts caused by the Kerr effect and by free-carrier refraction. We show that omitting two-photon absorption (TPA) as a cause of attenuation and accounting only for free-carrier absorption (FCA) as done in the derivation appropriately models the pulse propagation in short or highly lossy silicon-based waveguides such as plasmonic waveguides with particular use for high-energy input pulses. Moreover, this formulation is also aimed at serving as a tool in discussing the role of FCA in its competition with TPA when used for continuum generation or pulse compression in low-loss silicon waveguides. We show that sech-shaped intensity pulses maintain their shape independently of the intensity or pulse width and self-induced FCA may act as an ideal limiter on them. Pulse propagation under self-induced free-carrier absorption exhibits some features of superluminal propagation such as fast or even backward travelling. We find that input pulses need to have a sufficiently steep front slope to be compressible at all and illustrate this with the FCA-induced pulse broadening for Lorentzian-shaped input pulses.

Nondecaying Bloch modes of a dissipative lattice

We consider a Bose-Einstein condensate in a periodic optical lattice having a periodic sublattice of dissipative sites (which we call the dissipative lattice). The optical analog of this system consisting of lossy optical waveguides arranged in a checkerboard order is also presented. Using numerical simulations within the mean-field theory we demonstrate the existence of nondecaying (or propagating, in optical applications) Bloch-like coherent modes, which are eigenstates of the non-Hermitian operator with the dissipative lattice serving as a spatially periodic potential. The nondecaying modes have the Bloch index confined to the boundary of the Brillouin zone of the dissipative lattice. Besides the numerical simulations for a finite lattice, we use the Fourier space perturbation theory for a weak dissipative lattice and the single-band tight-binding approximation for a strong lattice to show that the effect is a universal property of the dissipative lattice.

Analysis of difference frequency generation and cascaded second-harmonic generation and difference frequency generation in lossy quasi-phase-matched semiconductor waveguides

The analytical expressions of converted wave power for difference frequency generation (DFG), cascaded second-harmonic generation and difference frequency generation (cSHG/DFG) processes have been obtained under the non-depletion approximation in lossy waveguides. It is shown that the analytical results and the numerical simulation with depletion agree very well for lossy waveguides. Employing the analytical solutions, the formulas of optimized waveguide lengths in lossy waveguides are obtained for DFG and cSHG/DFG processes. After designing an AlGaAs quasi-phase-matched ridge waveguide, we investigate and compare the characteristics of the second-order nonlinear effects with and without waveguide loss, such as conversion efficiency, conversion bandwidth, pump wavelength tolerance and temperature stability in detail.

Transition from slow and frozen to superluminal and backward light through loss or gain in dispersion-engineered waveguides

We describe the generic effects of loss or gain on pulse propagation in photonic-crystal and plasmonic waveguides that support ``frozen'' or ``in-band'' slow light at dispersion inflection points in the absence of loss (or gain). Using an analytical perturbation theory, we find that propagating and evanescent modes hybridize when loss exceeds a certain threshold, resulting in a reduced attenuation rate and switching from slow to superluminal velocity. Numerical simulations for photonic-crystal waveguides reveal the dynamic nature of this transition with forward-backward pulse velocity oscillations for loss above the threshold. Importantly, we show that the light intensity is enhanced close to the input end of the waveguide even under strong material losses, indicating the potential for slow-light enhancement of optical effects, even in such lossy waveguides.

Directional-Coupler-Based Polarization Splitting in Asymmetric Metal/Multi-Insulator Configuration for Optical Nanocircuitry

An ultra-compact polarization splitter based on asymmetric metal/multi-insulator directional coupler is proposed and investigated analytically and numerically. A large modal birefringence of >;0.69 is estimated, enabling a clean polarization selective directional coupling within 5.6. The insertion loss of and the splitting and extinction ratios of <;-0.32 are predicted. Modified coupled-mode equations for a coupled lossy waveguide system are also presented.

THE REAL-VALUED TIME-DOMAIN TE-MODES IN LOSSY WAVEGUIDES

The time-domain studies of the modal fields in a lossy waveguide are executed. The waveguide has a perfectly conducting surface. Its cross section domain is bounded by a singly-connected contour of rather arbitrary but enough smooth form. Possible waveguide losses are modeled by a conductive medium which fills the waveguide volume. Standard formulation of the boundary-value problem for the system of Maxwell’s equations with time derivative is given and rearranged to the transverse-longitudinal decompositions. Hilbert space of the real-valued functions of coordinates and time is chosen as a space of solutions. Complete set of the TE-time-domain modal waves is established and studied in detail. A continuity equation for the conserved energetic quantities for the time-domain modal waves propagating in the lossy waveguide is established. Instant velocity of transportation of the modal flux energy is found out as a function of time for any waveguide cross section. Fundamental solution to the problem is obtained in accordance with the causality principle. Exact explicit solutions are obtained and illustrated by graphical examples.

A High-Order Compact 2D FDFD Method for Waveguides

A high-order compact two-dimensional finite-difference frequency-domain (2D FDFD) method is proposed for the analysis of the dispersion characteristics of waveguides. A surface impedance boundary condition (SIBC) for the high-order compact 2D FDFD method is also given to model lossy metal waveguides. Four transverse field components are involved in the final eigenequation. Numerical examples are given, which show that this high-order compact 2D FDFD method is more efficient than the low-order compact 2D FDFD method and has a less storage cost.

Periodicity-suppressing effect of periodic lossy-dielectric-loaded cylindrical waveguide

Periodic dielectric-loaded waveguide is one of the diaphragmatic waveguides. For the excellent mode-selective propagation ability, it is of value for applications in gyrotron-traveling-wave amplifiers (gyro-TWT), accelerators, and other microwave propagation systems. This paper focuses on studying the application of the strong lossy-dielectric-loaded periodic waveguide in millimeter-wave gyro-TWT. It is revealed that due to the lossy property of the dielectric, the energy in the dielectric slots is absorbed effectively and the high order Bloch harmonics induced by the periodicity of the structure are suppressed, which changes the discrete spectrum under lossless condition into a continuous one. As a result, the periodicity of the system is severely suppressed and a mode in the hollow region could be approximated by a fast wave mode in an empty waveguide. These results bring specific guidance for the applications of the lossy dielectric-loaded waveguide in gyro-TWTs and other devices.

Quantum entanglement in coupled lossy waveguides

We investigate the viability of coupled waveguides as basic units of quantum circuits. We study entanglement when the waveguides are fed in by light produced by a down-converter working either in low gain limit or under large gain. We present explicit analytical results for the measure of entanglement in terms of the logarithmic negativity for a variety of input states. We also address the effect of loss on entanglement dynamics of waveguide modes. Our results indicate that the waveguide structures are reasonably robust against the effect of loss and thus quite appropriate for quantum architectures as well as for the study of coherent phenomena like random walks. Our analysis is based on realistic structures used currently.

Beam-wave interaction analysis of gyrotron-traveling-wave tube based on a lossy dielectric-lined waveguide

A lossy ceramic-loaded waveguide is positive to promote the stability and performance of the gyrotron traveling-wave amplifier (gyro-TWT). In this paper we develop the electron cyclotron maser linear theory on the basis of the field expressions in the lossy ceramic-loaded waveguide. By systematic numerical calculation, we find that both the field patterns and the dispersion curves have one-to-one mapping relation between the modes in ceramic-loaded waveguide and that in empty cylindrical waveguide. With the Laplace transform, the linear theory could calculate the field profile and the threshold of an absolute instability oscillation. The linear theory is also used to study the influence of the magnetic, current and ceramic layer on the amplification characteristics. These results are promotive to the application of the lossy ceramic in a gyro-TWT and the development of the high stable gyro-TWT.

A MODE BASED APPROACH FOR CHARACTERIZING RF PROPAGATION IN CONDUITS

We propose a mode based approach for developing a parametric model to characterize RF propagation in conduits. The model considers a conduit as a lossy waveguide and deflnes the total received power as the sum of powers excited in propagating modes. The model's parameters are estimated from both the physical properties of the conduit material and an empirical data set. Underground conduits have signiflcant value as wireless communication channels for condition based monitoring within the conduit. An enabler for this wireless sensor network application is based on characterizing the expected coverage range of wireless transceivers operating in the 2.4GHz ISM band. Previous studies on modeling RF propagation in underground conduits have focused on conduits with diameters larger than 1.05m. This motivated our measurement campaign to collect empirical data from underground conduits with varying diameters from 0.30m to 1.37m. The empirical data is used to predict the mode coupled powers which are model parameters that are analytically intractable. We observe that the proposed model provides a good estimate of received power in terms of contribution from dominant propagating modes.

Influence of Mode Loss on the Feasibility of Grating-Assisted Optical Fiber Surface Plasmon Resonance Refractive Index Sensors

In this paper, the influence of mode loss on the feasibility of grating-assisted optical fiber surface plasmon resonance (SPR) refractive index (RI) sensors is investigated. The loss of surface plasmon polarition (SPP) mode plays a key role in the design and implementation of such sensors. It is demonstrated through simulation that the grating length should be smaller than or comparable with the propagation length of SPP mode in order to achieve effective coupling. The loss of SPP mode is the severe limiting factor for the implementation of the grating-assisted SPR-RI sensors. More generally, in order to achieve effective mode coupling with the help of waveguide grating, the grating length is bounded by the shortest propagation length of the modes in lossy waveguides.

Enhancement of optics-to-THz conversion efficiency by metallic slot waveguides

A metallic slot waveguide, with a dielectric strip embedded within, is investigated for the purpose of enhancing the optics-to-THz conversion efficiency using the difference-frequency generation (DFG) process. To describe the frequency conversion process in such lossy waveguides, a fully-vectorial coupled-mode theory is developed. Using the coupled-mode theory, we outline the basic theoretical requirements for efficient frequency conversion, which include the needs to achieve large coupling coefficients, phase matching, and low propagation loss for both the optical and THz waves. Following these requirements, a metallic waveguide is designed by considering the trade-off between modal confinement and propagation loss. Our numerical calculation shows that the conversion efficiency in these waveguide structures can be more than one order of magnitude larger than what has been achieved using dielectric waveguides. Based on the distinct impact of the slot width on the optical and THz modal dispersion, we propose a two-step method to realize the phase matching for general pump wavelengths.

Exact step-coupling theory for mode-coupling behavior in geometrical variation photonic crystal waveguides

In this paper, an exact step-coupling theory is developed to describe the modes coupling behavior and lightwaves propagation for the power exchange between the waveguide modes in geometrical variation photonic crystal waveguide. The exact step-coupling theory provides a general description of the mode-coupling mechanism for light waveguide with geometrical variation with complete set of equations and solutions. The coupling equations of the exact step theory are derived and compared with the scattering matrix method, where simulation results show good agreement with an error of less than 2.2%. Subsequently, the coupling equations are applied to different case studies such as slab tapered waveguide and lossy ``turn-on'' waveguide. The transmission spectrum and field pattern distribution show that the lossy waveguide has a large radiation loss with an average transmission efficiency of less than 5%. The slab tapered waveguide can have more than 90% transmission efficiency with the convex curvature. The exact step-coupling theory can be applied to a vast range of geometrical variation photonic crystal based waveguides and it has quick and accurate convergence simulation results.

Attenuation and mode profile determination of leaky/lossy modes in multilayer planar waveguides by a coupling simulation method

Leaky planar waveguides are critically important to the operation of present day and future integrated photonic circuits. However, to incorporate these waveguides successfully into practical photonic circuits requires an accurate knowledge of their attenuation and mode profile in operation. In contrast with previous numerical methods for obtaining leaky waveguide characteristics, which usually involve complicated algorithms to solve for the complex roots of boundary conditions, the transverse transmission/reflection (TTR) method presented here provides a straightforward and simple approach by simulating the corresponding coupled-waveguide structure. By adding a high-index layer adjacent to the cover to enable the coupling, the transmission/reflection coefficients are shown to be definitively expressed in the form of a Lorentzian that is directly related to the complex propagation constant of leaky/lossy mode. The TTR method simultaneously determines the mode profile of the leaky/lossy mode via the angle of incidence for resonant transmission/reflection. In the present work, the TTR method is applied to an antiresonant reflection optical waveguide (ARROW), a lossy waveguide structure, and a waveguide structure that is simultaneously leaky and lossy.

Modal Transition and Reduction in a Lossy Dielectric-Coated Waveguide for Gyrotron-Traveling-Wave Tube Amplifier Applications

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A metal cylindrical waveguide coated with an inside layer of lossy dielectric which affects the propagation characteristics of a guided electromagnetic mode is investigated for gyrotron-traveling-wave tube (gyro-TWT) amplifier applications. This paper reveals a series of novel phenomena. The dispersion curve of a higher order mode has a turning point during its evolvement from the fast wave region to the slow wave region. An electromagnetic mode in the lossy dielectric-coated waveguide exhibits a transverse partial-standing-wave distribution. The dielectric loss induces modal transition which results in the dispersion curves of a pair of nearby modes crossing each other and interchanging mode structures. Modal reduction caused by strong dielectric loss merges a pair of nearby modes into one. In this one merged mode, the dielectric-coated waveguide is equivalent to a conventional cylindrical waveguide with imperfect conducting wall. This improved understanding of lossy dielectric-coated metal cylindrical waveguide is of value and usefulness for application toward gyro-TWTs capable of high-power and wide bandwidth. </para>

Reflection and Scattering of Electromagnetic Waves in Spatial Grids Consisting of Multiple Lossy Waveguides

Medical diagnosis technologies using X-ray and optical wave transmitted tomogra- phy have been rapidly developed with computer processing. X-ray and optical transmitted fleld characteristics in CT, that depend on biomedical absorption due to bio-molecules and atoms, yield biomedical information of human body tissues and cells. However, X-ray and optical trans- mitted fleld signals at receiving output of diagnosis CT sensors are disturbed by scattering waves in random bio-medical inhomogeneities around objects of body tissues and cells. Before sig- nal processing by computer, hardware tools of spatial flltering of disturbing scattering flelds for absorption characteristics in bio-medical tissues and cells are very useful system functions. Spatial flltering of scattered flelds by grid arrays consisting of transversely multiple lossy waveg- uides is one of excellent device function for spatial flltering in X-ray and optical diagnosis. Spa- tial fllter of grid arrays consisting of multiple lossy waveguides is waveguide array consisting of waveguides with transparent cores and lossy clads. Each waveguide has core size, clad size and waveguide length. Scattered flelds in random bio-medical media are incident on input plane of spatial fllter array, and coupled to lower modes of low losses for small scattering angles and higher modes of large losses for large scattering angles, in lossy waveguide array. Low angle scattered flelds couple to lower modes and large angle scattered flelds couple to lossy higher modes. Scattered flelds coupled to lossy higher modes are flltered in lossy grid arrays. Mode characteris- tics in lossy grid arrays excited by scattered fleld in random bio-medical media are discussed by mode expansion methods using boundary condition at input plane of lossy grid array, and flltered flelds at output plane of lossy grid array are shown by integral equations using Green's dyadics. Mode characteristics and flltered flelds are also investigated by the Wiener-Hopf method with spectral functions.

Loss-Induced Modal Transition in a Dielectric-Coated Metal Cylindrical Waveguide for Gyro-Traveling-Wave-Tube Applications

The mode identification principles, mode structures, and propagation characteristics of electromagnetic modes in a metal cylindrical waveguide coated with an inside layer of lossy dielectric have been investigated for gyro-traveling-wave-tube applications. For the first time, the loss-induced modal transition is revealed, in which the dispersion curves of a pair of nearby modes cross each other, and their mode structures interchange. The relations among the dispersion curves, mode structures, and propagation attenuations are also presented. The distinctive discriminations of propagation properties between different modes enable us to explore many promising applications using lossy dielectric-coated waveguides.

Statistical Characteristics of Transmitted Nano-meter Electromagnetic Waves in Random Bio-medical Tissues for X-Ray Diagnostic Images

Medical image diagnosis and computer aided diagnosis using X-rays are very im- portant technical tools for physiological diagnosis. Scattered X-ray flelds disturb medical image signal properties given by X-ray transmission properties through biological structures concerned with X-ray absorption efiects due to biological characteristics. In this paper, in order to improve image diagnosis, statistical analysis of X-ray is discussed for X-ray propagation, attenuation and scattering in random inhomogeneous biological media. Spatial flltering characteristics of grid structure for scattering X-rays particularly, for ofi-axis X-rays are shown for image X-ray projec- tion by mode theory of lossy waveguides for higher modes. Spatial grid fllters consist of waveguide arrays of graded index inhomogeneous waveguides.