Waveguide In Terms Research Articles

Generalized transfer matrix theory of electronic transport through a graphene waveguide

In the effective mass approximation, electronic property in graphene can be characterized by the relativistic Dirac equation. Within such a continuum model we investigate the electronic transport through graphene waveguides formed by connecting multiple segments of armchair-edged graphene nanoribbons of different widths. By using appropriate wavefunction connection conditions at the junction interfaces, we generalize the conventional transfer matrix approach to formulate the linear conductance of the graphene waveguide in terms of the structure parameters and the incident electron energy. In comparison with the tight-binding calculation, we find that the generalized transfer matrix method works well in calculating the conductance spectrum of a graphene waveguide even with a complicated structure and relatively large size. The calculated conductance spectrum indicates that the graphene waveguide exhibits a well-defined insulating band around the Dirac point, even though all the constituent ribbon segments are gapless. We attribute the occurrence of the insulating band to the antiresonance effect which is intimately associated with the edge states localized at the shoulder regions of the junctions. Furthermore, such an insulating band can be sensitively shifted by a gate voltage, which suggests a device application of the graphene waveguide as an electric nanoswitch.

Multiantenna Capacities of Waveguide and Cavity Channels

Multiple-input-multiple-output (MIMO) capacity of waveguide and cavity channels is investigated using the modal expansion technique. Rectangular and circular waveguides and cavities are studied in details. Approximate expressions for the number of modes and for the capacity are given. A MIMO system architecture is suggested for a waveguide channel, which achieves the full capacity by making use of the mode orthogonally (or near orthogonality) using an eigenmode modulator at the Tx end and a spatial correlation receiver at the Rx end. Various practical limitations (e.g., nonideal waveguides and modulators, using discrete sensors instead of continuous, one-dimensional sensors instead of two-dimensional, etc.) and their impact on the capacity are discussed. It is demonstrated that long cavities are equivalent to waveguides in terms of capacity. The concept of spatial capacity is introduced to characterize the limits on the transmission rates that are due to both electromagnetic and information-theoretic considerations, which can be evaluated in a closed form for ideal waveguides and cavities. It follows that the traditional single-mode transmission is optimum in terms of capacity in the small signal-to-noise ratio region only.

Optimal Design of Matched Load by Immune Micro Genetic Algorithm

Genetic algorithm (GA) is an efficient approach for optimize,combining GA with immune operator and micro technique, immune micro genetic algorithm (IMGA) is developed. In the process of the algorithm, simulated annealing, niche skill and shuffle scheme are added, too. IMGAcan optimize the initial population and efficiently reduce the memory requirement. Comparing with simple genetic algorithm (SGA), the operating time of the presented method is cut down obviously and computing efficiency is improved dramatically. It is successful to design the short matched load of rectangular waveguide in terms of IMGA. The methodology may be extended, in principle, to other similar problems.

Continuous-wave laser operation at 1.3 μm in Nd3+-doped Zn:LiNbO3 channel waveguides

This work reports continuous laser action around 1.3 μm at room temperature in LiNbO3:Nd3+ channel waveguides, fabricated by the two-steps Zn diffusion technique. A threshold of 64 mW and a slope efficiency of around 0.5% were obtained in an 8 μm wide channel guide. These results have been compared with laser oscillation around 1.08 μm in LiNbO3:Nd3+ waveguides in terms of the launched pump powers, taking into account the emission cross section values of the F3/24→4I13/2 and F3/24→4I11/2 transitions.

Classical versus quantum structure of the scattering probability matrix: chaotic waveguides.

The purely classical counterpart of the scattering probability matrix (SPM)/S(n,m)/(2) of the quantum scattering matrix S is defined for two-dimensional quantum waveguides for an arbitrary number of propagating modes M. We compare the quantum and classical structures of /S(n,m)/(2) for a waveguide with generic Hamiltonian chaos. It is shown that even for a moderate number of channels, knowledge of the classical structure of the SPM allows us to predict the global structure of the quantum one and, hence, understand important quantum transport properties of waveguides in terms of purely classical dynamics. It is also shown that the SPM, being an intensity measure, can give additional dynamical information to that obtained by the Poincaré maps.

Acoustic field of a vertical cylindrical antenna in an inhomogeneous waveguide with an impedance lower boundary

An expansion of the field of a vertical antenna located in an inhomogeneous waveguide in terms of the normal waves of a homogeneous reference waveguide is obtained. The frequency dependence of the radiation resistance is analyzed numerically for various antenna depths and sound velocity profiles. Variations in the radiation resistance are correlated with the variations in the sound velocity.

Theory of design parameters for quasi-phase-matched waveguides and application to frequency doubling in polymer waveguides

A theory of dispersion in single-mode symmetric waveguides is presented for phase-matching second harmonic generation (SHG) of the fundamental modes, based on the approximate analytical waveguide theory of Botez. The theory is used to derive new equations for the maximum phase-matching distance allowed when there are random fluctuations in waveguide thicknesses, under critical and noncritical phase-matching conditions. The theory is also used to calculate the overlap integral as a function of the waveguide parameters V/sup /spl omega// and V/sup 2/spl omega//. A new expression is derived for the efficiency of SHG in waveguides in terms of waveguide parameters that can be used to optimize SHG. Theoretical results are presented for typical LiNbO/sub 3/ and polymer waveguides. Quasi-phase-matched (QPM) waveguides are fabricated from nonlinear optical (NLO) polymers using the techniques of periodic poling and bleaching, and channel waveguides are printed by the bleaching of the NLO polymers. The NLO polymers are characterized for their refractive indexes, optical loss, NLO coefficients, and bleaching characteristics. Phase-matched SHG results are presented for the different fabrication methods over a distance of 0.5 cm, and an assessment is given of the relative strengths and weaknesses of the different fabrication approaches.

Scattering from a cylindrical waveguide with rectangular corrugations

Electromagnetic scattering from a circular cylindrical waveguide with rectangular corrugations is considered in this paper. An analysis method based on dyadic Green's functions and Fourier transforms is used to get the field in terms of modal currents induced on the corrugation openings. The fields are expressed through a series of modal eigenfunctions in the corrugations, and are integrated to get the unknown expansion coefficients. This analysis method can easily be extended to find the dispersion relations of the corrugated waveguide. The series solution obtained is analytic and suitable for numerical computation. Numerical results are presented to illustrate the scattering behavior of a corrugated cylindrical waveguide in terms of frequency and waveguide geometry.

High-confinement waveguides for mid-IR devices

We analyze the use of heavily doped semiconductor layers as a means to tightly confine light. The very low refractive index ( n≈1) that is present just above the plasma frequency renders large index steps possible. Slab waveguide structures with InP or GaAs cladding layers doped to n D ∼10 19 cm −3 are analyzed in the mid-IR wavelength range, λ=4– 15 μm . The performance of the waveguide in terms of achieved overlap with an active region of a given thickness versus the waveguide absorption, is compared to waveguides based on surface plasmons. The calculated results indicate that these plasma effect waveguides should be favourable in this respect for wavelengths λ≈6–10 μm using heavily doped InP as a cladding. In GaAs efficient use of the plasma effect is limited to wavelengths λ≳9 μm . Guidelines for required doping levels are given.

Nonstationary Model Problems for Waveguide Open Resonator Theory

ABSTRACT The paper takes a fresh look at rigorous approaches and algorithms for model problems of electromagnetic theory of waveguide discontinuities. Numerical experiment is still the main tool for investigation the resonant wave scattering phenomena, which are characteristic for such structures. The mathematical approaches employed for such purposes have to fit certain requirements for accuracy, efficiency and versatility, for ability to be focused on particular details of various practically interesting regimes. The approaches for analysis of transient processes in waveguide open resonators considered herein satisfy all these requirements. They rely on the description of scattering properties of discontinuities in regular waveguides in terms of transform operators related to “evolutionary basis” of nonstationary signals, which are qualitatively the same for all guiding structures. The prototypes of such approaches in frequency domain (FD) have been verified by large body of boundary value problems (BVP) and their efficiency has been demonstrated. Their modification for time domain (TD) has been carried out for the first time.

Spectral measurement of the film–substrate index difference in proton-exchanged LiNbO_3 waveguides

We report the spectral characterization of proton-exchanged lithium niobate (PE:LiNbO(3)) waveguides in terms of the variation of the refractive-index difference between the waveguiding layer and the substrate. The dispersion of the extraordinary refractive-index increase (deltan(e)) is measured from 405 to 1319 nm with several light sources. Two types of proton-exchanged waveguide, prepared under different conditions, are studied. These measurements should be of use in the optimization of PE:LiNbO(3) waveguides for nonlinear optical applications, particularly in second-harmonic generation in the blue-green wavelength region.

Mode-matching analysis of a waveguide junction formed by a circular and a larger elliptic waveguide

The paper presents the formulation of a waveguide junction formed by an elliptic and a circular waveguide using the mode-matching method. The junction studied is concentrically joined with the cross-section of the circular waveguide completely enclosed by the elliptic waveguide. By expressing the modal fields in both waveguides in terms of Bessel functions, exact closed-form expressions for evaluating the coupling integrals are obtained. Theoretical results compare well with measurements and published results.

Determination of ion-exchanged channel waveguide profile parameters by mode-index measurements

We report a method for characterizing ion-exchanged channel waveguides in terms of the diffusion depth, width, and surface-index change from mode-index measurements. The method is then used to study the effect of the diffusion time and the mask width on these parameters in K(+)-Na(+)-exchanged guides in BK7 glass. It is observed that the mask width affects the waveguide dimensions in ways not reported in earlier studies. Our results reveal that the diffusion depth depends on the mask opening and saturates with time to a maximum value for a given mask width. The lateral-diffusion rate and the side diffusion beneath the mask are independent of the mask width, which indicates that the lateral electric field associated with the metallic mask plays an important role in such diffusion. We discuss the implications of this behavior to modeling and fabricating integrated optical devices.

Mathematical refinements of excitation conditions in coupled waveguides

The picture of the electromagnetic field evolution along coupled waveguides in terms of the modes of the isolated primary waveguides and of supermodes of the overall coupler, is critically refined. A transformation matrix method, applied to a supermode set, enables us to express the electromagnetic field in a directional coupler by means of the modes of the isolated waveguides. A vectorial space is associated with the set of solutions of the Maxwell equations, where the orthogonal basis is given by normalized supermodes and another space in which the normalized vector basis is given by the normalized modes. Among the representations of two-coordinate systems, there is an angular rotation that does not maintain the orthogonality of the basis. Therefore, there are two characteristic rotation angles that allow a complete description of the light power exchange between the two guides and of the mode orthogonality. Many examples are included. Finally, a brief extension to N -coupled waveguides is reported.

Wide-angle propagating beam analysis based on the generalized Douglas scheme for variable coefficients

The generalized Douglas scheme is applied to the finite-difference beam-propagation method (FD-BPM) with a (1, 1) Padé approximant operator. A truncation error of O(Deltax)(4) is found in the transverse direction. To assess the accuracy of the scheme, we investigate the propagation error of the fundamental mode in a two-dimensional tilted waveguide in terms of the coupling efficiency. The present scheme offers further improvement in the analysis of wide-angle beam propagation compared with the conventional FD-BPM.

A complete set of normal modes in three-layered electron waveguides

Mathematical expressions are given for guided and radiation modes including nonpropagating parts in three-layered electron waveguides, in addition to gaining a clear understanding of the difference between electron and optical waveguides in terms of normal modes. According to the dispersion diagrams, it is shown that a complete set of normal modes for electron waveguides depends on the electron's total energy and that in the quantum-well-type waveguide the discrete modes and the continuous modes radiating to one side of the film are reduced to nonpropagating modes in a certain energy range, whereas a complete set of normal modes for optical waveguides always consists of three types of modes: guided modes, substrate radiation modes, and substrate-cover radiation modes. In addition, the discrete modes and the continuous modes radiating to one side of the film are not reduced to nonpropagating modes in the entire range of frequencies. Also, it is shown that the guided modes always exist in electron waveguides composed of arbitrarily different Al-concentration ratios in the film, substrate, and cover regions, and a new waveguiding quantum-step-type structure, which utilizes the total reflection at both the potential rise and drop is proposed, whereas in optical waveguides, the guided modes exist only in the structure in which the refractive index of the film is higher than those of the substrate and of the cover.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Silica-based integrated optic components for telecommunications applications

Waveguide devices have been produced comprising germania-doped silica cores and silica claddings on silica substrates, using microwave plasma assisted chemical vapor deposition and reactive-ion etching. This all-silica structure offers the maximum compatibility between fibers and waveguides in terms of both optical and physical properties. The all-silica philosophy is extended to the use of laser-cut silica V grooves in the construction of input/output fiber arrays. An important feature of this approach is that it enables fiber/waveguide interfacing by CO 2 laser welding of the respective silica blocks without the introduction of any significant additional loss. Both the waveguide fabrication and laser-welding processes are well suited to large-scale production of low-cost components.

Analysis of a bending dielectric slab waveguide by integral equation formulation

AbstractThe waveguide bend is an indispensable circuit element in realizing optical integrated circuits. In this paper, the bend in a dielectric slab waveguide is analyzed numerically by means of the integral equation formulation. When a numerical solution of an integral equation is attempted by the boundary‐element method, two problems exist: 1) how one can connect the field distributions expressed by the boundary‐element method with those by the eigenmodes of the waveguide. In this paper, instead of representing the field at the input/output boundary of the bend in the slab waveguide in terms of the boundary‐element method, a new method is proposed in which the field distributions expressed in terms of the eigenmodes are substituted directly into the integral equation; and 2) the treatment of the radiation mode. In this paper, a discretization method based on the sampling theorem is used. As an example, numerical calculations have been carried out for a conventional bend and a reflection bend in a dielectric slab waveguide. It is confirmed that the results agree well with data obtained by other methods. It is pointed out that good results can be obtained for such cases as a structure with large difference in index of refraction between the core and cladding and the one with a large bend angle, which were difficult to analyze by the conventional methods. Further, it is shown that the bending loss of the reflection bend is substantially smaller than that of the conventional bend.

A critical study of DGF at the source region in circular waveguides (I)

This is the second part of our work on the dyadic Green’s functions (DGF) at the source region in circular waveguides. With the application of the main results of the first part, a systematic investigation on the general properties of DGF at the source region is given. The dyadic version of the boundary conditions for Maxwell’s equations is derived and the completeness of the expansion of the transverse current source in circular waveguides in terms of the vector eigenfunctionsM andN is proved. It is shown that the electric and the magnetic DGF satisfy the basic dyadic equations.

Frequency and pulse characteristics of fiber-optic waveguides

An investigation was made of the response function, transfer function, and corresponding transmission band width of various types of fiber-optic waveguide, as a function of the waveguide length and the conditions under which radiation was coupled in. It was found that by studying fiber-optic waveguides in terms of their response and transfer functions, the fullest estimate can be made of their information properties.