Showing papers on "Guided wave testing published in 1993"

Microoptical sensor and method

Abstract: A microoptical sensor and a corresponding method for detecting chemical substances in a sample. The sample is brought into contact with the waveguiding layer of a planar optical waveguide. Coherent light is coupled into the waveguiding layer, propagates therein as a guided wave, and the latter is decoupled from the waveguiding layer. Both, coupling and decoupling of the light wave guided in the planar waveguide is effected by means of a multi-diffractive grating situated in the plane of the layer, i.e. a grating structure comprising two or more fundamental frequency components for in-coupling and out-coupling, respectively. The multi-diffractive grating coupler results in a directional separation of decoupled light from reflected, transmitted and directly diffracted sub-beams, respectively. This makes possible a background-free detection of guided light decoupled from the waveguiding layer, although the regions on the waveguiding layer in which coupling and decoupling of the guided light wave take place overlap in part.

An efficient 2-D FDTD algorithm using real variables (guided wave structure analysis)

Abstract: A two-dimensional finite-difference-time-domain (FDTD) method is proposed for the full-wave analysis of arbitrarily shaped guided wave structures. By using a phase shift beta Delta z along the z-direction (propagation direction), and assuming the limiting case of Delta z approaching zero, the propagation constant of hybrid modes is calculated using a two-dimensional mesh with a truly two-dimensional grid. When the field equation is multiplied by an additional factor j, only real variables are needed in the computation, leading to a very fast algorithm. >

An Approach to Guided Wave Mode Selection for Inspection of Laminated Plate

Abstract: Overall philosophy of composite material inspection is discussed with a sample problem presentation to illustrate the utility of guided wave analysis and the use of field distributions across the thickness of a specimen to predict its sensitivity to specific anomalies in the material. Guided Lamb-type waves in a three-layered laminated compos ite plate with different anomaly content is studied in this article. Our work is focused pri marily on the detection of interlaminar bond layer imperfections and cracks normal to the interfaces. A Lamb wave mode selection criteria is discussed with respect to defect sen sitivity analysis. Selection of appropriate modes and frequencies is suggested by an ap proach based on an analysis of the modal displacement and power distribution across the multilayered structure as opposed to the previously utilized approach based on a numerical analysis of the dispersion relations for different boundary conditions and effective material properties.

Optical properties of semiconductor superlattices in the far infrared

Abstract: In the far infrared, superlattice dielectric functions can be well represented by an effective-medium model. The overall symmetry of a superlattice is uniaxial, and each of the principal components of the resulting dielectric tensor typically contains a different series of resonances. Here we examine some consequences that are of importance to many of the spectroscopy experiments that are undertaken. We describe sharp Brewster modes near longitudinal frequencies in p-polarized oblique incidence reflectivity and give details of guided wave and surface modes, including interface polaritons at the superlattice–vacuum interface. After discussing the basic optical principles we examine their application to the measurement of phonon confinement parameters.

1‐to‐12 surface normal three‐dimensional optical interconnects

Abstract: We present a surface normal optical interconnect with a 1‐to‐12 collinear fan‐out. Two types of polymer‐based holograms were fabricated to provide a collinear 1‐to‐12 fan‐out from guided mode to substrate modes and twelve 1‐to‐1 surface normal interconnects. Fluctuation of up to 7.2 dB for the 1‐to‐12 fan‐out hologram was observed due to the oscillating and the film saturation effects of the transmission hologram. Diffraction efficiency better than 70% was observed for all the total internal reflection holograms. The result reported herein greatly enhanced optical signal processing capability of guided wave optical interconnects. The advantages of free space optical interconnect, such as global interconnect, three dimension, massive fan‐out/fan‐in capabilities, and surface normal optical interconnect, can be realized using the demonstrated architecture. The coupling from waveguide to fiber can be realized from the surface rather than the edge of a photonic integrated circuit.

An improved 2D-FDTD algorithm for hybrid mode analysis of quasi-planar transmission lines

Abstract: A significantly improved two-dimensional finite-difference time-domain (FDTD) method is proposed for the full-wave analysis of guided wave structures. By using a phase shift beta Delta z along the z-direction (propagation direction) and assuming the limiting case of Delta z, the propagation constant of hybrid modes can be calculated by using a two-dimensional mesh with a truly two-dimensional grid size. By appropriately arranging variables, only a real impulse response is involved. A novel grading scheme allowing a gradually nonequidistant mesh in three dimensions is introduced. >

Numerical simulations of fast MHD waves in a coronal plasma. IV: Impulsively generated nonlinear waves

Abstract: The temporal evolution of ducted waves in coronal loops (represented by smoothed slabs of enhanced gas density embedded within a uniform magnetic field) is studied in the framework of cold magnetohydrodynamics by means of numerical simulations The numerical results show that there is an energy leakage from the slab, associated with the propagation of wave packets which exhibit periodic, quasi-periodic and decay phases Even for weak slabs the nonlinearity can play a significant role, leading to wave breaking and shifted time signatures in comparison to the corresponding signatures of linear waves The quasi-periodic phase possesses the strongest amplitudes in an event, making this phase the most significant for observations

Numerical simulations of fast MHD waves in a coronal plasma: I.Periodic linear pulsations

Abstract: The temporal evolution of ducted waves under coronal conditions is studied in the framework of linearized low β MHD by means of numerical simulations. Coronal loops are represented by smoothed slabs of enhanced gas density embedded within a uniform magnetic field. The simulations show that for a smoothed density profile there is an energy leakage from the slab, associated with the propagation of sausage and kink waves. Wave energy leakage in the kink wave is generally small, whereas the wave energy in sausage waves leaks more strongly for long wavelengths and smoother slabs

New type of guided waves in birefringent media

Abstract: The existence of waves guided by thin dielectric films deposited over a positive birefringent crystal for waveguide parameters below usual cutoff is discussed. This additional kind of guided wave has a hybrid nature and occurs in properly tailored waveguides when a suitable orientation of the crystal optical axis, relative to the waveguide axis, is taken. The dependence of the allowed orientations on various waveguide parameters has been analyzed. Noticeable fast variations, with potential interest for switching applications, have been found. >

Anisotropic dielectric waveguides

Abstract: The guided modes of anisotropic waveguides are known to be leaky when degenerate radiation modes of the orthogonal polarization exist. These losses can be quite large when the principal axes of the dielectric tensor are not aligned with the waveguide axis. We calculate the leakage rate in waveguides that have arbitrarily oriented dielectric tensors, using a perturbation expansion that is valid for weak guidance and weak anisotropy A relatively simple expression for the loss is obtained and is applied to model cases drawn from planar and fiber waveguides with step- and graded-index profiles. The scaling of the loss with profile height and anisotropy is discussed, including the practical case of strongly asymmetric planar waveguides.

Observation of the midfrequency enhancement of tone bursts backscattered by a thin spherical shell in water near the coincidence frequency

Abstract: A prominent feature predicted for the backscattering of tone bursts by thin spherical shells is an enhancement of a guided wave contribution in the midfrequency range [L. G. Zhang et al., J. Acoust. Soc. Am. 91, 1862–1874 (1992)]. This feature may be useful for certain inverse problems and is associated with a strongly coupled slightly subsonic wave denoted by some authors as the a0− wave. The present research gives a comparison between a ray approximation and experiments in which tone bursts having carrier frequencies in the range 35

Full-wave analysis of guiding structures using a 2-D array of 3-D TLM nodes

Abstract: A TLM approach to the full-wave analysis of guided wave structures is introduced. Instead of real pulses as in the conventional TLM method, complex pulses are used. Therefore a nonreciprocal phase shift in the z-direction can be introduced and used to connect the z arms in a 3-D node directly. As a result, the 3-D array of 3-D nodes, normally required in the TLM method to calculate the propagation and attenuation constant, is reduced to only one mesh unit in the z direction (a 2-D array of 3-D nodes). The propagation constant is determined by choosing a value and then calculating the frequency at which this value is valid from the Fourier transform of the impulse response. Losses are found by computing the exponential decay of time harmonic solutions at the eigenfrequencies of the structure. >

The bidiffractive grating coupler

Abstract: A novel optical device for coupling a light beam to a planar waveguide structure and for decoupling the guided wave after a certain distance of propagation is demonstrated. The beam coupler comprises two superimposed uniform dielectric diffraction gratings with different periodicities and/or different orientations incorporated into a planar surface‐wave supporting structure. The bidiffractive coupler has translational invariant coupling efficiency and is ideally suited for optical surface probing using electromagnetic surface mode propagation over short distances only.

Analysis and design of broadband grating couplers

Abstract: The authors have assessed the ability of thin-film dielectric gratings to convert an incident guided wave into outgoing beams whose widths and intensities are maintained nearly constant over a large wavelength band. For this purpose, they have improved and extended a previously developed analytical approach and used it to investigate the pertinent coupling characteristics of typical planar gratings having symmetric rectangular or blazed triangular profiles. The results reveal that, by judiciously designing the grating configuration, it is possible to achieve broadband coupling operation over a wavelength ratio of 2:1 or larger. It is also found that rectangular gratings can offer wider bandwidths, but they couple energy almost equally into the cover and the substrate beams. By contrast, blazed gratings allow somewhat narrower bandwidths, but they can transfer energy almost entirely into either one of those two beams. >

A liquid level sensor using the absorption of guided acoustic waves

Abstract: This liquid level sensor consists of a vertically movable 10-cm-long hollow cylinder with an impedance matching conic section suspended at one end of a few-meters-long magnetorestrictive wire. The wire is automatically wound up or unwound such that the cylindrical sensor element at the bottom of the wire is immersed by just a small and measurable distance into the liquid whose level is sought. Acoustic waves are launched in the wire by a pulse of current in a coil and are transmitted to the hollow cylinder through the cylindrical horn. The carrier frequency of the wave trains and the thickness of the tube are chosen so that the elastic waves strongly leak into the liquid when the hollow cylinder is immersed. The level is then determined by measuring the time-of-flight of the echo reflected at the junction between the wire and the horn, followed by taking into account a small correction term computed from the attenuation of the end echo. >

Fibre Optic Magnetic Field Sensors Utilizing Iron Garnet Materials

Abstract: This thesis deals with the subject of fibre optic magnetic field sensors utilizing iron garnet materials. Such materials exhibit a large Faraday rotation which make them advantageous for application in compact mag­netic field sensors. After an introduction, in which fibre optic sensors and optical methods to measure electric current are reviewed, the original research work is summarized. A system for the measurement of the magneto-optic properties of trans­parent materials is described. Measurement results, showing the influence of temperature, magnetic field direction and sample treatment on the magneto-optical proper­ties of YIG-crystals, are presented. The proper­ties of thin magneto-optical waveguiding films have also been studied using different light coupling methods. Measurement results obtained for holo­graphic grating, prism and edge (end-fire) light coupling to different substituted YIG films are presented. It is shown that the launching method may affect the properties to be measured. The design and performance of several versions of extrinsic guided wave fibre optic magnetic field sensors are then reported. The sensors employ substi­tuted YIG (Yttrium Iron Garnet, Y3Fe5O12) thin film waveguides as sensing elements. Polari­zation maintaining fibres were used as feed and return to provide two signal channels. The signals were combined in a balanced measure­ment system, providing insensitivity to both fluctuations in optical power and loss. Sensors have been made both with separate fibres to guide the light to and from the sensing element and with a single fibre for both functions. The two fibre version, although less ”elegant”, is found to have a better performance. This version also makes it possible to determine both the magnitude and sign of the magnetic field. Measurement results indicate a usable measurement range of at least several mT with a noise equivalent magnetic field level of less than 8 nT/root(Hz). The design and performance of multimode fibre optic magnetic field sensors utilizing the Faraday effect in an epitaxially grown thick (YbTbBi)IG film is also described. This type of sensor is found to be linear over a range from 27 mT to less than 270 nT. Sensor prototypes suitable for current monitoring in high voltage transmission lines have also been developed.

Planar bianisotropic waveguides

Abstract: This paper presents an analysis of several planar bianisotropic waveguides including grounded slabs and strip lines. The bianisotropic properties of the materials are prescribed in a special but practical form so that the propagating waves are either transverse electric or transverse magnetic modes. Certain unique characteristics of the waves are discussed including mode elimination, unidirectional propagation, and the existence of complex modes. Both analytical and numerical results of guided wave properties are discussed.

Universal diagrams for TE waves guided by thin films bounded by saturable nonlinear media

Abstract: It is shown that universal V-b diagrams provide a powerful tool when analyzing the stationary waveguiding properties of the TE waves guided by a thin film bounded by a saturable nonlinear substrate or cladding. For a wide class of nonlinearities, the allowed and forbidden regions of these diagrams, for a stationary guided propagation to occur, display a universal pattern, the marginal loci separating different allowed regions from the forbidden ones being simple functions of only the asymmetry measure of the waveguide and the saturation value of the nonlinear permittivity. Relevant information for device design purposes is summarized on a few diagrams, so general waveguiding properties can be immediately read-off from them, and threshold power-independent values of the normalized thickness of the waveguide for a particular kind of guided wave to be allowed are obtained. Qualitative information concerning both the guided power and the stability of guided waves is also included in the diagrams. >

Electrostatically actuated integrated optical nanomechanical devices

Abstract: We demonstrated that active integrated optical (IO) devices based on the IO nanomechanical effect can be actuated by electrostatic forces. In particular, we demonstrated: (1) an intensity modulator, (2) the deflection, and (3) the focusing and defocusing of a guided wave in a planar waveguide.

Characterization by guided wave of instabilities of optical coatings submitted to high-power flux: thermal and third-order nonlinear properties of dielectric thin films.

Abstract: We study the reversible refractive index variations of optical thin films submitted to a high-power light flux. As a preliminary step, we study the thermorefractive coefficient ?n/?T and the laser damage threshold of our materials. From the hypothesis of a localized optical Kerr-type effect, we use the m-line technique to estimate the nonlinear refractive-index coefficients n(2) of TiO(2), Ta(2)O(5), and ZnS films with continuous illumination. Very large values of n(2) are found for the films obtained by conventional evaporation condensation. On the other hand, the study of the change in the decoupling direction of a low-power light guided in a layer disturbed by a pulsed YAG laser gives a first analysis of the phenomena versus time.

Vector wave equation 2-D-FDTD method for guided wave problems

Abstract: A compact FDTD algorithm for the full wave analysis of inhomogeneous wave guiding structures, using a two dimensional mesh is proposed. The formulation is based on the vector wave equation, and in contrast with previous approaches, allows for the formulation of the algorithm in a real domain only. Moreover, since only transverse electric fields are used, two real, instead of six complex components have to be updated and stored, and since they are both defined at the same mesh nodes, the treatment of dielectric inhomogeneities is simplified. Numerical examples validating the method are presented. >

Stationary nonlinear waves guided by thin films bounded by nonlinear diffusive media

Abstract: Stationary TE-polarized nonlinear waves, guided by thin films that are bounded by nonlinear diffusive media, are studied numerically for a variety of waveguide structures. We calculate both the guided-wave power as a function of effective index and the power-dependent field distributions. The nonlinear guided-wave dispersion relations are found to move to higher powers with increased diffusion length. For an asymmetric waveguide composed of two nonlinear media with unequal diffusion lengths, we predict the existence of two distinct asymmetric solutions.

A time-domain propagating scheme for simulation of dynamics of optical guided-wave devices

Abstract: A simple and efficient propagating scheme for electromagnetic wave propagation in time domain is demonstrated. It is capable of tracking a pulsed wave directly in time domain and can be used for the simulation of the dynamic behaviors of optoelectronic and fiber-optic guided-wave devices. >

Three-dimensional rigorous analysis of dielectric grating waveguides for general cases of oblique propagation

Abstract: A rigorous analysis of wave guidance by dielectric grating waveguides is presented for the general case that both the grating vector and the direction of the guided wave have an arbitrary orientation in three dimensions. Guided modes then become TE-like and TM-like modes owing to the coupling between TE and TM waves. The analysis is formulated in a unified matrix form so that guidance properties can be obtained accurately in any level by systematic matrix calculations. The calculated results indicate that peculiar stop bands and leaky waves appear as a result of coupling between TE and TM waves.

Leaky Rayleigh waves

Abstract: In an elastically isotropic semiinfinite medium having a free surface with a weak harmonic corrugation, the interaction between the Rayleigh wave guided along the free surface and the shear vertical and pressure bulk elastic waves propagating in the substrate is investigated for the propagation directions lying in a plane perpendicular to the average free surface and containing the grating vector. The governing equations of this interaction are deduced and are shown to be consistent with the requirements of reciprocity and power conservation. The theory is applied to determine the characteristics of the output coupler in which the Rayleigh wave guided along the free surface radiates the bulk elastic waves in the substrate as well as those of an input coupler in which the shear and the pressure bulk elastic waves in the substrate incident on the corrugated region of the free surface launches the Rayleigh wave. Techniques for optimizing the performance of the input coupler to achieve maximum excitation efficiency of the Rayleigh wave are examined. The characteristics of these transducers are presented with the help of some illustrative numerical results.

Exact dispersion relations for TM waves guided by thin dielectric films bounded by nonlinear media

Abstract: Exact dispersion relations for TM waves guided by thin dielectric films, surrounded on one or both sides by media with intensity-dependent refractive indices, have been derived. Numerical results for a symmetric structure are in complete agreement with those obtained by the finite-element method.

Finite-difference time-domain approach to nonlinear guided waves

Abstract: The finite-difference time-domain method (FDTD) is shown to be applicable to nonlinear guided wave problems. The approach used is easy to implement and the results of propagating a guided mode in a nonlinear waveguide indicate good accuracy.

Two-dimensional equations for guided electromagnetic waves in dielectric plates surrounded by free space

Abstract: Two‐dimensional governing equations of successively higher‐order approximations for guided electromagnetic (EM) waves in an isotropic dielectric plate surrounded by free space are deduced from the three‐dimensional Maxwell’s equations by expanding the EM vector potential in a series of trigonometric functions of a thickness coordinate in the plate and in exponentially decaying functions of a thickness coordinate in the upper and lower halves of free space. By further satisfying the continuity conditions of the EM field at the interfaces between the plate and free space, a single system of two‐dimensional governing equations is obtained. Solutions and dispersion relations are obtained from the two‐dimensional approximate equations. Dispersion curves are computed and compared with the corresponding ones obtained from the solutions of the three‐dimensional Maxwell’s equations for the transverse electric (TE) and transverse magnetic (TM) waves of the first four modes and for values of the refractive index n=...

An explanation of some E-plane scan blindnesses in single-polarized tapered slot arrays

Abstract: It has been found that a guided wave on a corrugated surface is a reasonably good predictor of some of the scan blindnesses observed in E-plane scan of tapered slot antenna arrays. The dependence of the blindness angle on the difference between array element spacing and thickness and its insensitivity to /spl epsi//sub r/ indicate that the metallization of the stripline antenna is the dominant factor in the blindness and not the dielectric substrate. The simple model presented here is, however, only a partial solution to the problem of predicting and eliminating scan blindnesses. It is noted that, in order to properly account for the loading effects of the periodic slots along the y direction, a moment method solution for the modes of such a structure should be developed. >