Showing papers on "Guided wave testing published in 1994"

Guided waves by electromagnetic gratings and non‐uniqueness examples for the diffraction problem

Abstract: We consider the two-dimensional scalar problem of the diffraction of a plane wave by an infinite grating of conducting bodies immersed in a periodical dielectric medium. A Fredholm-type formulation is derived and studied. The existence of a solution is proved and some uniqueness results are established. A detailed description of the guided modes of the grating is carried out. Finally, various non-uniqueness examples for the diffraction problem are exhibited.

Seismic guided waves trapped in the fault zone of the Landers, California, earthquake of 1992

Abstract: A mobile seismic array of seven stations was deployed at 11 sites along the fault trace of the M7.4 Landers earthquake of June 28, 1992, with a maximum offset of 1 km from the trace. We found a distinct wave train with a relatively long period following the S waves that shows up only when both the stations and the events are close to the fault trace. This wave train is interpreted as a seismic guided wave trapped in a low-velocity fault zone. To study the distribution of amplitude of the guided waves with distance from the fault trace and also their attenuation with travel distance along the fault zone, we eliminated source and recording site effects by the coda normalization method. The normalized amplitudes of guided waves show a spectral peak at 3–4 Hz, which decays sharply with distance from the fault trace. Spectral amplitudes at high frequencies (8–15 Hz) show an opposite trend, increasing with distance from the fault trace. The normalized amplitudes of guided waves at 3–4 Hz also show a systematic decrease with hypocentral distance along the fault zone, from which we infer an apparent Q of 50. In order to confirm the existence of the guided waves, a dense array of 31 stations was deployed at one of the 11 sites. The resultant records revealed unequivocal evidence for the existence of guided waves associated with the fault zone. By modeling the waveforms as S waves trapped in a low-velocity waveguide sandwiched between two homogeneous half-spaces with velocity Vs = 3.0 km/s, we infer a waveguide width of about 180 m, a shear velocity of 2.0–2.2 km/s, and a Q of ∼50. Hypocenters of aftershocks with clear guided waves show a systematic distribution both laterally and with depth delineating the extent of the low-velocity fault zone in three dimensions. We find that the zone extends to a depth of at least 10 km. This zone apparently continues to the south across the Pinto Mountain fault because guided waves are observed at stations north of the Pinto Mountain fault for earthquakes with epicenters south of it. On the other hand, the zone appears to be discontinuous at the fault bend located about 20 km north of the mainshock epicenter; guided waves were observed for stations and epicenters which are located on the same sides of the fault bend but not for those on the opposite sides.

A guided wave inspection technique for nuclear steam generator tubing

Abstract: The utility of guided ultrasonic waves for increasing the efficiency and sensitivity of nuclear steam generator tubing evaluation by nondestructive means is explored. The basic theoretical background of guided waves in tubes is presented, with major emphasis placed on those results with direct relevance to NDE applications. The possibility of increasing the inspection range of the guided wave modes as well as their sensitivity to defects by proper choice of mode and frequency is also discussed.

Utilization of guided elastic waves for the characterization of circumferential cracks in hollow cylinders

Abstract: The scattering of guided elastic waves from a circumferential crack in an isotropic hollow cylinder is investigated analytically using the S‐parameter formalism. A general formula, analogous to that of [B. A. Auld, Acoustic Fields and Waves in Solids (Krieger, Malabar, FL, 1990), 2nd ed., Vol. II, p. 200], is derived for the scattered amplitude of an arbitrary mode due to the incidence of an arbitrary mode. The amplitudes are expressed as an integral of the inner product of the crack opening displacement caused by the incident mode and the traction field of the scattered mode. Using a simple approximate form of known elastostatic solutions for the crack opening displacements due to pure tension, a formula is derived for the scattering of guided wave modes of arbitrary circumferential order due to the incidence of axisymmetric wave modes with large wavelength and uniform stress distribution. These results are analyzed and conclusions drawn on the possibility of crack sizing using relatively simple scattering amplitude information.

Excitation of Guided Waves in Generally Anisotropic Layers Using Finite Sources

Abstract: The excitation of guided wave modes in generally anisotropic layers by finite sized strip sources placed on the surfaces of the layer is examined. The general problem of arbitrarily applied harmonic surface tractions is first solved using the normal mode expansion technique in conjunction with the complex reciprocity relation of elastodynamics. This general solution is then specialized to loading situations modelling those commonly used to excite guided waves in layers for use in nondestructive evaluation. The amplitudes of the generated modes are written as the product of an “excitation function” which depends only on the distribution of the applied tractions and an “excitability function” which depends only on the properties of the specific mode(s) being excited and which determines how receptive the modes are to the applied tractions. Expressions are obtained for the −9 dB wave number and phase velocity bandwidths (σβ and σν respectively) which determine the widths of the wavenumber or phase velocity excitation spectra at the −9 dB generation point. Finally, the problem of transient loading is addressed by superimposing time harmonic solutions via an integration over the dispersion curves of the layer.

Optical waveguide device

Abstract: An optical waveguide device for coupling an external wave with a guided wave by means of a grating coupler formed on an optical waveguide device which lies on a substrate The grating coupler is further coated with a cladding layer An external wave is transformed into a guided wave after being diffracted by a first grating coupler, and travels along the waveguide to a second grating coupler The guided wave is then subjected to diffraction by means of the second grating coupler, whereupon a minus first-order wave is diffracted only toward the substrate, whilst other waves of higher order are respectively diffracted toward the cladding layer and the substrate The wave diffracted toward the cladding layer is totally reflected from a cladding layer boundary furtherst from the substrate, thereby passing through the substrate The diffracted wave thus reflected interferes with the other wave of the same order diffracted to the substrate, thereby weakening each other

Guided-wave electro-optic beam deflector using domain reversal in LiTaO/sub 3/

Abstract: We have fabricated and demonstrated the operation of electro-optic beam deflectors using domain reversal in planar waveguides fabricated on LiTaO/sub 3/ substrates. The planar waveguides were made using proton exchange on z-cut substrates, and proton exchange with subsequent rapid thermal annealing was used to form the inverted domains. The deflection sensitivity of the device was measured to be about 6.7 mrad/kV. Calculations indicate that the deflection sensitivity should be increased by a factor of 25 to 30 with improvements in device geometry. >

Ultrasonic guided wave inspection concepts for steam generator tubing

Abstract: Some very exciting and promising results have been obtained with respect to the utilization and guided wave techniques for inspecting steam generator tubing. In addition to some theoretical considerations that were studied recently, work has been carried out in special probe design and development. This special probe is used in demonstration of law detection feasibility, and in understanding the conceptual development of a complete flaw detection system. This includes transducer, pulser-receiver system, and appropriate signal processing and pattern recognition software for reliable inspection. Ultrasonic NDE techniques have progressed quite rapidly during this decade for two principal reasons: advanced signal processing, and the use and understanding of multi-mode ultrasonic wave propagation. Both concepts are useful in the proposed work on guided wave propagation in steam generator tubing. These new directions go beyond the use of normal beam longitudinal waves and angle beam shear waves for inspection. Guided waves such as surface and Lamb waves can be used to monitor larger volumes of material with greater efficiency. The generation of these waves, however, is more complex. Theoretically one can produce a large number of modes in a structure with a simple loading arrangement. However, the generation of sufficient amounts of energy inmore » a specific mode strongly depends on several factors. They include the loading system, angle of attack, probe frequency, frequency bandwidth, and a whole host of special transducer design and instrumentation parameters.« less

Dispersion and attenuation of acoustic guided waves in layered fluid-filled porous media

Abstract: The analysis of acoustic wave propagation in fluid‐filled porous media based on Biot and homogenization theories has been adapted to calculate dispersion and attenuation of guided waves trapped in low‐velocity layered media. Constitutive relations, the balance equation, and the generalized Darcy law of the modified Biot theory yield a coupled system of differential equations which governs the wave motion in each layer. The displacement and stress fields satisfy the boundary conditions of continuity of displacements and tractions across each interface, and the radiation condition at infinity. To avoid precision problems caused by the growing exponential in individual matrices for large wave numbers, the global matrix method was implemented as an alternative to the traditional propagation approach to determine the periodic equations. The complex wave numbers of the guided wave modes were determined using a combination of two‐dimensional bracketing and minimization techniques. The results of this work indica...

Geometrical optics design and aberration analysis of a focusing grating coupler

Abstract: The geometrical optics design of a focusing grating coupler (FGC) is discussed for the situation in which the shape of the guided and free-space pencil and the diffraction geometry are general. A function that describes the so-called optical path-length defect in a general point of the FGC yields the spatial frequency components after differentiation. The wavefront diffracted by the FGC is analyzed at large values of the numerical aperture of the free-space wave by means of ray tracing. Spatial variation of the wave amplitude over the wavefront due to the progressive diffraction of the guided wave is not taken into account. Tolerances on the wavelength drift of the source, the waveguide properties, and the source position are given for an integrated optical disk pickup equipped with an FGC.

Integrated-optic focal-spot intensity modulator using electrooptic polymer waveguide

Abstract: A novel integrated-optic focal-spot intensity modulator comprising an electrooptic (EO) phase-distribution modulator and a focusing grating coupler (FGC) in an EO polymer planar waveguide on a Si substrate is proposed and demonstrated. The integrated phase-distribution modulator is made up of three-finger top electrodes and a planar bottom electrode sandwiching the waveguide. The electrodes are used for both the poling process and the modulation of the phase distribution of the guided wave. The FGC couples the guided wave to a beam focused on a point in free space. The intensity distribution of the focused spot is modulated by altering the wavefront of the guided wave. The bottom metal electrode is extended to the place under the FGC and serves as a reflection film so that the output coupling occurs only into the direction of the air for increasing the efficiency. The device was designed and fabricated with an active polymer p-NAn-PVA (p-nitroaniline n-polyvinyl alcohol). An extinction ratio of the focal-spot intensity was higher than 3 dB at a modulation voltage of 30 V for 3-mm-long electrodes. The frequency response was measured to be flat over 2 MHz. The EO effect induced by poling showed no significant relaxation over 300 h in an unconditioned room (20-30/spl deg/C). >

Integrated-optical acoustical sensors

Abstract: We have investigated the feasibility of a novel integrated-optical (IO) microphone. Sound-pressure variations acting on a small membrane directly modulate the phase of a guided wave in a (planar) optical waveguide by the IO nanomechanical effect. The phase modulation is transformed into an intensity modulation in an IO difference (or polarimetric) interferometer.

Lithium niobate guided-wave beam former for steering phased-array antennas.

Abstract: We present the theoretical investigation, design, and simulation of a novel guided-wave optical processor for L-band-transmission beam forming in a linear array of phased active antennas. The proposed configuration includes two contradirectional surface acoustic-wave transducers, and it is based on a Y-cut, X-propagating Ti:LiNbO3 planar waveguide supporting the lowest-order modes of both polarizations (TE0 and TM0) at the free-space wavelength λ = 0.85 μm. A detailed comparison between the processor we propose and other optical and electronic architectures reported in the literature is carried out, exhibiting a number of significant advantages in terms of weight, total chip size, and power consumption, when the number of antenna elements is greater than 50.

Surface wave leakage phenomena in coupled slot lines

Abstract: Surface wave leakage phenomena of coupled slot lines are investigated with spectral domain analysis. When the guided mode in the coupled slot lines changes from bound mode to leaky mode, there is a frequency range, called "spectral gap," where only nonspectral solutions exist. If the width of the center conductor or slots is increased beyond a certain value this spectral gap may disappear, resulting in a frequency range where both bound mode and leaky mode can exist simultaneously. >

Surface and guided-wave polariton modes of magnetoplasma films in the Voigt geometry

Abstract: We discuss retarded electromagnetic modes of a film containing one or two species of carriers in the presence of an in-plane magnetic field. It is assumed that the film is characterized by a gyrodielectric tensor containing cyclotron and plasma response. We deal with the Voigt geometry (propagation transverse to the field) for both a symmetric film and a film on a substrate. A detailed discussion is given of the guided-wave modes that occupy the bulk continuum region. For the symmetric film, dispersion curves of both surface-type and guided-wave modes are reciprocal, omega (-q)= omega (q), but field profiles are different for +q and -q. For the film on a substrate both the dispersion curves and the field profiles are non-reciprocal. Numerical illustrations are given for the parameters of CMT, Cd1-xHgxTe.

Compact microwave antenna suitable for printed-circuit fabrication

Abstract: A compact microwave antenna suitable of printed-circuit fabrication is comprised of a non-radiating guided wave structure for supporting the propagation of captured surface-guided waves; a series of dipoles in proximity to the guided waves supported by the non-radiating guided wave structure, wherein such dipoles are excited by the guided waves, thereby generating a desired radiation field, wherein the dipoles are independent of electrical transmission lines, and wherein the length and spacing of the dipoles are selected to achieve a desired angle of radiation from the surface of the non-radiating guided wave structure and a desired radiation frequency; and excitation means, in communication with the non-radiating guided wave structure, for exciting the reactive medium with electromagnetic waves to generate the captured surface-guided waves.

Leaky guided wave propagation along imperfectly bonded fibers in composite materials

Abstract: Leaky guided modes propagating along embedded fibers in a composite material can be used for characterizing the fiber-matrix interface. This principle can be applied to real composites containing small-diameter fibers by using laser interferometric detection of very fine lateral resolution on the order of a few microns. The main purpose of this paper is to develop the analytical tools needed to assess the sensitivity of guided wave inspection to interface properties in composite materials. Typically, the sound velocity is much lower in the matrix than in the fiber and the guided modes are strongly attenuated by leaking their energy into the matrix as they propagate. As a result, the velocity of the lowest-order axisymmetric longitudinal mode decreases while its attenuation increases with increasing interfacial stiffness between the fiber and the matrix. It is shown that loose fibers can be readily identified from early signals produced by fast guided modes. In the case of a well-bonded fiber-matrix interface, these guided modes are slowed down and strongly attenuated by the loading of the matrix depending on the fiber diameter and the interfacial stiffness of the interface. Interestingly, the relative difference between the well-bonded and free fibers is greater at low frequencies. Therefore, good sensitivity to the sought interfacial stiffness can be achieved at a few MHz, i.e., when the fiber diameter is still much smaller than the acoustic wavelength. Our analytical results show that the leaky guided mode technique is mainly sensitive to the transverse interfacial stiffness of the fiber-matrix interface. At typical ultrasonic frequencies between 1 and 20 MHz, the technique works best in the 1011–1013 N/m3 interfacial stiffness range which is one or two orders of magnitude lower than the optimal sensitivity range of the more conventional bulk velocity and reflection methods.

Guided-wave characterization techniques for the comparison of properties of different optical coatings

Abstract: The specific behavior of optical thin films very often leads to limitations of optical system performance. Accurate characterization techniques for evaluating film properties are necessary to understand this behavior. Characterization techniques based on the propagation of guided waves in the thickness of the films appear to be very useful. We report our particular way to determine the refractive index and the thickness of both isotropic and anisotropic thin films. Guided-waves techniques are sensitive enough to detect slight variations of thin film optical constants, so we use them to study the variations of refractive index versus temperature. From this we can obtain the thermorefractive coefficients ∂ n /∂ T of our layers. Moreover, we can obtain, in some cases, the nonlinear refractive index coefficient. We also measure guided-wave attenuation and laser damage threshold with a digital imaging system. These means, dependent on guided waves, are used in combination for a comparative analysis of TiO 2 and Ta 2 O 5 layers made by different eposition techniques (conventional evaporation, ion assisted deposition and ion plating).

Guided waves in a jointed composite plate

Abstract: Reflection of guided waves by a thin region of bonding material at the interface between two composite plates has been investigated in this paper. The study is motivated by the need to develop a quantitative ultrasonic technique to characterize properties of joints between two plates, which may be laminated and anisotropic. Although there have been some recent studies that have examined the reflection and transmission of surface waves at the interface between two plates, they consider the two plates to be semi‐infinite in thickness. Thus the mode conversion of waves into higher guided modes of the plates are not taken into account. In this paper, a model study of the interaction of the A0 and S0 guided wave modes with the joint material is presented. It is shown that as the frequency increases the coefficient of reflection shows resonant peaks at the cutoff frequencies of higher guided modes. These peaks become increasingly pronounced as the thickness of the joint increases. Another interesting feature is...

Volume diffraction effects in computer-generated guided-wave holography

Abstract: An integrated-optics equivalent of any one-dimensional computer-generated hologram can, in principle, be formed by a patterning a (scaled) cross-sectional slice of the surface relief profile on the waveguide. Because of the small effective-index modulation, the thickness of such a computer-generated waveguide hologram must be far greater than that of its free-space counterpart. Considerable volume diffraction effects are thus introduced. An analysis of such effects is carried out with the thin-grating-decomposition method, using Fourier-plane-grating multiple beam splitters as an example. It is shown that the index-modulation profile must be reoptimized to obtain a good reconstruction fidelity in the presence of volume effects, and methods of achieving this are introduced.

Low-loss fiber-chip coupling by InGaAsP/InP thick waveguides for guided-wave photonic integrated circuits

Abstract: A generic waveguiding structure is reported with low coupling loss (/spl les/1 dB/facet) with lensed fibers and low propagation loss (/spl les/1 dB/cm). This structure is suitable for low loss guided-wave photonic integrated circuits including active elements such as switches, and passive interconnections such as mirrors, and allows low polarization dependent operation. Which is required for instance in wavelength demultiplexers. This structure based on a single epitaxy is of interest for low cost photonic switching fabrics. >

Light beam splitting apparatus

Abstract: When a laser beam enters into a grating coupler at a right angle, a guided wave with a wave mode corresponding to an incident angle can be generated. Although the guided wave is a light beam in single mode, the waveguide mode is deflected by passing through a step-wise structure and is converted into several guided waves having several modes. When the guided waves are radiated from a straight output grating coupler, the radiant angle is dependent upon the waveguide mode, so beams are radiated in different directions according to their respective waveguide modes. Therefore, a guided wave in zero-order mode outputs a radiant beam having a larger radiant angle, while a guided wave in first-order mode outputs a radiant beam having a smaller radiant angle.

Analysis of nonlinear grating couplers by singular perturbation technique

Abstract: The properties of the nonlinear output and input grating couplers are analyzed by using the singular perturbation technique with the multiple space scales. We first introduce the perturbation parameter concerned with the nonlinear parameter and the grating depth. After the wave functions are expanded, the perturbation solutions to satisfy the equivalent boundary conditions are derived. From the solvability condition to have nontrivial solutions on each perturbation order, the nonlinear equations to describe the power leakage of the guided wave due to the second-order coupling to the first-order waves are obtained. The dependence of the radiated field and the input efficiency on the power are discussed numerically. >

Double-grating thin-film devices based on second-order Bragg interaction

Abstract: Four coupled‐mode equations describing the characteristics of the second‐order Bragg interaction are derived for the transverse electric mode of a dielectric film waveguide with periodic corrugations on both the film‐cover and the film‐substrate interfaces. The characteristics of the frequency‐selective filter, output coupler, input coupler, and distributed feedback laser are obtained. Proper choice of the design parameters leads to a frequency‐selective filter radiating neither in the cover nor in the substrate, a highly efficient output coupler radiating only in the cover, and an efficient input coupler exciting a guided wave only in one direction. Similarly, in the distributed feedback laser, a suitable choice of the parameters of the two gratings provides a mechanism for controlling the oscillation characteristics of the laser. The attractive feature of the double‐grating design is that the characteristics of the thin‐film devices are not very sensitive to small deviations of the grating parameters from their designed values.

Characterization of aircraft joints using ultrasonic guided waves and physically based feature extraction

Abstract: The guided wave technique is an emerging inspection procedure for the inspection of adhesively bonded lap splice joints and tear straps in aging aircraft. Due to inspection of joint locations with area scanning techniques, the evaluation of a joint region with a line scanning makes the guided wave technique a potential candidate for fast joint inspection and probably for further data evaluation for characterization of the defected imperfections. An approach to characterize or classify the inspection region is done by feature selection in order to acquire physically based information. Results are presented from lab specimens and from measurements on a Boeing 737-222 specimen manufactured in 1968

Non-axisymmetric guided waves in a composite cylinder with transversely isotropic core

Abstract: SUMMARY Ultrasonic guided waves in a composite cylinder have been studied by using a Rayleigh-Ritz-type finite-element representation. Although the method can be applied to general anisotropic cylinders, the analysis has been confined here to the case of a narrow transversely isotropic solid cylindrical core surrounded by a thick coaxial isotropic cylinder. Numerical results are presented for the dispersion of the first two modes when the circumferential wavenumber n = 1 and 3. It is found that, for the particular case of a graphite core in a magnesium cylinder, the two modal branches approach very closely (pinch) at the Rayleigh wave speed of magnesium. In the presence of a soft interface material, the pinch frequency is found to shift downwards. The effect of this interface material on the dispersion is also discussed.