G.W. Farnell

PC software for SAW propagation in anisotropic multilayers

A software package that provides an interactive and graphical environment for surface acoustic wave (SAW) and plate-mode propagation studies in arbitrarily oriented anisotropic and piezoelectric multilayers is described. The software, which runs on an IBM PC with math coprocessor, is based on a transfer-matrix formulation for calculating the characteristics of SAW propagation in multilayers that was originally written for a mainframe computer. The menu-driven software will calculate wave velocities and field variable variations with depth for any desired propagation direction: the graphics capability provides a simultaneous display of slowness or velocity and of SAW Delta v/v coupling constant curves, and their corresponding field profiles in either polar or Cartesian coordinates, for propagation in a selected plane or as a function of one of the Euler angles. The program generates a numerical data file containing the calculated velocities and field profile data. Examples illustrating the usefulness of the software in the study of various SAW and plate structures are presented.<<ETX>>

Leaky Modes in Weakly Guiding Fiber Acoustic Waveguides

The analysis of leaky modes in a fiber acoustic waveguide with small differences between the acoustic parameters (Vr-, Vs, p) of its core and infinitely thick cladding is presented. These modes become ideally guided if both the shear-velocity and density differences vanish. The propagation constant of leaky modes is complex and has a small imaginary part that accounts for the attenuation of acoustic fields and power along the fiber. Aside from the attenuation coefficient, the trans- mission properties of leaky modes such as phase and group velocities, cutoff frequencies, and power flow are essentially the same as those of ideally guided modes that are independent of shear wave velocities. Furthermore, all modes are predominantly longitudinal if the differ- ence between the longitudinal velocities of the core and cladding is small. Their phase velocities lie between the two longitudinal velocities and they may not be of torsional or Stoneley wave types. The atten- uation coefficient, however, depends strongly on the shear velocity dif- ference t = lVSz - V~ssll/Vs, and is proven to be of U(t2) (subscripts 1 and 2 refer to the core and cladding regions, respectively). Waveguide materials and aspects of coupling techniques for longitudinal modes are also discussed.

Analysis of Weakly Guiding Fiber Acoustic Waveguide

Abslmct-The analysis of a fiber acoustic waveguide with infinitely thick cladding is carried out under the conditions of weak guidance, which prescribe small differences between the shear moduli p and the densities p of the core and cladding materials pll /pl << I , [pz p , / / p , << I ) . Dispersion and field equations are derived. It is found that all guided modes are predominantly shear and are essentially independent of the longitudinal velocities. Exact and approximate dispersion characteristics for several lower order modes of flexural, radial-axial, and torsional types are compared. For relative shear modulus and density differences less than ten percent, exact and approximate results are in good agreement at all frequencies. Expressions for cutoffs, group velocity and power flow are derived based on simplified dispersion and field equations. The analysis of weakly guiding fihers further reveals that, under the ideal conditions p2 = p , and pz = p , , there exists another set of modes with real propagation constants and with phase velocities between the longitudinal velocities of the core and cladding. These modes, however, are leaky under the nonideal conditions of weak guidance.

Phase variation of focused surface acoustic wave

Converging and diverging surface acoustic waves (SAW) have been generated and detected using a noncontacting method. A yttrium aluminum garnet laser together with an axicon lens produces a focused optical ring which excites both converging and diverging SAW on the test sample. An optical interferometer is used to measure the vertical displacement of the SAW. Variations of the SAW pulse waveforms in different positions along the radial direction of the ring are given. It is found that about 10 SAW wavelengths away from the focus, a 90° phase difference exists between the Fourier components of the SAW going toward the focus and of that having passed through and moving away.

Interactive PC software for SAW propagation in anisotropic multilayers

A software package which provides an interactive, graphical, and computational environment for surface-acoustic wave (SAW) and plate-mode propagation studies in arbitrarily oriented anisotropic and piezoelectric multilayers is described. The software, which runs on an IBM PC with maths coprocessor, is based on a transfer-matrix formulation for calculating the characteristics of SAW propagation in multilayers, originally written for a mainframe computer. The menu-driven software will calculate wave velocities and field variable variations with depth for any desired propagation direction; the graphics capability provides a simultaneous display of slowness or velocity and of SAW Delta v/v coupling constant curves and their corresponding field profiles in either polar or Cartesian coordinates for propagation in a selected plant or as a function of one of the Euler angles. The program generates a numerical data file containing the calculated velocities and field profile data. Examples illustrating the usefulness of the software in studying various SAW structures are presented.<<ETX>>

Analysis of liquid‐core cylindrical acoustic waveguides

Transmission properties of guided modes in liquid‐core cylindrical acoustic waveguides are investigated. It is observed that for each value of the circumferential order n the first mode becomes of Stoneley‐type above a certain frequency. There are two modes without cutoff frequencies, in contrast to only one mode in solid‐core waveguides. They are the first modes with n=0 and 1. The application of liquid‐core waveguides in sensor devices is pointed out and some preliminary experimental results are reported.

Cutoff Conditions in an Acoustic Fiber with Infinitely Thick Cladding

Absfmcf-Exact cutoff conditions for all modes in a fiber acoustic waveguide with infinitely thick cladding are derived. The cutoff conditions for radial-axial and flexural modes with circumferential orders equal to and grealer than unity (n 2 I) are presented in compact determinant forms. Special attention is paid to the cutoff analysis of the first flexural mode F, ,. It is proved analytically that this mode always exhibits a zero cutoff frequency and is therefore an all-pass mode. The analysis presented here is sufficiently general to cover such rare cases as Stoneley waves. Cutoff frequencies for the first ten lower order modes in several fiber samples are also provided.

Fiber Acoustic Waveguide : A Sensor Candidate

Sensor development plays a key role in the field of nondestructive evaluation and process control. The annual fiber optic sensor market alone is a multimillion dollar business (1). Acoustic waves are about five orders of magnitude slower than optical waves and can also be guided in cladded glass fibers, similar to optical fibers, with low loss and low dispersion (2–7). Fiber acoustic waveguides are believed to be a very attractive and basic component for further sensor development (8). In this paper a brief theoretical description of a weakly guiding acoustic fiber (7) is given. The material selection criteria for the core and the cladding of the fiber guide, the properties of single-mode operation, and some sensing mechanisms for temperature and pressure variations are discussed. The acoustic waveguide with a liquid core is also considered.

Optical fiber sensor based on differential spectroscopic absorption

A material sensor based on differential spectroscopic absorption is proposed. The presence of a foreign material in a medium embodying the fiber sensor results in power attenuation at some particular wavelengths. This attenuation, which may be used for measuring the amount of the lossy material, is theoretically analyzed for the case of single-mode operation. A sensitivity analysis is carried out, and some design considerations are discussed.

Laser Generation of Annular Converging Surface Acoustic Waves

Converging surface acoustic waves have been generated and detected using a noncontacting technique. A YAG laser together with an axicon lens produces a focussed optical ring which excites both converging and diverging SAW on the test sample. An optical interferometer is used to measure the vertical displacement of the SAW. Variations in the surface acoustic waveforms due to different optical power, beam width and shape, as well as the anisotropic and dispersive properties of the samples are analyzed. The focussing effect and the resolution of the converging surface waves are illustrated. Applications of this technique, using a full or partial optical e xcitation ring, for characterization as well as nondestructive testing of materials are discussed.