Francis Haine

Simultaneous generation of longitudinal and shear bulk ultrasonic waves in solids

The ability to generate acoustic longitudinal and shear bulk waves in solid materials using a single transducer is a suitable feature required in several devices and systems involving ultrasonic waves. The problem addressed here is that of the, possibly wideband, multimode buffer delay line, usable, for example, in non-destructive evaluation applications. The solution considered for this purpose is to use rotated, Y-cut, single-layer or two-layer transducers. The theory pertinent to this kind of transducer is briefly recalled and the results of the numerical analysis using two different delay line materials and three different piezoelectric materials are reported in detail. One experimental realization is also described.

Acousto-optic low-frequency shifter.

Here a large deflection angle, low optical frequency-shift acousto-optic device is presented. This is realized by two successive acousto-optic interactions in the same cell. The relevant parameters of operation are analyzed in detail. A practical case with paratellurite material is then considered. Results from numerical computations leading to practical design parameters are given and compared with experimental ones.

Influence of the thickness and the attenuation coefficient of a backing on the response of transducers

Abstract In this paper, the characteristic mechanical impedance of an absorbing medium (backing) is expressed in terms of its thickness TB, and the tangent of the phase angle (tan δ = m, δ is the loss angle). In fact, the backing has been considered as a lossy transmission line. Then, using the model of Masons circuit, the electrical impedance and radiated power of an in-air backed transducer will be expressed in terms of tan δ and TB. It is shown that the characteristics of such transducers do not change if the thickness is scaled down and the product mTB stays constant. The same properties have been obtained for backed transducer bars using a finite element method (FEM) analysis.

Application of the angular spectrum approach to compute the far-field pressure

The angular spectrum decomposition of an acoustic source can be used to compute the radiation pressure in three dimensional radiation problems. Then, the aim of this paper is to demonstrate that the discrete far-field pressure can be computed from the two-dimensional (2D) discrete Fourier transform (DFT) of the normal velocity. In fact, using the asymptotic expression of the Rayleigh integral, it will be shown that the analytical far-field pressure is given by the 2D Fourier transform of the normal velocity. Using the discrete form of this expression, guidelines for selection of the sampling interval and the size of baffle in which the source is mounted will be given. The case of a rectangular source mounted in a rigid baffle and radiating into water will be considered.

On the design of multimode ultrasonic transducers for acoustooptic correlation applications

The acoustooptic correlation using interaction of light beams with two collinear ultrasonic waves with crossed polarizations is studied. The main characteristics of acoustooptic interaction in optically isotropic materials are first recalled and its application to acoustooptic correlation explained. The problem of simultaneous generation of bulk longitudinal and shear waves by a single transducer is then addressed. Theoretical results involving some optically isotropic materials are presented and an experimental realization is described.

Statistical study of an acousto-optic correlator in terms of its bandwidth

An acousto-optic correlator (AOC) associated with pseudorandom sequences (PRS), as used in spread spectrum receivers, is considered. Simple expressions are derived for the output signal-to-noise ratio of an AOC in two cases: when the frequency response H(f) of the correlator is equal to rect(f/?f) or equal to sinc(?f/?f), where rect(x) is the rectangle function, sinc(x) = sinx/x and ?f is the correlator bandwidth. These simple expressions are given in terms of the ratio a of the code bit rate Fc of the PRS to the bandwidth Af of the AOC. It will be shown that when H(f) = rect(f/?f) and a is greater than 2, a loss results in the output signal-to-noise ratio approximately equal to 1/? with respect to an ideal (infinite bandwidth) AOC. When H(f) = sinc(?f/?f) the loss equals 1.5/?.

Acousto-optic detection of Bleustein-Gulyaev surface waves propagating on cubic crystals

The interaction of Bleustein-Gulyaev (BG) waves with light propagating in the SAW sagittal plane is investigated theoretically for crystal cuts of gallium arsenide (GaAs) and bismuth germanium oxide (BGO). The diffraction of the transmitted beam is studied versus the angle between the lights wavevector and the normal to the SAW supporting plate (the depth direction along which the SAW attenuates). The experimental results obtained with a BGO crystal are then reported and compared with the above theory.