Donna C. Hurley

Optical transducer and method of use

An optical transducer, such as used in an ultrasound system, includes a signal laser which generates an optical signal the frequency of which varies in correspondence with acoustic energy incident on the transducer. An optical cavity in the signal laser is disposed such that incident acoustic energy causes compression and rarefaction of the optical cavity, and this displacement varies optical frequency generated by the laser. A laser pump coupled to the lasing medium is adapted to apply selected levels of excitation energy appropriate to the generation and detection of acoustic pulses. The signal laser alternatively is adapted such that the refractive index of the optical cavity is varied in correspondence with the incident acoustic energy to modulate the optical frequency of the light generated by the signal laser. A piezoelectric device is disposed to receive the incident acoustic energy and generate a corresponding electrical signal that is applied to an electro-optic cell in the optical cavity, or alternatively, to conductors to generate an electric field across the lasing medium.

Measurements of surface‐wave harmonic generation in nonpiezoelectric materials

Methods for exciting surface waves suitable for harmonic generation experiments in nonpiezoelectric materials have been investigated. Such methods are needed for improved characterization of engineered surfaces. Unlike conventional ultrasonic techniques, nonlinear experiments require the ability to produce finite‐amplitude, spectrally pure surface waves. Comb structures were selected from the methods evaluated, because they appear to offer the greatest potential for harmonic generation experiments on arbitrary substrates. The out‐of‐plane component of the surface waves was measured using a Michelson interferometer. The apparatus enabled direct measurement of absolute displacements with a spatial resolution of approximately 50 μm. Using this combination of excitation and detection techniques, the evolution of narrowband surface waves in the 1–10 MHz range was studied. Displacement amplitudes as large as 35 nm at a fundamental frequency of 10 MHz were observed. The spatial behavior of the fundamental and se...

Resonant scattering phenomenon by in‐plane periodic cracks

Two isotropic elastic halfspaces, with periodic domains of perfect bonding and with perfect disbonds (cracks) in between, are considered. In such a system, a free‐interface vibration may exist, the frequency of which depends on the crack period and width. This vibration occurs when a bulk wave is scattered by the cracks. There is a deep drop in the transmission coefficient of the wave through the interface, and a corresponding rise in the reflection coefficient, dependent on frequency and the propagation direction of the incident wave. Steel and aluminium halfspaces are considered in each of three combinations, with either perfect or sliding contact between cracks. Numerical results are obtained by extending an existing theory [E. Danicki, J. Acoust. Soc. Am. 100, 2942–2948 (1994)] to account for near‐normal incidence. This phenomenon is important for understanding the operation of comb transducers, which may be used for the ultrasonic investigation of materials. Analysis of the interface wave can aid in ...

Laser‐ultrasonic measurements of thin‐film properties using surface acoustic waves

High‐frequency, laser‐ultrasonic methods to evaluate the mechanical properties of thin films have been developed. The approach is based on the optical generation and detection of surface acoustic waves (SAWs) to determine the frequency dependence of the phase velocity (dispersion relation). Broadband or variable‐frequency, quasi‐plane wave SAWs are generated by a line‐focused, 200‐ps pulsed laser. The out‐of‐plane SAW displacement amplitudes are measured using a path‐stablized Michelson interferometer with line‐focus detection. The apparatus incorporates differential photodiode detection with a −3‐dB bandwidth of 40 kHz–800 MHz. With this system, several displacement waveforms over a range of propagation distances are acquired and used to determine the SAW dispersion. Dispersion relations up to 200 MHz are presented for a variety of specimens, including a series of Si wafers with TiN films 0.2–1.3 micrometers thick and containing residual stress. The results illustrate the applicability of this technique ...

Characterization of ultrasonic contact transducers using a Michelson interferometer

Accurate measurements of velocity, attenuation, and other ultrasonic properties of materials involve diffraction corrections that can significantly alter the measured values. Although diffraction calculations require knowledge of the transducer’s effective radius, this parameter is difficult to determine precisely for contact transducers. Here, an experimental method to quantitatively characterize longitudinal contact transducers is presented. Narrowband (toneburst) electronics excite the transducer, and a Michelson interferometer measures the out‐of‐plane displacements on the sample’s opposite side. With a resolution of approximately 10 μm, this optical technique allows transverse spatial scans of the displacement field to be obtained. Furthermore, the on‐axis position may be located with the interferometer and the transducer excitation frequency varied. The frequency dependence of the on‐axis amplitude is measured using a sample whose thickness is approximately half the nearfield distance at the transducer’s center frequency. The shape of the transverse scans and the observed minimum in the frequency dependence permit the transducer’s effective radius to be calculated. Measurements are presented for transducers in the 1‐ to 10‐MHz range with nominal radii between 2.5 and 6.4 mm. The results are compared to a numerical diffraction model to evaluate the accuracy and precision of this approach.

Eddy Current Arrays for Defect Detection

What was originally an inspection technique for large-scale defects has become an increasingly miniaturized method. Originally, the use of eddy currents for nondestructive evaluation was limited to gross defects in massive structures such as railroads and ship hulls; today, eddy current techniques are routinely used to detect sub-millimeter cracks. Unfortunately, the trend towards miniaturization creates a practical dilemma. To achieve greater resolution and sensitivity, eddy current probes must be made smaller. As probes become smaller, the amount of time needed to completely cover an inspection area escalates. As with other NDE modalities, one solution proposed to resolve the conflict between productivity and sensitivity is the use of eddy current arrays. An array of many elements could easily decrease the required inspection time by an order of magnitude without sacrificing the high-resolution capabilities of smaller probes. The advantages of array inspection are thus quite attractive, and interest is increasing. However, practical implementation of eddy current arrays requires careful attention to a number of details, including elementto-element uniformity, size versus sensitivity, and electrical interactions (crosstalk).