Laser generation of narrow-band surface waves

Abstract

Tone-burst-like narrow-band surface waves are generated in the thermoelastic regime by illuminating the surface of a solid with an array of laser-generated line sources. The laser line array is formed by a system of lenses and an optical diffraction grating that provide for flexible and easy control of the line-array parameters. It is shown experimentally, consistent with theory, that the generation of narrow-band surface waves can be controlled by adjusting the line-array parameters such as the number of line sources in the array, the width of each line source, and the separation distance between them. Certain optimum generation conditions are experimentally determined whereby the amplitude of the narrow-band surface waves can be increased by a factor of N (the effective number of lines in the array) over the corresponding broadband signals that would be generated using only a single line source. A laser-interferometric system is used to detect the generated surface waves. By suitably bandpass filtering the detected signal, the greater part of the white noise in the system can be eliminated, leading to a significant improvement in the signal-to-noise ratio and hence the sensitivity of the whole laser ultrasonic system.

Video Ultrasonics by Pulsed TV Holography: A New Capability for Non-Destructive Testing of Shell Structures

“…The same types of waves are generated for lower power densities (thermoelastic regime), although their amplitudes are much lower than in the ablation regime. Laser generation of SAW can be improved by selecting adequate spatial and temporal irradiance distributions on the part surface, enabling the possibility to generate narrowband SAW (Huang et al 1992;Yamanaka et al 1993;Kenderian et al 2003;Edwards et al 1992;Murray et al 1997). …”

Characterization of Laser Ultrasonic Sources Using a Sagnac Interferometer

“…These modes are realized by focusing a pulsed laser beam onto a specimen surface and forming a specific illumination area such as a focal spot or a line for broad-band generation, or an array of lines for narrow-band generation [2,3]. To provide flexibility of operation and to provide the ability of remote placement of equipment we have used high power optical fibers to deliver laser energy on to the testing area.…”

“…For generation of a narrow-band signal using a linear array of sources, the main frequency of the generated toneburst is determined by the following relationship [2]: (2) where d is the spacing of the lines of the array, and CR is the Rayleigh velocity in the material. For aluminum with a Rayleigh velocity of 2900 mis, a spacing of 0.58 mm would be needed for generation at 5 MHz.…”

Fiberized Sagnac Interferometer for Ultrasound Measurement

“…In earlier papers, the present authors have discussed an LBU system which employs a diffraction grating for illumination of a line-array to generate narrow-band surface waves and Lamb waves [4], and a fiberized heterodyne dual-probe laser interferometer to measure signals [3]. This paper reports progress toward the development of a robust low cost fiberized Sagnac laser interferometer suitable for field applications.…”

“…These include non-contact generation and detection, remote placement of equipment using fiber-optics, easy scanning, absolute displacement calibration, both broad band, and narrow band signal generation, wide frequency band measurements, and applicability to curved surfaces. Both laser generation of ultrasound and the subsequent detection of the ultrasonic waves using a laser interferometry are areas of active research [1][2][3][4][5][6].…”