LASER GENERATION OF NARROW-BAND SURFACE-WAVES

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.