Measurement of ultrasound attenuation by laser ultrasonics

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Abstract
Laser ultrasonics is a technique which uses lasers to generate and detect ultrasound. Determination of the absolute ultrasonic attenuation is difficult since the diffraction corrections cannot be precisely calculated because of the poor knowledge of the laser beam’s intensity distributions. We show that there are two cases where this knowledge is not generally required, which allows precise determination of the attenuation. When the spherical wave approximation holds, i.e., for small laser spot sizes, thick samples, or low ultrasonic frequencies, the diffraction corrections can be precisely calculated. When the plane wave approximation holds, i.e., large laser spot sizes, thin samples, or high ultrasonic frequencies, the diffraction corrections are negligible. Both cases are illustrated with experimental data taken on ceramic and steel samples.