There is now a growing interest in the use of laser-generated ultrasound for nondesructive evaluation of materials [1,2]. In the case of nonmetals, the laser-ultrasonic displacement signals result from temporal convolution between the optical penetration, the laser pulse duration and the laser spot extension [3,4]. The temporal information prsent in the first longitudinal arrival called "the precursor" depends on this three parameters [4,5]. A temporal boradening of the precursor with increasing the optical penetration have been already observed experimentally [6]. It has been shown that a measurement of the full width at half maximum (FWHM) of the precursor allows an evaluation of the optical absorption coefficient of the material at the excitation wavelength [7].
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