Laser-ultrasonic Optical Characterization of Nonmetals

There is now a growing interest in the use of laser-generated ultrasound for nondestructive 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 present in the first longitudinal arrival called "the precursor" depends on this three parameters [4,5 ]. A temporal broadening 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]. To evaluate this coefficient with a good reliability one needs to take into consideration the two other parameter effects (the extension spot size effect and the pulse duration effect). In this paper, first we present experimental and theoretical studies of the optical penetration on the acoustic waveforms. As the laser wavelength was fixed (1064 nm), we have used Schott glasses with identical thermomecanic properties and different optical absorption coefficients. Characteristic curves (in the I-d and the 2-d regimes) that relates the FWHM of the precursor to the optical absorption coefficient have been plotted. Llinear variation regions have been identified on these curves. We have shown that it is possible to deduce the optical absorption coefficient in these regions by fitting the curves. Optical absorption coefficients of the Schott glasses have been deduced using this approach. Results were in good agreement with data given by the manufacturer and data calculated using a theoretical approach [5].