Experimental comparison between optical spectroscopy and laser-ultrasound generation in polymer-matrix composites

Laser ultrasound is a technique based on lasers to generate and detect ultr asound. The generation mechanism involves several parameters among which on e of the most important is the optical penetration depth. This letter prese nts a comparison between the amplitude of ultrasonic waves generated by an optical parametric oscillator and optical penetration depth spectra measure d by photoacoustic spectroscopy in the 3.0 to 3.5 mum wavelength range for three different composite samples. The laser-ultrasound amplitude spectra c losely track the photoacoustic spectra. The results presented in this lette r experimentally demonstrate why the 3.0-3.5 mum wavelength range generates more efficiently generates ultrasonic waves in the ultrasonic frequency ra nge of interest for the inspection of polymer-matrix composites than the 10 .6 mum wavelength of the CO2 laser. (C) 2001 American Institute of Physics.