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 ultrasound. The generation mechanism involves several parameters among which one of the most important is the optical penetration depth. This letter presents a comparison between the amplitude of ultrasonic waves generated by an optical parametric oscillator and optical penetration depth spectra measured by photoacoustic spectroscopy in the 3.0 to 3.5 μm wavelength range for three different composite samples. The laser-ultrasound amplitude spectra closely track the photoacoustic spectra. The results presented in this letter experimentally demonstrate why the 3.0–3.5 μm wavelength range generates more efficiently generates ultrasonic waves in the ultrasonic frequency range of interest for the inspection of polymer-matrix composites than the 10.6 μm wavelength of the CO2 laser.