Laser generated ultrasound: efficiency and damage thresholds in carbon fibre reinforced composites

Laser techniques are beginning to be used for noncontact ultrasonic inspection of carbon fibre reinforced composites in industry. The ultrasound is normally generated with a high-power pulsed laser and is detected with a laser interferometer. Other non-contact detection methods can also be used, such as simpler laser beam deflection techniques and air coupled transducers. The authors compare the generation efficiency and damage thresholds of a fundamental Nd:YAG laser (1.06 μm), and a TEA CO₂ laser (10.6 μm) which is normally preferred for carbon-fibre reinforced composites). A Nd:YAG laser with an optical parametric oscillator (OPO) tunable up to 4 μm is also examined. The laser energy is absorbed within the optical absorption depth; the temperature rise is affected by the wavelength and laser pulse duration. It is essential to remain in the thermoelastic regime in order not to damage the material. A modified Michelson interferometer is used to detect the absolute displacement of the ultrasound. Optical beam deflection techniques and air-coupled transducers are also evaluated as detectors