A non-contact scanning system for laser ultrasonic defect imaging

A non-contact scanning system is described which is capable of producing two-dimensional ultrasonic images of defects in opaque materials. Lasers are used both for generation and detection of ultrasound. Short-duration high-power ( approximately 2 MW) laser pulses are used to generate ultrasound on the surface of a sample. Such ultrasound in the form of steps or pulses propagates through the sample and after interaction with defects are monitored by a 50 cm confocal Fabry-Perot interferometer. The interferometer uses back-scattered laser light from the sample's surface, which can be diffuse in nature. System sensitivity is sufficient for applications on optically rough surfaces. Two-dimensional scans have been performed, leading to computer-generated images of artificial defects in aluminium.