Gas-coupled laser acoustic detection

Abstract

The investigation, development and application of a laser-based ultrasonic inspection system to the problems of evaluating polymer/graphite composite materials has been realized. The use of lasers to generate and detect ultrasonic waveforms in materials provides a means to detect material properties remotely. The study consisted of three main aspects: 1) A confocal Fabry-Perot (CFP) based system has been devolved which uses light reflected from the CFP interferometer to derive the ultrasonic signal. This allows higher frequency components of the detected waveforms to be discerned when compared to a CFP-based system using light transmitted through the CFP interferometer. 2) Thermoelastic and ablative laser generation of acoustic pulses in polymer/graphite composite materials has been investigated. Thermoelastic generation of ultrasound occurs when thermal energy deposited by a pulsed laser creates a localized expansion in the material. Ablative generation of ultrasound results from the creation of a plasma above the surface when the laser pulse surpasses an intensity threshold. 3) A novel technique, designated Gas-Coupled Laser Acoustic Detection (GCLAD), has been realized, in which the ultrasonic wave is detected optically after it has been transmitted from sample to air. This technique has the advantage of being independent of surface reflectivity and optical smoothness, and has comparable sensitivity to the CFP-based system.

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