Laser Based Ultrasonics for Materials Characterization an Overview

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Abstract
Although the basic mechanisms by which optical beams might be used to generate and detect ultrasonic signals have been understood for several decades, it has been only in the last 15 years or so that the most concerted efforts have taken place to investigate and develop laser-ultrasonic methods for materials characterization. Owing to the noneontact and remote sensing capability of laser-ultrasonics, initial uses of these methods have been as direct replacements of contact transducers to facilitate measurement and inspection in hostile environments or under conditions where conventional coupling to the specimen surface is either undesirable or impossible. Technological advances have overcome some of the problems of limited sensitivity so that commercial laser-ultrasonics systems are being built and demonstrated for industrial application on such diverse materials as graphic/epoxy composites on one hand and hot steel extrusions on the other. As our understanding of the details of the physical mechanisms of laser-ultrasonics improves, the range of applications for materials characterization expands beyond simply that which might be performed with contact ultrasonic transducers. For example, the ability to focus laser beams to diameters below 10 microns has permitted studies of grain orientation and ectopic grain growth in single crystal or few-grained materials. The large sensitivity bandwidth (often over 100MHz) and short source duration has made it possible to measure dispersion relations in a range of materials and geometries. Added to the time and frequency characteristics of laser-ultrasonics is its ability to perform absolute surface displacemnent measurements thereby enabling one to perform couplant-independent attenuation measurements and gather high fidelity acoustic emission and acousto-ultrasonic information. Only recently demonstrated is the use of extremely short (picosecond) laser pulses to gather throughthickness ultrasonic data for characterization of the elastic properties of thin films. Still to come from laser-ultrasonics is further development of these unique measurement capabilties already demonstrated and perhaps exploitation in the future of the spectroscopic nature of the photo-acoustic source and mechanism.

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