Fiber optic detection of laser generated ultrasound for materials characterization

A fiber optic Michelson interferometer with an active homodyne demodulation system was used to detect ultrasound generated by a Q-switched Nd:YAG laser. The sensing arm of the interferometer was adhered to or embedded in the materials under investigation. Insonification occurred either at the surface or inside the test specimens (In situ generation was accomplished using an embedded fiber optic delivery system). The ultrasonic transients so generated and detected were used to determine the velocity of certain bulk wave modes propagating in specimens of aluminum, epoxy, and graphite/epoxy. The experimentally determined velocities are reported and compared to those in the literature. Ultrasonic velocity is related to or is an indicator of a number of material properties and hence is a very useful parameter for materials characterization.