Detection of Acoustic Second Harmonics Using a Laser Interferometer

Non-linear acoustic measurements can provide information on the microstructure or internal state of stress of materials and offer great potential for the nondestructive characterization of materials [1]. But non linear effects are difficult to measure, especially in industrial environments. For instance, changes in acoustic velocity with temperature or with applied stress are very small. The detection of acoustic harmonics [2], the interaction of multiple acoustic wavefronts [3], or of acoustic-radiation-induced static strain [4] requires sensitive transducers calibrated for absolute amplitude measurements to allow die determination of the third order elastic constants. Among these techniques, the detection of second harmonics offers potential for industrial applications because it requires no applied stress, no (slow) change in temperature and only one acoustic source.