VELOCITY AND ATTENUATION COEFFICIENT MEASUREMENT TECHNIQUES USING THE INTERFEROMETRIC MODE OF A 100 MHz SCANNING LASER ACOUSTIC MICROSCOPE.

An interferogram is produced when the Scanning Laser Acoustic Microscope is operating in its interferometric mode. The interferogram contains the phase and amplitude information of the wavefield, which has penetrated through the specimen of known thickness. The interferogram is visualized on a monitor as equal phase wavefronts and digitized with an A/D converter. The individual raster lines of the interferogram appear as sinusoids. If there has been a localized change in the specimen velocity the phase of the adjacent lines would have been shifted relative to each other in that area. The amplitude of the sinusoids along the specimen is an indication of the acoustic pressure from which the relative changes can be evaluated for the localized attenuation coefficient. The bending of the equal phase wavefronts carries the information of the relative ultrasonic velocity variations along the specimen. When the specimen is immersed into saline, in which the ultrasonic velocity is known, these relative values can be transformed into absolute ones based on Snell's Law. The attenuation coefficient can also be determined from the same interferogram data.