Precise Measurement in Laser Ultrasonics by Phase Velocity Scanning of Interference Fringes

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Abstract
In laser ultrasonics, the characteristics of acoustic waves, such as mode selectivity and directivity, are not as good as in piezoelectric techniques, and this is a problem in achieving high measurement precision. Thus, we proposed the phase velocity scanning method for selective generation of surface acoustic waves (SAW), and bulk acoustic waves (BAW). For SAW generation, a thermoelastic source of interference fringes is scanned at the phase velocity of the SAW. For SAW velocity measurement, the ratio of the measured SAW frequency and the wave number of the scanning interference fringes is employed. The velocity measurement is very precise (better than 0.1%) because of the amplitude enhancement of desired SAW and suppression of spurious SAW and BAW. For BAW generation, the interference fringes are scanned at considerably higher velocity, and excellent directivity of longitudinal and shear waves has been realized for the first time.