| 摘要 |
Optical interferometric apparatus for detecting and measuring discontinuities in structural materials from ultrasonic stress waves at their surfaces. The apparatus includes an acousto-optic (a/o) modulator which shifts a portion of a laser beam in frequency, producing a modulated beam of light. This modulated light beam is deflected and passes through an adjustable lens to the surface being measured. The light scattered by the surface is focused by the lens on an end mirror of the laser which produced the original beam, and is divergingly reflected therefrom. The reflected, scattered light is heterodyned with the unshifted portion of the laser beam in a photodiode to produce a phase modulated signal at the acoustic frequency of the a/o modulator. Surface displacement can be measured as a function of instantaneous phase shift, and surface velocity as a function of rate of change of phase shift.
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