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Proceedings Paper

Laser-based ultrasonics for QNDE applications
Author(s): Pavel A. Fomitchov; Lewis S. Wang; J. Scott Steckenrider; Sridhar Krishnaswamy; Jan D. Achenbach
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Paper Abstract

Surface and plate acoustic waves are commonly used to nondestructively investigate the near-surface region of a solid component for cracks and other defects. An attractive method of generation and detection of ultrasonic signals is laser based ultrasonics (LBU). Because it is non-contact, LBU can be implemented for inspection of limited access components using optical fibers, requiring only a small cross-sectional area for access. The work presented here employs optical fibers to remotely generate and detect ultrasound with energy focused into a selected narrow frequency band. The generation system uses a binary diffraction grating to separate the single laser beam into 10 equal but spatially separated laser beams which are focused into 10 individual fibers, thereby maximizing optical throughput to the component surface. In addition, a low noise fiberized Sagnac interferometer for ultrasonic signal measurement is discussed. The main advantage of this interferometer is improved signal-to-noise ratio, which has been achieved using an optical frequency shifting technique for biasing to quadrature and for elimination of parasitic interference between sampling beams and other unwanted beams in the interferometer. The Sagnac interferometer is truly path-matched, and as such is insensitive to low frequency thermal fluctuations and vibration noise. An acousto-optic modulator is included in the Sagnac loop, and serves a two-fold purpose: frequency shifting and quadrature biasing. Experimental results are presented for both the fiber-optic ultrasound generating array and the Sagnac interferometer.

Paper Details

Date Published: 20 March 1997
PDF: 11 pages
Proc. SPIE 2921, International Conference on Experimental Mechanics: Advances and Applications, (20 March 1997); doi: 10.1117/12.269810
Show Author Affiliations
Pavel A. Fomitchov, Northwestern Univ. (United States)
Lewis S. Wang, Northwestern Univ. (United States)
J. Scott Steckenrider, Northwestern Univ. (United States)
Sridhar Krishnaswamy, Northwestern Univ. (United States)
Jan D. Achenbach, Northwestern Univ. (United States)

Published in SPIE Proceedings Vol. 2921:
International Conference on Experimental Mechanics: Advances and Applications
Fook Siong Chau; C. T. Lim, Editor(s)

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