Laser-Based Ultrasound (LBU) systems are now entering their maturity years by penetrating the factory in both the areas of non-destructive testing and process control. A LBU system can be used for the on-line characterization of a paper web in a paper mill. Compared to contact techniques, LBU is able to generate and detect on a paper web both symmetric and antisymmetric waves with a non-contact tool which is the spot of the generation laser. This provides all at once a rich amount of data about the stiffness properties of the sheet. To demonstrate this concept we made some experiments on static paper first, our ultimate goal being to monitor the stiffness properties of a paper web, on-line, at industrial speeds. A photorefractive interferometer using the two-wave mixing method with a continuous electric field applied to a BSO crystal has been built for ultrasound detection. Results are presented on different paper grades, using a Nd:YAG laser for generation. Both A₀ (anti- symmetric) and S₀ (symmetric) modes of Lamb waves have been detected with acceptable signal to noise ratio in single laser shot. The dispersive nature of A₀ wave is clearly visible as well as the higher frequency content of S₀ wave.

Date Published: 4 February 1999
PDF: 12 pages
Proc. SPIE 3589, Process Control and Sensors for Manufacturing II, (4 February 1999); doi: 10.1117/12.339967

Published in SPIE Proceedings Vol. 3589:
Process Control and Sensors for Manufacturing II
David M. Pepper, Editor(s)