Ultrasound Probes Laser Damage

Kevin Robinson

Researchers at Lawrence Livermore National Laboratory are developing an ultrasound method to assess laser-induced damage to the vacuum optics in high-peak-fluence laser systems, such as fusion research facilities. The work also may help to prevent the shattering of optical windows under vacuum tension.

An ultrasound analysis technique may answer questions about laser-induced damage to optics in high-peak-fluence systems, such as in fusion research acilities. Courtesy of the University of Rochester.

Using a gel compound, the researchers mounted a piezoelectric transducer to the edge of an experimental window. They gradually damaged the sample with a 1053-nm neodymium-phosphate glass, zigzag slab regenerative amplifier laser system.

When excited with a voltage spike, the transducer, 280 mm from the damage site, sent an ultrasonic signal into the window. Part of the signal was reflected to the transducer, which converted it into an electrical signal for readout on a digital oscilloscope. As the damage progressed, the acoustic signal echoed more strongly at the damaged locations.

As they fired the laser, the researchers recorded changes in both the direction and intensity of the reflected acoustic energy. The optical damage mechanism seems to be related to the phenomena that affect the depth and subsurface shape of the damage site. Therefore, they suggest, the mechanism may be undetectable to optical diagnostics looking through the window normal to the defect diameter.

The research team, which published the results of its work in the May 28 issue of Applied Physics Letters, believes that the method holds potential for monitoring high-fluence laser damage and that it also may shed more light on how laser damage happens. The group hopes to correlate its acoustic data with optical images of the damage sites, with an eye toward creating better methods of monitoring laser damage.