Experimental and simulated ultrasonic characterization of complex damage in fused silica

Abstract

The growth of a laser-induced, surface damage site in a fused silica window was monitored by the ultrasonic pulse-echo technique. The laser damage was grown using 12-ns pulses of 1.053-m wavelength light at a fluence of 27 J/cm 2 . The ultrasonic data were acquired after each pulse of the laser beam for 19 pulses. In addition, optical images of the surface and subsurface damage shape were recorded after each pulse of the laser. The ultrasonic signal amplitude exhibited variations with the damage size, which were attributed to the subsurface morphology of the damage site. A mechanism for the observed ultrasonic data based on the interaction of the ultrasound with cracks radiating from the damage site was tested using twodimensional numerical simulations. The simulated results exhibit qualitatively similar characteristics to the experimental data and demonstrate the usefulness of numerical simulation as an aid for ultrasonic signal interpretation. The observed sensitivity to subsurface morphology makes the ultrasonic methodology a promising tool for monitoring laser damage in large aperture laser optics used in fusion energy research.

Horizontal Shear Wave Imaging of Large Optics

“…The full surface area of the vacuum side of each window must be inspected. Previous work used 5 MHz longitudinal waves to successfully detect and locate the damage, but suffered from mode conversions and multiple echoes from defects as well as inaccurate sizing [2]. Another approach utilized 1 MHz point-like sources and tomographic reconstruction algorithms, but showed poor resolution, poor signal-to-noise, and inaccurate sizing [3].…”

Ultrasonic monitoring of laser damage in fused silica

“…1,2 These systems rely upon optical components on the order of 1 m in size which must operate at peak intensities as high as 6ϫ10 9 W/cm 2 ͑ϳ20 J/cm 2 at 3 ns/pulse͒ at 1.053 m. 3 In addition, these systems contain a number of vacuum vessels along the laser beamline that may contain spatial filters that serve to remove high spatial frequency noise from the beam, or may contain the laser target at the end of the propagation path. 4 Laser damage on the vacuum side of these optics, which can be under significant tensile stress, constitutes a potential threat of catastrophic failure.…”

“…Ongoing efforts are being directed towards enhancing the capability to correlate acoustic data with defect shape by using cameras to monitor changes in the defect profile during growth. 9 In addition, computer simulations are being applied to modeling the effects of various pit/crack combinations on the acoustic signal. Correlation of the experimental data with simulated results may provide additional insight regarding the evolution of the laser damage morphology, and the application of the acoustic technique to reliably monitoring this class of damage.…”

Ultrasonic Pulse-Echo Subwavelength Defect Detection Mechanism: Experiment and Simulation

“…FDTD is good at simulating transient wave propagation and scattering phenomena. (11)(12)(13)(14) The PML absorbs the outgoing wave at the truncated computational domain so that a simulated free space can be achieved. (10,15) The study presented in this contribution aims at a better understanding of the fundamental ultrasonic pulse-echo subwavelength defect detection mechanism through both experimental and numerical modeling approaches.…”