New laser-damage evaluation techniques boost testing capabilities

A defect-driven damage model, along with a raster-scan testing approach, expand the scope of laser-induced damage testing (LIDT). This technique was developed at Lawrence Livermore National Laboratory (Livermore, CA) in an effort to scale reduced test areas to damage characterization of large optics prior to the construction of NIF. n this technique, individual laser pulses are indexed in a serpentine raster pattern in which individual pulses are separated, in distance, by the 90% point of the Gaussian peak for a chosen focused spot diameter. This 90% separation is used in both thex and y direction of the raster scan. The scan is conducted over a predefined portion of the optic under test, and the onset of damage is observed either using a scatterometer making a real-time measurement or using inspection of the surface via microscope before and after irradiation. The advantage of this technique is that, essentially, when the footprints of the Gaussian beams are overlapped, the convolution of the overlapped beams provides a bumpy flat-top beam over the chosen scan area. The scan area can be any size up to the full aperture of the optic under test. If desired, the test can be designed, providing complete coverage of the clear aperture and therefore a 100% confidence level that all film defects have been interrogated. Specific variants of the test procedure can be performed, depending on the requirements for the test and the desired result. In applications where the test area is a large percentage of the clear aperture, the raster test is performed over the same area over and over. This provides a measurement known as a conditioned test, or R:1, where energy is ramped in the same area.