Functional damage thresholds of hafnia/silica coating designs for the NIF laser

Studies into the functional damage thresholds of hafnia/silica thin film coatings for the NIF laser have been conducted on two different-sized substrates: 50-mm-diam test substrates and full-sized (412 x 412 mm) NIF mirror substrates. For both studies, the optics were raster scanned by Q-switched Nd:YAG lasers emitting 1064-nm light with 10- ns pulse lengths. The coatings tested were primarily high reflectors, although polarizing beam splitter and anti- reflective thin films were tested on the small substrates. Tests were performed to find the functional damage threshold: the minimum fluence at which a damaged optic degrades the performance of the NIF laser, or, experimentally, the minimum fluence at which a damaged site begins to grow. Thus, the concern is with finding not only the fluence that caused a pit (for example) but also the fluence at which that pit begins to grow with subsequent laser shots. After a site begins a growth phase, the growth rate is measured as a function of fluence. This provides some information for predicting the optic lifetime for given operating limits of the laser.