Growth of laser-induced damage during repetitive illumination of HfO2-SiO2 multilayer mirror and polarizer coatings

As designers want to increase the peak fluence of high power laser, it becomes necessary to tolerate some damage on mirrors and polarizers. To quantify how the different types of damage morphologies initiate and grow during repetitive illumination, hafnia-silica multilayer mirror and polarizer coatings were laser damage tested. The coatings were prepared bye-beam evaporation and irradiated with a 3-ns-pulse at 1064 nm. The morphology of laser-induced damage was recoded after each shot to determine the types of damage that cause massive unstable failure and lower the optic's functional damage threshold. The results of the tests were summarized on damage stability maps plotting the average damage size as a function of the number of shots for fluences ranging from 10 to 40 J/cm(2). The maps indicate that the commonly observed damage morphologies (i.e. pits, flat bottom pits, scalds and outer layer delamination) have distinct growth behaviors and influence the value of the functional damage threshold differently. While pits are stable up to fluences as high as 40 J/cm(2). flat bottom pits can grow during repetitive illumination above a critical fluence of about 35 J/cm(2). Scalds are formed in the first shot and never grow at fluences below 40 J/cm(2). Finally, delaminates are highly unstable and have the potential for damaging the coating catastrophically above 15 J/cm(2). The results show that the delaminate damage morphology should be prevented. This knowledge has allowed coatings development efforts to focus on eliminating the origin of such damage morphology.