A laser preconditioning process for improving the laser damage threshold, and the search for subtle laser damage from long-duration laser exposure for IBS thin films

Laser damage testing of ion beam sputtered (IBS) films for 1064 nm, Q-switched, low repetition rate operation typically involves the measurement of small physical defects that can be detected visually or photographically. For this kind of damage the small absorber model adequately describes most of the observations. We discuss why the damage threshold for IBS anti-reflection (AR) coatings is lower than for high reflectors and polarizers. We report on a new process technique that eliminates the physical damage to IBS AR coatings for fluences up to 21 J/cm2, possibly higher. The success of this process and the lower damage for AR's indicates that the small absorbers are at the substrate/coating interface or are in the subsurface region of the substrate. We also discuss ongoing experiments to look for subtle forms of laser damage involving changes of the optical parameters of the thin film materials, under long-term exposure to Q-switched 1064 nm radiation and continuous 355 nm radiation.