Damage tests for 355 nm laser mirror by using a new, variable spot size optical system (VariSpot®)

We report preliminary measurements on the catastrophic damage threshold and also on the maximum fluence/irradiance for non-damage of a 355 nm high reflectance mirror, by using a new optical arrangement. Our goal was two-fold: to determine the ability of a new, variable spot size (zoom) optical system (named VariSpoe) to be used in laser-induced damage measurements, and also to characterize the damage properties of the mirror under test. The classical measurement scheme maintains a constant spot size on the target by using a fixed-focus optical system, and the laser fluence/irradiance on the target is varied by appropriately attenuating the laser beam before the focusing optics. In contrast, our scheme maintains constant the energy/power of the incoming beam before the focusing optics and uses a zoom-type optical system to continuously change the spot size and correspondingly the fluence/irradiance on the target, while keeping constant the working distance (from optics to target). A 355 nm beam representing the third harmonic of an electro-optically Q-switched, unstable cavity Nd:YAG laser was used as incoming beam. Its spatial diagnostics was done by using a CMOS-type beam profiler and following the definitions and recommendations of the ISO 11146 standard. The VariSpot optical system represents a prototype with a working distance of approximately 100 mm and delivering a round and adjustable spot size with diameters from 0.085 mm to 4.3 mm at that distance for the specific 355 nm incoming laser beam. The results show the advantages of using the VariSpot system for such type of measurements and also reveal reasonable good damage properties of the tested mirror. We briefly discuss the major sources of errors in the obtained results and also suggest several ways to improve the future measurements using similar arrangements.