Laser-induced damage of CaF2 optics at 193 nm

Laser-induced damage (LID) tests were conducted on CaF₂ optics at 193 nm using ISO S-on-1 method with the S varying from a standard 200 to 10³, 10⁴, and 10⁵ shots/site and fluences ranging from 0.1 J/cm² to 4.0 J/cm². Using a flat-top beam profile and a beam footprint of 250 μm × 250 μm, absorption-derived LID was observed on the standard 200-on-1 test. Defect-initiated LID was detected by increasing the pulse count with a reduced fluence. The absorption-driven LID was attribute to subsurface damage and two-photon absorption. The former was eliminated by using a FemtoFinish polishing process. The latter was experimentally determined by using laser calorimetric measurement. Improved crystal bulk and surface finishing quality were confirmed by X-ray diffraction and laser calorimetric measurement. Accelerated lifetime damage test (ALDT) was further conducted with an increased pulse count up to 10⁶ shots/site. The results confirm an enhanced lifespan prediction of the demanding laser optics.