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.
Laser-induced damage threshold (LIDT) tests were performed at 1064 nm and 20 ns. Nodule defects were identified as the LIDT-limiting factor. The results suggest that the scale of the nodules is associated with the size of defects residing on the aluminum substrate surface. 3D finite-difference time-domain (FDTD) simulation was employed to calculate the electric field intensity (EFI) enhancement at the nodular defects with a seed diameter ranging from 0.35 μm to 2.5 μm. A direct linkage between the EFI enhancement and laser-induced damage morphology was established. Additional LIDT tests were conducted on surface modified aluminum substrate by using Corning aluminum process (CAP). The surface modification led to a 10x increase of the LIDT. Finally, LIDT of the multiband mirrors was predicted based on the absorption-driven damage and defect-driven damage. The results suggested that a combination of the CAP-modified Al6061 and low defect deposition process of the dielectric enhanced layers lead to high laser durability.
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