Functional LIDT: influence of the LIDT testing process on the reflected beam properties

The evaluation of the LIDT of optical devices is critical for running high power laser facility safely and in a reliable way. The most accepted methodology to assess the fluence limit of an optical coating lies in the observation of the irradiated area and the evaluation of the presence of any permanent changes. The damage can be catastrophic, or i.e. just a faint discoloration of the coating without affecting the main function. However, especially for the latter case, the damage observation ex-situ by Differential Interference Contrast (DIC) microscopy relies mostly on the estimation from an operator which can introduce errors. In this contribution, we report on the LIDT evaluation of mirrors deposited by evaporation with our new ELIAS coating system across 1200 mm aperture and aluminum mirrors deposited by three magnetron sources (DCMS, RFMS and HiPIMS). The 1k-on-1 ultrafast LIDT process at 1064 nm was used to evaluate the presence of damage via scattered light probing and by observing the change of reflected beam power. These two approaches have been compared. We emphasize that the reflectivity of samples remains, in some cases, unchanged even if the damage is detected by ex-situ DIC microscopy. For example, protected gold mirror showed a 1k-on-1 LIDT of 680 mJ/cm2 while the drop of power of more than 2% from the reflected beam is reported at fluence ≥ 733 mJ/cm2. This discrepancy can be explained by the delamination of the protective layer. Moreover, we report that the discoloration observed from the oxidation of aluminum coating during LIDT test doesn’t substantially affect the power reflected by a mirror. For example, discoloration of aluminum mirrors has been observed for a fluence ≥ 260 mJ/cm2 while the drop of power of 2% of the reflected beam is observed at a fluence ≥ 288 mJ/cm2.