Analysis of the air-vacuum effect in dielectric coatings

As a consequence of the ongoing interest for deployment of laser systems into space, optical coatings have to be developed which allow for reliable long term operation under vacuum conditions. Extensive laser damage tests for space qualification of laser optics have recently been performed at the DLR and LZH laser damage test facilities in the IR, VIS, and UV spectral range within the ESA-ALADIN (Atmospheric Laser Doppler Instrument) test campaign. These tests have consistently revealed the degradation of the LIDT values for e-beam evaporated dielectric coatings under vacuum environment, which occurred independently of wavelength and type of coating (HR or AR) and other parameters. Dense coatings like IAD-based coatings, on the other hand, did not show this effect. Water desorption and diffusion processes seem to mediate the degradation under vacuum exposure.