Analysis of the air – vacuum effect in dielectric coatings

Abstract

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 LZH and DLR test facilities in the IR, VIS, and UV spectral range within the ESA-ALADIN (Atmospheric Laser Doppler Instrument) test campaign. These tests have revealed consistently the degradation of the LIDT values for e-beam evaporated dielectric coatings under vacuum environment, which occurred independently of wavelength and type of coating. 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. In this paper we perform an analysis of the air–vacuum effect for different types of coatings (highly reflective, anti-reflective, and partial reflective) at different wavelengths (355 nm, 1064 nm), being manufactured at the LZH coating facility. In the analysis, various effects like thermal conductivity or mechanical stress are considered. Stress is induced mechanically onto coatings and its influence on the LIDT values is measured. Spectral vacuum shift measurements were performed to identify the involved timescales for water adsorption and desorption processes. High-resolution microscopy is used to monitor the surface properties and porosity of the coatings under investigation.