Laser-induced molecular contamination testing

Kurzfassung

Laser technique is becoming increasingly important for space applications. Complex laser systems are used both for earth observation and for planetary exploration. For long-term operation optical components have to satisfy high requirements concerning precision and reliability. Before being used in space all optical components have to be tested extensively. For standardized determination of laser damage threshold ISO 21254 should be applied. For characterization of optics for space applications corresponding tests should be performed under vacuum conditions. But beside laser damage threshold a different topic should be noted: laser-induced molecular contamination (LIMC). LIMC denotes the interaction of laser radiation, especially in case of high fluences and short wavelengths with volatile molecules and the resulting formation of deposits on optical components. LIMC proved to be particular critical, if the laser system is operated under vacuum conditions and could considerably reduce the functionality of the whole laser system. It should be noted that LIMC is primarily induced by molecular and to a much lesser extent by particle contamination. Particle contamination and its impacts on performance are described in ISO 14644-9:2012. Molecular contamination is mainly caused by organic materials and silicones, e.g. glues, adhesives, insulating material, or circuit boards due to stronger outgassing rates compared to inorganic materials. The outgassing can be reduced but not totally prevented by selection of suitable materials and preconditioning e.g. bake-out at elevated temperature well above the planned operating temperature. The outgassing behavior of materials is generally characterized by the parameters CVCM (collected volatile condensable material), TML (total mass loss), RML (recovered mass loss), VCM (volatile condensable material) and WVR (water vapour regained). Definitions and corresponding measuring specifications for these quantities can be found in ECSS-Standard Q-ST-70-02C, ASTM-E595-07 and ASTM-E1559. This document outlines the test procedure for investigations of laser-induced molecular contamination in order to compare the growth of laser induced depositions on optical surfaces for different molecular contamination materials.