Laser-Induced Deposition of Contaminants on Space Optics-A Chemical and Microstructural Analysis.

Abstract

The European Space Agency’s ADM-AEOLUS satellite , due to be launched into a 440km low earth orbit in 2008, will have the Atmospheric Laser Doppler Wind Instrument (ALADIN) as its sole payload. The aim of ALADIN is to accurately measure the Earth’s global wind profiles from space by measuring the Doppler shift from light backscattered from clouds and aerosols using a pulsed, high energy, frequency tripled (355nm), Nd:YAG space-based laser operating in vacuum. One key issue for the mission, is the deposition of laser-induced contaminants via the interaction of outgassing products from materials used in the construct of the laser; the optics within the instrument; and the laser itself, in the vacuum environment of space. In order to mitigate the risk of instrument performance degradation due to this mechanism an extensive programme of experiments has been undertaken in order to try and define a selection criterion for materials that can be used within the instrument and to understand the mechanisms behind the formation laser-induced contamination layers on optics. Various types of substrates and a variety of optical coatings have been subjected to laser irradiation under various conditions (e.g. in air, vacuum and an oxygen atmosphere), in the presence of outgassing from various material samples and partial pressures of aromatic contaminants, at various fluences and at different wavelengths. Visual observations have shown the presence of deposits in the laser impact area on a number of samples. These deposits also become strongly absorbing at the laser wavelengths and are accompanied by subsequent ablation when subjected to further pulsed irradiation. Various post-test analyses, including chemical depth profiling using Secondary Ion Mass Spectrometry (SIMS), Sputtered Neutral Mass Spectrometry (SNMS), X-ray Photoelectron Spectroscopy (XPS), Rutherford Backscattering Spectroscopy (RBS) and Transmission Electron Microscopy (TEM) have been used to examine the chemistry and the microstructure of virgin glass (non irradiated) and that of the laser irradiated areas. The results indicate that under certain conditions some of the optics are susceptible to extensive damage, which we believe is due to local melting of the glass in the area of the laser spot