Laser-induced contamination on space optics

Kurzfassung

Laser-based systems like LIDAR (Light Detection and Ranging) are widely used in space for detailed earth atmosphere observations as well as planetary surveying. The suited LIDAR systems comprise high energy pulsed lasers in the IR, green or UV spectral range. In the past, several of these missions have suffered from anomalous performance loss or even failure after short operation times. This degradation was due to selective contamination of laser-exposed optical surfaces fed by the outgassing of organic materials and interaction of the intense laser light with volatile molecules and the surface of the optics. Deposits tend to built up favourably when the laser system is operating in the UV at 355 nm, and even ultrathin molecular layers on the nanometer thickness level reduce the performance and the lifetime of the optical components noticeably. In this paper comprehensive investigations of laser-induced deposit formation are reported. Epoxy, polyurethane and silicone based polymeric materials were used as contamination source. Onset and dynamic of deposit formation was monitored in-situ and online by UV induced fluorescence imaging. This method turned out to be very sensitive and allowed the detection of nanometer thick depositions. Online transmission measurements provided information about the influence of deposits on the optical performance. The growth mechanisms were found to be different for low and high fluence irradiations. Structure and composition of the deposits were analyzed by White Light Interferometry, Nomarski, Fluorescence, and Atomic Force Microscopy, respectively. Time of Flight Secondary Ion Mass Spectroscopy was used for chemical analysis. Influence of the optical surface roughness, coating, and ambient atmosphere on the deposit formation was investigated and mitigation methods are discussed.