Aspects of laser optics qualification for space applications

Abstract

As a consequence of the ongoing interest for deployment of laser systems into space, suitable optical components have to be developed and must be extensively space qualified to ensure reliable, continuous, and autonomous operation. The exposure to space environment can adversely affect the longevity of optics, mainly coatings, and lead to system degradation. An increased operational risk is due to the air-vacuum effect, which can strongly reduce the laser damage resistance of optical coatings. For this purpose, a vacuum laser damage test bench has been developed and is operated at the DLR laser labs. In extensive test campaigns, all damage-prone optics of the ALADIN laser system (being the laser source of the upcoming ESA ADM Aeolus mission) were tested under operative conditions at the fundamental and at the harmonic wavelengths of Nd:YAG. Further operational risks are due to operation under high vacuum. In the past, several space-based laser missions have suffered from anomalous performance loss or even failure after short operation times. This degradation is due to selective contamination of laser-exposed optical surfaces fed by outgassing constituents. These volatile components are omnipresent in vacuum vessels. Various organic and inorganic species were tested at our facilities for their criticality on deposit built-up. Finally, active optical components like Q-switch crystals or frequency converter crystals can also suffer from bulk absorption induced by high-energy radiation (gray tracking) and dehydration. To analyze these effects, an ultra-high vacuum phase matching unit was set up to test various combinations of SHG and THG frequency converters.