Laser qualification testing of space optics

Abstract

Laser optics being used in space laser systems are usually exposed to high vacuum conditions under the absence of air or oxygen. In the past, several space-based laser missions have suffered from anomalous performance loss or even failure after short operation times. To mitigate the risks involved in these long-term operational conditions, a laser damage test bench has been developed and is operated at the German Aerospace Establishment to test laser optics in the IR, VIS, and in the UV spectral range. The testing is performed under application oriented conditions, i.e. under high-vacuum using dry pump systems. The main goal of the test campaign is to identify the critical components in terms of their laser damage threshold for very high pulse numbers applied per site. Characteristic damage curves according to ISO 11 254 are evaluated for each component under investigation for up to 10 000 shots per site. The characteristic damage curves are used for the estimation of the performance at very high pulse numbers. The typical behavior found was a sharp drop in LIDT for small pulse numbers followed by a smooth decrease for larger pulse numbers (laser fatigue effect).