Stray Light Issues

Stray light in gravitational wave interferometric detectors is a possible source of limitation for the low frequency performance of these instruments. It has to be “controlled,” i.e., the instrument has to be carefully designed and baffled in order to overcome this potential issue. We will see in this chapter that this problem has already been identified and taken into account in the design phase of the detectors in the 1980s. However, all the first- generation instruments had to deal with unexpected issues regarding stray light. Commissioners have set up systematic investigations to trace the various noise sources and fix optical setups. The new generation of advanced detectors will actually be even more sensitive to displacement noise due to stray light because of the increase of power stored in the cavities and the enhanced sensitivity to amplitude field fluctuation. We will review all kinds of mechanisms that can lead to “stray light noise,” we will see how to quantify it, and mitigate it. Despite all the precautions that have been taken, we will see that the detectors will actually be affected by this noise at least for a limited period of their data taking time.