Degradation of optical coatings induced by high-intensity, 515 nm, sub-picosecond burst-mode laser beam

We are presenting our observations regarding laser induced degradation of coated optics exposed to the burst-mode pulsetrain of the optical pump-probe laser systems (PP-laser) at the European X-ray Free Electron Laser (EuXFEL). Degradation happens on HR and AR coatings from various suppliers during the course of prolonged exposure to the 515 nm, 800 fs burst-mode radiation. Since the lasers typically operate 24/7, coatings degrade within a few weeks to several months, depending on the requested burst lengths and number of pulses and hence integrated burst energies. Reduced performance of pointing- and pulse energy-stability and a drop of average power of the non-collinear optical parametric amplifiers (NOPAs) result. As user facility, the EuXFEL strives to maximize up-time and to reliably provide high quality x-ray and optical laser beam lines for user experiments, which is compromised to some extent by the observed coating degradation. In this report we show our investigations into the effect so far. In collaboration with the coating manufacturer Layertec GmbH, an improved mirror coating was found, which prolongs optics life time by nearly a factor of 3.