Origin of light scattering in ytterbium doped calcium fluoride transparent ceramic for high power lasers

The origin of light scattering defects was studied in transparent 6 at% Yb:CaF2 ceramics. Samples were synthesized by a soft chemistry route followed by sintering and hot pressing which leads to highly transparent ceramics with low scattering losses (0.016 cm−1 at 1200 nm). Light scattering defects were studied using scanning transmission electron microscopy (STEM) and high angle annular dark field-STEM (HAADF-STEM) techniques. Energy dispersive X-ray spectroscopy showed a 50% increase in Yb3+ concentration at grain boundaries. A 3–5 nm thick oxygen rich phase was detected at some grain boundaries by both HAADF-STEM and EDS. The origin of the oxygenized grain boundaries was traced to a 2–15 nm thick oxygenized shell present on the starting powders. Analysis of high resolution HAADF-STEM images revealed that Yb3+ substitutes into the fluorite lattice as clusters rather than individual ions, but the types of clusters could not be identified by this imaging technique.