Optical properties of Yb3+ doped Lu3Al5O12 crystal fibers grown by μ-pulling down technique

Yb doped Lu3Al5O12 single crystal fibers has been grown with various Yb3+ ion concentrations: 2%, 5%, 10%, 15%, and 50% Yb doping levels. The diameter of the fibers is around 3 mm and their length is up to 110 mm. The fibers have been annealed in air at 1400 degrees C for 24 h to remove Yb2+ produced during the growth in the fibers. Structural characterization by x-ray diffraction method has been performed to confirm the monocrystalline phase and the orientation of the fibers. Absorption measurements have been performed on different pieces of the fibers to check the homogeneity of the distribution of Yb3+ along the fiber length. The results indicate that the distribution coefficient is close to 1. The fluorescence lifetime was measured for different Yb3+ concentrations. Results are in agreement with the literature and can be interpreted as a mixture of the radiation trapping and concentration quenching effects. The emission cross section has been calculated both from absorption spectra and from emission data. The two different independent methods give similar results, but emission data suffer from strong reabsorption effects on some of the lines. On the zero phonon line at high doping levels a central dip appears that can be interpreted with a reabsorption model.