Fluorescence mapping of oxidation states of Cr ions in YAG crystal fibers

We have demonstrated the use of laser scanning confocal microscope for mapping the refractive index as well as Cr3+ and Cr4+ fluorescence in Cr:YAG crystal fibers with 1 μm spatial resolution. The corresponding sensitivities for Cr3+ and Cr4+ fluorescence intensities are 1.6×1017/cm3 and 7.5×1015/cm3, respectively. The share of Cr4+ ions in the total Cr concentration is found to be proportional to the ratio of Ca to Cr concentrations. However, only less than 1% of the Ca2+ ions become active in charge compensation. The concentration of Cr4+ ions is higher near the center of the crystal fiber when the core diameter is less than 300 μm. In addition, stress-induced striations created near the fiber center disappear when the fiber diameter is smaller than 500 μm.