Local structure around Yb 3+ ions in Yb-P-doped silica glasses and its P concentration dependence were elucidated by EXAFS measurements. Yb-O coordination number was found to change from 4 → 8 → 6 → 7 ∼ 8 with increasing the atomic ratio P/Yb. This strange change can be corresponded to the local structure changes in the order of YbPO 4 -like, Yb(PO 3 ) 3 -like to YbP 5 O 14 -like local structures. This correspondence is supported by various optical measurements, such as the Rayleigh/Mie scattering, Raman scattering, absorption cross-section, CT transition, and X-ray induced defect formation, which were also found to depend on P/Yb. These results provide a guideline of the suitable P concentration in RE-P-doped silica glasses for various applications.

The local structure of Yb3+ ions in Yb-doped silica glass was investigated by extended X-ray absorption fine structure, the Rayleigh scattering and optical absorption measurements. It was found that the Yb coordination number increases from 3 to 4.5 with increasing Yb concentration. Simultaneously, the absorption cross-section of Yb3+ ion decreases and the Rayleigh scattering coefficient increases. These results suggest that local structure observed in Yb2O3 or Yb2Si2O7 crystals appears in high-Yb samples. Furthermore, photodarkening by X-ray irradiation was measured, and the correlation to the local structure change was clarified.

We present the first demonstration of visible laser oscillation in the Dy3+-doped silica fiber pumped by a 451nm InGaN laser diode. It was found that Ge-co-doping plays the following important roles of laser oscillation: (1) to reduce the Rayleigh scattering loss, (2) to suppress the X-ray-induced and pump-induced photodarkening (PD), and (3) to increase lasing slope efficiency. In a fiber with 0.46wt% Dy, 1.8 wt% Ge, and 0.54wt% Al, the slope efficiency is 22.0 % at 582.5 nm, and the maximum output power is 18.4 mW.

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X-ray induced photodarkening and its suppression by Ge-codoping were investigated in Al-doped, Dy-doped, and Dy-Al-doped silica glasses. Absorption bands of 4f-4f5d transition of Dy2+ were determined in Dy-Al-doped silica glass fabricated by Si nano-powders as a reducing agent, and it was found that pair generation of Dy2+ and Al-OHC was occurred in X-ray irradiated Dy-Al-doped silica glass. Ge-codoping suppresses the above-mentioned pair generation and instead forms the pair of GEC and GLPC+. Similar suppression effect of Ge-codoping was observed in Al-Ge-doped silica glass, where pair generation of Al-OHC and Al-E’ center was suppressed. To investigate the stability of defects, annihilation processes during thermal annealing was measured. Al-OHC and Dy2 + are destabilized by Ge-codoping in Al-doped and Dy-Al-doped silica glasses, while GEC is stabilized in Al-Ge-doped and Dy-Al-Ge-doped silica glasses. Based on these results, local structure models around Al, Ge, and Dy ions and possible mechanisms of photodarkening were proposed