Origin of Radiation-Induced Darkening in Yb3+/Al3+/P5+-Doped Silica Glasses: Effect of the P/Al Ratio.

Yb3+/Al3+/P5+-co-doped silica glasses with different P/Al ratios were prepared using the sol-gel method combined with high-temperature sintering. The evolution of composition-dependent color centers caused by X-ray irradiation in these glasses was correlated with their structural changes, which are controlled by the P/Al ratio. Nuclear magnetic resonance (NMR) and Raman spectra have been used to characterize the glass network structure, and advanced pulse electron paramagnetic resonance (EPR) has been employed to study the local coordination atomic structures of Yb3+ ions in pristine glasses as a function of the P/Al ratio. Si- (Si-E'), Al- (Al-E', Al-ODC, AlOHC), P- (P1, P2, POHC), and Yb-related (Yb2+) color centers in irradiated glasses have been observed and explained by optical absorption and continuous wave-EPR spectroscopies. The formation mechanisms of these centers, the structural models of glasses, and the relationship between them were proposed. Direct evidence confirms that the formation of Yb2+ ions induced by radiation is highly dependent on the coordination environment of Yb3+ ions in glasses. In addition, the glass network structure significantly affects the generation of oxygen hole color centers (AlOHCs/POHCs) caused by radiation. These results are useful in understanding the microstructural origin and the suppression mechanism of the radiodarkening effect by phosphorus co-doping in Yb3+-doped silica fibers.