Insights into the photodarkening mechanism of Tb3+/Al3+ co-doped silica glasses

•The photodarkening mechanism of Tb3+/Al3+ co-doped silica glasses was revealed.•Influence of Al3+ on optical properties of Tb3+ in silica glasses was studied.•X-ray radiation induced defects in Tb3+/Al3+ co-doped silica glasses were studied.•Valence changes behavior of Tb3+ ions and defects in silica glasses were revealed. In this work, Tb3+/Al3+ co-doped and Al3+ single doped silica glasses are taken as the research objects, and X-ray is used as the simulated light source to study the photodarkening mechanism. Detailed optical properties of the glasses before and after X-ray irradiation and the results of electron paramagnetic resonance (EPR) indicate that the photodarkening effect is attributed to the valence changes of Tb3+ ions and the energy transfer between Tb3+ ions and related defects. In addition, this work revealed the relationship between valence changes behavior of Tb3+ ions and various defects in glass matrix materials, deepening the understanding of the photodarkening mechanism in Tb3+ doped oxide glass. We envisage that our finding provides a depth-theoretical support for the synthesis of Tb3+-doped oxide matrix glass fibers with excellent optical and anti-photodarkening performance.