Spectroscopic properties of Tm3+/Al3+ co-doped sol–gel silica glass

•The high thulium doped silica glass was realized.•The thulium doped silica glass has a large stimulated emission cross section.•Samples have a long fluorescence lifetime and a low hydroxyl content.•The crossrelaxation was used to explain the optimum thulium doping concentration.

Tm3+/Al3+ co-doped silica glass was prepared by sol–gel method combined with high temperature sintering. Glasses with compositions of xTm2O3–15xAl2O3–(100 − 16x) SiO2 (in mol%, x = 0.1, 0.3, 0.5, 0.8 and 1.0) were prepared. The high thulium doped silica glass was realized. Their spectroscopic parameters were calculated and analyzed by Judd–Ofelt theory. Large absorption cross section (4.65 × 10−21 cm2 at 1668 nm) and stimulated emission cross section (6.00 × 10−21 cm2 at 1812 nm), as well as low hydroxyl content (0.180 cm−1), long fluorescence lifetime (834 μs at 1800 nm), large σem × τrad (30.05 × 10−21 cm2 ms) and large relative intensity ratio of the 1.8 μm (3F4 → 3H6) to 1.46 (3H4 → 3F4) emissions (90.33) are achieved in this Tm3+/Al3+ co-doped silica glasses. According to emission characteristics, the optimum thulium doping concentration is around 0.8 mol%. The cross relaxation (CR) between ground and excited states of Tm3+ ions was used to explain the optimum thulium doping concentration. These results suggest that the sol–gel method is an effective way to prepare Tm3+ doped silica glass with high Tm3+ doping and prospective spectroscopic properties.