Controllable structural tailoring for enhanced ∼2  µm emission in heavily Tm³⁺-doped germanate glasses

Heavily Tm³⁺-doped glass fibers are urgently desired for ∼2µm single-frequency fiber lasers and high-repetition-rate mode-locked fiber lasers. Here the structure of glass networks was tuned through controlling the numbers of non-bridging oxygens and bridging oxygens by adjusting the composition of the glasses, hence increasing the Tm³⁺ doping concentration of germanate glasses. The increased flexibility of the glass networks favors the distribution of Tm³⁺ ions to decrease fluorescence quenching, which was confirmed by the experimental and theoretical results. A heavily Tm³⁺ (9.8×10²⁰ions/cm³)-doped germanate glass was successfully fabricated without quenching by tuning the components of the glass. To the best of our knowledge, the Tm³⁺ ion doping concentration is the highest reported level in Tm³⁺-doped glasses and fibers. The results suggest that the heavily Tm³⁺-doped germanate glass is highly promising for fabricating ∼2µm glass fibers with high gain per unit length.