Fig. 1. Small signal absorption spectrum and pump absorption at 830 nm of the BEDF. (a) Small signal absorption spectrum of the BEDF; (b) pump absorption coefficient varied with pump power of the BEDF at 830 nm

Fig. 2. Schematic for experimental measurement. (a) Schematic for backward fluorescence measurement of the BEDF under 830 nm laser pumping with various thermal treatments; (b) backward propagating on-off gain and excited state absorption measurement scheme of the BEDF under 830 nm laser pumping with various thermal treatments

Fig. 3. Effects of thermal treatment processes (cooling rate) and temperature on BEDF near-infrared fluorescence spectra. (a) Effect of cooling rate on fluorescence spectra; (b) effects of cooling temperature on fluorescence spectra (the inset shows peak intensity at 400‒600 ℃)

Fig. 4. Effect of quenching times and temperature on peak fluorescence intensity of BAC-Al. (a) Thermal quenching process for BAC-Al; (b) peak fluorescence intensity versus quenching times and temperature

Fig. 5. Effect of heating time and quenching times on peak fluorescence intensity of BAC-Al. (a) Effect of heating time on BAC-Al (the inset shows fluorescence spectra at τ_dwell=2 min); (b) effect of quenching times at different τ_dwell on BAC-Al

Fig. 6. Effect of heating time on fluorescence spectra shape of BEDF. (a) Comparison of fluorescence spectra; (b) fluorescence intensity of BACs-Al versus heating time

Fig. 7. Energy levels of BAC-Al

Fig. 8. Effect of heating time on small signal gain spectra of BEDF. (a) On-off gain spectra of BAC-Si and Er³⁺ with different τ_dwell (the left inset shows transmission spectra of signal with pump on and off at τ_dwell=20 min, and the right inset shows ESA spectra of BAC-Al); (b) on-off gain and ESA versus τ_dwell

Fig. 9. Fluorescence decay curves with heating time (the inset shows decay curve of fluorescence lifetime with heating time)

Fig. 10. Relationship between heat induced loss and heating time. (a) Variation in heat induced loss spectra with τ_dwell; (b) variation in α_t/α_uns with different τ_dwell; (c) radial profile of light intensity for different τ_dwell