Fig. 1. XRD specturm of TW$x$ glasses with different WO₃ content

Fig. 2. DSC curve of TW$x$ glass

Fig. 3. Raman spectrum of TW$x$ glasses with different WO₃ content

Fig. 4. Absorption spectrum of TWEY$y$ glass samples

Fig. 5. Integral absorption intensity at ⁴I_11/2 energy level of TWEY$y$ glass samples

Fig. 6. Absorption cross section of TWEY$y$ glass samples

Fig. 7. Fluorescence spectrum of TWEY$y$ glass samples (inset: relationship between integrated intensity and Yb₂O₃ concentration)

Fig. 8. (a) Upconversion spectrum and (b) Upconversion intensity of TWEY$y$ glass samples versus Yb₂O₃ concentration

Fig. 9. Variation of the ratio of upconversion and fluorescence, the ratio of integrated intensity of red and green emission peaks with the concentration of Yb₂O₃

Fig. 10. Energy transfer mechanism between Er³⁺/Yb³⁺ ions