Fig. 1. Absorption spectra of Yb³⁺/Tm³⁺ co-doped phosphate glasses

Fig. 2. Upconversion luminescence properties of Yb³⁺/Tm³⁺ co-doped phosphate glasses with different Tm³⁺ doping mole fractions. (a) Upconversion emission spectrum; (b) intensity of emission peak at 476 nm; (c) intensity of emission peak at 650 nm; (d) intensity of emission peak at 793 nm

Fig. 3. Upconversion luminescence of Te⁴⁺-doped Yb³⁺/Tm³⁺ co-doped phosphate glass. (a) Upconversion emission spectrum; (b) intensity of emission peak at 476 nm; (c) intensity of emission peak at 650 nm; (d) intensity of emission peak at 793 nm

Fig. 4. Schematic diagram of Yb³⁺ and Tm³⁺ energy levels and upconversion luminescence paths