Fig. 1. Partial energy level diagrams of Ho³⁺ and Pr³⁺ and corresponding transitions related to ~3 µm laser

Fig. 2. Absorption spectrum of Ho³⁺/Pr³⁺ co-doped AlF₃-based fiber

Fig. 3. Schematic of the experimental set-up

Fig. 4. Laser output power at λ≈2. 87 μm as a function of absorbed pump power at λ≈1.15 μm

Fig. 5. Temporal dependence of output power over a period of 45 min. The inset shows the experimental set-up