Fig. 1. Energy levels and emission spectra of Er³⁺, Ho³⁺ and Dy³⁺ at 3-5 μm band ^[15-20]. (a) Mid-infrared emission spectra; (b) Corresponding energy level transition process and pumping excitation mode

Fig. 2. (a) Experimental setup of the Dy-doped ZrF₄ fiber laser pumped at 1.69 μm; (b) Tunable laser output spectrum^[42]

Fig. 3. Schematic diagram of the 985 nm and 1 973 nm dual-wavelength cascaded pumped Er³⁺ fluoride doped 3.5 μm fiber laser; (b) Tran-sition process of dual wavelength pumping^[30]

Fig. 4. (a) Energy level diagram of red light LD pumping doped Er³⁺/Dy³⁺: ZBLAN fiber; (b) Output power evolutions^[34]

Fig. 5. Schematic diagram of 3.92 μm fiber laser system at room temperature^[37]. (a) Experimental setup; (b) Energy level diagram; (c) Output power

Fig. 6. (a) Experimental setup of the Fe²⁺: ZnSe crystal-based Q-switched Er³⁺: ZBLAN fiber laser; (b) Average output power and wave-length tunability^[53]

Fig. 7. (a) Experimental setup of the gain-switched Dy³⁺-doped fiber laser at 3.24 µm; (b) Output power^[54]

Fig. 8. Dy³⁺: ZBLAN ring oscillator with NPR mode-locked^[58]. (a) Sche-matic diagram of the cavity and energy level of the ring fiber laser; (b) Pulse autocorrelation trace