Fig. 1. Energy level and mid-infrared emission spectra of Er^3+[18]. (a) Energy level and transitions involved in mid-infrared laser emission; (b) corresponding emission spectra in the wavelength regions of 2.8 μm and 3.5 μm

Fig. 2. Energy transfer upconversion process in heavily Er-doped fluoride fiber

Fig. 3. Schematic of 41.6 W 2.8 μm Er-doped fluoride fiber laser^[21]

Fig. 4. Energy level transitions involved in the 2.8 μm/1.6 μm cascaded emission

Fig. 5. High-efficiency Er-doped fluoride fiber laser based on 2.8 μm/1.6 μm cascaded emission^[40]. (a) Experimental setup; (b) 2.8 μm and 1.6 μm output power curves; (c) absorption spectrum of ⁴I_13/2→⁴I_9/2 transition

Fig. 6. Energy level transitions involved in 1.6‒1.7 μm pumping scheme

Fig. 7. 2.8 μm Er-doped fluoride fiber laser pumped at 1.7 μm^[41]. (a) Experimental setup; (b) 2.8 μm output power curve; (c) 2.8 μm laser spectrum

Fig. 8. Energy level transitions involved in 0.98 μm+2 μm dual-wavelength pumping scheme

Fig. 9. 3.5 μm all-fiber Er-doped fluoride fiber laser^[66]. (a) Experimental setup; (b) 3.5 μm output power curve

Fig. 10. Tunable Er-doped fluoride fiber laser^[71]. (a) Experimental setup; (b) fluorescence spectrum of Er³⁺ and tuning output power curve