Fig. 1. Typical structure of mid-infrared rare earth ion doped fiber laser

Fig. 2. DSC curves for AYF fiber undoped core and cladding composition^［51］, where T_g is glass transition temperature, T_x is crystallization temperature, and ΔT=T_x-T_g

Fig. 3. Transmission spectra of typical AlF₃-based glasses before immersion and after drying^[51]

Fig. 4. Raman spectra of AYF and ZBLAN glasses, and the dashes represent the multiple peaks' fitting results^[51]

Fig. 5. Laser test results^[51]. (a) Laser output power as a function of pump power; (b) laser spectrum at the highest pump power of ~20 W; (c) temporal dependence of the maximum output power at λ≈3 μm

Fig. 6. Laser output power at 2864 nm as a function of absorbed pump power at 1150 nm^[57]

Fig. 7. Laser spectra obtained at output powers of 5, 367,604, and 977 mW (inset: laser spectrum obtained at an output power of 977 mW)^[57]

Fig. 8. Spectra of ZBYA glass samples^[31]. (a) Raman spectrum of undoped ZBYA glass sample; (b) transmission spectrum of 2.0 mm thick ZBYA glass

Fig. 9. DSC curves of ZBYA and ZBLAN samples and ZBYA fiber loss^[31]. (a) DSC curves of ZBYA and ZBLAN glass samples and characteristic temperatures; (b) ZBYA fiber loss at 1570 nm

Fig. 10. Results of ZBYA glass immersion experiment^[31]. Transmission spectra of (a) ZBLAN and (b) ZBYA glasses after leaching in deionized for different time; (c) spectral comparison of ZBLAN and ZBYA glass samples after 24 h leaching in deionized; (d) loss of weight comparison of ZBLAN and ZBYA glass samples after leaching in deionized water

Fig. 11. Laser spectra (both the fiber lengths are 1.8 m)^[31]. (a) 1.2 μm optical spectra for various pump powers; (b) 2.9 μm laser spectra of 0.5% (mole fraction) Ho³⁺-doped ZBYA fiber

Fig. 12. Basic physicochemical properties of fluoroindate glass^[55]. (a) Raman spectra of fluoroindate glass and ZBLAN glass; (b) differential scanning calorimetry curves in the temperature range of 100‒600 ℃; (c) refractive index curves of cladding and core glasses; (d) transmission spectra of core glass before and after immersing in deionized water

Fig. 13. Linear regression calculation results of sample glass damage threshold, and the inset shows laser-induced damage structure arrays under different laser powers on the fluoroindate sample^[55] (D—diameter of the damaged crater; E—incident pulse energy)

Fig. 14. 2.7 μm of Er-doped fluoroindate glass luminescence characteristics^[55]. (a) Absorption cross section of the ⁴I_15/2 →⁴I_11/2 transition; (b) emission spectra of glasses with different Er³⁺concentrations obtained within the wavelength region of 2400‒3000 nm; (c) emission cross-section of the ⁴I_11/2→⁴I_13/2 transition; (d) luminescence decay curves of the ⁴I_13/2 energy level

Fig. 15. Laser spectra^[55]. (a) Output power as a function of the launched pump power, the inset shows laser spectrum centered at 2.7 μm of Er³⁺-doped fluoroindate glass fiber for a pump power of 1.5 W; (b) dependence of 2.7 μm laser slope efficiency on the fiber length of Er³⁺-doped fluoroindate glass fiber

Fig. 16. Laser output power as a function of launched pump power in the simulation and experiment^[87]

Fig. 17. Energy level diagram of Ho^3+[87] (GSA—ground state absorption; ESA—excited state absorption; ETU—energy transfer up-conversion; CR—cross relax-ation, SE—stimulated emission; NR—nonradiative relaxation)

Table 1. Maximum phonon energies and infrared cut-off wavelengths of several main glass materials

Table 2. Research progress of mid-infrared continuous-wave fiber laser using commercial fluoride glass fibers

Table 3. Thermal and mechanical properties of typical fluoride glass materials