Fig. 1. Schematic diagram of molten core method

Fig. 2. (a) Schematic diagram of twice drawing^[39]; (b)-(g) Optical micrograph of the cross-section for YAG, Yb:YAG, Nd:YAG, Er:YAG, Er/Yb:YAG, Tm:YAG-SiO₂ fiber

Fig. 3. Energy level structure of Nd³⁺

Fig. 4. (a) Schematic diagram of single-frequency laser; (b) Longitudinal mode characteristics by F-P interferometer^[47]

Fig. 5. (a) Schematic diagram of single-frequency laser; (b) Output power with respect to pump power; (c) Output spectrum^[49]

Fig. 6. Energy level structure of Yb^{3+ [54]}

Fig. 7. (a) Schematic diagram of single-frequency laser; (b) Output power with respect to pump power; (c) Output spectrum^[56]

Fig. 8. (a) Schematic diagram of experiment; (b) Internal structure; (c) Self-heterodyne signal with Lorentzian fitted linewidth; (d) Operation interface of Labview;(e) Prototype; (f) Output power stability^[57]

Fig. 9. (a) Schematic diagram of amplifier device; (b) Output power and backward optical power with respect to pump power; (c) Laser linewidth before and after amplification^[58]

Fig. 10. (a) Schematic diagram of single-frequency laser; (b) Output power with respect to pump power; (c) Self-heterodyne signal with Lorentzian fitted linewidth^[60]

Fig. 11. (a) Molten core method based on CO₂ laser-heated; (b) Output power with respect to pump power^[62]

Fig. 12. Energy level structure of Er^3+[54]

Fig. 13. (a) Schematic diagram of single-frequency laser; (b) Output power with respect to pump absorption power; (c) Longitudinal mode characteristics by F-P interferometer; (d) Typical trace of single pulse^[39]

Fig. 14. (a) Schematic of the co-melt in tube method; (b) Output spectrum; (c) Longitudinal mode characteristics by F-P interferometer^[54]

Fig. 15. Energy level structure of Tm³⁺

Fig. 16. (a) Schematic diagram of single-frequency laser; (b) Output spectrum; (c) Longitudinal mode characteristics by F-P inter-ferometer; (d) Output power with respect to pump power and pump absorption power