Fig. 1. Comparison of laser wavebands generated by typical fiber gas laser and fiber laser^[1]

Fig. 2. Diagrams of energy level transition. (a) Diagram of energy level transition of C₂H₂ molecules pumped with P branch absorption lines; (b) diagram of energy level transition of CO₂ molecules pumped with R branch absorption lines

Fig. 3. Diagram of fiber gas laser based on population inversion

Fig. 4. 3 μm laser generation from acetylene-filled Kagome HCF pumped by OPO^[2]. (a) Experimental setup; (b) output 3 μm laser spectrum

Fig. 5. Single-pass configuration experiment using acetylene-filled anti-resonant HCF pumped by tunable diode laser ^[4]. (a) Diagram of experimental setup; (b) laser pulse energy varying with absorbed pump pulse energy at different pressure

Fig. 6. Ring-cavity configuration experiment using acetylene-filled anti-resonant HCF pumped by diode laser^[6].(a) Diagram of experimental setup; (b) output spectra for different pump wavelengths

Fig. 7. Single-pass configuration experiment of fiber acetylene gas CW laser output^[7]. (a) Diagram of experimental setup; (b) output laser power as a function of absorbed pump power at different pressure

Fig. 8. Experiment for measuring output beam quality of fiber acetylene gas laser^[8]. (a) Experimental setup; (b) M² value corresponding to different output pulse energy

Fig. 9. Experiment of OPO pumping CO₂-filled silver plating capillary^[1]. (a) Diagram of experimental setup; (b) output spectrum and energy level transition principle

Fig. 10. Experimental setup and results of CO₂ laser based on anti-resonant HCFs^[13]. (a) Diagram of experimental setup; (b) schematic diagram of energy level transition; (c) output spectrum; (d) output power at 4 μm varying with absorbed pump power

Fig. 11. Experiment of thulium-doped fiber amplifier pumping HBr-filled anti-resonant HCF^[14]. (a) Diagram of experimental setup; (b) output spectrum and energy level transition principle

Fig. 12. Experimental setup and output spectrum of CW light pumped I₂ vapor fiber gas laser^[5]. (a) Experimental setup;(b) output laser spectrum

Fig. 13. Experiments of 3 μm laser radiation from electrically excited He-Xe gas based on anti-resonant HCF^[41]. (a) Diagram of experimental setup and output signals for different fiber lengths; (b) output laser spectrum

Fig. 14. Coupling by inserting tapered solid-core fiber into HCF^[47]

Fig. 15. Coupling efficiency of tapered fiber and anti-resonant HCF varying with waist diameter^[48]. (a) Coupling efficiency of tapered fiber HI-1060 and ice-cream anti-resonant HCF at 1064 nm varying with waist diameter. Inset is output near field from HCF; (b) coupling efficiency of tapered SMF-28 and node-less anti-resonant HCF at 1568 nm varying with waist diameter

Table 1. Common gas media and related parameters in mid-infrared fiber gas laser

Table 2. Research progress of fiber gas lasers based on population inversion