Fig. 1. Characteristics of CO₂. (a) Three normal vibration modes of CO₂; (b) absorption line spectrum of CO₂ around 2 μm; (c) diagram of CO₂ molecular energy level transition of 4.3 μm emission pumped at 2 μm

Fig. 2. Experimental setup. Inset is cross-sectional scanning electron micrograph of experimental hollow-core fiber

Fig. 3. Diagram of thulium-doped fiber amplified tunable 2 μm laser diode

Fig. 4. Characteristic diagrams of 2 μm tunable fiber amplifier. (a) Seed laser wavelength varies with temperature control voltage V_Tec (V_bias is 1.2 V, and red star indicates tunable range); (b) thulium-doped fiber output power of 2 μm laser varies with 793 nm pump power; (c) output spectrum varies with 793 nm LD pump power at a center wavelength of 2000.6 nm (corresponding to the absorption line R(30)); (d) fine spectrum near the center wavelength of 2000.6 nm; (e)

Fig. 5. Measured output optical spectra corresponding to two absorption lines

Fig. 6. 4 μm laser output power and optical-optical conversion efficiency of the two absorption lines versus coupling power. (a) Measured 4 μm laser output power varies with coupled pump power under various CO₂ pressures at R(30) absorption line; (b) optical-optical conversion efficiency varies with coupled pump power under various CO₂ pressures at R(30) absorption line; (c) measured 4 μm laser output power varies with coupled pump power under various CO₂ pressures at R(

Fig. 7. Comparison of power characteristics of two absorption lines. (a) Maximum output powers of two absorption lines under different CO₂ pressures through 5 m hollow-core fiber; (b) measured 4 μm output power as a function of coupled pump power under 500 Pa CO₂ gas pressure at two absorption lines