Fig. 1. Characteristics of HBr. (a) Schematic diagram of energy level transition of HBr molecules depicting 4 μm laser; (b) absorption spectrum of H⁷⁹Br molecules at 2 μm band; (c) emission spectrum of H⁷⁹Br molecules at 4 μm band

Fig. 2. Tunable narrow linewidth 2 μm fiber pump source

Fig. 3. Output characteristics of the tunable 2 μm fiber pump source. (a) Tunable output spectrum; (b) fine spectrum at R(2) absorption line; (c) measured wavelength change with the normalized tuning voltage; (d) output laser power change with the power of 793 nm LD

Fig. 4. Characteristics of the pump laser linewidth. (a) Experimental setup for the measurement of the pump laser linewidth by F-P interferometer; (b) measured results of laser linewidth

Fig. 5. Experimental setup (inset is the electron micrograph of hollow fiber cross section)

Fig. 6. Absorption linewidth of HBr and the wavelength stability of pump source. (a) Measured HBr molecule absorption linewidth around R(2) absorption line at 1.8 mbar and 4.1 mbar; (b) residual pump power with respect to time when tuned at center of R(2) absorption line

Fig. 7. Output 4 μm spectrum(inset is energy transition diagram of HBr for P(4) and R(2) emissions when pumped at R(2) absorption line)

Fig. 8. Characteristics of 4 μm laser output power. (a) Measured output 4 μm power and (b) optic-to-optic efficiency varies with incident pump power at different pressures; (c) measured 4 μm power and (d) optic-to-optic efficiency varies with coupled pump power at different pressures; (e) absorbed pump power as a function of coupled pump power; (f) measured absorbed pump power and the maximum output 4 μm laser power as a function of HBr pressures

Table 1. Characteristics of rare-earth-doped CW fiber lasers with emission wavelength above 2 μm

Table 2. Absorption lines at 2 μm band and corresponding emission lines at 4 μm band of H⁷⁹Br