Fig. 1. Characteristics of tunable 1958 nm pump system. (a) Structure of pump system with two stage amplification; (b) absorption spectrum of H⁷⁹Br molecule at 2 μm band and energy level transition process corresponding to R(5) absorption line pumping; (c) output wavelength varying with V_tec; (d) output 1958 nm laser power varying with pump power of main amplification stage; (e) pump spectra at different output pump powers

Fig. 2. Schematic diagram of experimental equipments, upper right illustration is scanning electron microscope picture of HCF cross section

Fig. 3. Characteristics of output spectra at different gas pressures. (a) Relationship between output spectra and pressure at incident pump power of 2.7 W; (b) relationship between output spectra and pressure at incident pump power of 7.8 W

Fig. 4. Characteristics of output power. (a) Output laser powers at different pressures when pumping H⁷⁹Br isotope, discrete points are measured data, and solid lines are corresponding fitted lines; (b) conversion efficiency varying with coupled pump power at different pressures when pumping H⁷⁹Br isotope, discrete points are measured data, and solid lines are corresponding fitted lines; (c) maximum output power, residual pump power and absorbed pump power varying with pressure when pumping H⁷⁹Br and H⁸¹Br isotopes; (d) output power varying with incident pump power and absorbed pump power when pumping H⁷⁹Br and H⁸¹Br isotopes at pressure of 550 Pa

Fig. 5. Mode field profiles measured by scanning method. (a) Experimental setup for measurement of mode field profile of 4 μm laser; (b) intensity distribution of three-dimensional mid-infrared laser spot; (d) intensity distribution of two-dimensional mid-infrared laser spot