Fig. 1. (a) Structure, (b) optical path and (c) refractive index distribution of the Yb³⁺ doped LMA TCF

Fig. 2. Output spectrum varies with (a) amplifier gain fiber length and (b) core diameter

Fig. 3. Variations in the (a) polymer clad temperature with backward pump power, temperature difference between core and polymer clad at a total pump power of 5 kW

Fig. 4. (a) Variations in the core and polymer clad temperature with backward pump power for different thermal conductivities; (b) Variations of core temperature with thermal conductivity

Fig. 5. Variation of power upper limit with external temperature for different 2^nd pump power ratios

Fig. 6. Schematic diagram of the home-made triple-cladding narrow linewidth fiber laser

Fig. 7. Comparison of the upper limit for the thermal effect power under different coupling methods

Fig. 8. Output power characteristics of the home-made high-power triple-clad narrow linewidth fiber laser

Fig. 9. Spectral characteristics of the home-made high-power triple-cladding narrow linewidth fiber laser