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Thermal effects are critical constrains for developing high-powerthulium-doped fiber lasers (TDFLs). In this paper, we numerically investigatethe lasing and thermal characteristics of the TDFLs under different pumptransitions. Our results show, the widely-used pump transition$^3H_6\rightarrow^3H_4$, taking advantages of high-power high-efficiency laserdiodes at $\sim$0.8 $\mu$m, may not be a superior choice for directlyoutputting multi-kilowatt at 2 $\mu$m because of severe thermal problems.Meanwhile, using other pump transitions resulting 2-$\mu$m emissions,especially the in-band pump transition $^3H_6\rightarrow^3F_4$, will decreasethe generated heat to a large extent. By optimizing the power filling factor ofthe gain fiber, we find a 2-$\mu$m TDFL cladding-pumped at 1.9 $\mu$m will leadto the laser slope efficiency close to its quantum efficiency (95\%). Theinduced ultra-low quantum defect would be of great importance for powerscaling. We thus propose tandem-pumped TDFLs for reducing the heat at highpowers and discuss the related issues of design. Besides, we also explore thedifferences of the thermal characteristics between laser and superfluorescentoperations, which will contribute to deepening the understanding of the thermaleffects in high-power thulium-doped fiber amplifiers.

Influences of pump transitions on thermal effects of multi-kilowatt thulium-doped fiber lasers