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

TL;DR

This study numerically investigates how different pump transitions affect thermal management in high-power thulium-doped fiber lasers, revealing that in-band pumping reduces heat and enhances efficiency, with implications for power scaling.

Contribution

It introduces a detailed numerical analysis of pump transition effects on thermal behavior in TDFLs and proposes tandem-pumping strategies for improved high-power performance.

Findings

In-band pump transition $^3H_6\rightarrow^3F_4$ significantly reduces heat generation.

Optimized cladding-pumped TDFL achieves near-quantum efficiency (95%).

Tandem-pumping effectively lowers thermal issues at high powers.

Abstract

Thermal effects are critical constrains for developing high-power thulium-doped fiber lasers (TDFLs). In this paper, we numerically investigate the lasing and thermal characteristics of the TDFLs under different pump transitions. Our results show, the widely-used pump transition $^3H_6\rightarrow^3H_4$, taking advantages of high-power high-efficiency laser diodes at $\sim$0.8 $\mu$m, may not be a superior choice for directly outputting 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 decrease the generated heat to a large extent. By optimizing the power filling factor of the gain fiber, we find a 2-$\mu$m TDFL cladding-pumped at 1.9 $\mu$m will lead to the laser slope efficiency close to its quantum efficiency (95\%). The induced ultra-low quantum defect would be of great importance for power scaling. We thus propose tandem-pumped TDFLs for reducing the heat at high powers and discuss the related issues of design. Besides, we also explore the differences of the thermal characteristics between laser and superfluorescent operations, which will contribute to deepening the understanding of the thermal effects in high-power thulium-doped fiber amplifiers.