In this article, we demonstrate great potential of lightly-erbium(Er ³⁺ )-doped ZrF ₄ fiber in efficient power scaling at ∼2.8 μm, when excited at ∼1.7 μm. Core-pumped by a random Raman fiber laser at 1.69 μm, the 1.5 mol.% Er ³⁺ -doped ZrF ₄ fiber in a F-P cavity has experimentally yielded an output power of >6 W at 2.783 μm with a slope efficiency of up to 58.4% (with respect to the absorbed pump), close to its Stokes limit of 60.7% (1.69/2.783). To our knowledge, this is the highest efficiency of Er ³⁺ -doped ZrF ₄ fiber lasers in the mid-infrared. Further power scaling is only limited by the current pump power. Based on the validated numerical model, the dominant role of the excited state absorption (ESA) process (i.e.,

$^4{\mathrm{I}}_{13/2}{ \to }^4{\mathrm{I}}_{9/2}$

) in exciting efficient ∼2.8 μm laser has been highlighted, and the resonant ground state absorption process (i.e.,

$^4{\mathrm{I}}_{15/2}{ \to }^4{\mathrm{I}}_{13/2}$

) is also essential. The simulation results indicate that the pump wavelength closer to the ESA peak can yield a higher slope efficiency. In addition, heat load analyses via numerical modeling reveals that the

$^4{\mathrm{I}}_{9/2}{ \to }^4{\mathrm{I}}_{11/2}$

multiphonon relaxation is the main heat source of the system. By taking fiber optical damage limit into account simultaneously, a high-power lightly-Er ³⁺ -doped ZrF ₄ fiber laser arrangement has been proposed, which theoretically enables 100-W-class output under a slope efficiency of >50% and a relatively low heat load.

J. L. Shi

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