Design of a radiation-balanced fiber laser via optically active composite cladding materials

Although the output power of commercial fiber lasers has been reported to exceed 500 kW, the heat generated within fiber gain-media has limited the generation of higher laser powers due to thermal lensing and melting of the gain-media at high temperatures. Radiation-balanced fiber lasers promise to mitigate detrimental thermal effects within fiber gain-media based on using upconverted, anti-Stokes photoluminescence to extract heat from the optical fiber's core. In this paper, we experimentally demonstrate that Yb(III) ions within $ {\text{YLiF}_4} $YLiF4 (YLF) microcrystals are capable of cooling the cladding of optical fibers. We also present a design for radiation-balanced fiber lasers using a composite fiber cladding material that incorporates YLF nanocrystals as the active photonic heat engine. YLF crystals have the potential to form composite cladding materials to mitigate thermal gradients within the core and cladding based on anti-Stokes photoluminescence. Analytical models of heat transfer within the fiber are presented where the electric-field amplitude within the fiber core is responsible for both the heating of the core and the excitation of Yb(III) ions for anti-Stokes laser refrigeration in the cladding.