Laser refrigeration of optical fibers via optically-active composite cladding materials

Although the output power of current commercial fiber lasers has been reported to exceed 500kW, 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 core. In this manuscript, we demonstrate that Yb(III) ions within YLiF₄ (YLF) crystals are capable of cooling both the core and cladding of optical fibers. We also demonstrate a novel radiation balanced fiber laser design using a composite fiber cladding material that incorporates YLF nanocrystals as the active photonic heat engine. YLF crystals are mixed with the cladding material to mitigate thermal gradients within the core and cladding. 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 also the excitation of Yb(III) ions for anti-Stokes laser refrigeration in the cladding.