Yttrium aluminium garnet doped fiber lasers operating at 2 μm

Rare earth doped yttrium aluminium garnet represents a key photonic material for a construction of solid state and fiber lasers. We prepared active optical fibers by incorporating Ho-doped yttrium aluminium garnet nanocrystals into the porous silica frit. During frit sintering, the nanocrystals reacted with the host silica matrix to form the transparent yttrium aluminosilicate glass making the preform core. The maximum refractive index of preform was 1.463. The preform was drawn into the optical fiber with the standard outer diameter of $125 \mu \mathrm{m}$ and the core diameter of $16 \mu \mathrm{m}$. The absorption coefficient for the electronic transition ${ }^{5} \mathrm{I}_{7} \rightarrow{ }^{5} \mathrm{I}_{8}$ of $\mathrm{Ho}^{3+}$ ions localized at 1950 nm reached a value of $10.8 \mathrm{~dB} \cdot \mathrm{m}^{-1}$. The corresponding radiation lifetime followed the single-exponential decay curve providing the value of 0.750 ms. The fiber was successfully tested in the fiber laser set-up providing the laser threshold 0.374 W and slope efficiency of $55.39 \%$. Further modification of the chemical composition and processing parameters can improve the achieved laser properties and allows preparation of radioluminescent optical fibers for distributed sensors of harmful radiation.