Blue Upconversion Characteristics of Thulium-doped Silica Fiber with High Germania Content

Intense blue upconversion luminescences were observed in a high-NA silica fiber doped with Tm 3+ and 25mol% GeO 2 by pumping with a 630-670nm laser. The intensities of two bands due to the 1 D 2 → 3 F 4 at 455nm and 1 G 4 → 3 H 6 transition at 480nm were changed with the pumping wavelength, power, and the fiber length. These phenomena were ascribed to the population inversion of the 3 F 4 excited state to the ground state and large population of the 3 H 4 , that are the intermediate excited levels for the 3 G 4 and 1 D 4 upconversions, respectively. The nonlinear power dependence of the population could be due to the relatively low-phonon-energy, high Tm 3+ -concentration and high pump-power density in the core of this high-NA fiber. The peak shift of the 480nm-band with the fiber length and a two-photon process of 1.2 µm luminescence band are also discussed.