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 Tm3+ and 25mol% GeO2 by pumping with a 630-670nm laser. The intensities of two bands due to the 1D2 → 3F4 at 455nm and 1G4 → 3H6 transition at 480nm were changed with the pumping wavelength, power, and the fiber length. These phenomena were ascribed to the population inversion of the3F4 excited state to the ground state and large population of the 3H4, that are the intermediate excited levels for the 3G4 and 1D4 upconversions, respectively. The nonlinear power dependence of the population could be due to the relatively low-phonon-energy, high Tm3+-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.