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.