BLUE UP-CONVERSION CHARACTERISTICS OF THULIUM-DOPED SILICA FIBER WITHHIGH GERMANIA CONTENT

Intense blue upconversion luminescences were observed in a high-NA sil ica fiber doped with Tm3+ and 25mol% GeO2 by pumping with a 630-670nm laser. The intensities of two bands due to the (1)D2 --> (3)F4 at 455n m and (1)G4 --> (3)H6 transition at 480nm were changed with the pumpin g wavelength, power, and the fiber length. These phenomena were ascrib ed to the population inversion of the (3)F4 excited state to the groun d state and large population of the (3)H4, that are the intermediate e xcited levels for the (1)G4 and (3)D2 upconversions, respectively. The nonlinear power dependence of the population could be due to the rela tively 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 mu m lumin escence band are also discussed.