Ultraviolet-induced color centers and annealing in thulium-doped fluorozirconate optical fibers

We have investigated the dynamics of color center formation (darkening) in a 1000 ppm thulium-doped ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) optical fiber by ultraviolet (285, 300, 351, 364 nm) and infrared(1112 nm) exposures. We observe that color centers are simultaneously created and photobleached by the UV light and that upon exposure to 1112 nm the color centers are thermally activated. We make the hypothesis that the mechanism responsible for the formation of color centers is a charge transfer to trap sites following the ionization of the glass matrix by an excitation from the 5d level of the thulium ion by similar to 55 000 cm(-1). Also, thermal annealing of the color centers is investigated with the use of a standard stretched-exponential model. The annealing results are consistent with the assumption of a distribution of energies of color center states. (C) 1998 Elsevier Science B.V. All rights reserved.