Photobleaching of thulium-doped ZBLAN fibers with visible light

Photobleaching of darkened thulium-doped ZBLAN fibers is observed after short exposure to visible light. The bleaching process is characterized by a stretched-exponential function of time with an exponent $\beta$ independent of both the bleaching intensity and the thulium concentration. The bleaching rate $1/\tau$ is also shown to scale linearly with the bleaching intensity and to have a $1/3$-power dependence on the thulium concentration. Incomplete and slow recovery of the color centers is observed in previously bleached fibers, suggesting the presence of at least two types of defect.