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Phosphosilicate and germanosilicate optical fibers doped with Tb3+ are darkened rapidly by exposure to 488 nm light, resonant with the 7F6 yields 5D4 transition of Tb3+. The induced absorption decreases monotonically throughout the visible, becoming negligible at around 750 nm, and can be bleached out by exposure to 514 nm light. In phosphosilicate fibers the bleaching goes virtually to completion, but in germanosilicate fibers it is hindered by the gradual formation of Ge-based color centers. The darkening and bleaching processes occur over comparable timescales, and require three and two photons respectively. For the optical intensities used, the bleaching of the induced loss is photonic, not thermal in nature, but thermal processes may become important at higher intensities. Other argon laser lines are able to induce darkening to a certain extent, dependent on the overlap with the 7F6 yields 5D4 band, and the equilibrium absorption state of the fiber depends on the competition between the darkening and bleaching processes. The absorption state, once established, is stable at room temperature over a timescale of months. The darkening is believed to occur by photo-ionization of Tb3+, with trapping of the released electron at sites in the network; the absorption may be due either to these electron traps or to charge transfer bands between Tb3+ and Tb4+. Refractive index changes associated with the absorption changes may be useful for side-writing fiber gratings with blue-green light, and the reversibility of the absorption changes may have application in optical memories.
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