Ultraviolet-induced paramagnetic centers and absorption changes in singlemode Ge-doped optical fibers.

We investigated the laser-energy-density dependence of absorption changes and paramagnetic centers induced by a cw Ar(+)laser operating at 5.1 eV, in both unloaded and H(2)-loaded singlemode Ge-doped optical fibers. The induced absorption is measured in the blue and near ultraviolet spectral range by using the 3.1 eV photoluminescence, ascribed to Ge lone pair center (GLPC), as an in situ probe source. We find that the Ge(1) center (GeO-(4) ) is induced upon UV exposure by electron trapping on GeO(4) precursors, where the free electrons are most likely produced by ionization of GLPC. Ge(1) is responsible of optical transmission loss of the fiber in the investigated range. Hydrogen loading strongly influences the generation efficiency of the several observed paramagnetic defects, leading in particular to passivation of radiation-induced Ge(2) centers.