Temperature dependence of UV-induced defects in thulium-doped ZBLAN fibers

The thulium-doped ZBLAN fiber has been identified as a promising candidate for the generation of coherent blue radiation.1 Yet Tm-ZBLAN blue fiber lasers have been shown to suffer from a progressive deterioration of their performances as they operate for several hours, apparently because of an increase in the absorption of the fiber at the laser wavelength. This problem, which has been clearly observed with a pumping source at 1.12 μm, is thought to be related to the formation of defect centers induced by UV radiation generated as a by-product of the upconversion process occuring in the thulium ions.2,3 The induced absorption can be bleached by exposure to visible light. However, the observed photobleaching is only temporary and a permanent recovery of the pristine condition of the glass can only be achieved by a thermal treatment. Moreover, the darkening as well as the bleaching processes taking place in these fibers have been shown to be well described by stretched-exponential (S-E) functions.4 Thus, it appears that a better understanding of these defects, which is crucial for the development of practical Tm-ZBLAN blue fiber lasers, relies on the study of their UV-induced formation as well as their temperature dependence. We report on these two mechanisms and, through an S-E analysis of the temperature dependence of the defects, show to what extent they are related.