Ultraviolet-enhanced photosensitivity in cerium-doped aluminosilicate fibers and glasses through high-pressure hydrogen loading

Abstract

We have studied the photosensitivity of both hydrogen-loaded and unloaded Ce3+-doped aluminosilicate fibers. Refractive-index changes as great as 1.5×10-3 have been achieved in the treated samples. The thermal stability of gratings appears to depend not on whether the fiber is treated but rather on the UV cumulative fluence used for the inscription. The change in refractive index follows a power law dependence on exposure time and does not saturate for exposure times as long as ∼2 h. In contrast, the changes in the absorption spectra saturate after a few seconds of exposure time. This observation and others that we report show that the color-center model does not fully explain the refractive-index change. As is observed in germanosilicate fibers, exposure of the hydrogen-loaded fiber to UV light increases the hydroxyl content according to a power law dependence as a function of time. This shows that enhancement of the fiber’s photosensitivity is strongly related to hydrogen-assisted bond breaking within the glass network. Microscopic inspections of gratings written in the cores of hydrogen-loaded preforms have shown corrugations embedded in a valley. The depth of the valley and the heights of the corrugations are more important in the hydrogen-loaded sample than in the case of an unloaded preform. This difference is closely correlated with the enhancement of the fiber’s photosensitivity.