Decrease of fluorescence in optical fiber during exposure to pulsed or continuous-wave ultraviolet light

Abstract

Abstract We exposed optical fibers to UV light and simultaneously measured the intensity of the blue fluorescence from the fiber core. Two silica glass fibers with different core dopants were investigated: a germanium-doped fiber and a germanium-boron-codoped fiber. The fibers were exposed transversely to pulsed or continuous-wave 244 nm light for times ranging from a few minutes to over an hour. For all UV intensities and exposure times used, the fluorescence decreased during UV exposure. For a given fiber, the fractional decrease in fluorescence seen from the side of the fiber was dependent only on the total UV fluence. The side-collected fluorescence from the germanium-doped fiber decreased to 60% of its initial value after exposure to 3600 J/cm 2 fluence, while the fluorescence from the germanium-boron-codoped fiber decreased to 40% of its initial value after the same fluence. We compared the temporal characteristics of the fluorescence radiated transversely to the exposed region with that which was collected from the end of the fiber. The temporal characteristics of the fluorescence guided down the core and collected from the end were masked by photodarkening that occurred on the same time scale. We relate the observed fluorescence decrease to competing theories of its origin and relationship to photoinduced refractive index changes.