DECAY OF ULTRAVIOLET-INDUCED FIBER BRAGG GRATINGS

We report measurements of thermally induced decay of fiber Bragg gratings patterned by ultraviolet irradiation in germanium-doped silica fiber. The decay is well characterized by a ''power-law'' function of time with a small exponent, which is consistent with the rapid initial decay followed by a substantially decreasing rate of decay. We propose a decay mechanism in which carriers excited during writing are trapped in a broad distribution of trap states, and the rate of thermal depopulation is an activated function of the trap depth. This model is consistent with the observed power-law behavior. An important consequence of this mechanism is that the decay of the induced index change can be accelerated by increasing temperature. A decelerated-aging experiment verifies this prediction. This result demonstrates that it is possible to preanneal a device incorporating ultraviolet-induced refractive-index changes, wiping out the portion of the index change that would decay over the lifetime of the device, and keeping only the very stable portion of the index change.