DECAY OF ULTRAVIOLET-INDUCED FIBER BRAGG GRATINGS

We report measurements of thermally induced decay of fiber Bragg grati ngs patterned by ultraviolet irradiation in germanium-doped silica fib er. The decay is well characterized by a ''power-law'' function of tim e with a small exponent, which is consistent with the rapid initial de cay 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 depopu lation is an activated function of the trap depth. This model is consi stent with the observed power-law behavior. An important consequence o f 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-in dex changes, wiping out the portion of the index change that would dec ay over the lifetime of the device, and keeping only the very stable p ortion of the index change.