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.
Long-period fiber gratings are used to f latten the gain spectrum of erbium-doped fiber amplifiers. A broadband amplifier with <0.2-dB gain variation over 30 nm is presented. We also show that a chain of amplifiers can be equalized, leading to a bandwidth enhancement by a factor of 3.
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