Complex decay behavior of fiber Bragg gratings: further evidence of three-energy-level system of photosensitivity in germanium-doped optical fibers

The field of fiber Bragg gratings written from the side of an optical fiber has progressed enormously. A much needed phenomenological model of the effect has yet to come. In a recent paper, we have proposed a three-energy-level system, supported by the observation of (I) a negative index change following the initial positive index change, (II) the growth rate of the index change being proportional to the intensity of the writing beam, and (III) the saturated index change being independent of the intensity of the writing beam. We have used the model to successfully predict the grating growth and to explain the amplification of gratings with a subsequent uniform exposure. We have also confirmed an earlier observation that the negative index change is much more stable than the positive index change. In this paper, we report on measurement of the complex decay of fiber Bragg gratings predicted by the three-level model