Abstract

As fiber Bragg gratings progress from the research to the commercial environment there is increasing interest in their long-term stability, in particular determining means by which the stability can be enhanced. Reports^1,2 on the time decay of the refractive index difference (Δn) at elevated temperatures indicate that accelerated aging of the gratings should result in more stable gratings at room temperature. In the model proposed by Erdogan et al.,¹ electrons ionised by the UV processing are trapped at defects with a range of activation energies. Thermal detrapping of these centers occurs with a probability that is a function of the trap depth. Accelerated annealing is then expected to cause the shallow traps to empty leaving only the deeper, more stable traps, thereby enhancing the stability of the remaining grating. From this it is expected that the aging characteristics will differ for different fibers and for varying preconditioning, such as hydrogen loading, but that the annealing-decay of Δn for weak, strong, and saturated gratings should be identical. If, however, during the writing process, the weak traps are filled first, it would be expected that the grating stability would be a function of the grating strength. We have studied the dependence of annealing behavior on grating strength to test this hypothesis and so shed light on the nature of the index change mechanism that occurs during both the writing and annealing processes.

© 1996 Optical Society of America