Photoinscription of Bragg gratings within a germanosilicate fiber subjected to a high static electric field.

The kinetics of Bragg grating growth in germanosilicate fibers subjected to a high static electric field are compared with those obtained without any electric field. The gratings were written by exposure of the fiber core to laser light at 244 or 193 nm. These experiments gave some clues about the mechanisms responsible for both the photosensitivity in germanosilicate fibers and the nonlinear second-order UV-induced susceptibility in silica glasses. The refractive-index modulation proved to be significantly higher in the fibers subjected to an electric field. Furthermore, the change in the fiber's mean effective refractive index as a function of exposure time was not monotonic. This evolution can be explained by the assumption that some electric-field-induced diffusion of electron trapped centers [Ge(1) and Ge(2)] from the fiber core is involved.

Analysis of color-center-related contribution to Bragg grating formation in Ge:SiO 2 fiber based on a local Kramers -Kronig transformation of excess loss spectra.

UV-induced excess losses have been measured at various pulse energy densities and exposure times in germanosilicate optical fiber preform cores. The corresponding refractive-index changes have been determined through a Kramers -Kronig analysis. Because of the nonlinear behavior of the excess losses as a function of both exposure time and fluence per pulse, one should be careful when comparing the refractive-index modulation deduced from such measurements with that obtained from Bragg grating reflectivity. Indeed nonlinear effects such as saturation imply that it is necessary to take into account the local character of the change in absorption to calculate the evolution of the refractive-index modulation accurately as a function of the exposure time. Implications of these results are discussed.

Ultraviolet-induced permanent Bragg gratings in cerium-doped ZBLAN glasses or optical fibers.

A holographic method was used to write refractive-index gratings in ZBLAN fluoroziroconate glasses or fibers doped by a Ce concentration of 5000 or 10,000 parts-in-10(6) weight. Direct pumping of the (2)F(5/2)-5d transitions of Ce(3+) ions near 245 nm resulted in a change in the refractive index. The photoinduced change partly recovered on a time scale of several hours at room temperature. The remaining change in the refractive index looked stable on a time scale of a month. This permanent change reaches 2 x 10(-5) at 1560 nm.