Study of thermal annealing effect on Bragg gratings photo-inscribed in step-index polymer optical fibers

摘要

In this paper, both non-annealed and annealed trans-4-stilbenemethanol-doped step-index polymer optical fibers were photo-inscribed using a 325 nm HeCd laser with two different beam power densities reaching the fiber core. In the high-density regime where 637 mW/mm~2 are used, the grating reflectivity is stable over time after the photo-writing process but the reflected spectrum is of limited quality, as the grating physical length is limited to 1.2 mm. To produce longer gratings exhibiting more interesting spectral features, the beam is enlarged to 6 mm, decreasing the power density to 127 mW/mm~2. In this second regime, the grating reflectivity is not stable after the inscription process but tends to decay for both kinds of fibers. A fortunate property in this case results from the possibility to fully recover the initial reflectivity using a post-inscription thermal annealing, where the gratings are annealed at 80 °C during 2 days. The observed evolutions for both regimes are attributed to the behavior of the excited intermediate states between the excited singlet and the ground singlet state of trans- and cis-isomers as well as the temperature-dependent glassy polymer matrix.