Study of the Impact of Built-in Stress and Strain on the Brillouin Gain Spectrum in Acoustically Anti-Guiding Fibers via Annealing

The impact of fiber draw conditions on the Brillouin gain spectra in acoustically anti-guiding optical fibers is investigated. The draw process can lead to residual stresses that remain in the fiber, ones that can modify both its optical and acoustic guiding properties. As shown herein, such stresses can render a nominally acoustically anti-guiding fiber into one that is guiding. A set of thermal annealing experiments are performed to study the change of the material properties, namely the acoustic velocity profile, which are reflected in the Brillouin scattering spectrum of those fibers. Various mechanisms are responsible for the change under different annealing conditions and are studied separately, but as is shown, the relaxation of residual viscoelastic axial stress dominates at annealing temperatures of 800°C or less. To better understand the impact of annealing, modeling of the acoustic modes was performed, incorporating the cross-sectional stress profiles of the fibers. The results enhance the understanding of the material properties and the impact of stress, and guides in the fabrication of Brillouin scattering-suppressed optical fibers for high-power laser applications.