Physical mechanisms of femtosecond laser induced refractive index change in direct-written mid-infrared fiber Bragg gratings

Femtosecond laser inscription of fluoride Fiber Bragg Gratings (FBGs) has enabled the realization of all-fiber laser sources at mid-infrared wavelengths. However, many fundamental aspects of the fabrication process remain unknown, which hinders further development. In this paper, we for the first time reveal the physical mechanisms that underpin the laser-induced permanent refractive index change in the periodic grating elements that constitute an FBG. Quadriwave lateral shearing interferometry and micro-reflectivity measurements are introduced as techniques for the full characterization of the refractive index change profile of the grating structure. The fundamental processes that eventually result in a permanent index change were found to differ significantly between the chemically closely related fluoroindate and fluorozirconate glass families. In fluoroindate fibers, the index change can almost entirely be attributed to elemental migration, whereas in fluorozirconate fibers the involved processes are more intricate and include the selective migration of fluorine ions as well as the formation of F-centers, more specifically unpaired electrons trapped in anion vacancies. In addition, in fluorozirconate fibers, the formation of dynamic stress regions in-between the grating planes was observed, more specifically a build-up of higher stress in areas adjacent to positive index-change regions and of lower stress in areas adjacent to negative index-change regions, which for the first time explains the unusual annealing behavior of mid-IR FBGs. © 2024 Author(s).