(INVITED) On the evolution of nanoparticles in nanoparticle-doped optical fibers

The doping of dielectric nanoparticles (NPs) into the cores of silica fibers doped with a rare earth (RE) has proven to be an effective approach for controlling the spectroscopic performance of the rare earth. However, little is understood about the chemical and structural evolution of the NPs during fiber preform fabrication, or how any process-induced changes in the NPs affect the optical properties of the resultant fiber. In this work, rare-earth-doped alkaline-earth (AE) fluoride [RE:(AE)F₂, RE = Yb, Eu; AE = Ca, Sr, Ba] nanoparticles were synthesized, suspension-doped in a silica preform fabricated using modified chemical vapor deposition (MCVD), and their evolution from as-synthesized to processed into a final preform were thoroughly studied. A thermal profile of the preform during fabrication was developed based on optical-pyrometer temperature measurements and used for time/temperature/structural correlations. A series of characterization methods, including x-ray diffraction, electron microscopy, optical spectroscopy, and thermochemical analyses were employed to study the NP phase and structure evolution. The fluoride NPs are shown to react with the core-glass soot, eventually oxidizing and amorphizing under the thermal treatment associated with the preform fabrication. This work sets the foundation for understanding the composition and structure of RE:(AE)F₂ NPs in silica optical fiber cores, and aids in the understanding and tailoring of the optical properties of the resultant fiber.