Synthesis and characterization of alkali/alkaline earth-doped fiber optic silica preforms

The search for an ultra-low loss optical fiber has been driven by the discovery that certain multicomponent glasses possessed lower Rayleigh scattering losses than silica. Typically, these glasses cannot be employed in fiber optic applications because they are fabricated by conventional melting and processing techniques that introduce large amounts of impurities into the materials. High purity processing techniques such as chemical vapor deposition are required to truly realize the potential of these glasses as fiber optic materials.; The Modified Chemical Vapor Deposition (MCVD) process was employed in the thesis work because of the flexibility and species confinement available with this processing method. The multicomponent glass compositions investigated in the thesis work include: Na₂O-Al₂O₃-SiO ₂, CaO-Al₂O₃, & MgO-Al₂O₃ -SiO₂. Novel vapor delivery approaches, based on the current organometallic and chloride vapor delivery of rare earth metals, were devised to fabricate the multicomponent glasses evaluated in this work. Thermodynamic data were used to predict the feasibility of the MCVD processing of the glasses. Initial work on the Na₂O-Al₂O₃-SiO₂ system was unsatisfactory. The minimal amounts, <0.40 mol%, of sodium achievable in doped preforms proved that the Na₂OAl₂O ₃-SiO₂ system could not be adequately synthesized by MCVD processing. However, the moderately high dopant levels, 1–5 mol% CaO & MgO, achieved in the MCVD fabrication of CaO-Al₂O₃-SiO ₂ & MgO-Al₂O₃-SiO₂, demonstrated the suitability of these latter systems as fiber optic materials.; The first successful MCVD fabrication and fiberization of the alkaline earth doped silica glasses achieved in this thesis work represents a milestone in ultra-low loss glass research. The modification of the silica glass structure with minor dopant levels, <10 mol%, resulted in noticeable changes in the optical properties of the glass. The CaO-Al₂O₃-SiO ₂ glass system produced waveguide properties superior to the current GeO₂-SiO₂ glass fiber including a smooth index profile, improved ability to tailor the index profile, comparable scattering losses, and a rare earth host glass with enhanced solubility. The MgO-Al₂O ₃-SiO₂ possessed lower OH absorption at 1.39 μm than the Rutgers GeO₂-SiO₂ glass fiber due to the dampening of the fundamental OH vibration. The demonstration of alkaline earth aluminosilicate glass compositions as viable optical fiber compositions offers many areas of opportunity for future applications.