Date of Award

5-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

School of Materials Science and Engineering

Committee Member

Prof. John Ballato, Committee Chair

Committee Member

Prof. Stephen Foulger

Committee Member

Prof. Liang Dong

Committee Member

Prof. Philip Brown

Abstract

Glass optical fibers have generated significant commercial and research interest in the fields of communications, lasers and sensors since their successful development in the 1970s. Since then, higher performing optical fibers have arisen due to new and evolving demands necessitating the community to occasionally rethink the materials from which optical fibers are made. Although chemical vapor deposition (CVD)-based methods dominate due to their ability to make extremely low loss optical fiber, it is limited in the range of materials, hence properties, that can be brought to bear on modern problems. Accordingly, the method for fiber fabrication has proven to be a very useful technology from which fruitful knowledge and fiber performance has emerged. Not only does this technique allow the study of new and unusual glass optical fibers but it has also provided the opportunity of fabricating crystalline core analogs as well. Crystals, because of their regular structure, are very attractive fiber waveguide materials; particularly for electro-optic functionalities. The fabrication of crystalline oxide core phases using the molten core method is further intriguing because of the high quench speed (~m/min compared to mm/h for standard conventional crystal fiber growth techniques), which usually leads to amorphous phases. The possibility of fabricating both phases (crystals and glasses) whilst using conventional optical fiber drawing techniques is thus an attractive feature of the molten core method. The thermodynamic-kinetic interplay offered by said method is the central topic of this dissertation. The questions of where does the thermodynamic takes over the kinetics when one draws fibers using the molten core method? and can one control crystal formation during fiber draw? will be investigated. For that purpose, the bismuth germanate and bismuth silicate system will be explored for their interesting electro-optic and nonlinear optic phases (Bi4Ge3O12/Bi4Si3O12 crystals and bismuth oxide glass).

Chapter I provides a background on optical fiber history and the principal optical fiber fabrication techniques. Additionally, the fundamental origin of nonlinearities in materials are described as are a few nonlinear applications.

Chapter II investigates the fabrication of Bi4Ge3O12 (BGO) crystalline core fibers in borosilicate glass cladding. Phase pure BGO crystalline core fibers were demonstrated. It is shown that one needs to control the inherent core-clad interaction, which incorporates glass cladding compounds and prevents one to retain a stoichiometric melt in order to obtain a single phase. Nonetheless, the glass cladding compounds (SiO2 notably) are found incorporated into the crystal structure and do not prevent the crystallization processes from taking place.

Chapter III explores the understanding of eulytine crystal formation during fiber draw in borosilicate and soda-lime silicate glass claddings. Homogeneous nucleation is investigated and refuted as a crystallization mechanism. Instead, heterogeneous nucleation is demonstrated as a pathway for crystal growth. The reaction 2Bi2SiO5 + SiO2 → Bi4Si3O12 for crystal growth is also considered but could not be identified as the mechanism.

Chapter IV studies the necessary processing conditions using the molten core method in order to fabricate bismuth-containing glass optical fibers. The materials’ processing conditions are shown to affect the fiber core structure, where a low density precursor powder is necessary to achieve a glass core phase as a result of a volume expansion effect. Furthermore, it is found that fibers fabricated from a borosilicate glass cladding are impractical due to cracks as a result of the CTE mismatch between core and cladding, and, soda-lime silicate glass cladding provides a better match. Finally, the thermo-reduction behavior of bismuth oxide is studied and it is shown that bismuth metallic nanoparticles are formed during fiber draw. The use of an oxidizing agent such as CeO2 is shown to have no relevant impact on the formation of these nanoparticles.

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