Title

Thermal processing and failure of microstructured optical fibers

Date of Completion

January 2007

Keywords

Applied Mechanics|Engineering, Mechanical

Degree

Ph.D.

Abstract

In this study several engineering problems related to the fabrication and failure of microstructured optical fibers are addressed. The first of these problems is the flow of material during the drawing of these fibers. The fiber evolution is driven by either prescribing velocity or a force at the ends of the fiber, and the free surfaces evolve under the influence of surface tension, internal pressurization, inertia and gravity. We use the fact that ratio of the typical fiber radius to the typical fiber length is small to perform an asymptotic analysis of the full three-dimensional Navier-Stokes equations similar to earlier work on non-axisymmetric (but simply connected) fibers. A numerical boundary integral solution to the multiply-connected steady state drawing problem is formulated based on the solution to the Sherman-Lauricella equation. The effects of different drawing and material parameters like surface tension, gravity, inertia and internal pressurization on the drawing is examined and extension of the method to non-isothermal evolution is presented. The second problem, is the heat transfer during the fabrication of the microstructured optical fibers. Although we do not solve the conjugate and coupled heat transfer problem, we present a Monte-Carlo ray tracing method to determine the overall absorption in the complex structure of the fiber and make suggestions on the use of a few different methodologies to solve the conjugate heat transfer problem. The last of these problems is associated with the failure of these fibers during usage. The last of these problems is related to the failure of these fiber during use. These fibers are commonly coated with hermetic coatings to protect them from harsh environments. It is well known that fibers coated with a brittle hermetic coating has a lower inert strength when compared to that of uncoated fibers. The nature of this strength degradation is examined and a new mechanism is proposed to explain this behavior. We proposed that the film cracks first and these cracks will propagate into the substrate under certain conditions and that these conditions dominate fiber failure. The determination of these conditions required the development of a new fracture mechanics based superposition scheme to determine the influence of external loads on thin film cracking. The existing solution for the case of a crack tip in the substrate is modified to provide a solution of greater accuracy. Crack arrest is examined and parameters for determining the possibility of crack arrest are presented. It was found that the mechanical properties of the thin films and the substrate affect the failure significantly. The mechanical properties of thin amorphous carbon films deposited on glass was measured using nano-indentation. Finally, experiments were performed to measure the strength of fibers with and without carbon coatings. The proposed failure model was checked with experimental data and this verified that the proposed model explains the failure of fibers with thin coatings.^