Master Thesis Defense by Heidi Lundegaard Sørensen – Niels Bohr Institute - University of Copenhagen

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Master Thesis Defense by Heidi Lundegaard Sørensen

In this progress report we present a theoretical and experimental study of subwavelength-diameter tapered optical fibers. We have constructed an all-computer controlled heat-and-pull setup with which we can taper a standard optical fiber to a sub-micron waist size. The fiber diameter can thereby be decreased by a factor of 250 in a controlled way. The fibers are characterized according to their optical and spatial properties. The former are obtained by analyzing the transmission signal of light recorded during the pulling procedure, while the latter are carried out by analyzing CCD and SEM images of the fibers. To tailor the tapered fibers for specific applications it is important to have full control of the resultant fiber shape from the pulling procedure. We have therefore developed a model for the fiber shape consisting of the continuity equation of the fiber flow together with an axially varying fiber velocity profile dependent on the momentary fiber shape. While the continuity equation can be obtained from simple considerations of the fiber flow, the fluid dynamical properties of the fiber motion had to be analyzed to derive the velocity profile of the fiber. The model yields results in very good agreement with the measured fiber shape. By applying this model the fiber shape can thus be predicted for arbitrary pulling conditions.