Fabrication of microstructured optical fibers-part II: numerical modeling of steady-state draw process

By combining theoretical, numerical, and experimental analyses, this paper examines the continuous draw process that underpins the fabrication of microstructured optical fibers (MOFs) with the aim of quantifying the impact of material properties and drawing conditions on the hole structure in the finished fiber. First, by treating the continuous draw process as a steady-state isothermal extensional flow of a Newtonian material, three-dimensional (3-D) modeling clearly demonstrates how a combination of force effects can lead to dramatic hole deformation in the neck-down region-i) surface tension contributing to hole size collapse (particularly if the fiber contains small holes and is drawn slowly over a long distance), while ii) viscous effects are the major contributor to hole shape changes (particularly in cases where different size holes are in close proximity within the overall structure). Then the central role of the neck-down region in hole deformation is examined via nonisothermal numerical analysis. Results indicate that the shape of the neck-down region is highly sensitive to the viscosity profile and thus to temperature gradients. Finally, it is shown that predicted hole deformations agree well with experimental measurements made in drawing polymethylmethacrylate MOFs.