Hole distortion and drift in extruded microstructured optical fiber glass preforms: Part II - Optimization

In Part I of this computational study, scalar variables were introduced to quantify the quality of the cross-sectional geometry of a glass preform used to draw Microstructured Optical Fibers (MOF). These non-dimensional variables measure the processing induced distortions that occur during extrusion of the preform. That study investigated flow through two die designs, and showed that the distortions, which can limit the use of this technology, are a strong function of friction that accompanies interface slip. Furthermore, it was observed that a short welding chamber length, while producing a complex flow, might allow for an optimization of the die to produce a desired final geometry. An extensive sensitivity analysis set the stage for the current optimization study, which includes two approaches. First, the radii of the circular feed holes in the two die designs were altered. Within the computational domain, there are 17 and 13 feed holes in the two designs. Second, the five circular pins that create the holes in the preform were modified in both shape and position. Results show that the DAP parameter, which quantifies the degree of circularity of the holes, can be used to guide optimization of the feed hole size and pin shape to drastically reduce hole distortion. Furthermore, for the examples considered, the drift of the holes from their desired positions was simultaneously improved.