The VAD preform stretching process: A numerical model for evaluation of internal distortion

Making an optical preform by the Vapor Axial Deposition (VAD) method requires stretching the consolidated boule into a rod so that additional cladding may be added. The effect of such stretching on a preform, other than to reduce its diameter, has not been determined, although distortions of the layer structure within the doped glass have been observed. In an effort to identify the effect of processing parameters, the stretching of pure silica blanks is examined using a finite-element fluid flow model modified to incorporate radiative heat transfer. The model, which predicts the measured neckdown profile well, also calculates the full two-dimensional flow field within the glass. Using this flow field and a tracking routine, the relative displacements of points in the glass during stretching can be determined. With this capability to study the effects of material and processing parameters, it is found that distortion is very sensitive to the initial shape of the layer structure, moderately sensitive to the width of the hot zone, and insensitive to the maximum temperature.