Polymer Optical Fiber Manufacturing - Fiber Drawing

A schematic of the fiber drawing system is shown below. A preform is fed into the furnace at a specified speed. The preform is heated within the furnace and becomes soft allowing it to be pulled into a fiber. The fiber exits the furnace at a given draw speed with a time averaged fiber diameter that is governed by the conservation of mass. Downstream of the furnace a laser diameter gauge measures the fiber diameter.

Unfortunately, thermal and operational perturbations can affect the fiber diameter uniformity. In order to achieve sub-micron diameter tolerance a robust and stable drawing system is required. This has motivated considerable investigation of the fiber-drawing environment.

Numerical Model

The steady-state drawing environment is much more complicated than the initial heating case previously investigated. The flow and shape of the necking polymer is determined by the polymer's rheology. The polymer's rheology is in turn dependant on the heat transfer within the furnace, which is sensitive to the shape of the polymer. This creates a highly complex, coupled, non-linear problem that necessitates the calculation of:

This task is tackled by the commercial finite element package FIDAP. FIDAP uses the method of spines to construct the polymer/air interface. Using this method the free surface shape can be solved simultaneously with the flow and temperature fields using the fully coupled Newton-Raphson solution method. The adjacent figure illustrates the deforming mesh of the necking polymer (in red) and shows the calculated temperature contours (left) and flow field (right).

The figure below shows the predicted and experimentally measured draw force for a range of furnace wall temperatures. Because of PMMA's highly temperature dependent viscosity the draw force decreases by over 80% when the furnace wall temperature is increased 15C. Typically, an error of 1C in the predicted polymer temperature will translate into a 10% error in the predicted draw tension. For this reason considerable care is required in modeling the fiber drawing environment. Special attention should be focused on correctly assigning the thermal boundary conditions and ensuring there is a sufficient number of radiating elements.

Below are plots of the measured and numerically predicted free-surface shapes for a range of preform feed speeds. At higher mass through-put, the polymer progresses further before it heats sufficiently to deform and neck-down to a fiber.