Flow control for optical fiber fabrication using the double crucible technique

A double crucible fiber fabrication system employing a rod to control flow of the core material to produce a core of desired diameter. The rod fits closely within the inner crucible and moves at a speed necessary to produce a core material flow rate which will produce a desired core diameter. Control of the rod speed produces good control of core diameter for a wide variety of different core materials. It is also possible to control the rod speed in order to compensate for capillary effects and leakage of core material. Furthermore, control of the rod speed allows fabrication of a fiber having a core diameter which varies as desired throughout a single fiber.