Process for collapsing an optical fibre preform tube

摘要

The specification describes a process and apparatus (not claimed) for collapsing preform tubes in Modified Chemical Vapor Deposition processes. The problem of bubble formation during tube collapse was found to be attributable to excess dopant vapor pressure emitted from the hot zone during the final stage of tube collapse. This excess pressure is controlled by cooling (via means 63) the tube (61) in advance of the torch (62), thereby decreasing the viscosity of the dopant vapor and increasing its transport rate. This is found to reduce internal tube pressure and eliminate bubble formation in the collapsed preform.