While collapsing the central duct in hollow glass optical fiber preforms, an etchant consisting of a mixture of oxygen and a fluorocarbon compound from the group C.sub.2 F.sub.6, C.sub.3 F.sub.8 and N-C.sub.4 F.sub.10 is passed through the duct. In particular, the etchant is passed through the duct only...http://www.google.fr/patents/US4793843?utm_source=gb-gplus-shareBrevet US4793843 - Method of manufacturing an optical fiber preform
