Optical preform fabrication

An arrangement for introducing an auxiliary gaseous medium into the interior of an exhaust tube (11) that is joined to a downstream end of a substrate tube (10) from which a primary gaseous medium carrying glass soot particles during a chemical vapour deposition phase of an optical preform fabrication process emerges into the exhaust tube comprises a gas injection tube (13) which is partly received in the exhaust tube with radial spacing therefrom except at a contact zone and has a free end disposed at the region of merger (14) of the exhaust tube with the substrate tube. The auxiliary gaseous medium flows through the gas injection tube into the merger region for mixing thereat with the emerging flow of the primary gaseous medium. Relative circumferential movement is effected between the contact zone and at least the exhaust tube to release any soot present at the contact zone for entrainment in the mixed flow through the spacing and out of the exhaust tube. The gas injection tube lies at the bottom of the interior of the exhaust tube which rotates during the deposition phase in one sense, while the gas injection tube is being rotated in the opposite sense, so that soot accumulations are scraped off both the internal surface of the exhaust tube and the external surface of the gas injection tube as these surfaces move past the contact zone.