Abstract: An arrangement for introducing an auxiliary gaseous medium into the interior of an exhaust tube that is joined to a downstream end of a substrate tube from which a primary gaseous medium carrying glass soot particles during a chemical vapor deposition phase of an optical preform fabrication process emerges into the exhaust tube comprises a gas injection tube which is partly received in the exhaust tube with radial spacing therefrom except at a contact zone and having a free end disposed at the region of joinder of the exhaust tube with the substrate tube. The auxiliary gaseous medium flows through the gas injection tube into the joinder 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.

Abstract: An arrangement for collapsing tubular formations into optical preforms includes a control arrangement which controls the pressure build-up at the downstream end of the tubular formation during all of but the last one of the collapsing increments to control the flow of a gaseous medium through the interior of the tubular formation from an upstream end to and beyond the downstream end of the tubular formation. The magnitude of the back-up pressure is controlled by a set point controller in dependence on the actual value of the back-up pressure, the set point controller being operative for counteracting any deviation from the desired pressure. The set point controller controls the flow-through cross-section of a control valve that admits a pressurized inert gas to a region situated downstream of the downstream end of the tubular formation and also downstream of the point at which the pressure build-up is being measured.

Abstract: An arrangement for collapsing tubular formations into optical preforms includes a control arrangement which controls the ratio of hydrogen to oxygen in an oxyhydrogen burner flame during the collapsing operation toward overabundance of hydrogen when it is desired to increase the degree of evaporation of glass from the outer surface of a tubular proportion being collapsed. This arrangement is used for bringing the ratio of the cross-sectional area of the core region to that of the cladding region in an optical preform resulting from the collapse into or close to a predetermined range, regardless of the initial value of this ratio in the original tubular formation, by evaporating more or less of the glass from the outside of the tubular formation during the collapse.