Manufacture of optical fibre preforms

A method of manufacturing a glass optical fibre preform in which a coating composed essentially of oxide material is formed on the interior surface of a glass substrate tube by causing a chemical reaction to take place between oxygen and the vapour of at least one compound capable of reacting with oxygen to produce the desired coating material, wherein an inner tube which has a multiplicity of perforations through its wall is supported coaxially within the substrate tube, an annular space being provided between the perforated tube and the substrate tube, and a gaseous mixture consisting of oxygen and each said vapour, and optionally an additional carrier gas, is caused to flow into the said annular space, at least each said vapour being introduced into the annular space by being passed into the inner tube so as to emerge through the perforations into said space, the gas pressure in the inner tube being maintained higher than that in the said annular space so as to cause gas to flow through the perforations into said annular space, while at least a major part of the length of the substrate tube is maintained at an elevated temperature, and a chemical reaction is caused to take place throughout the said annular space by generating energy in said space, whereby a coating of solid material resulting from the said reaction is formed simultaneously on the whole of the interior surface of the heated length of the substrate tube, while residual gases and gaseous reaction products are withdrawn from said annular space, the perforations in the said perforated inner tube being varied in respect of at least one of the features consisting of diameter, number and distribution along the length of the tube as required to control the rates of gas flow through the perforations and deposition of the coating so that the coating formed on the interior surface of the substrate tube is of uniform thickness both radially and longitudinally.