Patent ID: 8859057

Claim:
A method for manufacturing an optical preform by means of a plasma chemical vapour deposition process, the method comprising the steps of: providing a device for applying electromagnetic microwave radiation in a plasma inside a substrate tube, the device comprising: i) spaced-apart inner and outer cylindrical walls defining an annular cavity therebetween, the inner cylindrical wall having a circumferential applicator slit extending in a full circle around the circumference of the inner cylindrical wall in a plane perpendicular to a cylindrical axis of the annular cavity; ii) an elongate microwave guide arranged with a first end in communication with the annular cavity and a second end in communication with a microwave generating means such that microwaves are supplied from the microwave generating means to the annular cavity through the elongate microwave guide; iii) means for supplying a cooling gas through the elongate microwave guide to a position near the applicator slit; and iv) a hollow glass substrate tube arranged in an area enclosed by the annular cavity such that the hollow liner tube and the annular cavity are coaxially aligned, the hollow liner tube having a plurality of openings therethrough circumferentially arranged around a cylindrical axis of the hollow liner tube, the plurality of openings being laterally aligned with the circumferential applicator slit about the circumferential axis of the annular cavity; carrying out a plasma chemical vapour deposition process for depositing one or more doped or undoped layers of silica on the interior surface of the hollow substrate tube while supplying a cooling gas to the interior of the annular cavity using the means for supplying the cooling gas, wherein the cooling gas is supplied through the interior of the elongate microwave guide; and subjecting the substrate tube to a thermal contraction treatment so as to form a solid preform.