Patent ID: 7734135

Claim:
A method for manufacturing optical preforms, in which one or more layers of glass, doped or undoped, are deposited onto the internal surface of a hollow substrate tube, which deposition is effected by supplying one or more reactive gas mixtures of glass-forming compounds to the interior of the hollow substrate tube and subsequently generating a non-isothermal plasma in the hollow substrate tube, after which the hollow substrate tube provided with layers of glass via a deposition process is subjected to a contraction process for the purpose of forming a massive rod, from which optical fibres are drawn, characterized in that the contraction process comprises the steps of: i) providing a hollow substrate tube enveloped by a protective tube, which protective tube is stationary with respect to the hollow substrate tube, with the hollow substrate tube being enveloped by the protective tube along substantially the entire length thereof, ii) providing a resonator which surrounds the protective tube, iii) supplying a plasma-forming gas to the annular space present between the outer circumference of the hollow substrate tube and the inner circumference of the protective tube, iv) generating a non-isothermal plasma in said annular space, v) reciprocating the resonator in a longitudinal direction with respect to the protective tube for the purpose of contracting the hollow substrate tube into a massive rod.