Method for the manufacture of a quartz glass preform for an optical fiber
The invention relates to a method for manufacture of a quartz glass preform for an optical fibre consisting of the following steps: preparation of a hollow cylinder made of porous quartz glass which exhibits an inner layer with a doping substance which increases the refractive index of quartz glass and an outer layer surrounding the inner layer, with a lower refractive index, and collapse of the hollow cylinder characterised by collapse of the porous hollow cylinder onto a quartz glass rod containing the doping substance.
1. A method for the manufacture of a quartz glass preform for an optical fiber, comprising the following method steps:
providing a hollow cylinder of porous quartz glass having an inner layer with a doping substance increasing the refractive index of quartz glass, and an outer layer having a lower refractive index than the inner layer and enveloping the inner layer; and
collapsing the hollow cylinder, wherein the porous hollow cylinder is collapsed onto a quartz glass rod containing the doping substance, and,
wherein said quartz glass rod and inner layer contain the doping substance in approximately identical concentrations, and the difference of refractive indices of the quartz glass rod and the inner layer is 0.001 maximum.
2. A method according to claim 1 , wherein said quartz glass rod has a radially homogeneous refractive index profile and a refractive index higher by at least 0.003 than undoped quartz glass.
3. A method according to claim 1 , wherein the doping substance is germanium dioxide and that the germanium dioxide concentration of the quartz glass rod is at least 3% by weight.
4. A method according to claim 1 , wherein the porous hollow cylinder contains fluorine.
5. A method according to claim 1 , wherein the porous hollow cylinder is subjected to a chlorine treatment by heating it in a chlorine containing atmosphere at a temperature above 700° C.
6. A method according to claim 5 , wherein the outside diameter of the quartz glass rod and the inside diameter of the porous hollow cylinder are selected such that an annular gap between the porous hollow cylinder and the quartz glass rod inserted therein will remain during the chlorine treatment.
7. A method according to claim 1 , wherein the quartz glass rod has an outside diameter of 5 mm or less.
8. A method according to claim 1 , wherein the quartz glass rod is manufactured according to the VAD method.
9. A method according to claim 1 , wherein the quartz glass rod is obtained by stretching a rod preform.
10. A method according to claim 1 , wherein a core rod is obtained by collapsing the hollow cylinder onto the quartz glass rod, the core rod being jacketed with a jacket tube of quartz glass.