Method for making a preform as well as a fibre obtained from such a preform

Abstract

The present invention relates to a method for producing a perform, which is substantially free of PH impurities, for an optical fibre, wherein one or more quartz layers, which may or may not be doped, are deposited on the internal surface of a quartz glass support tube, in which method a furnace is move axially with respect to the support tube, and after the quartz layers have been deposited the support tube is contracted into a bar-shaped perform while being heated, wherein the furnace comprises an electrical resistance furnace, in which furnace the support tube is rotated, with the space between the rotating support tube and the furnace being washed with an inert gas.

1 . A method for producing a preform, which is substantially free of OH impurities, for an optical fibre, comprising: depositing one or more quartz layers on the internal surface of a quartz glass support tube, and in which a furnace is moved axially with respect to the support tube, the furnace comprising an electrical resistance furnace, and after the quartz layers have been deposited the support tube is contracted into a bar-shaped preform while being heated, wherein the support tube is rotated, and the space between the rotating support tube and the furnace being washed with an inert gas during contraction of the support tube.  
     
     
         2 . The method of  claim 1 , wherein the temperature of the electrical resistance furnace is set so that the support tube , while being heated, will have a temperature at which the viscosity of the support tube ranges between 101 Pa.s and 10 6.65  Pa.s.  
     
     
         3 . The method of  claim 1 , wherein the contraction of the support tube is carried out in a atmosphere wherein the humidity level is low.  
     
     
         4 . The method of  claim 1 , wherein the electrical resistance furnace effects rotation-symmetrical heating of the support tube.  
     
     
         5 . The method of  claim 1 , wherein the axial length along which the heating of the support tube by means of the electrical resistance furnace takes place ranges between 5 cm and 20 cm.  
     
     
         6 . The method of  claim 5 , wherein the axial length along which the heating of the support tube by means of the electrical resistance furnace takes place ranges between 7 cm and 15 cm.  
     
     
         7 . The method of  claim 1 , wherein the support tube is internally washed with a gas.  
     
     
         8 . The method of  claim 7 , wherein said internal washing of the support tube is carried out with an oxygen-containing gas.  
     
     
         9 . The method of  claim 1 , wherein the rotation of the support tube takes place at a rotational speed of more than 5 revolutions per minute.  
     
     
         10 . The method of  claim 9 , wherein the rotation of the support tube takes place at a rotational speed ranging between 15 and 35 revolutions per minute.  
     
     
         11 . The method of  claim 1 , wherein the inert gas that is used for washing the space between the rotating support tube and the electrical resistance furnace is a gas selected from one of argon, helium, nitrogen, and a mixture thereof.  
     
     
         12 . The method of  claim 1 , wherein the deposition of the quartz layers on the internal surface of the support tube takes place by using a Plasma Chemical Vapour Deposition (PCVD) process.