Silica-based optical fibers and multi-pass sintering

Abstract

A process produces a glass overcladding tube from a silica gel body. The process includes passing the gel body through a hot zone under conditions that cause partial sintering of the gel body and repassing the gel body through the hot zone under conditions that further sinter the gel body into a glass overcladding tube.

What is claimed is:  
     
         1 . A process, comprising: 
 passing a silica gel body through a hot zone under conditions that cause partial sintering of the gel body; and    repassing the gel body through the hot zone under conditions that further sinter the gel body.    
     
     
         2 . The process of  claim 1 , wherein the passing and repassing comprises: vertically moving the gel body through the hot zone.  
     
     
         3 . The process of  claim 1 , further comprising: 
 passing the gel body through the hot zone under conditions that significantly purify the gel body without shrinking the gel body.    
     
     
         4 . The process of  claim 3 , further comprising: 
 treating the silica gel body to cause dehydroxylation prior to performing the passing.    
     
     
         5 . The process of  claim 1 , wherein the gel body has a tubular shape; 
 the passing causes at least a 1 percent shrinkage in a diameter of the gel body; and    the repassing causes at least another 1 percent shrinkage in the diameter of the gel body.    
     
     
         6 . The process of  claim 5 , wherein one of the passing and the repassing causes at least a 5 percent shrinkage of the diameter of the gel body.  
     
     
         7 . The process of  claim 1 , further comprising: 
 forming a sol comprising silica particles; and    casting the gel body from the sol.    
     
     
         8 . The process of  claim 1 , wherein both the passing and repassing include vertically moving the hot zone along the gel body.  
     
     
         9 . The process of  claim 8 , wherein the passing and repassing include regulating a temperature of the hot zone to be at least 1300° C.  
     
     
         10 . The process of  claim 8 , further comprising: 
 inserting a core-cladding rod into the further sintered gel body; and heat collapsing the further sintered gel body onto the rod to produce a preform.    
     
     
         11 . The process of  claim 9 , wherein the passing and repassing produce a preform having a level of [OH] impurities of less than 2 parts per million.  
     
     
         12 . The process of  claim 1 , wherein the repassing includes producing a transparent silica-glass overcladding tube.  
     
     
         13 . A process, comprising: 
 subjecting one end of a cylindrical silica gel body to a hot zone until the end is at least partially sintered; and    vertically passing the gel body through the hot zone to sinter the gel body by causing the partially sintered end to enter the hot zone last.    
     
     
         14 . The process of  claim 13 , wherein the partially sintered end has a diameter at least 1 percent smaller than the diameter of the end prior to the subjecting.  
     
     
         15 . The process of  claim 13 , further comprising: 
 producing the silica gel body from a sol comprising silica particles; and    wherein the gel body has a tubular form.    
     
     
         16 . The process of  claim 14 , wherein the passing includes raising the gel body through the hot zone in a direction opposite to the direction of gravity.  
     
     
         17 . The process of  claim 13 , wherein the passing produces a silica glass tube.  
     
     
         18 . A manufacture, comprising: 
 a preform having a central core, a cladding layer, and an overcladding layer; the core, cladding layer, and overcladding layer each comprising silica-glass, the preform having an OD variation of 10 −1  percent or less at one longitudinal position.    
     
     
         19 . The manufacture of  claim 18 , wherein the length and outer diameter of the preform are at least as great as 1200 mm and 90 mm, respectively.  
     
     
         20 . The manufacture of  claim 18 , wherein the overcladding layer has less than 2 parts per million of hydroxide impurities.  
     
     
         21 . The manufacture of  claim 20 , wherein the overcladding layer has less than 0.2 parts per million of hydroxide impurities.  
     
     
         22 . The manufacture of  claim 19 , wherein the preform has an OD variation of 5×10 −2  percent or less at one longitudinal position.  
     
     
         23 . The manufacture of  claim 18 , wherein the inner diameter of the overcladding layer varies by less than 1 percent over the length of the preform.