Method and apparatus for fabricating silica based glass

Abstract

A method for fabricating fluorine doped, silica based glass and fiber by depositing a layer of high purity silica soot on a core rod while rotating the core rod places the rod of pure fused silica or doped fused silica and silica soot in a furnace having a lining of Al 2 O 3 , elevates the temperature in a fluorine rich atmosphere to establish the proper differential in the indices of refraction between the core start rod and the deposited layer, and heats the resulting rod at consolidation temperatures in an atmosphere of helium to form a preform. The apparatus includes a furnace having a lining of high purity alumina that is resistant to chemical etching and change normally due to the fluorine and chlorine.

I claim:  
     
         1 . A method of making a fluorine doped, silica based glass comprising the step of: 
 a. fluorinating silica soot in a structure having a surface of aluminum oxide facing the soot, wherein the purity level of the aluminum oxide is sufficient to prevent the formation of a devitrified surface layer on said glass.    
     
     
         2 . The method of  claim 1  wherein said surface of aluminum oxide has an aluminum oxide content of at least 99.97%.  
     
     
         3 . The method of  claim 1  wherein said surface of aluminum oxide has a bulk density of at least 3.99 grams per cubic centimeter, and has a sodium content of less than one part per million, and has a iron content of less than one part per million.  
     
     
         4 . The method of  claim 1  wherein said surface of aluminum oxide has a potassium content of less than one part per million, a calcium content of no more than approximately one part per million, a silicon content of no more than approximately twenty-eight parts per million, and a magnesium content of no more than approximately two hundred and fifty parts per million.  
     
     
         5 . The method of  claim 1 , wherein said structure is a muffle, and wherein said muffle consists of aluminum oxide having a visual transmittance of 96%.  
     
     
         6 . The method of  claim 1  wherein the fluorination step is conducted in the presence of helium within said structure.  
     
     
         7 . The method of  claim 1  further comprising the step of drying the soot in the presence of chlorine prior to said step of fluorinating the soot.  
     
     
         8 . The method of  claim 1  further comprising the step of drying the soot in the presence of chlorine during step of fluorinating the soot.  
     
     
         9 . The method of  claim 1  further comprising the step of rotating said soot.  
     
     
         10 . The method of  claim 1  further comprising the step of elevating the temperature within said structure prior to and during said fluorinating step to increase the reactivity of said fluorine, and maintaining said elevated temperature during said fluorinating step.  
     
     
         11 . The method of  claim 1  further comprising the step of elevating the temperature of said structure to a consolidation temperature after said fluorinating step for a time sufficient to consolidate said soot into a glass.  
     
     
         12 . The method of  claim 1  wherein the pressure within said structure is substantially equal to the ambient pressure outside the structure.  
     
     
         13 . The method of  claim 1  further comprising the step of increasing the pressure within the structure to a level higher than the ambient pressure outside the structure.  
     
     
         14 . A method of making a fluorine doped, silica based glass comprising the steps of: 
 a. placing a preform of soot into a structure which at least partially encloses said preform and which structure comprises a surface facing said preform of soot and in which said surface comprises aluminum oxide of sufficient purity to prevent the formation of a devitrified surface layer on said glass;    b. rotating said preform of soot;    c. elevating the temperature within said structure to a first level sufficient to increase the reactivity of fluorine;    d. fluorinating said preform of soot while maintaining said first level temperature; and,    e. heating the preform of soot to a temperature within the consolidation temperature range for a time sufficient to cause the preform of soot to consolidate into a glass.    
     
     
         15 . A structure for fluorinating silica being formed into a glass comprising: 
 a. a furnace;    b. a muffle positioned juxtaposed said furnace and comprising at least a portion capable of being heated by said furnace;    c. means for placing reactive fluorine within said muffle; and,    d. a surface on said muffle portion comprising aluminum oxide facing said reactive fluorine, wherein the purity level of the aluminum oxide is sufficient to prevent the formation of a devitrified surface layer on said glass.