Method of making optical fiber having depressed index core region

Abstract

Disclosed is a method of making an optical fiber preform having at least one annular region of depressed refractive index. A tube of silica doped with fluorine and/or boron is overclad with silica soot. A core rod is inserted into the overclad tube and the resultant assembly is heated while chlorine flows between the tube and the core rod to clean the adjacent surfaces. When the soot sinters, the tube collapses onto and fuses to the rod. The resultant tubular structure is formed into an optical fiber which exhibits low attenuation as a result of the low seed count at the interface between the inner core and the region that is doped with fluorine and/or boron.

1 . A method of making a fiber comprising: 
 making a fluorine doped glass tube via a process, which involves: 
 forming a tubular porous glass preform having an aperture therethrough,  
 placing the porous glass preform in a furnace and heating the preform while injecting a fluorine-containing gas into the furnace to form a fluorine doped porous tube, and  
 heating the fluorine doped tube to sufficiently consolidate the tube into a glass fluorine doped tube,  
 positioning a glass rod within the fluorine doped glass rod; and  
 collapsing and sintering the tube onto the rod to form a core glass preform.  
   
     
     
         2 . The method of  claim 1 , further comprising prior to the collapsing and sintering step, depositing a layer of glass particles on said tube to form a soot coated assembly, and said collapsing and sintering step comprises heating the soot coated assembly to a temperature sufficient to sinter said coating thereby generating a radially-inwardly directed force that facilitates said tube collapsing onto and fusing to said rod.  
     
     
         3 . The method of  claim 1 , further comprising prior to the collapsing and sintering step, depositing a layer of glass particles on said tube to form a soot coated assembly, and said collapsing and sintering step comprises heating the soot coated assembly to a temperature sufficient to sinter said coating thereby generating a radially-inwardly directed force that facilitates said tube collapsing onto and fusing to said rod.  
     
     
         4 . The method of  claim 1 , further comprising, prior to said collapsing step, flowing a centerline gas through said tube, said centerline gas comprised of at least 50% chlorine.  
     
     
         5 . The method of  claim 1 , further comprising, prior to said positioning step, redrawing said glass tube to have an internal diameter which is narrower than the internal diameter of said tube prior to said redrawing step.  
     
     
         6 . The method of  claim 1 , wherein said fluorine doping step comprises doping the tube with an amount of fluorine which is sufficient to give said tube a Δ-value of less than −0.5% with respect to silica, where Δ a−b =(n a   2 −n b   2 )/(2n a   2 ), n a  being the refractive index of the fluorine-doped glass and n b  being the refractive index of silica.