Methods for modifying ovality of optical fiber preforms

Abstract

Methods for modifying preform core ovality during and subsequent to the formation of an optical fiber preform. Prior to MCVD deposition on a starting tube, the outer diameter of the starting tube is altered by etching or a like process to modify its ovality. Additionally, after MCVD deposition forms the core rod, but prior to overcladding of the core rod, the code rod may be etched to change its ovality. Both methods may be used independently or in combination to modify the ovality and reduce PMD of optical fiber drawn from the core rod. An additional method includes etching the cladding material of a core rod having an oval or elliptical core such that the cladding material mirrors the shape of the oval core. During drawing, the perform created there from is placed under a surface tension, or pulled in a manner to generate a circular or near perfect circular optical fiber having desired ovality and low PMD.

That which is claimed:  
     
         1 . A method of modifying the ovality of an optical preform, comprising: 
 providing a starting tube having a wall, said wall having an exterior surface and an interior surface and wherein said interior surface defines a hollow region of said starting tube; and    etching said wall until the exterior of said wall comprises a substantially circular cross section.    
     
     
         2 . The method of  claim 1 , wherein etching said wall comprises etching said wall using heating means.  
     
     
         3 . The method of  claim 2 , wherein said heating means comprises a plasma torch.  
     
     
         4 . The method of  claim 1 , further comprising the step of measuring the ovality of said starting tube prior to etching the wall of said starting tube to determine the ovality of said starting tube.  
     
     
         5 . The method of  claim 1 , wherein etching said wall comprises etching said wall only wherein the exterior of said wall is substantially oval in cross-section.  
     
     
         6 . The method of  claim 5 , further comprising rotating said starting tube as said starting tube is etched.  
     
     
         7 . The method of  claim 6 , further comprising removably mounting said starting tube to a lathe to etch said wall.  
     
     
         8 . A method of modifying the ovality of an optical perform, comprising: 
 providing a core rod, wherein said core rod comprises a core and a cladding layer, and wherein said cladding layer has a non-uniform thickness;    mounting said core rod on a lathe having at least two rotors;    rotating said core rod using said at least two rotors; and    subjecting said core rod to a heat source,    wherein said heat source alters the geometry of the cladding layer such that optical preform ovality is modified and wherein one of said at least two rotors is phase-shifted from another one of said at least two rotors after said core rod is mounted on said lathe.    
     
     
         9 . The method of  claim 8 , further comprising the step of etching said cladding layer of said core rod using said heat source.  
     
     
         10 . The method of  claim 9 , wherein subjecting said core rod to a heat source comprises subjecting said core rod to a plasma torch.  
     
     
         11 . The method of  claim 9 , further comprising the step of measuring the ovality of said core rod prior to etching the core rod.  
     
     
         12 . The method of  claim 11 , wherein said step of measuring comprises measuring the ovality of said cladding layer.  
     
     
         13 . The method of  claim 9 , wherein the step of etching comprises etching said core rod only where the cladding layer is substantially oval in cross-section.  
     
     
         14 . The method of  claim 9 , wherein the step of rotating comprises rotating said core rod as said core rod is etched.  
     
     
         15 . The method of  claim 9 , wherein the step of rotating comprises the step of rotating said core rod in clockwise and counterclockwise directions.  
     
     
         16 . The method of  claim 9 , wherein the angular velocity of said rotors is the same.  
     
     
         17 . The method of  claim 9 , wherein the step of etching produces a core rod having less than 5% core ovality.  
     
     
         18 . The method of  claim 9 , wherein the step of etching produces a core rod suitable for producing optical fiber having less than 0.1 ps/sqrt (km) of polarization mode dispersion.  
     
     
         19 . A method of fabricating an optical fiber, comprising: 
 providing a core rod, wherein said core rod comprises a core and a cladding layer, and wherein said cladding layer has a non-uniform thickness;    mounting said core rod on a lathe having at least one rotor;    rotating said core rod using said at least one rotor;    subjecting said core rod to a heat source to alter the geometry of the cladding layer such that the cladding layer has a substantially uniform thickness; and    drawing optical fiber from said core rod, wherein a surface tension is applied to the optical fiber during the drawing of the optical fiber to alter the geometry of said optical fiber.    
     
     
         20 . The method of  claim 19 , wherein said surface tension is applied to reduce the ovality of the optical fiber.  
     
     
         21 . The method of  claim 20 , wherein said surface tension is applied to reduce the ovality of the optical fiber such that it has less than 5% core ovality.  
     
     
         22 . The method of  claim 20 , wherein said surface tension is applied to reduce the ovality of the optical fiber such that it has less than 0.1 ps/sqrt(km) of polarization mode dispersion.