Method Of Fabricating Optical Fiber Using An Isothermal, Low Pressure Plasma Deposition Technique

Abstract

An isothermal, low pressure-based process of depositing material within a substrate has been developed, and is particularly useful in forming an optical fiber preform results in creating an extremely narrow reaction zone within which a more uniform and efficient deposition will occur. Sets of isothermal plasma operating conditions have been found that create a narrow deposition zone, assuring that the deposited material is clear glass rather than soot particles. The exhaust end of the tube is connected to a vacuum system which is in turn connected to a scrubber apparatus for removal and neutralization of reaction by-products. The operating conditions are selected such that the hot plasma does not transfer a substantial amount of heat to the substrate tube, where the presence of such heat has been found to result in vaporizing the reactant material (creating soot) and developing hot spots.

1 . An isothermal, low pressure deposition apparatus comprising
 a chamber;   a resonant coil;   an RF generator coupled to the resonant coil, wherein the resonant coil is disposed to surround a portion of the chamber, the resonant coil creating an electro-magnetic field within the chamber, the resonant coil and chamber configured to provide for relative movement therebetween;   a chemical reactant delivery system for introducing selected chemical reactants into the chamber through a first end termination, the chemical reactants interacting with the electro-magnetic field to create an isothermal plasma within the chamber; and   a vacuum-based exhaust system for maintaining a sub-atmospheric pressure within the chamber and creating a narrow deposition zone for reactant products of the at least one chemical reactant upstream of the created isothermal plasma.   
     
     
         2 . An isothermal, low pressure deposition apparatus as defined in  claim 1  wherein the apparatus further comprises a first seal for coupling the chemical reactant delivery system to the chamber and a second seal for coupling the vacuum-based exhaust system to the chamber. 
     
     
         3 . An isothermal, low pressure deposition apparatus as defined in  claim 1  wherein the vacuum-based exhaust system maintains a pressure of about 10 Torr within the chamber. 
     
     
         4 . An isothermal, low pressure deposition apparatus as defined in  claim 1  wherein the resonant coil comprises a concentrator coil for shaping the electro-magnetic field into a relatively small volume within the chamber. 
     
     
         5 . An isothermal, low pressure deposition apparatus as defined in  claim 1  wherein the apparatus further comprises a scrubber module coupled to the vacuum-based exhaust system to remove and neutralize reaction by-products. 
     
     
         6 . An optical preform fabrication apparatus comprising:
 a reactant delivery system to introduce reactants into an upstream side of a substrate tube;   a pressure-control system to maintain sub-atmospheric pressure within the substrate tube; and   a resonant coil configured to generate an isothermal plasma field within the substrate tube, such that the introduced reactants deposit on the substrate tube inner wall in a narrow zone upstream of the isothermal plasma field.   
     
     
         7 . The apparatus of  claim 6  wherein the resonant coil comprises a shaped, concentrator coil configured to produce a shaped electro-magnetic field occupying a relatively small volume. 
     
     
         8 . The apparatus of  claim 6  wherein the resonant coil is configured to create a reaction zone that is less than approximately one centimeter in length.