Fiber Bragg Gratings in Carbon-Coated Optical Fibers and Techniques for Making Same

Abstract

A technique is described for fabricating one or more optical devices in a carbon-coated optical fiber. A photosensitive optical fiber is provided having a hermetic carbon coating. Further provided is a laser having a beam output that is configured to inscribe one or more refractive index modulations into the optical fiber through the hermetic carbon layer while leaving the hermetic carbon layer intact. The laser is used to inscribe one or more optical devices into the optical fiber through the hermetic carbon layer.

What is claimed is: 
     
         1 . A method for fabricating one or more optical devices in a carbon-coated optical fiber, comprising:
 providing a photosensitive optical fiber having a hermetic carbon coating and a secondary outer coating over the hermetic carbon coating, wherein each of the hermetic carbon coating and the secondary outer coating has a respective absorption of light at a given wavelength that allows a laser operating at the given wavelength to inscribe one or more optical devices into the fiber through the secondary outer coating and the hermetic carbon coating, while leaving them intact;   providing a laser having a beam output that is configured to inscribe one or more refractive index modulations into the optical fiber through the hermetic carbon layer and the secondary outer coating while leaving the hermetic carbon layer and the secondary outer coating intact; and   using the laser to inscribe one or more optical devices into the optical fiber through the hermetic carbon layer and the secondary outer coating.   
     
     
         2 . The method of  claim 1 ,
 wherein the one or more optical devices comprise one or more fiber gratings.   
     
     
         3 . The method of  claim 2 , wherein a phase mask technique is used to inscribe the one or more gratings into the fiber. 
     
     
         4 . The method of  claim 2 , wherein a holographic technique is used to inscribe the one or more gratings into the fiber. 
     
     
         5 . The method of  claim 1 ,
 wherein the one or more optical devices are written into the fiber during draw, subsequent to the application of the carbon coating and secondary outer coating onto the fiber.   
     
     
         6 . The method of  claim 1 ,
 wherein the carbon coating and secondary outer coating are applied to the fiber during draw, and   wherein the one or more optical devices are inscribed into the fiber during a post-draw process.   
     
     
         7 . The method of  claim 1 , wherein the secondary outer coating comprises a polymer. 
     
     
         8 . The method of  claim 1 ,
 wherein the laser is an ultraviolet laser, and   wherein each of the hermetic carbon coating and the secondary outer coating has a respective ultraviolet absorption that is sufficiently low so as to allow the ultraviolet laser to inscribe one or more optical devices into the fiber through the secondary outer coating and the hermetic carbon coating.   
     
     
         9 . The method of  claim 1 ,
 wherein the laser is a femtosecond laser, and   wherein the secondary outer coating has an infrared absorption that is sufficiently low so as to allow the femtosecond laser to inscribe one or more optical devices into the fiber through the secondary outer coating and the carbon coating.   
     
     
         10 . The method of  claim 1 ,
 wherein the carbon layer has a thickness ranging from 0.02 to 0.08 μm.   
     
     
         11 . A method for fabricating a photosensitive, carbon-coated optical fiber, comprising:
 doping a fiber to have a high degree of photosensitivity;   applying a hermetic carbon coating to the fiber cladding; and   applying a secondary outer coating over the hermetic carbon coating,   wherein the secondary outer coating has an absorption at a given wavelength of light that is sufficiently low so as to allow the one or more optical devices to be inscribed into the fiber, in a post-secondary-coating process, through the secondary outer coating and the carbon coating, using a laser operating at the given wavelength.   
     
     
         12 . The method of  claim 11 ,
 wherein the fiber core or fiber core and part of fiber cladding are doped with one or more rare-earth dopants.   
     
     
         13 . The method of  claim 12 ,
 wherein the one or more photosensitive dopants comprises germanium.   
     
     
         14 . The method of  claim 13 ,
 wherein hydrogen or deuterium loading is used to increase the photosensitivity of the fiber.   
     
     
         15 . The method of  claim 14 ,
 wherein hydrogen or deuterium is loaded into the fiber subsequent to the application of the carbon coating by heating the fiber to a temperature at which the hydrogen or deuterium to be loaded passes through the carbon coating.   
     
     
         16 . A method for fabricating a photosensitive, carbon-coated optical fiber, comprising:
 doping a fiber to have a high degree of photosensitivity;   applying a hermetic carbon coating to the fiber cladding, wherein the carbon layer is hermetic to both water and hydrogen, and wherein the carbon layer has a thickness that is sufficiently transparent to light at a given wavelength so as to allow an optical device to be written into the fiber through the carbon layer; and   using a laser operating at the given wavelength to write an optical device into the fiber.   
     
     
         17 . A method for fabricating one or more optical devices in a carbon-coated optical fiber, comprising:
 providing a photosensitive optical fiber having a hermetic carbon coating and a secondary outer coating over the hermetic carbon coating;   providing a laser having a beam output that is configured to inscribe one or more refractive index modulations into the optical fiber through the hermetic carbon layer while leaving the hermetic carbon layer intact;   in a selected region of the fiber, stripping off the secondary outer coating, while leaving the carbon coating intact; and   using the laser to inscribe one or more optical devices into the optical fiber through the hermetic carbon layer.   
     
     
         18 . The method of  claim 17 , further including:
 after using the laser to inscribe one or more optical devices into the optical fiber, applying a protective coating over the selected region of the fiber.   
     
     
         19 . A system for mass producing carbon-coated optical fiber gratings, comprising:
 a draw tower;   a furnace assembly at the top of the draw tower for receiving an optical fiber preform, and for heating the preform so as to allow an optical fiber to be drawn therefrom;   a carbon reactor assembly located in the draw tower below the furnace assembly for receiving the drawn fiber and for applying thereto a hermetic carbon layer;   a grating inscription assembly located in the draw tower below the carbon reactor assembly for receiving the carbon-coated optical fiber and for using a laser to inscribe gratings into the carbon-coated optical fiber through the hermetic carbon coating, while leaving the fiber's carbon coating intact; and   an outer coating applicator assembly for applying an outer coating over the carbon-coated optical fiber and the gratings inscribed therein.   
     
     
         20 . A system for mass producing carbon-coated optical fiber gratings, comprising:
 a draw tower;   a furnace assembly at the top of the draw tower for receiving an optical fiber preform, and for heating the preform so as to allow an optical fiber to be drawn therefrom;   a carbon reactor assembly located in the draw tower below the furnace assembly for receiving the drawn fiber and for applying thereto a hermetic carbon layer;   an outer coating applicator assembly for applying a secondary outer coating over the carbon-coated optical fiber, wherein the secondary outer coating has an absorption at a given wavelength that is sufficiently low so as to allow one or more optical devices to be written into the optical fiber through the carbon layer and secondary outer coating; and   a grating inscription assembly located in the draw tower below the outer coating application assembly for receiving the optical fiber and for using a laser to inscribe gratings into the optical fiber through the carbon coating and second outer coating, while leaving the fiber's carbon coating intact.