Optical fiber Bragg grating coating removal detection

Abstract

An optical corrosion sensor employs an optical fiber Bragg grating 20 embedded within an optical fiber 18. The grating 20 has a coating 40 made of a material, such as aluminum, which corrodes or can otherwise be removed. The coating 40 exerts forces 46 radially inward around and along the grating 20 so as to cause the wavelength bandwidth of the grating reflectivity profile to become broader and to be shifted relative to its uncoated condition. Also, the forces on the grating 20 are reduced when the coating corrodes, thereby causing the wavelength bandwidth and shift of the reflectivity profile of the grating to narrow and to return to its uncoated condition.

We claim:  
     
         1 . An optical sensor, comprising: 
 an optical fiber;    a fiber grating embedded within said optical fiber, said grating having a reflection wavelength bandwidth of a reflectivity profile for reflecting incident light;    a coating of a material having a predetermined thickness and being around the circumference and along the length of said fiber grating;    said coating exerting forces radially inward around and along said grating so as to cause said wavelength bandwidth of said reflectivity profile of said grating to become broader than it would be without said coating; and    said forces on said grating being reduced when said coating is at least partially removed, thereby causing the wavelength bandwidth of said reflectivity profile of said grating to narrow.    
     
     
         2 . The sensor of  claim 1  wherein said optical fiber comprises a fiber core and a cladding surrounding said fiber core.  
     
     
         3 . The sensor of  claim 1  wherein said forces from said coating are non-uniformly distributed around and along said grating and disrupt a periodic refractive index variation of said grating, thereby causing the broadening of said wavelength bandwidth of said reflectivity profile.  
     
     
         4 . The sensor of  claim 1  wherein said forces from said coating also cause a peak reflection wavelength of said grating to exhibit a wavelength shift from a value that said peak reflection wavelength would be at without said coating and wherein said wavelength shift is reduced when said coating is at least partially removed.  
     
     
         5 . The sensor of  claim 4  wherein said forces from said coating exert an overall average force around and along said grating thereby causing said wavelength shift.  
     
     
         6 . The sensor of  claim 1  wherein said coating comprises aluminum.  
     
     
         7 . The sensor of  claim 1  wherein the removal of said coating comprises corrosion of said coating.  
     
     
         8 . A method for making an optical sensor, comprising: 
 obtaining an optical fiber with a fiber grating embedded therein;    applying a coating to said fiber grating around the circumference of and along the length of said grating;    said coating being applied to said grating such that said coating exerts non-uniform forces around and along said grating;    said forces causing said wavelength bandwidth of a reflectivity profile of said grating to become broader than it would be without said coating; and    said forces on said grating being reduced when said coating is at least partially removed, thereby causing the wavelength bandwidth of said reflectivity profile of said grating to narrow.    
     
     
         9 . The method of  claim 8 , wherein: 
 said coating exerts an overall average force around and along said grating thereby causing a peak reflection wavelength of said grating to exhibit a wavelength shift from a value that said peak reflection wavelength would be at without said coating and wherein said wavelength shift is reduced when said coating is at least partially removed.    
     
     
         10 . The method of  claim 8  wherein said coating comprises aluminum.  
     
     
         11 . The method of  claim 8  wherein said step of applying said coating comprises vapor deposition.  
     
     
         12 . The method of  claim 8  wherein said step of applying said coating comprises freeze coating.  
     
     
         13 . The method of  claim 8  wherein the removal of said coating comprises corrosion of said coating.