Optical fiber sensors based on pressure-induced temporal periodic variations in refractive index

Abstract

A optical fiber sensor for measuring temperature and/or pressure employs temporally created long period gratings. The gratings may be produced by a periodic change in the refractive index of the fiber along the fiber longitudinal axis caused by periodically spaced compressive and/or expansive forces or by spaced-apart unbalanced forces that cause periodic fiber micro-bending. Pressure and temperature are determined by measuring changes in both the wavelength at which light is coupled from a mode guided by a core to a different mode and an amount of such coupling. The gratings are created intrinsically and extrinsically. Single and multiple core fibers are used.

1 . A optical fiber sensor comprising: 
 an optical fiber, the optical fiber having a first core and a cladding;    a light source connected to an end of the optical fiber;    a tube surrounding a portion of the optical fiber, the tube having an interior surface and an exterior surface, the interior surface of the tube being bonded to the optical fiber at a plurality of spaced-apart bonding locations, wherein a long period grating is produced in a portion of the first core surrounded by the tube when a pressure is applied to the exterior surface of the tube; and    a processor connected to the optical fiber, the processor being configured to determine an amount of attenuation of the light source by the long period grating.    
   
   
       2 . The optical fiber sensor of  claim 1 , wherein an entire circumference of the optical fiber is bonded to the tube at each bonding location.  
   
   
       3 . The optical fiber sensor of  claim 2 , wherein the circumference is perpendicular to the core such that opposing balanced forces are exerted on a core of the fiber at each bonding location, the balanced forces creating changes in the refractive index of the core at each bonding location.  
   
   
       4 . The optical fiber sensor of  claim 1 , wherein the bonding locations are periodically spaced.  
   
   
       5 . The optical fiber sensor of  claim 1 , wherein the bonding locations are chirped.  
   
   
       6 . The optical fiber sensor of  claim 1 , wherein light guided by the first core is coupled to a non-guided mode in the cladding by the long period grating.  
   
   
       7 . The optical fiber sensor of  claim 1 , wherein the optical fiber further comprises a second core and wherein light in the first core is coupled to a guided mode in the second core.  
   
   
       8 . The optical fiber sensor of  claim 7 , wherein the first and second cores are concentric.  
   
   
       9 . An optical fiber sensor comprising: 
 a first core; and    a cladding surrounding the first core, the cladding having an exterior surface, the exterior surface having a portion including a plurality of spaced-apart circumferential grooves formed therein.    
   
   
       10 . The optical fiber sensor of  claim 9 , wherein each of the grooves has a direction perpendicular to the core, whereby a change in an index of refraction of the first core is created in areas of the core corresponding to areas of the cladding between the grooves when the portion is exposed to a pressurized fluid.  
   
   
       11 . The optical fiber sensor of  claim 9 , wherein each of the grooves has a direction that forms an acute angle with a direction of the first core, whereby a series of microbends is formed in the first core when the portion is exposed to a pressurized fluid.  
   
   
       12 . The optical fiber sensor of  claim 11 , wherein the groove has a first wall, a second wall and a bottom surface and the first wall is perpendicular to the bottom surface.  
   
   
       13 . The optical fiber sensor of  claim 9 , further comprising a second core, the second core being positioned such that light guided by the first core is coupled to the second core when the portion is exposed to the pressurized fluid.  
   
   
       14 . The optical fiber sensor of  claim 13 , wherein the second core is surrounded by the cladding.  
   
   
       15 . The optical fiber sensor of  claim 13 , further comprising a second cladding surrounding the second core.  
   
   
       16 . An optical fiber sensor comprising: 
 an optical fiber, the fiber having at least a first core and a first cladding, the first cladding;    a first plate, the first plate having a first plurality of ridges formed thereon and pressed against the optical fiber; and    a second plate, the second plate having a second plurality of ridges formed thereon and pressed against the optical fiber;    wherein the first plurality of ridges and the second plurality of ridges are offset such that a series of microbends in the core is created when the first plate and the second plate are exerting forces on the fiber, the microbends causing light guided by the first core to be coupled to a second mode not guided by the first core.    
   
   
       17 . The sensor of  claim 16 , wherein the second mode is a cladding mode.  
   
   
       18 . The sensor of  claim 16 , wherein the optical fiber further comprises a second core and the second mode is a mode guided by the second core.  
   
   
       19 . The sensor of  claim 18 , wherein the second core surrounds the first core.  
   
   
       20 . The sensor of  claim 18 , wherein the second core is spaced apart from the first core.  
   
   
       21 . The sensor of  claim 20 , wherein the second core is surrounded by a second cladding.  
   
   
       22 . The sensor of  claim 20 , wherein the second core is surrounded by the first cladding.