Distributed optical fiber sensor with controlled power loss

Abstract

Distributed fiber optic chemical and physical sensors provide a relatively highly uniform response over the length of the fiber by, for example, varying such properties as the core/cladding index of refraction ratio to compensate for the non-linearity in sensitivity due to the loss of higher order modes in multi-mode fibers. The variation of the ratio changes the absorption coefficient of the fiber and can be used to compensate for any non-linearity in response. Other techniques for compensation also are disclosed.

What is claimed is:  
     
         1 . An optical fiber, said fiber having a core and a sheath, said sheath having at least one parameter that varies from an input end of said fiber to an output end thereof in a manner to maintain a constant power loss per unit length over the length of said fiber.  
     
     
         2 . An optical fiber as in  claim 1  wherein said core is fabricated in a manner to be sensitive to a target chemical.  
     
     
         3 . An optical fiber as in  claim 1  wherein said sheath includes a cladding and said cladding is fabricated in a manner to be sensitive to a physical quantity.  
     
     
         4 . An optical fiber as in  claim 1  wherein said core is fabricated in a manner to be sensitive to a physical quantity.  
     
     
         5 . An optical fiber as in  claim 1  wherein said sheath includes a cladding and said cladding is fabricated in a manner to be sensitive to a target chemical.  
     
     
         6 . An optical fiber as in  claim 1  wherein said one parameter comprises an increase in the diameter of said core from said input end to said output end.  
     
     
         7 . An optical fiber as in  claim 1  wherein said one parameter comprises the core/cladding refractive index ratio.  
     
     
         8 . An optical fiber as in  claim 1  wherein said one parameter comprises an increase in the absorption coefficient of said fiber from said input end to said output end.  
     
     
         9 . A multi-mode optical fiber, said fiber having a core and a cladding, said cladding being permeable and including a composition sensitive to a target chemical, said fiber having at least one parameter that varies as a function of position in said fiber to maintain uniform the level of sensitivity of the reaction between said composition and said target chemical.  
     
     
         10 . An optical fiber as in  claim 9  wherein said at least one parameter comprises the core/cladding refractive index ratio and said ratio increases as a function of distance from an input end of said fiber.  
     
     
         11 . An optical fiber as in  claim 9 , wherein said cladding includes said composition therewithin.  
     
     
         12 . An optical fiber as in  claim 9  including a coating on said cladding wherein said coating includes said composition.  
     
     
         13 . An optical fiber as in  claim 10  wherein said cladding includes said composition therein, said fiber including means for introducing light into said input end.  
     
     
         14 . An optical fiber as in  claim 12 , said fiber including means for introducing light into said input end.  
     
     
         15 . An optical fiber as in  claim 13  also including a light sensor at an output end thereof.  
     
     
         16 . An optical fiber as in  claim 14  also including a light sensor at an output end thereof.  
     
     
         17 . An optical fiber, said fiber having a core and a cladding, said cladding being fabricated to be sensitive to a physical quantity, said fiber having at least one parameter that varies from an input end to an output end in a way calculated to make the power loss vary in a controlled way over the length of the fiber.  
     
     
         18 . An optical fiber as in  claim 17  wherein said one parameter comprises an increase in the diameter of said core from said input end to said output end.  
     
     
         19 . An optical fiber as in  claim 17  wherein said one parameter comprises the core/cladding refractive index ratio.  
     
     
         20 . An optical fiber as in  claim 17  wherein said one parameter comprises an increase in the scattering coefficient of said fiber from said input end to said output end.  
     
     
         21 . A distributed fiber optic sensor comprising a multi-mode fiber having a core and a permeable cladding, said cladding including a composition responsive to an external material to generate a light signal characteristic of that response, said fiber having at least one parameter that varies as a function of position within the fiber to compensate for any non-linear power loss over the length of said fiber.  
     
     
         22 . An optical fiber as in  claim 21  wherein said one parameter comprises an increase in the diameter of said core from said input end to said output end.  
     
     
         23 . An optical fiber as in  claim 21  wherein said one parameter comprises the core/cladding refractive index ratio.  
     
     
         24 . An optical fiber as in  claim 21  wherein said one parameter comprises an increase in the absorption coefficient of said fiber from said input end to said output end.  
     
     
         25 . An optical fiber as in  claim 21  wherein said composition is characterized by an increase in scattering coefficient from an input to an output end of said fiber.  
     
     
         26 . An optical fiber as in  claim 21  also including a light sensor at an output end thereof.  
     
     
         27 . An optical fiber as in  claim 26  said fiber having a light source at the input end thereof.