Patent ID: 11933646
Assignee: WUHAN UNIVERSITY OF TECHNOLOGY
Field: Measurement (Instruments)
Classification: CPC G  H  Y | IPC G  H

Claim 8:
9. The demodulation method of the interferometric demodulation system for the large capacity fiber grating sensing network according to claim 7, wherein: in the S5, calibrating the initial center wavelength Ji of the ith FBG, the FBGi in the grating array sensing network, and heating the any interference arm of the Mach-Zehnder interferometer by the heating device, a length of a heated optical fiber is LH, and demodulating a phase shift of the each FBG simultaneously; under an influence of the temperature, changing the refractive index n and the arm length difference N, so a relationship between a phase difference change amount Δφd and a temperature change amount ΔT is shown as follows:, Δφ
    d
   
   
    Δ
    ⁢
    T
   
  
  =
  
   
    
     2
     ⁢
     π
    
    
     λ
     i
    
   
   ⁢
   
    (
    
     
      N
      ⁢
      
       
        Δ
        ⁢
        n
       
       
        Δ
        ⁢
        T
       
      
     
     +
     
      
       nL
       H
      
      ⁢
      α
     
    
    )
   
  
 

wherein: Δn/ΔT is a thermal-optical coefficient of a fiber core material, representing a rate of the refractive index with the change of the temperature, and α is a thermal expansion coefficient of the fiber core material;
the fiber core material commonly used in engineering is fused quartz, and a relationship between the refractive index n and a wavelength of a transmitted signal in fiber is:, n
   ⁡
   (
   λ
   )
  
  ≈
  
   
    1
    +
    
     
      0.7
      
       λ
       2
      
     
     
      
       λ
       2
      
      -
      4679.15
     
    
    +
    
     
      0.41
      
       λ
       2
      
     
     
      
       λ
       2
      
      -
      13512.06
     
    
    +
    
     
      0.9
      
       λ
       2
      
     
     
      
       λ
       2
      
      -
      
       10
       9
      
     
    
   
  
 

wherein: λ is a wavelength variable;
calculating a ratio of a phase shift (Δφd)i of the ith FBG to a phase shift (Δφd)r of the reference grating, and obtaining:, (
     
      Δφ
      d
     
     )
    
    i
   
   
    
     (
     
      Δφ
      d
     
     )
    
    
     r
      
    
   
  
  =
  
   
    
     
      
       (
       
        Δφ
        d
       
       )
      
      i
     
     
      Δ
      ⁢
      T
     
    
    ⁢
    
     
      Δ
      ⁢
      T
     
     
      
       (
       
        Δφ
        d
       
       )
      
      
       r
        
      
     
    
   
   =
   
    
     
      λ
      r
     
     
      J
      i
     
    
    [
    
     
      
       N
       ⁢
       
        
         Δ
         ⁢
         
          n
          ⁡
          (
          
           J
           i
          
          )
         
        
        
         Δ
         ⁢
         T
        
       
      
      +
      
       
        n
        ⁡
        (
        
         J
         i
        
        )
       
       ⁢
       
        L
        H
       
       ⁢
       α
      
     
     
      
       N
       ⁢
       
        
         Δ
         ⁢
         
          n
          ⁡
          (
          
           λ
           r
          
          )
         
        
        
         Δ
         ⁢
         T
        
       
      
      +
      
       
        n
        ⁡
        (
        
         λ
         r
        
        )
       
       ⁢
       
        L
        H
       
       ⁢
       α
      
     
    
    ]
   
  
 

since N(Δn/ΔT)<<nLHα, after simplifying and integrating, obtaining:, (
     
      φ
      d
     
     )
    
    i
   
   
    
     (
     
      φ
      d
     
     )
    
    r
   
  
  =
  
   
    
     λ
     r
    
    ⁢
    
     n
     ⁡
     (
     
      J
      i
     
     )
    
   
   
    
     J
     i
    
    ⁢
    
     n
     ⁡
     (
     
      λ
      r
     
     )
    
   
  
 

wherein: λr represents a center wavelength of the reference grating, Δn(Ji)/ΔT and Δn(λr)/ΔT represent a change rate of the refractive index of the fiber core with the temperature when the light with wavelengths of Ji and λr transmit in optical fiber; n(Ji) and n(λr) represent the refractive index of the fiber core when the light with the wavelengths of Ji and λr transmit in the optical fiber; and
since the center wavelength λr of the reference grating is known, the initial center wavelength of all FBG in the grating array sensing network can be obtained by calculating a ratio of phase shifts.