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

Claim 7:
8. The demodulation method of the interferometric demodulation system for the large capacity fiber grating sensing network according to claim 7, wherein: a pulse width t and a period T of the pulse optical signal in the S2 satisfy a following formula:, t
   <
   
    
     2
     ⁢
     nl
    
    c
   
  
  ,
  
   
    and
    ⁢
       
    T
   
   >
   
    
     2
     ⁢
     nL
    
    c
   
  
 

wherein: n is a refractive index of a fiber core in the grating array sensing network, and c is a speed of light in a vacuum, l is a distance between two adjacent gratings in the grating array sensing network, and L is a total length of the grating array sensing network;
in the S5, the reflected optical signal of an FBG which is an ith FBG in the grating array sensing network, after passing through the 1×2 coupler and arriving at the 3×3 coupler, forms two beams of light {right arrow over (E1)} and {right arrow over (E2)}, the {right arrow over (E1)} and the {right arrow over (E2)} are respectively represented as follows:, {
  
   
    
     
      
       
        E
        1
       
       →
      
      =
      
       
        E
        →
       
       ⁢
       
        cos
        ⁡
        (
        
         
          
           
            2
            ⁢
            π
            ⁢
            c
           
           
            n
            ⁢
            
             λ
             i
            
           
          
          ⁢
          t
         
         +
         
          φ
          1
         
        
        )
       
      
     
    
   
   
    
     
      
       
        E
        2
       
       →
      
      =
      
       
        E
        →
       
       ⁢
       
        cos
        ⁡
        (
        
         
          
           
            2
            ⁢
            π
            ⁢
            c
           
           
            n
            ⁢
            
             λ
             i
            
           
          
          ⁢
          t
         
         +
         
          φ
          2
         
        
        )
       
      
     
    
   
  
 

wherein: |{right arrow over (E)}| represents an amplitude of the two beams of light, λi is a central wavelength of the FBGi, φ1 and φ2 represents a phase of two optical signals;
the two beams of light are interfered in the 3×3 coupler to form a reflected light interference signal {right arrow over (Ec)};

{right arrow over (Ec)}={right arrow over (E1)}+{right arrow over (E1)}

then an interference signal strength I:

I={right arrow over (Ec)}·{right arrow over (Ec)}=[{right arrow over (E1)}+{right arrow over (E2)}]·[{right arrow over (E1)}+{right arrow over (E2)}]=2E2+2E2 cos(φd)

wherein: {right arrow over (Ec)}, {right arrow over (E1)} and {right arrow over (E2)} denote a conjugation of {right arrow over (Ec)}, {right arrow over (E1)} and {right arrow over (E2)} respectively, and E denotes a light intensity of a single beam of light;
φd is a phase difference of the two optical signals:, φ
   d
  
  =
  
   
    2
    ⁢
    n
    ⁢
    π
    ⁢
    N
   
   
    λ
    i
   
  
 

wherein: N is the arm length difference of two optical fiber arms, and n is the refractive index of an optical fiber core in the grating array sensing network;
the two optical signals interfere in the 3×3 coupler through the optical fiber arms, an output intensity ratio of the 3×3 coupler is 1:1:1, and the phase difference of an each output signal is 2π/3, output signals I1, I2 and I3 of three output ends of the 3×3 coupler are represented as:, {
  
   
    
     
      
       
        
         
          I
          1
         
         =
         
          A
          +
          
           B
           ⁢
             
           
            cos
            ⁡
            (
            
             
              φ
              d
             
             -
             
              
               2
               ⁢
               π
              
              3
             
            
            )
           
          
         
        
       
      
      
       
        
         
          I
          2
         
         =
         
          A
          +
          
           B
           ⁢
             
           cos
           ⁢
           
            (
            
             
              φ
              d
             
             +
             0
            
            )
           
          
         
        
       
      
     
    
   
   
    
     
      
       I
       3
      
      =
      
       A
       +
       
        B
        ⁢
          
        cos
        ⁢
        
         (
         
          
           φ
           d
          
          +
          
           
            2
            ⁢
            π
           
           3
          
         
         )
        
       
      
     
    
   
  
 

wherein A is a direct current component of an interference signal and B is an alternating current component of the interference signal;
simultaneous equations I1, I2 and I3 can be obtained as follows:, φ
   d
  
  =
  
   arctan
   ⁡
   (
   
    
     
      I
      1
     
     +
     
      I
      2
     
     -
     
      2
      ⁢
      
       I
       3
      
     
    
    
     
      3
     
     ⁢
     
      (
      
       
        I
        1
       
       -
       
        I
        2
       
      
      )
     
    
   
   )
  
 

when N, n and λ are fixed, a relationship between a variation Δλi of λi and a variation Δφd of a phase difference φd is as follows:, Δλ
   i
  
  =
  
   
    -
    
     
      J
      i
      2
     
     
      2
      ⁢
      π
      ⁢
      nN
     
    
   
   ⁢
   
    Δφ
    d
   
  
 

wherein: Ji is an initial center wavelength of the FBGi, Δλi=λi−Ji; and
by monitoring a phase shift of a self-interference signal of an each FBG reflected signal, a relative change of the center wavelength is obtained, and a change of the center wavelength is proportional to a change of the temperature, the stress and the vibration of the object to be measured, and then the distributed sensing of the physical quantities is realized.