PSI - Issue 42

Valerii Matveenko et al. / Procedia Structural Integrity 42 (2022) 307–314 V.Matveenko, N.Kosheleva, G.Serovaev / Structural Integrity Procedia 00 (2022) 000 – 000

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2. Principle of operation of fiber-optic sensors Fiber-optic sensors can be divided into point and distributed sensors categories. The most common type of point FOSs are sensors based on fiber Bragg gratings, which are obtained by changing the refractive index profile at a certain length of the optical fiber core. FBG has a narrow reflection spectrum with a central wavelength λ , which depends on the effective refractive index of the core of the optical fiber n and the grating period Λ and is expressed by the ratio (Mao et al. , 2016): 2 n  =  (1) Under external influence on the Bragg grating (strain or temperature), the central wavelength of the reflected spectrum is shifted allowing to utilize FBG as a sensitive element. It is possible to inscribe several FBGs on one optical fiber and provide quasi-distributed measurements by wavelength-division multiplexing. To carry out measurements using Bragg grating sensors, an 8-channel HYPERION si255 interrogator by Micron Optics was used. In contrast to point FOSs, where only a small length of the optical fiber has strain and temperature sensitivity, where the grating is inscribed, in distributed FOSs the entire length of the optical fiber is used as a sensing element. Thus, no prior exposure of the optical fiber is required in order to change the refractive index. The principle of operation can be based on the measurement of Rayleigh, Raman or Brillouin scattering. In this paper, the possibilities of using distributed FOSs based on Rayleigh backscattering are considered. For distributed measurements, a high-resolution optical backscattering reflectometer OBR 4600 from Luna Innovations was used with the option of distributed measurement of strain and temperature. Rayleigh backscattering in an optical fiber is caused by random changes in the refractive index along the fiber core due to the presence of inhomogeneities in the structure of the material. For different optical fibers, the scattering along the length will be different, but it will be preserved from measurement to measurement (in the absence of external influences) for a single optical fiber and can be considered as a weak Bragg grating with a random grating period. The physical length and refractive index of the optical fiber are sensitive to various environmental parameters. In this case, temperature and strain will have a dominant influence on the spectral response of the Rayleigh backscattering. The swept-wavelength interferometry based technique allows for reliable distributed measurements of strain and temperature with a high spatial resolution of the millimeter range and a resolution of strain and temperature measurements up to 1 µε and 0.1˚ C. The maximum investigated optical fiber length for the owned device is 70 m (Luna Technologies Inc., 2013). To carry out measurements the Rayleigh scatter profile of the analyzed section of the optical fiber under test is divided into subsections of a given length (gage length) with a certain step (distance between sensors), which can be considered as separate sensors. In this case, the subsections may intersect, and the center of each subsection becomes the coordinates of the points on the diagram of distributed measurements of strain or temperature. Under an external influence on the optical fiber (strain, temperature) the spectrum in the frequency domain shifts. To calculate the strain or temperature, the spectrum shift of each subsection is estimated relative to the reference one and this shift value is multiplied by the coefficient of strain or temperature sensitivity of the optical fiber. The spectral shift (  ) of the measured region of the optical fiber is similar to the shift of the resonant wavelength (  ) of the Bragg grating and is associated with a change in strain and temperature as follows: T K K T    = −  =   +   , (2) where T K и K  are the temperature and strain sensitivity coefficients. For most optical fibers with germanium doped core: 6 1 6.45 10 С , 0.78 T K K − −  =  = . In the absence of mechanical strain, the temperature change can be expressed:

сK 



,

(3)

T  = −  =

K

T



0

T