PSI - Issue 28

N.A. Kosheleva et al. / Procedia Structural Integrity 28 (2020) 1883–1891 Author name / Structural Integrity Procedia 00 (2019) 000–000

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3. The principle of fiber Bragg grating operation Fiber-optic sensors are becoming a widely used tool for monitoring various constructions due to their light weight and small size, resistance to electromagnetic interference, corrosion resistance, accessibility, the ability to be embedded into a structure at the stage of its manufacturing, and the ability to remotely monitor civil structures in real time (Hong et al. , 2016; Khadour and Waeytens, 2018; R. H. Scott et al. , 2019). There are various types of fiber-optic sensors (Ramakrishnan et al. , 2016), among which one of the most used is FOSs based on fiber Bragg gratings (Di Sante et al. , 2014). A fiber Bragg grating is a section in the core of a single-mode optical fiber with a periodically changing refractive index. The principle of operation of the FBG is shown schematically in Fig. 3 and is based on measuring the Bragg wavelength λ B of the optical signal reflected from the sensor. This value depends on the effective refractive index of the optical fiber core n in the grating zone and the grating period Λ and is expressed by the following relation (Mao et al. , 2016): 2 B n    (1)

Fig. 3. The principle of FBG operation (Serovaev and Kosheleva, 2019).

The Bragg wavelength shifts under external action on the Bragg grating (strain or temperature). In case, when the optical fiber in the region of the Bragg grating is in, or close to uniaxial stress state, the following relation, expressing the relationship between the shift of the Bragg wavelength and the longitudinal strain component along the fiber could be used (Lammens et al. , 2011).



(2)

k S T

   

3

T



B

2 n p





1  

(   

)

(3)

k

11 p p

12

12

2

where B      – central wavelength shifts in the current  and initial B  moments of time, k and S T – strain and temperature sensitivities, Δ T – temperature change, 11 12 , p p – strain optic coefficients. For silica glass fiber 11 0.113 p  , 12 0.252 p  , n =1.458,  = 0.17. Thus, under the uniaxial stress state of the Bragg grating, the coefficient k ~0.78 (Kablov et al. , 2011). 3  – longitudinal strain component along the fiber,