Abstract: In order to study the non-uniform strain sensing characteristics of fiber Bragg grating (FBG), a circular through hole is opened on the surface of a rectangular cantilever beam with equal section using laser cutting technology, and the displacement is applied to construct non-uniform strain distribution. The surface strain distribution of cantilever beam is simulated and analyzed. Uniform FBG (UFBG) and apodized FBG (AFBG) are pasted on the non-uniform strain distribution area of the cantilever beam surface respectively, and experimental research on the non-uniform strain sensing characteristics of FBG is carried out. The experimental results show that with the increase of displacement, the central wavelength and bandwidth of the two FBGs increase, the peak power and side mode rejection ratio decrease. However, only the center wavelength shows good linearity with the displacement, with the displacement sensitivity of 0.1298 nm/mm and 0.1582 nm/mm for UFBG and AFBG respectively. Relatively speaking, the reflection spectrum of UFBG is more sensitive to the change of non-uniform strain distribution, which is beneficial to the sensing based on spectral shape, while AFBG shows a certain inhibitory or immune effect, which is beneficial to the sensing based on the central wavelength. In addition, in order to improve sensitivity and realize temperature compensation, the double FBG sensing method is selected for displacement measurement. The experimental results show that the displacement measurement sensitivity of double UFBG spectral bandwidth is 0.2826 nm/mm, and the displacement measurement sensitivity of double AFBG central wavelength difference is 0.3142 nm/mm.

Key words: fiber optics, fiber Bragg grating, non-uniform strain, cantilever beam, displacement measurement