Multimode Interference in Single Mode–Multimode FBG for Simultaneous Measurement of Strain and Bending

In this paper, a discriminating measurement of strain and bending based on single mode-multimode fiber Bragg grating (FBG) structure is investigated experimentally. Experimental results show that the reflected modes of single mode-multimode FBG exhibit different responses toward the bending of transmitting multimode fiber because of the effect of multimode interference, which can be used for ratiometrical detection of bending, meanwhile the wavelength of the reflected modes is immune from bending but sensitive to strain. Using the different responses of the reflected modes of single mode-multimode FBG to bending, strain and bending can be discriminated by measuring simultaneously the wavelength shift of FBG resonance and the reflected intensity ratio between different modes.