Fig. 1. Schematic diagram of femtosecond laser direct writing FBG and coupled waveguides in seven-core fiber

Fig. 2. Schematic diagram of single-channel measuring displacement sensing structure based on multi-core FBG and coupled waveguide

Fig. 3. Theoretical model and actual image of cross section of seven-core fiber. (a) Cross section of seven-core fiber with defined geometrical parameters; (b) cross-sectional microscopic image of seven-core fiber

Fig. 4. Microscopic images of femtosecond laser direct writing FBGs and coupled waveguides in seven-core fiber. (a) Overall distributions in x and y directions; (b) top and side view morphologies of waveguide 2; (c) top and side view morphologies of waveguide 3

Fig. 5. Reflection spectra measured by fiber circulator

Fig. 6. End-side light distributions of seven-core FBG and coupled waveguide structure detected by monochrome CCD imaging and color laser beam profiler. (a) Monochrome CCD imaging result; (b) color two-dimensional mode field distribution; (c) color three-dimensional mode field distribution

Fig. 7. Schematic diagram of displacement measurement system

Fig. 8. Directional responses of FBG2 and FBG3 and their displacement responses under most sensitive directional angle. (a) Reflection wavelength differences of FBG2 and FBG3 under different displacement directions; (b) displacement responses of FBG2 and FBG3 in direction of maximum displacement sensitivity

Fig. 9. Average reconstruction results of displacement direction and reconstruction errors in each direction under three repeated measurements. (a) Reconstructed direction versus applied direction; (b) reconstructed angle errors under different applied directions

Fig. 10. Displacement direction and displacement responses. (a) Displacement direction responses of FBG2 under different displacements; (b) displacement direction responses of FBG3 under different displacements; (c) displacement responses of FBG2 under different displacement directions; (d) displacement responses of FBG3 under different displacement directions