Fig. 1. Micrograph of cross section of multi-core optical fibers^[7]

Fig. 2. Four-core fiber fan in/out device based on fading core pulling cone. (a) Schematic diagram of the four-core fiber fan in/out device; (b) refractive index distribution of the double-clad transition fiber; (c) micrograph of cross section of the double-clad transition fiber; (d) transition of the mode field from the core to the inner cladding; (e) packaged four-core fiber fan-in-fan-out device

Fig. 3. Schematic diagram of three-core fiber bending

Fig. 4. Schematic diagram of axisymmetric triangular three-core fiber structure

Fig. 5. Schematic diagram of spiral multi-core fiber sensing principle

Fig. 6. Micrograph of cross section of axisymmetric triangular four-core fiber

Fig. 7. Distributed sensing based on scattered light

Fig. 8. Multi-core fiber orthogonal axis curvature measurements. (a) Cleaved face of a multi-core fiber; (b)‒(d)unstrained FBG reflection spectra for cores 1, 2, and 3,respectively^[64]

Fig. 9. Three-core fiber shape measurement example. (a) Relationship between curves P1‒P5; (b) relationship between Z and θ; (c) 3D curve display of C1^[77]

Fig. 10. Multi-core fiber continuous grating inscription. (a) Continuous fiber grating fabrication apparatus; (b) cross section of a twisted multi-core fiber; (c) side view of a twisted fiber^[57]

Fig. 11. Four-core fiber shape sensor^［92］

Fig. 12. Soft robot based on optical fiber shape sensing^[82]

Fig. 13. Twisted multi-core fiber shape sensing experiment^[104]

Fig. 14. Application of multi-core fiber shape sensing in civil engineering

Fig. 15. Experimental setup of the DATS system^[113]