Fig. 1. Reflective U-shaped thin-core micro-nano optical fiber sensor. (a) Schematic illustration of structure; (b) magnified image under a microscope; (c) He-Ne laser is launched into the fiber sensor

Fig. 2. Simulation analysis results of the optical field distribution in the U-shaped thin-core micro-nano optical fiber under different D and r conditions. (a) r=7 μm, D=4 mm; (b) r=7 μm, D=3 mm; (c) r=7 μm、D=2 mm; (d) D=2 mm, r=9 μm; (e) D=2 mm, r=7 μm; (f) D=2 mm, r=5 μm

Fig. 3. Fabrication process of the sensor. (a) Fusion of single-mode fiber and fine-core fiber; (b) preparation of thin-core micro-nano optical fiber; (c) bending into U-shaped; (d) control of the bending diameter

Fig. 4. Schematic diagram of the reflective U-shaped thin-core micro-nano optical fiber RI sensing system

Fig. 5. Under the conditions of D=2 mm and r=11.51 μm, spectrum of sensor in air and its corresponding spatial frequency spectrum. (a) Optical spectrum in air; (b) corresponding spatial frequency spectrum

Fig. 6. RI sensing experimental results of the sensor with different bending diameters at the same waist diameter. (a)(c)(e) Reflected spectra at different RI when bending diameters of D₁, D₂, and D₃; (b)(d)(f) fitting relationship curves between wavelength and RI when bending diameters of D₁, D₂, and D₃

Fig. 7. RI sensing experimental results of the sensor with different waist diameters at the same bending diameter. (a)(c)(e) Reflected spectra at different RI when waist diameters of r₁, r₂, and r₃; (b)(d)(f) fitting relationship curve between wavelength and RI: waist diameters of r₁, r₂, and r₃

Fig. 8. Stability test of the sensor. (a) Spectral variation within 1 h, inset is magnified view of the characteristic peak; (b) variation of the characteristic peak wavelength with time