Fig. 1. Numerical simulation results of phase variation sensitivity. (a) Numerical simulation results of phase variations under different external RI; (b) numerical simulation results of phase variation sensitivity in different diameters of taper waist when external RI changes

Fig. 2. Experimental setup for distributed optical fiber RI sensing using tapered fibers in OFDR

Fig. 3. Schematic of OFDR signal processing based on differential phase demodulation method

Fig. 4. Schematic of relative phase in the sensing area before and after changes in external RI. (a) Phase of reference signal; (b) phase of measurement signal; (c) distance domain signal in the sensing region of tapered fiber; (d) relative phase between reference and measurement signals before external RI change; (e) relative phase between reference and measurement signals after external RI change

Fig. 5. Relative phase unwrapping curves before and after radius denoising. (a) Relative phase unwrapping curves without radius denoising; (b) relative phase unwrapping curves with radius denoising, and the inset shows local phase noise in the phase unwrapping curve after denoising

Fig. 6. Variations in phase with distance in the tapered fiber sensing region. (a) Phase as a function of distance at different RI only with average denoisng when ΔX=340 μm; (b) phase as a function of distance at different RI only with wavelet denoisng when ΔX=68 μm; (c) phase as a function of distance at different RI with both average denoising and wavelet denoisng when ΔX=340 μm, and the red dashed box corresponds to central sensing region of tapered fiber, with length of 45 mm and taper waist diameter of 6 μm; (d) after average denoising and wavelet denoising, linear fitting curve between phase and RI when ΔX=340 μm

Fig. 7. RBS shifts changed with distance in the tapered fiber sensing region. (a) RBS shifts as functions of distance at different RI when ΔX=340 μm; (b) RBS shifts as functions of distance at different RI when ΔX=340 μm; (c) linear fitting result between RBS shift and RI when ΔX=340 μm