Fig. 1. Microscope image of TCF cross section with dimensions

Fig. 2. Proposed device. (a) Configuration; (b)‒(d) microscopic images of inscribed waveguides in start, middle and end parts; (e) light distribution of TCF with bridged waveguides; (f) microscopic image of three cores after FBG inscription

Fig. 3. Reflective spectrum of proposed device

Fig. 4. Measurement of fiber bending. (a) Diagram of bent TCF with FBGs; (b) definition of TCF’s orientation angle

Fig. 5. Experimental setup diagrams for characterizing directional curvature, temperature, and axial strain. (a) Curvature characterization; (b) temperature characterization; (c) axial strain characterization. O, R, and β represent circle center, radius, and central angle of curved fiber, respectively. L0 denotes initial length of fiber before bending or stretching

Fig. 6. Spectral variation of proposed device with orientation of 90° as increasing curvature from 0 to 8.4317 m^-1

Fig. 7. Curvature responses of peak 1, peak 2 and peak 3 in all orientation angles with step of 30°. Lines are respectively linear fittings for central wavelengths of three peaks with varying curvature

Fig. 8. Curvature sensitivities of peak 1 (red) and peak 3 (green) with respect to device orientation in polar coordinates. Angular coordinate denotes orientation angle, while radial coordinate corresponds to curvature sensitivity. Radial coordinate starts from -100 pm/m^-1 and crosses angular coordinate at 100 pm/m^-1, where minus sign stands for blue shift of resonant peak, and positive value means that resonant peak shifts to longer wavelength

Fig. 9. Wavelength interval between two FBG peaks employed as indicator for curvature sensing. (a) Curvature responses of wavelength interval between peak 1 and peak 3 in different orientation angles; (b) curvature sensitivities of wavelength interval against device orientation, with sinusoidal fitting expressed as y=-3.59+188.93×sinπ(x-2.86)179.27 (R2=0.9986)

Fig. 10. Reflection spectra evolution with varying temperature from 25 to 95 ℃

Fig. 11. Temperature responses of peak 1, peak 2 and peak 3, respectively

Fig. 12. Reflection spectra evolution with varying axial strain from 0 to 2000 µε

Fig. 13. Axial strain responses of peak 1, peak 2 and peak 3, respectively