Fig. 1. Schematic diagram of the superfine multiresonant TFBG-capillary sensor.

Fig. 2. Fabrication of the superfine multiresonant TFBG-capillary sensing device. (a) Micrograph of a TFBG probe and a capillary which are separated by a distance and well-aligned in priority to the insertion. (b) Micrograph of the pair of TFBG probe and capillary after the insertion. (c) Cross-section views of TFBG-capillary sensor with ODs of 381 μm, 700 μm, and 1000 μm, respectively.

Fig. 3. Schematic diagram of the experimental setup.

Fig. 4. Characteristics of the superfine multiresonant TFBG-capillary sensor. (a) Typical spectra of the TFBG-capillary sensors with different outer diameters and a bare TFBG. (b) Magnified view of the TFBG-capillary sensor and bare TFBG spectra. (c) Simulated spectrum of the TFBG-capillary sensor as a function of the outer diameter. (d) Evolution of the FSR of the cladding modes as a function of the outer diameter at around 1550 nm (single-mode fiber; grating pitch, 1117.24 nm; tilt angle, 12°).

Fig. 5. RI sensing performance of the superfine multiresonant TFBG-capillary sensor and a bare TFBG. (a) Spectral responses of the TFBG-capillary sensors and a bare TFBG to SRI. (b) Position of the cutoff point (marked by a red star) versus the surrounding RI.

Fig. 6. Sensing performance for small RI variation discrimination. (a) Spectrum changes of the bare TFBG as the RI increases from 1.35710 to 1.36144 with small increments. (b) Spectrum changes of the superfine multiresonant TFBG-capillary sensor as the RI increases from 1.35710 to 1.36144 with small increments. (c) Position of the cutoff point versus the RI for the bare TFBG sensor. (d) Position of the cutoff point versus the RI for the superfine multiresonant TFBG-capillary sensor.