Fig. 1. (a) RI distribution of DCF; (b) sensor head structure of SMS-DCF-SMF; (c) transmission spectrum of the DCF sensor head

Fig. 2. Dispersion curves of core mode and cladding modes for DCFs with different cladding diameters. (a) Outer cladding diameter is 125.0 μm; (b) outer cladding diameter is 103.6 μm; (c) outer cladding diameter is 82.0 μm; (d) outer cladding diameter is 60.4 μm

Fig. 3. Theoretical RI sensing response of DCFs with different cladding diameters

Fig. 4. (a) Sensor head structure of etched DCF; (b) relationship between DCF cladding diameter and HF etching time

Fig. 5. Transmission spectra of RI sensing response of DCFs with (a) 125, (b) 75, (c) 59 μm outer cladding diameters; (d) RI sensing response of resonant wavelength of DCF with different outer cladding diameters of 125, 75, 59 μm

Fig. 6. Consistency of RI sensing response of etched DCF

Fig. 7. (a) Sensor head structure of etched DCF with coated nanofilm; (b) SEM picture of the Al2O3 nanofilm coated optical fiber

Fig. 8. (a) Spectral RI sensing response of DCF with 59 μm outer cladding diameter and coated with 2000 cycles Al2O3 nanofilm; (b) RI sensing response of resonant wavelength of etched DCF with different Al2O3 nanofilm coating thicknesses