Fig. 1. LPFG structure fabricated using CO₂ laser

Fig. 2. PMCs of LPFG based on 80 μm single mode fiber

Fig. 3. Seawater refractive index as a function of salinity

Fig. 4. (a) PMCs of LPFG with LP_1,9 cladding mode and (b) seawater refractive index sensitivity as a function of grating period when refractive index changes from 1.33 to 1.34

Fig. 5. Schematic diagram of sensing probe and measuring device

Fig. 6. Refractive index response characteristics of LPFG with a period of 112 μm. (a) Transmission spectra of LPFG under various refractive indices; (b) resonance wavelength as a function of refractive index

Fig. 7. Refractive index response characteristics of LPFG with a period of 113 μm. (a) Transmission spectra of LPFG under various refractive indices; (b) resonance wavelength as a function of refractive index

Fig. 8. Refractive index response characteristics of LPFG with a period of 115.4 μm. (a) Transmission spectra of LPFG under various refractive indices; (b) resonance wavelength as a function of refractive index

Fig. 9. Refractive index response characteristics of LPFG with a period of 115.6 μm. (a) Transmission spectra of LPFG under various refractive indices; (b) resonance wavelength as a function of refractive index

Fig. 10. Performance test results of LPFG with a period of 115.4 μm. (a) Repeatability; (b) stability

Fig. 11. Temperature response characteristics of LPFG with a period of 115.4 μm. (a) Transmission spectra of LPFG under different temperature; (b) resonance wavelength as a function of temperature