Fig. 1. Diagram of HLFBG inscription based on femtosecond laser and phase mask method

Fig. 2. Micrograph of HLFBG along the direction of incident light

Fig. 3. Trasmission spectrum of HLFBG in air

Fig. 4. Relationship between refractive index of glycerol solution with concentration

Fig. 5. Trasmission spectrum of HLFBG in water

Fig. 6. Characteristics of transmission spectrum of HLFBG as a function of refractive index

Fig. 7. Schematic diagram of liquid phase diffusion process of glycerol-water solution by using HLFBG

Fig. 8. Evolution of HLFBG transmission spectra at different diffusion times

Fig. 9. Relationship between the central wavelength of cut-off mode for HLFBG and diffusion time

Fig. 10. Relationship between the concentration of glycerol solution and diffusion time