Fig. 1. Three types of refractive index modulation in glass induced by femtosecond laser[3]

Fig. 2. Schematic diagram of fiber Bragg grating fabricated by femtosecond laser holographic interferometry[4]

Fig. 3. Schematic diagram of fiber Bragg grating fabricated by femtosecond laser phase-mask technique[5]

Fig. 4. Spectra and PDL of (a) H2-loaded and (b) H2-free fiber Bragg gratings[8]

Fig. 5. Processing system of fiber Bragg grating fabricated by femtosecond laser point-by-point inscription

Fig. 6. (a) Processing principle of sampled fiber Bragg grating fabricated by femtosecond laser point-by-point inscription; (b) spectra of sampled fiber Bragg gratings with different sampling parameters[13]

Fig. 7. (a) Schematic diagram and (b) optical microscope image of the series fiber Bragg gratings fabricated by femtosecond laser point-by-point inscription; (c) schematic diagram and (d) optical microscope image of the parallel fiber Bragg gratings fabricated by femtosecond laser point-by-point inscription

Fig. 8. Schematic diagram of fiber Bragg grating fabricated by femtosecond laser line-by-line inscription

Fig. 9. Schematic diagram of fiber Bragg grating fabricated by femtosecond laser continuous scanning inscription[17]. (a) Top view; (b) side view

Fig. 10. Morphology of (a) Type I and (b) Type II femtosecond laser fiber Bragg gratings[18]

Fig. 11. Reflectivity evolution relationship of Type II fiber Bragg gratings with and without pre-annealing for continuous working 20 h at 1200 ℃[20]

Fig. 12. Spectral evolution of the negative-index fiber Bragg grating. (a) Transmission spectra and PDL; (b) transmission spectra of two orthogonal polarization modes (TE, TM)

Fig. 13. High temperature stability comparison between negative-index and Type II fiber Bragg gratings[8]

Fig. 14. (a) Morphology and (b) reflection spectrum of the sapphire fiber Bragg grating fabricated by femtosecond laser phase-mask technology[23]