Fig. 1. Transmission spectrum of a point-by-point inscribed FBG^[18]

Fig. 2. FBG preparation by point-by-point writing. (a)Experimental device; (b) microscope images and reflection spectra of FBGs with different periods written in different positions; (c) spectrum of FBGs with the same period written in different positions^[21]

Fig. 3. FBG array inscription in twist seven-core fiber^[23]

Fig. 4. FBG preparation by line-by-line writing. (a) Schematic of femtosecond laser line-by-line inscription; (b) microscopic of fourth-order FBG; (c) transmission spectrum of line-by-line inscribed FBG^[24]

Fig. 5. π phase shift grating. (a) Transmission spectrum of different polarization states; (b) curves of P₁-P₂ with different twist angles^[26]

Fig. 6. Line-by-line writing grating array. (a) Schematic diagram of the encoded FBG array with a 3-bit binary coding; (b) backscattering of FBG array with code 111^[28]

Fig. 7. Plane-by-plane writing grating array. (a) FBGs array^[29]; (b) TFBG spectrum with a tilt angle of 7°^[30]

Fig. 8. Grating pair on double-clad fiber co-doped with Er and Yb. (a) Spectrum of FBGs; (b) schematic of oscillator; (c) slope efficiency^[33]

Fig. 9. Experimental results. (a) Polarization dependent loss and insertion loss of 45° tilt grating; (b) schematic of NPR mode-locked fiber laser; (c) optical spectrum of single-soliton mode-locked fiber laser; (d) autocorrelation of single-soliton mode-locked fiber laser; (e) optical spectrum of noise-like mode-locked fiber laser; (f) autocorrelation of noise-like mode-locked fiber laser^[34]

Fig. 10. Experimental results. (a) Schematic of plane-by-plane inscription; (b) spectrum of type I FBG; (c) spectrum of type I CFBG; (d) spectrum of type II FBG^[35]

Fig. 11. Core-scanning technology. (a) Schematic of core-scanning; (b) FBG spectrum comparison of core-scanning and point-by-point ^[36]

Fig. 12. CFBG written by different methods. (a) Point-by-point; (b) core-scanning; (c) modified core-scanning spectrum of CFBG by point-by-point; (d) spectrum of CFBG by core-scanning; (e) spectrum of CFBG by modified core-scanning^[37]

Fig. 13. Twin-core FMFBG. (a) Experimental optical path; (b) partial enlarged view^[41]

Fig. 14. TMFBG. (a) Schematic of TMFBG inscription; (b) spectrum of TMFBG^[45]

Fig. 15. Experimental results. (a) Spectrum of FBG (blue is with coating, black is without coating);(b) slope efficiency and schematic of oscillator^[47]

Fig. 16. Writing FBG on the optical fiber without decoating. (a) Schematic of FBG inscription; (b) spectrum of FBG with repetition rate 1 kHz and exposure time 5 min; (c) spectrum of FBG with repetition rate 500 Hz and exposure time 10 min^[49]

Fig. 17. Femtosecond laser phase template scanning technology. (a) Schematic of phase mask scanning technology; (b) transmission spectra and transmission over length^[50]

Fig. 18. Phase mask scanning technology. (a) Transmission spectrum of FBG in EDF; (b) laser experiment setup; (c) slope efficiency^[51]

Fig. 19. Experimental results. (a) Spectrum of CFBG; (b) laser experiment setup; (c) slope efficiency^[53]

Fig. 20. Experimental results. (a) Spectrum of inner-cladding CFBG; (b) laser experiment setup^[54]

Table 1. Comparison of various femtosecond laser direct inscribing methods

Table 2. Comparison between phase mask writing methods

Table 3. Development of fiber gratings inscribed by using femtosecond laser

Table 4. Comparison between direct inscribing and phase mask assisted writing