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]