Fig. 1. Schematic diagram of fiber oscillator structure (LD: Laser diode, HRFBG: High reflector FBG, YDF: Ytterbium-doped fiber, OCFBG: Output-coupler FBG, CLS: Cladding light stripper, QBH: Quartz block head)

Fig. 2. (a) Schematic diagram of CTFBG structure; (b) Simulation of transmission spectra of TFBG and CTFBG with a tilt angle of 4°, and the chirp rate of CTFBG is 0.3 nm/cm^[33]

Fig. 3. (a) Writing system based on ultraviolet pulse laser and phase mask; (b) CTFBG structure inside the fiber^[35]

Fig. 4. With the increase of pump power, the output spectrum changes. (a) Spectrum before CTFBG is placed; (b) Spectrum after CTFBG is placed; (c) Comparison when the pump power is 3490 W^[37]

Fig. 5. (a) Schematic diagram of the heating principle of the fiber coating at the inner end of the CTFBG cavity; (b) Thermal image of the CTFBG without cladding corrosion treatment; (c) Schematic diagram of the stepwise corrosion treatment of the CTFBG cladding; (d) Thermal image of CTFBG after cladding corrosion treatment^[33]

Fig. 6. (a) Output spectrum before the system is connected to CTFBG; (b) Spectrum after the system is connected to CTFBG; (c) Difference between the SRS band spectra before and after CTFBG is added^[33]

Fig. 7. (a) Schematic diagram of fiber SBS suppression experimental system based on TFBG; (b) SBS suppression effect based on TFBG^[41]

Fig. 8. (a) Schematic diagram of LPFG structure; (b) LPFG simulation transmission spectrum^[44]

Fig. 9. Using LPFG to achieve suppression of SRS effect in double-clad pulsed fiber lasers^[46]

Fig. 10. Output spectra of the MOPA system. (a) Without and (b) with the 14/250 LPFG; (c) Difference between the output spectra of the system with and without the 14/250 LPFG^[44]

Fig. 11. Evaluation system of SRS in LPFG suppression oscillator^[47]

Fig. 12. Structure of PS-LPFG and the principle of suppressing laser spectral broadening^[16]

Fig. 13. (a) PS-LPFG system based on point-by-point scanning technology; (b) PS-LPFG spectrum produced^[16]

Fig. 14. (a) High-power MOPA system used to evaluate the performance of suppressing spectral broadening; (b) System output spectrum before PS-LPFG; (c) System output spectrum after PS-LPFG^[16]

Fig. 15. (a) Bidirectionally pumped all-fiber oscillator setup; (b) Reflectance spectra of HR-FBG (blue line) and LR-FBG (orange line)^[48]

Fig. 16. Schematic diagram of the refractive index change of the fiber when the femtosecond pulsed laser beam is incident from the left^[52]