Fig. 1. Schematic of the dual-structure fiber grating structure and the principle of mitigating SRS.

Fig. 2. Influence of the CTFBG’s loss rate on the dual-structure fiber grating. (a) Transmission and (b) reflection spectra of the CTFBG with different refractive index modulation amplitudes. The tilt angle of the CTFBG is 6°, the period is 395.5 nm, the chirp rate is 1 nm/cm and the length is 40 mm. (c) Transmission and (d) reflection spectra of the FBG. The period of the FBG is 390.5 nm, the chirp rate is 1 nm/cm, the length is 40 mm and the refractive index modulation amplitude is 0.0007. Calculated (e) transmission and (f) reflection spectra of the dual-structure fiber grating.

Fig. 3. Influence of the FBG’s FWHM on the dual-structure fiber grating. (a) Transmission and (b) reflection spectra of the CTFBG with the refractive index modulation amplitude of 0.007 (the other parameters are the same as those in Figure 2). (c) Transmission and (d) reflection spectra of the FBG at different lengths. The period of the FBG is 390.5 nm, the chirp rate is 1 nm/cm and the refractive index modulation amplitude is 0.0007. Calculated (e) transmission and (f) reflection spectra of the dual-structure fiber grating.

Fig. 4. Influence of the FBG’s center wavelength on the dual-structure fiber grating. (a) Transmission and (b) reflection spectra of the CTFBG (the parameters are the same as those in Figure 3). (c) Transmission and (d) reflection spectra of the FBG at different periods. The length of the FBG is 40 nm, the chirp rate is 1 nm/cm and the refractive index modulation amplitude is 0.0007. Calculated (e) transmission and (f) reflection specta of the dual-structure fiber grating.

Fig. 5. Design results of the dual-structure fiber grating’s (a) transmission and (b) reflection spectra. The center wavelength of the designed fiber grating is 1135 nm, the FWHM is approximately 13.1 nm and the loss rate is more than 40 dB.

Fig. 6. Schematic of the inscription process of the dual-structure fiber grating.

Fig. 7. (a) Transmission and (b) reflection spectra of the fabricated dual-structure fiber grating. The center wavelength of the grating is 1134.91 nm, the FWHM is approximately 13.9 nm and the loss rate is more than or equal to 35 dB (99.97%).

Fig. 8. High-power fiber MOPA system for evaluating the dual-structure fiber grating’s performance of mitigating SRS.

Fig. 9. Output spectra of the evaluation system (a) without a fiber grating, (b) with one single CTFBG, (c) with a dual-structure fiber grating and (d) comparison of these three cases when the pump power of the amplifier reaches 4800 W.

Fig. 10. Output power versus amplifier pump power of the three cases. The slope efficiencies of the system for the three cases are 68.8%, 65.1% and 65.2%, respectively. In addition, the M² values of the MOPA system for the three conditions (amplifier pump power of 4800 W) are 2.36, 2.41 and 2.42, respectively.

Fig. 11. (a) Transmission and (b) reflection spectra of the mismatched dual-structure fiber grating. (c) Output spectra of the evaluation system with the mismatched dual-structure fiber grating.

Fig. 12. Transmission spectra of (a) CTFBG-1, (b) CTFBG-2 and (c) CTFBG-3. The loss rates are 32, 29.5 and 30 dB, respectively.