Suppressing stimulated Raman scattering in kW-level continuous-wave MOPA fiber laser based on long-period fiber gratings.

Two long-period fiber gratings (LPFGs) used to separately suppress the stimulated-Raman-scattering (SRS) in the seed and amplifier of kW-level continuous-wave (CW) MOPA fiber laser are developed in this paper. A process that combines constant-low-temperature and dynamic-high-temperature annealing was employed to reduce the thermal slopes of 10/130 µm (diameter of core/cladding fiber) and 14/250 LPFGs, used in the seed and amplifier respectively, from 0.48 °C/W to 0.04 °C/W and from 0.53 °C/W to 0.038 °C/W. We also proposed a reduced-sensitivity packaging method to effectively reduce the influence of axial-stress, bending, and environmental temperature on LPFGs. Further, we established a kW-level CW MOPA system to test SRS suppression performance of the LPFGs. Experimental results demonstrated that the SRS suppression ratios of the 10/130 and 14/250 LPFGs exceed 97.0% and 99.6%, respectively.