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The average power of diode-pumped fiber lasers have developed deep into the kW regime in the past years. However, stimulated Raman scattering (SRS) is still a major factor limiting the further power scaling. To date, many methods for SRS suppression have been proposed in fiber systems, such as the application of large-mode-area (LMA) fibers or enlarging the fiber mode area, spectrally selective fibers, or lumped spectral filters like long-period gratings (LPGs). The enlarging of fiber mode area must be combined with controlling numerical aperture (NA) for the operation of fundamental mode. Otherwise it will leads to a decreased transverse modal instability (TMI) threshold in fiber lasers, which also limits further power scaling. It is quite difficult to realize by today’s material and manufacturing technologies of fibers. The designing of spectrally selective fibers is usually very complex. Besides, it is also not easy to manufacture such fibers and it is still limited by the maximum fiber core size that can be employed. The working principle of lumped filters is similar to that of spectrally selective fibers, but it is much easier to design and fabricate such filters. LPGs have good filtering properties, but the filtering characters of LPGs is instable for their high sensitivities to the environment variable such as temperature, strain or humidity. Here, we have demonstrated the mitigation of SRS in kilowatt-level diode-pumped fiber amplifiers using a chirped and tilted fiber Bragg grating (CTFBG) for the first time. The CTFBG is designed and inscribed in LMA fibers, matching with the operating wavelength of the fiber amplifier. With the CTFBG being inserted between the seed laser and the amplifier stage, a SRS suppression ratio of ~10 dB is achieved in spectrum at the maximum output laser power of 2.35 kW, and there are no reduce in laser slope efficiency and degradation in beam quality. This work proves the feasibility and practicability of CTFBGs for SRS suppression in high-power fiber lasers, which is very useful for the further power scaling.
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