Effective suppression of stimulated Raman scattering in direct laser diode pumped 5 kilowatt fiber amplifier using chirped and tilted fiber bragg gratings

We report here for the first time, to the best of our knowledge, a direct laser diodes pumped 5 kW 1080 nm fiber amplifier using chirped and tilted fiber Bragg gratings (CTFBGs) to suppress stimulated Raman scattering (SRS). With one or two CTFBGs, which are carefully designed and fabricated to match the Raman gain spectrum, inserted between the seed and the amplifier stage, effective SRS suppression is obtained. The maximum suppression ratio is about 20 dB with one CTFBG and 27.5 dB with two when the amplifier operates at 5 kW, with beam quality factor M² of ∼1.8. By further improving the performance of CTFBGs and using more CTFBGs in a series, a better SRS suppression effect could be achieved, which is very useful for further power scaling of all-fiber laser systems. (letter)