Experimental study of SRS threshold dependence on the bandwidths of fiber Bragg gratings in co-pumped and counter-pumped fiber laser oscillator

With increasing output power, stimulated Raman scattering (SRS) becomes one of the main limitations for high power fiber laser oscillators. In this work, the SRS threshold dependence on the bandwidths of fiber Bragg gratings (FBGs) has been experimentally investigated in co-pumped and counter-pumped ytterbium-doped all-fiber laser oscillators, respectively. In both co-pumped and counter-pumped laser oscillators, FBGs with respective bandwidths of 0.24 nm, 0.5 nm, 1 nm and 2 nm are used as the output coupler. It is found that with the increasing bandwidth of the output-coupler FBG (OC-FBG), the SRS threshold of the laser oscillator also increases and when the bandwidth of the OC-FBG reaches over ∼1 nm, the SRS threshold increases slowly. Compared with co-pumping configuration, a counter-pumped laser oscillator holds a higher SRS threshold when the same OC-FBG is applied. By employing a counter-pumping configuration and increasing the bandwidth of the OC-FBG to 2 nm, the output laser power is further scaled to 3 kW-level with a slope efficiency of 70%. The M2 factor is measured to be ∼1.3 at the maximum output power.