Experimental investigations on multi-kilowatt all-fiber distributed side-pumped oscillators

3 kW and 4 kW all-fiber distributed side-pumped oscillators, based on a distributed side-coupled cladding-pumped fiber, are demonstrated for the first time, to the best of our knowledge. Using three cascaded gain sections, the 2.95 kW output power with 77.4% slope efficiency and more than 52 dB suppression of stimulated Raman scattering (SRS) is obtained. However, the power scaling is mainly limited by the pump power. By adding one more gain section, the output power is scaled up to 3.96 kW, the slope efficiency is about 73.0% and the SRS suppression is about 48 dB. The transverse mode instability is observed in the 4 kW fiber oscillator, and its dependence on the output power is also measured in experiment.