Kilowatt Ytterbium-doped fiber laser is found widespread application in medical technology, industry and military areas. At present, most of the multi-kilowatt single-mode fiber lasers are achieved by tandem-pumped master oscillator power-amplifier (MOPA) system. When the laser output power reaches kilowatt, the output will be strongly affected by nonlinear effects in the amplifier. The Stimulated Raman Scattering effects is known as the major restrictions to the increase of output signal power. Up to now, Raman effects in conventional diode-pumped amplifier have been well studied while the Raman effects in tandem-pumped has not yet been thoroughly analyzed. In this paper, a theoretical analysis of Raman effects using numerical solution of steady-state rate equations in kilowatts tandem-pumped ytterbium-doped fiber amplifiers is presented. The numerical simulation describing output power characteristics and laser distribution along the fiber is carried under the co-directional end-pumping. Furthermore, an optimization of Raman effects is discussed, which provides a solid foundation for achieving a higher fiber laser output.
We have demonstrated a 1018 nm continuous wave fiber oscillator pumped by LDs operating at 976 nm. Three kinds of Ytterbium-doped dual-clad fibers, 15/130 μm fiber, 25/250 μm fiber and 30/250 μm fiber, are employed separately in the experiments. We achieve 67 W total output power with the 15/130 μm fiber for 100 W of pump power with a slope efficiency of 67%. And with the 25/250 μm fiber, 276 W total output power is generated for 372 W of pump power with a slope efficiency of 74%. In the end, 300 W output power with the slope efficiency of 81% is successfully achieved with the 30/250 μm fiber for 372 W of pump power. To the best of our knowledge, this is the highest output ever reached by a dual-clad fiber oscillator at this wavelength that ever reported in open detail.