Contents 1. Introduction
The operating wavelengths of rare-earth-doped fiber lasers depend on the emission band of the dopants. The wavelength extension of high-power fiber lasers (HPFLs) has drawn great attention of researchers. The operating wavelengths of high-power ytterbium-doped fiber lasers are commonly within 1030 to 1100 nm, especially for 10-kw-level fiber lasers. Compared with the 20-kW ytterbium-doped fiber laser reported in 2013 [1], there is still a blank need to fill in for the wavelength extension of HPFLs. Raman fiber lasers are an efficient method for filling in the gaps among the emission bands of rare-earth ions. Combining the gain of the ytterbium ions and Raman scattering, our previous study showed that ytterbium-Raman fiber lasers (Yb-RFLs) are promising sources for wavelength extension and also suitable for power boosting [2],[3]. They have set the record output power of quasi-4-kW for the fiber lasers beyond 1.1 μm [4]. However, power scaling-up of Yb-RFLs faces two problems. One is the high-order Stokes wave induced by high-order Raman scattering at high pump power level, and another is residual pump of Stokes wave in the output laser at low pump power level. Fortunately, adjusting Raman threshold according to the pump power enables us to solve these two problems at the same time in the Yb-RFL seed [5].
