120W monolithic Yb-doped fiber oscillator at 1150nm

We present experimental results on a high-power, diode-pumped, 1150 nm Yb-doped fiber (YDF) oscillator in an all-fiber configuration. A cut-back method is used to investigate the impacts of YDF length on the key performances of the oscillator, including parasitic lasing threshold, spectral evolution, and laser efficiency. Discrepancies of the 1150 nm laser's output characteristics with different fiber Bragg grating reflectivity are also analyzed. A record power of 120 W is obtained when a length of 7 m YDF is utilized, with a slope efficiency of 54.5% with respect to injected pump power. The threshold of parasitic lasing decreases with the YDF length and possible explanations of this unexpected phenomenon are discussed in detail.