Multi-stage amplification of laser near 980 nm enabled by absorption saturation of Yb-doped fiber

Multi-stage amplification is significant for up-scaling near-diffraction-limited laser power near 980 nm. However, its feasibility is unclear due to the 1030-nm amplified spontaneous emission (ASE) enhanced by high-power seed light injected into last-stage amplifier. This paper investigates the feasibility of multi-stage amplification near 980 nm by studying the effect of seed light. Interestingly, it is found that the 1030-nm ASE can always be well suppressed with unlimitedly enlarged seed power because of saturation absorption of Yb-doped fiber (YDF), as long as YDF is short enough. More importantly, the shortened YDF is no shorter than the optimum YDF length of pumped amplifier near 980 nm, and thus it will not lower the amplifier efficiency. Thus, multi-stage amplification near 980 nm is feasible without cost of pump efficiency. The experimental demonstration is also given by using a two-stage YDF amplifier, and the record 163.4-W power at 979 nm with M² factor smaller than 1.8 is achieved. This study can provide significant guidance for studying high-power high-brightness fiber lasers near 980 nm and their applications.