Investigation of multipass Yb-doped fiber amplifiers

We investigated both numerically and experimentally single-pass, two-pass, and four-pass Yb-doped core-pumped fiber amplifier configurations and compared their performance with respect to achievable pump-to-signal conversion efficiency. Despite numerical calculations predicting the highest efficiency in the four-pass setup, an experimentally highest conversion efficiency of 77% was achieved in the two-pass setup and only 49% in the four-pass setup. Reduced performance of the experimental four-pass fiber amplifier configuration was caused by excess coupling losses, which do not impose a fundamental limitation. At the same time, the four-pass fiber amplifier configuration demonstrated advantageous characteristics of high gain using shorter length of active fiber and lower noise figure compared with other tested configurations.