Power amplification characteristics of ultrafast pulses after the NPR port of Yb-doped fiber laser in all normal dispersion configuration

The power amplification characteristics of all-fiber Yb-doped double clad fiber amplifier seeded by the pulses extracted after the nonlinear polarization rejection (NPR) port of all-normal dispersion (ANDi) mode-locked Yb-doped fiber oscillator is reported. 21.8 W of amplified average power at 37 MHz repetition rate was obtained at pump power of 35 W corresponding to ~62% pump to signal power conversion efficiency. The single pulse energy of the amplified mode locked pulses was ~590 nJ. The effect of pre-amplifier signal strength on temporal and spectral profiles of amplified pulses were investigated experimentally. It was found that though stronger signal strength at the input of the power amplifier is useful to increase the threshold for onset of amplified spontaneous emission (ASE), however, it may lead to severe distortion in temporal profile after compression due to enhanced nonlinearity. We conclude that there is an optimum signal strength for generation of clean and smooth ultrashort pulses with low ASE contribution with in the operating range of the power amplifier. The compressed pulse duration of the amplified pulses was measured to be 280 fs. Keywords—All normal dispersion, mode locked, fiber amplifier, Yb-doped fiber laser.