High energy rectangular pulse generated in a low repetition rate all normal-dispersion Yb3+-doped fiber laser

With elongated cavity length, a 317 kHz low-repetition-rate rectangular pulse was demonstrated in an all-normal dispersion mode-locked (ML) ytterbium-doped fiber laser (YDFL) based on the nonlinear polarization evolution technique. As the pump power increased, the duration of the rectangular pulse broadened linearly without wave-breaking. At a maximum pump power of 240 mW, a rectangular pulse with a duration of 20.5 ns and highest pulse energy of 182 nJ was produced. Through an Yb3+-doped fiber amplifier with 4 W pump power, the energy of the rectangular pulse was further amplified up to 2.28 mu J. The duration of the rectangular pulse remained constant as the pump energy increased and revealed no pulse distortion under the maximum output energy.