Ultra-wide square pulses generation in a Yb-doped fiber laser based on nonlinear polarization rotation effect

An ultra-wide square pulse is demonstrated in a passively mode-locked Yb-doped fiber laser based on nonlinear polarization rotation effect. In our experiment, the pulse width can be tuned from 26 ns to 164 ns without wave-breaking when the pump power increases from 210 mW to 600 mW. And the peak power of the square pulse remains constant. The maximum output single pulse energy is 8.75 nJ at a pump power of 600 mW. To the best of our knowledge, this is the widest pulse in any mode-locked Yb-doped fiber laser.