  We firstly report mode-locked square-wave pulse in Yb-doped fiber laser based
on graded index multimode fiber of reverse suturable absorption. By adjusting
the pump power, the width of the square-wave can be tuned range from 350 ps to
52.6 ns with 3dB bandwidth spectrum of 0.73 nm. The supermode suppression ratio
exceeds ~ 65 dB, which indicates excellent mode-locked operation state, and we
also study the characteristic of pulse by the chirp measurement system. The
mode-locked square-wave pulse fiber laser can sever as high power light source
for industrial applications.
