  We theoretically and experimentally demonstrate a novel mode-locked
ytterbium-doped fiber laser with a saturable absorber based on nonlinear Kerr
beam cleanup effect. The saturable absorber was formed by a 2-m graded-index
multimode fiber and a single-mode fiber segment served as a diaphragm. With an
all-normal-dispersion-fiber configuration, the laser generated dissipative
soliton pulses with pulse duration of 26.38 ps and pulse energy more than 0.25
nJ; output pulses could be compressed externally to 615.7 fs. Moreover, the
self-starting mode-locking operation of this laser exhibited a high stability
with a measured signal-to-noise ratio of 73.4 dB in the RF spectrum.
