  The performance of fiber mode-locked lasers is limited due to the high
nonlinearity induced by the spatial confinement of the single-mode fiber core.
To massively increase the pulse energy of the femtosecond pulses, amplification
is obtained outside the oscillator. Recently, spatiotemporal mode-locking has
been proposed as a new path to fiber lasers. However, the beam quality was
either highly multimode and/or with low pulse energy. Here we present an
approach to reach high energy per pulse directly in the mode-locked multimode
fiber oscillator with a near single-mode output beam. Our approach relies on
spatial self-beam cleaning via the nonlinear Kerr effect and we demonstrate a
multimode fiber oscillator with M2<1.13 beam profile, up to 24 nJ energy and
sub-100 fs compressed duration. The reported approach is further power scalable
with larger core sized fibers and could benefit applications that require high
power ultrashort lasers with commercially available optical fibers.
