  Multimode fiber-based saturable absorbers enable mode-locking in lasers,
generating ultrafast pulses and providing an exceptional platform for
investigating nonlinear phenomena. Previous analyses in the continuous wave
(CW) limit showed that saturable absorption can be obtained due to nonlinear
interactions between transverse modes. We find experimentally that saturable
absorption can be achieved thanks to the interplay of single-mode fiber
nonlinearity and the wavelength-dependent linear transmission of the multimode
fiber, even with negligible intermodal nonlinearities. We further show that
even when intermodal nonlinearities are significant, the CW analysis may not be
sufficient for long multimode fibers. Understanding the underlying mechanisms
of multimode fiber-based saturable absorbers opens new possibilities for
developing programmable devices for ultrafast control.
