Mode-locking with slow and fast saturable absorbers-what's the difference?

Abstract
We investigate the differences in the dynamics of lasers mode-locked by fast and slow saturable absorbers. Slow saturable absorbers can already generate almost transform limited pulses much shorter than the recovery time of the absorber. If soliton-like pulse shaping is present in addition the pulses can be further compressed below the resulting net gain window until either the continuum breaks through or the pulses break up into multiple pulses, which sets a limit to the shortest pulsewidth achievable. Given a certain amount of saturable absorption, a comparison is made that results in an estimate for the shortest pulse achievable for a solitary laser stabilized by a fast or a slow saturable absorber. The theoretical results are compared with experiments.