  We demonstrate that the intrinsic properties of monolayer graphene allow it
to act as a more effective saturable absorber for mode-locking fiber lasers
compared to multilayer graphene. The absorption of monolayer graphene can be
saturated at lower excitation intensity compared to multilayer graphene,
graphene with wrinkle-like defects, and functionalized graphene. Monolayer
graphene has a remarkable large modulation depth of 95.3%, whereas the
modulation depth of multilayer graphene is greatly reduced due to nonsaturable
absorption and scattering loss. Picoseconds ultrafast laser pulse (1.23 ps) can
be generated using monolayer graphene as saturable absorber. Due to the
ultrafast relaxation time, larger modulation depth and lower scattering loss of
monolayer graphene, it performs better than multilayer graphene in terms of
pulse shaping ability, pulse stability and output energy.
