  The optical conductance of monolayer graphene is defined solely by the fine
structure constant. The absorbance has been predicted to be independent of
frequency. In principle, the interband optical absorption in zero-gap graphene
could be saturated readily under strong excitation due to Pauli blocking. Here,
we demonstrate the use of atomic layer graphene as saturable absorber in a
mode-locked fiber laser for the generation of ultrashort soliton pulses (756
fs) at the telecommunication band. The modulation depth can be tuned in a wide
range from 66.5% to 6.2% by varying the thickness of graphene. Our results
suggest that ultrathin graphene films are potentially useful as optical
elements in fiber lasers. Graphene as a laser mode locker can have many merits
such as lower saturation intensity, ultrafast recovery time, tunable modulation
depth and wideband tuneability.
