Solid-state Laser - Mode Locking

Mode Locking

Mode locking of solid state lasers has wide applications as large energy ultra-short pulses can be obtained. Like its counterpart, the fiber laser, there are three types of real saturable absorbers widely used as mode lockers: SESAM, SWCNT and graphene.

Particularly, graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. It is recently confirmed that the optical absorption from graphene could become saturated when the input optical intensity is above a threshold value. This nonlinear optical behavior is termed saturable absorption and the threshold value is called the saturation fluence. Graphene can be saturated readily under strong excitation over the visible to near-infrared region, due to the universal optical absorption and zero band gap. This has relevance for the mode locking of fiber lasers, where wideband tuneability may be obtained using graphene as the saturable absorber. Due to this special property, graphene has wide application in ultrafast photonics. Further, comparing with the SWCNTs, as graphene has a 2D structure it should have much smaller non-saturable loss and much higher damage threshold. Indeed, with an erbium-doped fiber laser, self-started mode locking and stable soliton pulse emission with high energy have been achieved.