Generation of Bright Pulses, Dark–Bright–Dark Pulses, and H-Shaped Pulse Rains in Fiber Lasers with CoTe2 Nanosheets as Saturable Absorbers for Ultrafast Photonics

In this paper, we report the use of CoTe2 nanosheet polyvinyl alcohol composites as saturable absorbers for pulse generation in erbium-doped fiber lasers. The saturable absorption characteristics of CoTe2 nanosheets have been investigated, and their modulation depth and saturation intensity are 3.64% and 1.25 MW/cm2, respectively. By controlling the pump power and polarization controller, conversion from dark–bright–dark pulses to h-shaped pulse rains can be achieved in the laser. This mode-locked state transition occurs because the nonlinear effect of the saturable absorber increases as the pump power increases. This is the first time that mode-locked operation in the 1.5 μm range has been achieved in a fiber laser using a CoTe2 nanosheet-based saturable absorber, and the first time that h-shaped pulse rains have been generated by using a real saturable absorber. Our experimental results show that CoTe2 nanosheets have great potential for application in ultrafast photonics.