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.