Preparation of ultrathin graphitic carbon nitride nanosheet and its application to a tunable multi-wavelength mode-locked fiber laser

Tunable multi-wavelength mode-locked ytterbium-doped fiber laser is realized by using ultrathin graphitic carbon nitride (g-C3N4) nanosheets as saturable absorber. g-C3N4 nanosheets with an average thickness of 3.5 nm are obtained through liquid-phase ultrasonic exfoliation of g-C3N4 powder prepared by high temperature sintering. Passively mode-locked ytterbium-doped fiber laser was obtained by using the g-C3N4 nanosheets coated D-shaped fiber as saturable absorber. It is interesting that tunable dual-, triple- and quadruple-wavelength mode-locked lasers were obtained by adjusting the pump power and polarization controller. And the maximum average output power of the mode-locked laser can reach 10.32 mW. The pulse width of the quadruple-wavelength mode-locked laser is 1.9 ns, with a working repetition of 11.2 MHz. To our best knowledge, this is the first time reporting this tunable multi-wavelength mode-locking phenomenon based on g-C3N4.