Mode-Locked Thulium-Doped Fiber Laser Using a Single-Layer-Graphene-Covered Tapered Fiber

For the first time, to the best of our knowledge, we demonstrate an all-fiber passively mode-locked thulium-doped fiber ring laser using evanescent field coupling in a tapered fiber with a single-layer graphene as a saturable absorber. The modulation depth limitation of a single-layer graphene is overcome by incorporating the graphene layer along the fiber longitudinal plane, effectively extending the interaction length between the single-layer graphene and the evanescent field. The fiber oscillator produces stable 0.98 ps soliton pulses with a pulse energy of 544 pJ, a 3 dB spectral bandwidth of 4.6 nm, and an RF signal-to-noise ratio of 70 dB.