Graphene Mode-Locked Fiber Laser at 2.8 <inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula>

Mid-infrared erbium (Er ³⁺ )-doped ZrF ₄ -BaF ₂ -LaF ₃ -AlF ₃ -NaF fiber laser mode-locked by multilayer graphene saturable absorber was demonstrated. Mode-locked pulses at 2.8 μm with an average output power of 18 mW at a repetition rate of 25.4 MHz, corresponding to a pulse energy of 0.7 nJ, were obtained. The pulsewidth was measured to be ~42 ps by a home-made autocorrelator. Our experiment has validated the mode-locking capability of graphene in the 3-μm wavelength region.