Fig. 1. Characterization of graphene oxide-COOH nanosheets. (a) SEM image, the inset shows graphene oxide-COOH solution; (b) Raman spectra.

Fig. 2. (a) Graphene oxide-COOH-PVA film attached on the end of fiber connector; (b) nonlinear transmission curve of graphene oxide-COOH-PVA film.

Fig. 3. Experimental setup of the fiber laser with graphene oxide-COOH as a mode locker.

Fig. 4. (Color online) (a) Pulse trains at the pump power of 40 mW: inset, single pulse profile of the Q-switching laser; (b) the evolution of pulse repetition rate and duration as pump power increases; (c) the evolution of output power and pulse energy with the pump power; (d) the output spectrum of Q-switching.

Fig. 5. (a) Output spectrum of mode-locking pulse; (b) output pulse train; (c) output autocorrelation trace with a pulse duration of 1.5 ps; (d) radio-frequency spectrum.