▎ 摘　　要

Graphene saturable absorbers (GSAs) have been widely applied in ultra-fast mode-locked fiber lasers. Thanks to the broadband advantage of graphene, we theoretically and experimentally demonstrate the variation of the modulation depth of GSA by employing the effect of cross absorption modulation. This method provides an easy and efficient way to modulate the characteristics of GSA. By varying the modulation power, we realize an all-fiber fundamental mode-locked fiber laser and a harmonic mode-locked fiber laser with tunable output pulse width. Results show that the output pulse widths of the two fiber lasers can be tuned more than 40%, and the lasers have high wide application potential on nonlinear optical bio-imaging and offer an advantageous front end for extreme-power laser technologies. (C) 2014 AIP Publishing LLC.