Bright and Dark Square Pulses Generated From a Graphene-Oxide Mode-Locked Ytterbium-Doped Fiber Laser oleh Rong-yong Lin, Yong-gang Wang, Pei-guang Yan, Ge-lin Zhang, Jun-qing Zhao, Hui-quan Li, Shi-sheng Huang, Guang-zhong Cao, Ji-an Duan