▎ 摘　　要

We demonstrate an ytterbium doped fiber (YDF) laser being able to generate dark pulses and Q-switched pulses by using a graphene oxide (GO) based composite as saturable absorber (SA). These operation regimes are realized for the first time in a 1-mu m band fiber laser as far as our best knowledge. In the fabrication of GO based saturable absorber (GOSA), a vertical evaporation process is included, which results in a GO concentration and thickness variable SA. The generated dark pulse has pulse duration in ns-scale. In the Q-switched operation, pulses with durations from 5.321 mu s to 2.655 mu s and repetition rates from 28.47 kHz to 58.95 kHz can be obtained as the launched pump power is changed. The transition from the dark pulse to Q-switchecl operations can be clearly seen as the pump power is increased to a certain value. The radio frequency (RF) and optical spectrum variations with respect to the change of pump power have also been analyzed in detail. (C) 2013 Elsevier B.V. All rights reserved.