Multilayer graphene for Q-switched mode-locking operation in an erbium-doped fiber laser Zhiteng Wang a,1 , Shou-En Zhu b,1 , Yu Chen a , Man Wu a , Chujun Zhao a,n , Han Zhang a,n , G.C.A.M. Janssen b , Shuangchun Wen a a Key Laboratory for Micro-/Nano-Optoelectronic Devices of Ministry of Education, College of Information Science and Engineering, Hunan University, Changsha 410082, China b Micro & Nano Engineering Lab, Department of Precision and Microsystems Engineering, Delft University of Technology, Delft 2628CD, The Netherlands article info Article history: Received 29 January 2013 Received in revised form 11 March 2013 Accepted 12 March 2013 Available online 4 April 2013 Keywords: Multilayer-graphene Saturable absorber Mode-locking Q-switching Erbium-doped fiber laser abstract We report the laser operation of Q-switched mode-locking (QML) in an erbium-doped fiber laser by using a multilayer graphene saturable absorber (SA), which consists of 22-layer of graphene fabricated by the chemical vapor deposition method. Based on our balanced twin detector measurement, the graphene sample is confirmed to show a saturable intensity of 3.375 MW/cm 2 and an absolute modulation depth of 40.27%. It is demonstrated that this graphene SA can readily produce high quality QML pulses. At the pump power of 391.9 mW, the stable mode-locked pulse train with the Q-switched envelope repetition rate of 16.98 kHz and the envelope width of 13:84 μs are achieved. The maximal main pulse peak power can reach up to 35.89 W. This verifies that the multilayer graphene can be still applied as an effective saturable absorber for passively Q-switched mode-locked operation. Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved. 1. Introduction Laser mode-locking operation can be classified into the two types: continuous wave mode-locking (CWML), that is, mode- locked pulses emitted from every round trip show uniform pulse features, such as pulse duration, pulse energy, chirp and peak power [1]; Q-switched mode-locking (QML), that is, a train of pulses with cavity repetition rate inside a giant Q-switched envelope [2]. The repetition frequency of CWML is typically in the order of megahertz, which matches with the laser cavity repetition rate [1], whereas QML possesses two characteristic repetition frequencies: one relates with the cavity length and the other is the Q-switching modulation frequency in the kHz region [3]. Laser pulses generated from CWML show the advantages of high pulse stability and low noise. While, much higher pulse energy could be achieved by QML laser cavity [4]. Therefore, the QML pulses may be attractive in nonlinear frequency conversion, precise fabrication of microstructures [5], and surgery [6]. The scheme used for the generation of QML pulses can be active (e.g., exploiting acousto-optic modulator [7]) or passive (e.g., using saturable absorber (SA) [8]). Passively QML features a more compact geometry and simpler setup than active, which requires additional switching electronics (acousto-optic modula- tor) [7]. Doped bulk crystals [9], semiconductor SA mirrors (SESAMs) [10] and carbon nanotubes (CNTs) [11] are the common SAs in passive Q-switching. However, the doped crystals are mostly used in solid-state laser. The doped crystals, as SAs in fiber laser, require extra elements (mirrors, lenses) to focus the fiber output into the crystal. Therefore it is limited in a small range. SESAMs have limited operation bandwidth, typically few tens nm [12], which are not suitable for broad-band tunable pulse genera- tion. Compared to SESAMs, CNTs can be considered as the broad- band SAs, operated on Yb-doped [13] and Er-doped [14] fiber laser. However, its operation wavelength is related to diameter and chirality of the CNTs [15]. Recently, graphene, a single layer of carbon in a hexagonal lattice, has been intensively researched due to its wonderful optical properties [16]. In laser photonics, graphene SA has been widely used as a broadband SA to passively Q-switch or mode lock the fiber laser or solid state laser, at different laser wavelengths ranging from 1 μm to 2 μm [17–28]. Conventional solitons [17–20], dissipative solitons [29], vector solitons [30], and Q-switched pulses [25,26] have been reported in fiber laser based on graphene SAs. In the present paper, we report on the QML pulses in a fiber laser cavity by using a multilayer graphene SA. The saturable intensity and the absolute modulation depth of the multilayer graphene SA is about 3.375 MW/cm 2 and 40.27%, respectively. Through a range of experiments with an erbium-doped fiber (EDF) laser, it has been demonstrated that this multilayer graphene SA Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/optcom Optics Communications 0030-4018/$ - see front matter Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.03.010 n Corresponding authors. E-mail addresses: cjzhao@hnu.edu.cn (C. Zhao), hanzhang@hnu.edu.cn (H. Zhang). 1 These authors contributed equally to this work. Optics Communications 300 (2013) 17–21