Contents lists available at ScienceDirect Optics Communications journal homepage: www.elsevier.com/locate/optcom Modelling of graphene Q-switched Tm lasers A.S. Yasukevich a , P. Loiko b , N.V. Gusakova a , J.M. Serres c , X. Mateos c,d, ⁎ , K.V. Yumashev a , N.V. Kuleshov a , V. Petrov d , U. Griebner d , M. Aguiló c , F. Díaz c a Center for Optical Materials and Technologies, Belarusian National Technical University, 65/17 Nezavisimosti Ave., Minsk 220013, Belarus b ITMO University, Kronverkskiy pr., 49, Saint-Petersburg 197101, Russia c Física i Cristal·lografia de Materials i Nanomaterials (FiCMA-FiCNA), Universitat Rovira i Virgili (URV), Campus Sescelades, c/ Marcel·lí Domingo, s/n., Tarragona E-43007, Spain d Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, 2A Max-Born-Str., Berlin D-12489, Germany ARTICLE INFO Keywords: Thulium laser Diode-pumped laser Q-switching Graphene ABSTRACT We report on a model of diode-pumped Thulium lasers passively Q-switched by a graphene saturable absorber applicable also for any other “fast” saturable absorber. It reasonably predicts the dependence of the pulse duration, pulse energy and pulse repetition frequency on the absorbed power. The model is applied in the present work for a Tm: KLuW microchip laser passively Q-switched with a multi-layer graphene saturable absorber. The laser generates ~1 W at 1926 nm with a slope efficiency of 39%. Stable 190 ns /4.1 μJ pulses are achieved at a pulse repetition frequency of 260 kHz. The potential of graphene for the generation of few-ns pulses at ~2 μm is discussed. 1. Introduction Thulium (Tm 3+ ) is a laser-active ion that provides an emission at ~2 μm due to the 3 F 4 → 3 H 6 electronic transition [1]. This emission finds applications in medicine (due to the strong absorption of water at this wavelength) and remote sensing of water and CO 2 in the atmo- sphere. A common approach to produce laser pulses from a solid-state laser is the passive Q-switching (PQS) technique, which is realized by the insertion of an appropriate saturable absorber (SA) into the laser cavity. Conventional “bulk” SAs for Tm lasers are based on zinc chalcogenides, Cr:ZnS and Cr:ZnSe, which enable the generation of Q-switched pulses with high energies at low pulse repetition frequen- cies (PRFs) [2,3]. Recently, novel SAs for Tm lasers based on carbon nanostructures have attracted a lot of attention, including graphene [4], graphene oxide [5] and single-walled carbon nanotubes [6]. Focusing on graphene, this material is composed of a single layer of carbon atoms arranged in a honeycomb lattice [7]. It shows broadband and almost wavelength-insensitive linear absorption [8] from ~0.6 μm up to at least ~3 μm and also broadband saturable absorption [9]. Consequently, graphene can be used as “universal” SA for near-IR lasers, including those based on Tm. It shows relatively low saturation intensity, reasonable non-saturable losses, high laser damage threshold and ultrafast recovery time [10–12]. In addition, the control of the modulation depth is possible by varying the number of graphene layers [9]. Graphene saturable absorbers (GSAs) may enable the generation pulses at high PRFs, typically ranging from hundreds of kHz to few MHz. Recently, PQS of bulk Tm lasers with graphene has been realized [4,13,14]. The potential of graphene for the generation of ns pulses in Tm lasers at ~2 μm has also been shown [15]. In the present work, we aimed at a theoretical description of PQS of Tm lasers by a GSA. It enables the prediction of the pulse character- istics as well as their dependence on the absorbed pump power. To verify the validity of our model, a laser based on a Tm:KLu(WO 4 ) 2 crystal (Tm:KLuW) is experimentally investigated. It belongs to the family of monoclinic double tungstates (DTs), which are very suitable hosts for Tm doping [16]. Tm-doped DTs offer intense and broad absorption and emission bands for different polarizations [17,18], and high doping concentrations are possible [19] without significant changes in the crystalline structure and spectroscopic properties. Efficient Tm-doped DT lasers operating in the continuous-wave (CW) [20–22] and the PQS [23–25] regime have been reported previously. As laser set-up, we selected the microchip geometry [26] where both the laser crystal and SA are placed in a compact plano-plano cavity. In particular for GSA, such a set-up is useful for the generation of shorter pulses in the ns range due to the significant reduction of the cavity roundtrip time [15,27]. http://dx.doi.org/10.1016/j.optcom.2016.12.023 Received 1 June 2016; Received in revised form 6 December 2016; Accepted 8 December 2016 ⁎ Corresponding author. E-mail addresses: xavier.mateos@urv.cat, mateos@mbi-berlin.de (X. Mateos). Optics Communications 389 (2017) 15–22 0030-4018/ © 2016 Published by Elsevier B.V. MARK