1484 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 48, NO. 11, NOVEMBER 2012 Experimental Study of the Nonlinear Dynamics of an Actively Q-Switched Ytterbium-Doped Fiber Laser Yuri O. Barmenkov, Member, IEEE, Alexander V. Kir’yanov, Member, IEEE, and Miguel V. Andrés, Member, IEEE Abstract—In this paper, a comprehensive experimental study of the dynamics of an actively Q-switched Yb-doped GTWave- based fiber laser is presented. It is shown that the appearance of the Q-switching regime in the laser is sensitive to both the repetition rate and the temporal width of the transparency window of an acousto-optical Q-switch modulator placed in the cavity. It is also shown that, at low repetition rates, a peculiar self-Q-switch regime induced by stimulated Brillouin scattering is observed in the laser, which interferes stochastically with the regular (true) active Q-switching mode. While increasing the Q-modulator repetition rate, the self-Q-switch pulsing steadily vanishes but the true Q-switching remains. The latter, however, becomes strongly subjected, while further increasing the Q-switch repetition rate, by the nonlinear laser dynamics effects. That is, the laser is allowed to operate at certain subharmonics of the repetition rate or in some specific regimes that occur at laser transients from one attractor to another. These transient regimes are characterized by alternating between pulse amplitude and energy, and pulse spacing or chaos, which is strongly influenced by the adjacent attractors’ properties. Index Terms—Active Q-switching, chaos, GTWave fiber, jitter, laser dynamics, stimulated Brillouin scattering, Yb-doped fiber. I. I NTRODUCTION Q -SWITCHED Ytterbium doped fiber lasers (QS-YDFLs) producing energetic pulses in a nanosecond range are attractive sources of light for using in many practical areas such as laser marking and cutting [1], nonlinear frequency conversion [2], [3], supercontinuum generation [4]–[6], laser- initiated ignition [7], etc. The important features of such lasers are a long interaction length of the pump light with the active fiber core, permitting to reach a high fiber gain, and single transversal mode operation, important for technological applications. It is frequently observed in actively and passively QS- YDFLs that output pulses have multi-peak structure with an Manuscript received June 7, 2012; revised September 1, 2012; accepted September 18, 2012. Date of publication September 28, 2012; date of current version October 16, 2012. This work was supported in part by the Ministerio de Economía y Competitividad under Project TEC2008-05490 of Spain, the Generalitat Valenciana under Project PROMETEO/2009/077 of Spain, and “Omics Research Ltd.” Hong Kong. Y. O. Barmenkov and A. V. Kir’yanov are with the Centro de Investigaciones en Óptica, León 35150, Mexico (e-mail: yuri@cio.mx; kiryanov@cio.mx). M. V. Andrés is with the Departamento de Física Aplicada ICMUV, Uni- versidad de Valencia, Burjassot 46100, Spain (e-mail: miguel.andres@uv.es). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JQE.2012.2220530 interval between the adjacent sub-peaks being equal to the cavity round-trip time [8]–[10]. Such shape of Q-switching (QS) pulses is fully explained by the model of two contra- propagating waves that travel through a long fiber cavity [8], [11]–[14]. Another feature of QS-YDFLs is pulse jittering observed mostly in passively QS lasers [15], [16], which deteriorates the regime quality. It is also known that in QS solid-state lasers, timing jitter is normally ruled by chaos [17]. Furthermore, the period dou- bling and deterministic chaos in the pulse energy distribution are observed in such lasers [18]. Recently, nonlinear dynam- ics of fiber lasers upon periodical modulation of intracavity loss (analog of active QS) was inspected theoretically [19] where a generalized multistability was shown to comprise the generation of QS pulses in multi-periodical and chaotic attractors. Despite a great number of researches that dealt with the dynamics of QS solid-state lasers exists, no comprehensive study – neither theoretical nor experimental – was made to date with fiber lasers. The present paper serves to fill, at least partially, the gap. Below, we present a detailed experimental analysis of the dynamics of an actively QS (AQS) fiber laser built using an Yb-doped GTWave (“twin” or “dual”) fiber [20]–[23]. Our aim was to make, on its example, a deeper insight into the details inherent in QS fiber lasers for getting a more complete picture of their nonlinear dynamics. We believe this would be important for further enhancement and optimization of fiber lasers of such kind. In the laser under study, an AQS unit is a standard acousto- optical modulator (AOM) with fiber outputs, which provides fast change in the cavity Q-factor. The GTWave active fiber [20] was chosen as one of the best solutions for simpli- fying an overall laser implementation and, at the same time, for clarifying the physics that stands behind AQS regimes. We demonstrate further that an AQS regime is mostly featured both by the AOM on/off repetition rate ( f m ) and the AOM transparency window (or “gate”). If the AOM gate is fixed while f m is increased from the lowest values, the laser consequentially passes through a set of regimes where QS pulses appear at the first, second and third sub-harmonics of the AOM repetition rate (so-called period-1 (P1), period-2 (P2), and period-3 (P3) attractors). Depending on the attractors’ numbers, the scenarios that may occur in the areas between the adjacent attractors upon 0018–9197/$31.00 © 2012 IEEE