Self-pulsing in a double-clad ytterbium fiber laser induced by high scattering loss A. Martı ´nez-Rios a, * , I. Torres-Go ´mez a , G. Anzueto-Sanchez b , R. Selvas-Aguilar c a Centro de Investigaciones en Optica, Lomas del Bosque 115, Colonia Lomas del Campestre, 37150, Leo ´ n, Guanajuato, Mexico b Departamento de Electro ´nica, FIMEE, Universidad de Guanajuato. A.P. 215-A 36730 Salamanca, Guanajuato, Mexico c Facultad de Ciencias Fı ´sico Matema ´ ticas, UANL, Cd. Universitaria, NL 66450, Mexico Received 18 June 2007; received in revised form 24 September 2007; accepted 1 October 2007 Abstract The characteristics of self-pulsing behavior in a double-clad ytterbium-doped fiber laser are presented. The self-pulsing generates a supercontinuum spectrum that mostly propagates through the multimode structure of the fiber. In fact, 90% of the total power, including the fundamental laser wavelength and the generated supercontinuum spectrum, propagates trough the first cladding. The experimental evidence indicates that the main mechanism for the self-pulsing is the high scattering loss in the double-clad fiber. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Fiber laser; Double-clad; Self-pulsing; Ytterbium; Scattering; Supercontinuum 1. Introduction Generation of high peak power pulses in double-clad fiber lasers is recognized as a desirable alternative to solid-state pulsed laser sources [1]. The most common approach to obtain a pulsed regime in fiber lasers is based on external bulk elements, such as acousto-optic or electro- optic modulators, that are inserted into the cavity to form an actively Q-switched fiber laser [2]. In contrast with solid- state lasers, fiber lasers require longer interaction lengths, which in conjunction with the high powers sometimes favors the onset of nonlinear effects such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), self-phase modulation (SPM), cross-phase modula- tion (XPM), and four-wave mixing (FWM) [3,4]. In this case, a direct coupling between the stimulated emission and the nonlinear effects is observed, which result in insta- bilities and the generation of unwanted Stokes waves that are detrimental for the laser performance [4,5]. On the other hand, to obtain higher pulse energies it is necessary to increase the gain volume of the active medium, which in the case of Q-switched double-clad fiber lasers results in an increase of the fiber length or larger doped core. The increase of fiber length instead, limits the mini- mum pulse width that can be obtained in the fiber laser, i.e., the longer the cavity length, the broader the pulse. A different approach to obtain Q-switched operation of fiber lasers exploit the fiber nonlinearities and their dynamics, allowing the generation of pulses in the order of a few nanoseconds and high peak powers [6,7]. Recently, the operation of a double-clad fiber laser with 10 kW peak power and pulse width of the order of 2 ns has been dem- onstrated [6]. In this case, several dynamic mechanisms contribute to the generation of the pulses, namely, distrib- uted Rayleigh scattering, cascaded stimulated Brillouin scattering (SBS), SBS amplification and noise [8]. It is clear that in this case the main mechanism for the passive Q- switching is based on the cascaded Rayleigh scattering and SBS that acts as a dynamic feedback element enhanced by the addition of few meters of standard single-mode fiber. The high peak power and short pulses generated through SBS initiate a cascade of nonlinear processes 0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.10.006 * Corresponding author. Tel.: +52 477 441 42 00; fax: +52 477 441 42 09. E-mail address: amr6@cio.mx (A. Martı ´nez-Rios). www.elsevier.com/locate/optcom Available online at www.sciencedirect.com Optics Communications 281 (2008) 663–667