Fiber laser switched by a long period grating interferometer as an intra-cavity loss modulator D. Sáez-Rodríguez a, ⁎, J.L. Cruz a , Yu. O. Barmenkov b , A. Díez a , M.V. Andrés a a Departamento de Física Aplicada y Electromagnetismo/ICMUV, Universidad de Valencia, Dr. Moliner 50, Burjassot 46100, Spain b Centro de Investigaciones en Óptica, Loma del Bosque 115, 37150, León, Guanajuato, Mexico abstract article info Article history: Received 4 November 2009 Received in revised form 17 March 2010 Accepted 18 March 2010 Keywords: Fiber laser Active switch Long period grating Fiber interferometer In this paper we present an actively switched fiber laser with an all-fiber long-period grating-based interferometer used as an intra-cavity loss modulator. The modulator consists of two equal long-period gratings written sequentially in the same piece of a double-clad optical fiber. One of the gratings is fixed onto a piezoceramic cylinder producing fast modulation of the interferometer transmission spectrum. The laser demonstrates a stable regime of pulsed emission at repetition rates in the range of tens of kHz. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Long period fiber grating (LPG) produces energy coupling between fiber modes traveling through the fiber in the same direction. LPGs were extensively studied for many practical applications such as band rejection filters [1] or, in cascaded configuration, as coaxial fiber interferometers [2]; LPGs have been also used for wavelength division multiplexing [3], many types of fiber sensors [4], gain equalizers in erbium doped fiber amplifiers [5], etc. Nowadays, because of surge of a practical interest in rare-earth doped fiber lasers and amplifiers, LPGs have been found some new applications as, for instance, tunable optical filters [6,7], pump power management in cladding pumped fiber lasers [8] and control of the operation wavelength [9]. In the framework of fiber laser applications, the main disadvantage of LPGs arises from the interaction of cladding modes with the surrounding medium. LPGs written in double cladding fibers overcome this problem and therefore found a number of new potential applications [10]. They have been studied for some special sensing applications, in which, for instance, thermal and mechanical stabilities are required [11]. Recently we have demonstrated a robust intensity all- fiber modulator based on two cascaded LPGs written in double-clad fiber [12]. Actively Q-switched fiber lasers can be implemented using an external transducer to modulate the Q-factor of the laser cavity. Usually bulk acousto-optic modulators or rotating mirrors are used to modulate the intra-cavity loss [13,14] while mangnetostrictive or piezoelectric transducers can be applied in all-fiber configurations to modulate fundamental properties of the electromagnetic field inside the laser cavity: phase modulation is a common way to change the spectrum of Bragg reflectors for Q-switching fiber lasers [15]; polarization modula- tion implemented with high birefringent fibers results in polarization dependent loss [16]; finally, direct modulation of intra-cavity loss has been achieved by interaction with the evanescent field in either a polished fiber or a tapered fiber [17,18], also by an acoustically induced microbending, which behaves as LPG, to couple energy from the fundamental PL 01 fiber mode to cladding modes [19,20] and, recently, by dynamic misalignment of the fiber cores [21]. Ramachandran and coworkers [22] have proposed a new concept of fiber laser where amplification is performed by high order cladding modes and emission is achieved through the fiber core, in that work the authors propose a conventional Fabry-Perot cavity based on two Bragg gratings and conversion to cladding modes is realised by two cascaded long period gratings. Though Ramanchandra's proposal still has theoretical character and has not been experimentally demon- strated (probably because of the lack of appropriate fiber) the passive device has been successfully operated [23]. In this paper we report the first demonstration, to the best of our knowledge, of a switched erbium-doped fiber laser (EDFL) based on intra-cavity loss modulation using a pair of LPGs written sequentially in the same piece of a double-clad fiber. These LPGs establish a Mach– Zehnder interferometer for fiber modes propagating through the core and cladding in the same direction. The modulator is driven by a piezoceramic cylinder held to one of the gratings. The laser demon- strates a stable regime of giant pulses in the range of repetition frequencies of 10 kHz. The technique proposed here is completely compatible with the concept of high order mode laser proposed in [22]. Optics Communications 283 (2010) 2892–2895 ⁎ Corresponding author. E-mail address: david.saez@uv.es (D. Sáez-Rodríguez). 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.03.041 Contents lists available at ScienceDirect Optics Communications journal homepage: www.elsevier.com/locate/optcom