IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 7, APRIL 1, 2009 417 Fiber Ring Laser Operated by Dynamic Local Phase Shifting of a Chirped Grating Ana González-Segura, Pere Pérez-Millán, José Luis Cruz, and Miguel V. Andrés, Member, IEEE Abstract—An ultranarrow linewidth erbium-doped fiber ring laser is presented. It is based on the filtering properties of a phase-shifted chirped fiber Bragg grating, which is inserted inside the cavity of the laser. A dynamic control of the phase shifting, which is induced by a magnetostrictive transducer, permits both tunable continuous-wave and actively -switched operation. The use of a chirped grating overcomes the limitations imposed by the narrow spectra of uniform gratings observed in previously reported ring lasers based on intracavity transmission filters. Index Terms—Fiber ring laser, phase-shifted Bragg gratings, -switched laser. I. INTRODUCTION U LTRANARROW linewidth fiber lasers are of high interest for their application in fields like optical communications, sensing, medicine, imaging, or laser ranging. Distributed Bragg reflectors (DBRs) or distributed-feedback (DFB) fiber structures based on phase-shifted Bragg gratings are commonly used for the fabrication of ultranarrow linewidth lasers [1], [2]. However, they have short lengths, which limit the power of the emission and makes lasing difficult if they have low doping concentra- tions, while high doping concentrations induce self-pulsation at moderate pumping powers due to nonlinear effects [3]. The use of Fabry–Pérot or ring structures prevent from the limitations mentioned above: the active DFB Bragg grating structure can be inserted inside a ring cavity [4]; however, the maximum achiev- able power is limited by the length of the grating. In order to get higher powers, extra active fiber can be used and an external filtering element is needed in order to select the emission wave- length [5], [6]. Phase-shifted Bragg gratings are intrinsically ultranarrow bandpass filters of very high selectivity. They are of interest in many applications, such as transmission-based channel selec- tion in optical communications or subnanostrain sensing [7]. In particular, if introduced inside a laser cavity, phase-shifted Manuscript received August 19, 2008; revised November 27, 2008. First published February 03, 2009; current version published March 13, 2009. This work was supported by the Ministerio de Educación y Ciencia of Spain (Grant TEC2005-07336-C02-01/MIC). The work of A. González-Segura and P. Pérez-Millán was supported by the FPI program. A. González-Segura is with the Physical-Chemistry Department, University of Valencia, 46100 Burjassot, Spain (e-mail: ana.gonzalez-segura@uv.es). P. Pérez-Millán is with the Universitat Politècnica de València, Nanopho- tonics Technology Center, 46022 Valencia, Spain (e-mail: ppmillan@ntc.upv. es). J. L. Cruz and M. V. Andrés are with the ICMUV-Department Física Aplicada, University of Valencia, 46100 Burjassot, Spain (e-mail: cruz@uv.es; miguel.andres@uv.es). Digital Object Identifier 10.1109/LPT.2009.2013129 gratings determine the bandwidth and modify the -factor of the cavity. Two main procedures are utilized to fabricate phase-shifted Bragg gratings: either during the fabrication of the grating (utilizing, for instance, phase-shifted phase masks [8]) or by a postprocess based on local exposure to ultraviolet (UV) radiation or thermal treatment [9], [10]). These pro- cesses lead to a static phase-shifted Bragg grating. Recently, we have proposed a technique for generating dynamic phase shifts based on a magnetostrictive transducer [11]. If inserted inside the cavity of a fiber ring laser, a phase-shifted grating behaves as a selector of the longitudinal modes, narrowing the natural linewidth of the laser, which can work even in single-longitudinal-mode regime, as Guy et al. demonstrated [5]. Compared to other schemes to fabricate ultranarrow linewidth lasers (DFB or DBR), such a ring configuration permits longer cavities, hence higher gains with low doping concentrations, preventing self-pulsation. In this letter, the mag- netostriction-based technique to change the grating phase-shift dynamically [11] is incorporated to the ring laser scheme of [5]. This increases the versatility of the laser significantly, since it can work either in a wavelength-tunable continuous-wave (CW) regime or in an actively -switched pulsed regime [12], adding pulsed light-based applications (as optical time-domain reflectometry (OTDR), or microprocessing of materials) to the list of potential uses of this type of narrow linewidth laser source. Chirped fiber Bragg gratings (CFBGs) have been used in order to overcome the limitations imposed by narrow spec- trum FBGs. First, unwanted laser oscillation at high pumping levels due to sidelobes of the external grating is avoided thanks to the wide stopband of the intracavity CFBG. Consequently, higher output powers can be obtained, as demonstrated in [5] (where wideband FBGs are used), compared to the low power achieved in [6] (where narrowband FBGs are used). Uniform apodized external gratings could be utilized also for this purpose [13], [14]. Second, the bandwidth of the external grating is wide enough to avoid the need of tuning its central wavelength to compensate environmental temperature changes. II. SETUP Fig. 1 shows the assembly of the fiber ring laser cavity used in our experiment. The light of a standard 980-nm pumping diode is launched through 4 m of low concentration erbium- doped fiber (EDF) through a wavelength-division multiplexer. The fiber used (Fibercore DF1500D-0980) had an absorption of 5.5 dB/m at 979 nm, a cutoff wavelength nm, and a numerical aperture of 0.23. The FBGs used were both chirped, fabricated by UV exposure through a uniform phase mask and a 1041-1135/$25.00 © 2009 IEEE