Eur. Phys. J. AP 6, 225–228 (1999) T HE EUROPEAN P HYSICAL JOURNAL APPLIED PHYSICS c  EDP Sciences 1999 Rapid Note A tunable Er 3+ –doped fiber laser with polarimetric and Mach-Zehnder filters N. Azami, A. Sa¨ ıssy a , M. De Micheli, G. Monnom, and D.B. Ostrowsky Laboratoire de Physique de la Mati` ere Condens´ ee b , Universit´ e de Nice-Sophia Antipolis, Parc Valrose, 06108 Nice Cedex, France Received: 22 December 1998 / Revised: 16 March 1999 / Accepted: 21 April 1999 Abstract. We show theoretically and experimentally that the wavelength tunability of an Er 3+ –doped fiber laser is improved by using a twofold filter combining a Mach-Zehnder interferometer and a polarimetric filter. This new filter allows us to tune continuously the wavelength from 1.53 to 1.56 μm without the discontinuity associated to the narrow transmission bands of a Mach-Zehnder tuning filter. PACS. 42.81.-i Fiber optics – 42.55.Wd Fiber lasers – 42.81.Gs Birefringence, polarization 1 Introduction The development of the optical fiber amplifier and os- cillator has inspired investigation into related technolo- gies for improving their properties, such as mode locking, modulation, sensors, and wavelength tuning and selection. Tunability of Er 3+ –doped fiber have received widespread attention, to tune the wavelength of these lasers in the range 1.52-1.60 μm various intra-cavity methods are available. A tuning range of 30 nm is currently obtained with fiber Bragg grating [1] or acousto-optical modulators [2,3]. Tuning filter based on integrated optic waveguides, as lithium niobate devices [4], are generally sensitive to the state of polarization (SOP) of the signal at the in- put of them. Mainly, these devices operate on a linearly polarized state (either a TM or TE mode). In tunable single-mode fiber (SMF) laser systems where such devices are used as filters, three conditions should be satisfied for optimal performance. (i) SOP of the fiber output light must be maintained to avoid system disruptions, the use of fiber with a well- defined birefringence overcome these polarization sta- bility problems. (ii) The signal SOP mismatch between the fiber and the polarization-sensitive component must be minimized. The polarization of light entering the device can be typically controlled by using a fiber polarization con- troller (FPC) [4]. (iii) In order to ensure low lasing threshold as well as to optimize both slope efficiency and spectral laser tun- a e-mail: saissy@naxos.unice.fr b UMR 6622 du CNRS ing range, the filter must be characterized by low loss, narrow bandwidth, and single-transmission peak on the gain band. Integrated Mach-Zehnder interferometer (MZI) on LiNbO 3 crystal is an interesting tuning element with a transmission spectrum easily tunable using low voltage. The gain spectrum of an Er 3+ –doped fiber result mainly from the combination of two bands around 1.53 and 1.55 μm, also the transmission spectrum of the MZI must induce large differential losses between these two bands in order to prevent any band hopping. As an increase of the free spectral range (FSR) of the filter give rise also to an increase of the transmission spectral width, the con- dition (iii) cannot be satisfied easily using only one MZI. Here we present a way to increase the tunability of a fiber laser by adding to the MZI filter a polarimetric tunable filter made from a FPC and a birefringent erbium-doped fiber. The birefringence of the fiber being choosed such that the FSR of the polarimetric filter is larger than the FSR of the MZI alone. This can be made non destruc- tively on a standard doped fiber by coiling the fiber under tension on a cylindrical former [5,6]. As a result, the com- bination of the MZI, the birefringent fiber, and the FPC act as a tuning filter with large FSR and small width of maximum transmission. The tunability of the polarimetric filter allows us to induce the large differential losses nec- essary to the selection of only one band, 1.53 or 1.55 μm, in the gain spectrum. In this paper, first we describe the tuning properties of the MZI-FPC system. Experiment on a tunable erbium- doped fiber laser is then presented and discussed.