ACTIVE OPTICAL FIBERS: NEW DESIGN AND ALTERNATIVE METHOD OF FABRICATION Philippe Roy 1 , Mathieu Devautour 1 , Laure Lavoute 1 , Sébastien Février 1 , Georges Humbert 1 , Philippe Leproux 1 , Aude Roy 1 , Raphaël Jamier 1 , Frédéric Gérôme 1 , Kay Schuster 2 , Jens Kobelke 2 , Stephan Grimm 2 , Carlos Pedrido 3 , Frédéric Sandoz 3 1 Xlim Institute, University of Limoges CNRS, 123 Avenue Albert Thomas, 87060 Limoges, France 2 IPHT, Institute of Photonic Technology, Albert-Einstein-Str. 9, 07743 Jena, Germany 3 Silitec Fibers SA, Route de la Gare 70, CH-2017 Boudry, Switzerland philippe.roy@xlim.fr Abstract: In this paper, we propose, through different examples, some alternate designs and methods of manufacturing for pushing the limits of active fiber optics. The novelty comes from modal filtering under laser operation, fabrication of fibers using silica powders and laser with extended spectrum of emission. 1. INTRODUCTION Fiber lasers are resolutely entering the market of lasers and this requires the development of fibers dedicated to specific applications. High-power lasers and some active fibers already demonstrate very large effective area and singlemode operation [1]. However, such fibers are based on PCF design and singlemode operation occurs only with refractive index of core close to that of pure silica. As ytterbium doping of silica matrix provokes an increase of refractive index, the average ytterbium doping level remains limited in such cores. With the aim of decreasing interaction length between light and material, and consequently non linear disturbance, we demonstrate some alternate design and method of fabrication. First, strong interaction between a highly ytterbium doped multimode core and a properly tailored microstructured cladding is studied and under laser operation, modal filtering is performed and singlemode operation is expected. Such a structure being very sensitive to slight variations of refractive index profile, we propose, in a second section, a new fabrication process based on direct sand vitrification for active fiber fabrication. MCVD drawbacks such as small doped diameter and poor refractive index homogeneity are suppressed as this method allows the fabrication of large and highly doped core with high index homogeneity. The fabrication process will be described and as a proof of concept, a highly efficient multimode continuous wave fiber laser has been demonstrated. In another field of applications, fiber based optical sources have demonstrated very nice capabilities for broad band emission like supercontinua. Up to now, low cost visible sources mainly rely on a nonlinear microstructured fiber pumped near its zero dispersion wavelength by a nanosecond microchip laser [2]. However, the available Spectral Power Density (SPD) at the output of the fiber remains limited by the low average power of microchip laser. To increase SPD while keeping low cost and compact pump source, we propose the use of a cladding- pumped nonlinear and rare-earth-doped microstructured fiber, both in amplifier and laser configuration. 2. LHDC FIBER LMA and highly doped core are required for short non linear interaction between light and active fiber in high power fiber amplifiers or short fiber laser cavities. In the proposed design, a Large and Highly rare-earth-Doped multimode Core (LHDC, see figure 1) interacts with a tailored cladding made of an array of high refractive index inclusions. When selective coupling (proper overlapping and phase matching) is operated between high order modes of the core and modes of the rods, the electric field of propagating modes spreads over the cladding and the overlap factor with gain medium is decreased. Under laser operation, mode selection occurs leading to the emission of one single mode. Figure 1: Image of fiber core surrounded by an array of high refractive index rods. 2.1 Theoretical approach The principle of the modal selection is based on a total internal reflection guidance regime modified by resonant coupling with modes of microstructured cladding. Depending on ytterbium doped core parameters, several methods can be used to estimate suitable refractive index profile of rods and cladding pitch. The anti-resonant reflecting optical waveguides (ARROW) theory gives a good estimation of