Fiber Bragg grating photowriting in microstructured optical fibers for sensing application based on refractive index measurement M.C. Phan Huy *a , G. Laffont a , V. Dewynter-Marty a and P. Ferdinand a P. Roy b , J-M. Blondy b and D. Pagnoux b W. Blanc c and B. Dussardier c a CEA-LIST, Centre d’Etudes de Saclay, 91191 Gif-sur-Yvette Cedex France b IRCOM / OGI, UMR-CNRS 6615, 123 Avenue Albert Thomas, 87060 Limoges Cedex France c LPMC / FOA, UMR-CNRS 6622, Université de Nice Sophia-Antipolis, Parc Valrose, 06108 Nice France ABSTRACT In this paper we present results relative to Fiber Bragg Grating (FBG) photowritten in two kinds of Ge-doped microstruc- tured optical fiber cores devoted to sensing applications. A cross comparison between theoretical and experimental mo- dal field patterns is carried out. We present the first values of spectral sensitivity of Bragg grating, in relation to the re- fractive index of the substance inserted into the holes. Keywords: microstructured optical fibers, Fiber Bragg Grating, sensors 1. INTRODUCTION In the last few years, due to their large potentialities in several fields of optics, such as non-linear effects, dispersion compensation, high power propagation or sensors 1 , photonic crystal fibers are more and more a matter of interest in R&D laboratories. Classically, we can distinguish fibers guiding light by photonic bandgap effect 2 and microstructured optical fibers (MOF) guiding by total internal reflection. For the latter, different process coexist for the Bragg grating photowriting 3,4,5 methods in Ge-doped photosensitive fibers 6 . In this study, we have designed and manufactured micro- structured optical fibers using a Ge-doped core devoted to Bragg grating photowriting in the prospect of refractometry applications. 2. DESIGN AND MANUFACTURING OF MICROSTRUCTURED FIBERS AND FBG PHOTOWRITING In order to refine the convenient design of microstructured fibers, we have implemented a simulation tool based on a scalar version of the localized functions method 7,8 (LMF). The decomposition of the index profile is adapted to take the doped core 8 into account. This tool allows us to accurately model the guided modes characteristics (effective index n eff , modal field patterns) taking into account the air-hole distributions and the refractive index of the substance present into the holes. The Ge-doped preform is fabricated by LPMC-Nice France using the MCVD technique. In order to reduce the diameter of such preform, two successive steps are necessary to reach the diameter aimed. A mechanical machining is used first to reduce the diameter from 120 mm to 2 mm, followed by a chemical attack in order to get a diameter close to 1 mm. We use then a conventional stack and draw process to fabricate the fibers. The first manufactured fiber is constituted by one ring of six air holes of diameter d ~ 15 μm and pitch Λ ~ 15.8 μm, surrounding a Ge- doped core of 11 μm (see Fig. 1a). Such fiber is multimode (d/Λ= 0.95). The second fiber fabricated, is a two-ring triangular of respectively 6 and 12 air holes diameter d ~ 3.1 μm and pitch Λ ~ 6.7 μm, surrounding a Ge-doped core of 5 μm (slightly decentered) * phanhuy@cea.fr ; phone +33 (0)1 69 08 29 22; fax : +33 (0)1 69 08 83 95