Long-Period Grating Thermal Sensitivity Dependence on the External Medium Refractive Index Ricardo C. Kamikawachi, Gustavo R. C. Possetti, Márcia Muller and José L. Fabris Federal University of Technology –Paraná, Curitiba, 80.230-901, Brazil Abstract  We report the thermal sensitivity dependence of long period gratings on the surroundings refractive index. For external refractive indexes ranging from 1.000 to 1.447 the grating thermal sensitivity changes from 0.040 ± 0.001 nm/ºC to 0.393 ± 0.015, respectively. The presented results point to the important behavior that must be considered when the device is intended to operates as a temperature sensor for different external media, or as a refractometer working in different temperatures. Keywords  Long period grating, optical sensor, temperature sensor, refractive index. I. INTRODUÇÃO LPGs are formed by inducing a periodic refractive-index modulation in the core of an optical fiber. The phase- matching condition causes light from fundamental guided mode to be coupled to forward-propagating cladding modes at distinct wavelengths, given by the following relation [1]: ( ) Λ − = m cl co m n n λ (1) where and represent the refractive-index of the guided mode and a LP co n m cl m cl n 0m cladding mode, respectively. The , and the grating period Λ can be affected due to changes in the external medium, such as strain, temperature or refractive index. As a result, the coupling wavelength (λ co n n m ) experiences a shift that can be used to measure the parameter being changed. The optical power coupled to the cladding modes are strongly affected by fiber imperfections, micro and macro bending, and by boundary condition at the cladding-external medium interface. Thus, the light coupled from core to the cladding modes leaks out the fiber, leaving several dips in the transmission spectrum, each one corresponding to a specific coupling governed by (1). ) Ricardo C. Kamikawachi, canute@cpgei.cefetpr.br, Gustavo R. C. Possetti, gustavo_possetti@pop.com.br , Márcia Müller, marcia@cefetpr.br, José L. fabris.fabris@cefetpr.br, Tel. +55-41-33104642, This work was partially supported by CAPES, CNPq and Fundação Araucária (Brazilian Agencies) For resonant wavelengths the transmission T though the core is [2]: ( 2 / cos 2 DL T = (2) where L is the grating length and D is the coupling coefficient. In accordance with Quin et al [3] the thermal sensitivity of LPG is due two factors: the thermal expansion effect and the thermo-optic effect. The thermal-expansion coefficient for silica is about 10 -7 ºC -1 [4], while the thermo-optic coefficient, α, is about 10 -5 ºC -1 [3]. Therefore the thermal sensitivity of LPG mainly depends on the thermal-optic coefficient given by [3]: ( ) T n n n n m cl co m cl co ∆ − ∆ − = 1 α (3) Temperature sensitivities of LPGs produced in single mode fibers are rather low, reaching only values between 0.04 and 0.1 nm/ºC [5]. Some techniques have been adopted to improve this temperature sensitivity. A significant enhanced sensitivity of 3.4 nm/ºC was achieved with a bare LPG inscribed in commercial Boron/Germanium co-doped fiber operating in the dispersion turning-point region [6]. A still higher sensitivity of 19.2 nm/ºC was obtained for a bare LPG immersed in a liquid with a high thermo-optic coefficient and index of refraction close to that of fiber cladding [7]. He et al [8] achieved a wavelength shift of 60 and 0.6 nm to temperature changes from 0 to 100 ºC, using acrylate-based polymer and silicone resin as recoating materials surround LPG, respectively. Recently Chormát et al [9] obtained sensitivities of 0.56 nm/ºC for the bare LPG fabricated in a graded-index optical fiber and 0.86 nm/ºC when the same grating is recoated with a polymer layer. In these works the temperature sensitivity change was obtained by properly doping the fiber core, by altering the fiber structure and geometry and by coating the LPG with a polymer layer or surround it with a temperature-sensitive liquid. In this work long period gratings were produced by the use of a point-by-point writing method, applying on a bare fiber