Objectives: The present study focused on the development of a optic fiber sensor for carbon dioxide (CO2), the measuring of variations of the refractive index of a CO2 medium undergoing controlled changes of temperature and pressure, and comparing this experimental refractive index with a calculated theoretical refractive index. Materials and methods: For the making of the optical sensor, we used samples of 48 to 56 micrometers inner diameter micro-structured optic fiber with a solid silica

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... red hollow center; and we selected dimethilformamide as the light conductor fluid. The fiber was filled with pressure; the connectrorization of the fiber was made with the fussion, the high-temperature afhesive, and the mechanical processes. We use a refractometer for measuring the refractive index during the temperature and pressure change processes in the CO2 chamber. Results: Profiles of the refractive index vs. temperature in cooling and heating cycles were obtained, as well as the punctual measures of the refractive index in five fixed thermodynamic points tested; the calculation for the theoretical refractive index for these points was obtained, and was compared with the experimental ones. The percentage of error of the experimental refractive index was estimated between 0.016% and 0.03%. Conclussions: We achieved the measuring of transmission of white light through a fluid in different phases, and we registered significant changes in the refractive index in each phase change using the fabricated optic sensor.