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A temperature-insensitive fiber-optic refractive-index sensor based on optically heated long-period grating (LPG) is presented. The LPG is manufactured in a short section of cobalt-doped optical fiber (COF), which can be thermally stabilized through a pump light. Two fiber Bragg gratings (FBGs) are manufactured in COF and single-mode fiber (SMF), respectively, to monitor the local sensor and surrounding medium temperature. Experimental results demonstrate that this fiber-grating-based miniature refractive-index sensor can efficiently eliminate temperature cross-sensitivity in the temperature range of 5~60°C.