A fiber optic sensor for simultaneous measurement of refractive index and temperature is presented. The sensing probe is achieved by introducing multimode interference inside a high birefringence fiber loop mirror resulting in a configuration capable of refractive index and temperature discrimination. The multimode interference peak is sensitive to the surrounding refractive index (90 nm/RIU) and slightly responsive to the temperature (0.005 nm°C). On the other hand, the birrefringent fiber loop mirror is highly sensitive to temperature (2.39 nm°C) and has no response to refractive index. Therefore, a temperature independent refractive index measurement can be made with a resolution of ±2.5x10-5. © 2012 SPIE.8421 Suhadolnik, A., Babnik, A., Mozina, J., Optical-fiber reflection refractometer (1995) Sensors and Actuators B Chemical, 29 (1-3), pp. 428-432Asseh, A., Sandgren, S., Ahlfeldt, H., Fiber optical Bragg grating refractometer (1998) Fiber and Integrated Optics, 17 (1), pp. 51-62Pereira, D.A., Frazao, O., Santos, J.L., Fiber Bragg grating sensing system for simultaneous measurement of salinity and temperature (2004) Optical Engineering, 43 (2), pp. 299-304Laffont, G., Ferdinand, P., Tilted short-period fibre-Bragg-grating-induced coupling to cladding modes for accurate refractometry (2001) Measurement Science & Technology, 12 (7), pp. 765-770Han, M., Guo, F.W., Lu, Y.F., Optical fiber refractometer based on cladding-mode Bragg grating (2010) Optics Letters, 35 (3), pp. 399-401Patrick, H.J., Kersey, A.D., Bucholtz, F., Analysis of the response of long period fiber gratings to external index of refraction (1998) Journal of Lightwave Technology, 16 (9), pp. 1606-1612Allsop, T., Reeves, R., Webb, D.J., A high sensitivity refractometer based upon a long period grating Mach-Zehnder interferometer (2002) Review of Scientific Instruments, 73 (4), pp. 1702-1705Esteban, O., Cruz-Navarrete, M., Gonzalez-Cano, A., Measurement of the degree of salinity of water with a fiber-optic sensor (1999) Applied Optics, 38 (25), pp. 5267-5271Fukano, H., Matsumoto, Y., Taue, S., High-sensitivity optical fiber refractive index sensor using multimode interference structure (2012) IEICE Electronics Express, 9 (4), pp. 302-306Jung, Y., Kim, S., Lee, D., Compact three segmented multimode fibre modal interferometer for high sensitivity refractive-index measurement (2006) Measurement Science & Technology, 17 (5), pp. 1129-1133Zhao, Y., Wu, H.K., Wang, Q., Advances of high-birefringence fiber loop mirror sensors (2011) Journal of Optoelectronics and Advanced Materials, 13 (5-6), pp. 457-465Frazao, O., Marques, B.V., Jorge, P., High birefringence D-type fibre loop mirror used as refractometer (2008) Sensors and Actuators B-Chemical, 135 (1), pp. 108-111Kang, J., Dong, X.Y., Zhao, C.L., Measurement of refractive index with a PM-LPG based sagnac loop sensor (2011) Spectroscopy and Spectral Analysis, 31 (4), pp. 902-905Zibaii, M.I., Frazao, O., Latifi, H., Controlling the sensitivity of refractive index measurement using a tapered fiber loop mirror (2011) Ieee Photonics Technology Letters, 23 (17), pp. 1219-1221Abbate, G., Bernini, U., Ragozzino, E., The temperature dependence of the refractive index of water (1978) Journal of Physics D: Applied Physics, 11 (8), pp. 1167-1172