The proposed sensing device relies on the self-imaging effect that occurs in a pure silica multimode fiber (coreless MMF) section of a single-mode-multimode-single-mode (SMS)-based fiber structure. The influence of the coreless-MMF diameter on the external refractive index (RI) variation permitted the sensing head with the lowest MMF diameter (i.e., 55 7mu;m) to exhibit the maximum sensitivity (2800 nm/RIU). This approach also implied an ultrahigh sensitivity of this fiber device to temperature variations in the liquid RI of 1.43: a maximum sensitivity of-1880 pm/°C was indeed attained. Therefore, the results produced were over 100-fold those of the typical value of approximately 13 pm/°C achieved in air using a similar device. Numerical analysis of an evanescent wave absorption sensor was performed, in order to extend the range of liquids with a detectable RI to above 1.43. The suggested model is an SMS fiber device where a polymer coating, with an RI as low as 1.3, is deposited over the coreless MMF; numerical results are presented pertaining to several polymer thicknesses in terms of external RI variation. © 2012 Optical Society of America.511632363242Kumar, A., Varshney, R.K., Antony, C.S., Sharma, P., Transmission characteristics of SMS fiber optic sensor structures (2003) Opt. Commun., 219, pp. 215-219Wang, Q., Farrell, G., Multimode-fiber-based edge filter for optical wavelength measurement application (2006) Microw. Opt. Technol. Lett., 48, pp. 900-902Wang, P., Brambilla, G., Ding, M., Semenova, Y., Wu, Q., Farrell, G., High-sensitivity, evanescent field refractometric sensor based on a tapered, multimode fiber interference (2011) Opt. Lett., 36, pp. 2233-2235Hatta, A.M., Semenova, Y., Wu, Q., Farrell, G., Strain sensor based on a pair of single-mode-multimode-single-mode fiber structures in a ratiometric power measurement scheme (2010) Appl. Opt., 49, pp. 536-541Silva, S., Frazäo, O., Viegas, J., Ferreira, L.A., Araújo, F.M., Malcata, F.X., Santos, J.L., Temperature and strainindependent curvature sensor based on a singlemode/ multimode fiber optic structure (2011) Meas. Sci. Technol., 22, p. 085201Soldano, L.B., Pennings, E.C.M., Optical multi-mode interference devices based on self-imaging: Principles and applications (1995) J. Lightwave Technol., 13, pp. 615-627Mohammed, W.S., Smith, P.W.E., Gu, X., All-fiber multimode interference bandpass filter (2006) Opt. Lett., 31, pp. 2547-2549Antonio-Lopez, J.E., Castillo-Guzman, A., May-Arrioja, D.A., Selvas-Aguilar, R., Wa, P.L., Tunable multimode-interference bandpass fiber filter (2010) Opt. Lett., 35, pp. 324-326Antonio-Lopez, J.E., Sanchez-Mondragon, J.J., Wa, P.L., May-Arrioja, D.A., Fiber-optic sensor for liquid level measurement (2011) Opt. Lett., 36, pp. 3425-3427Wu, Q., Semenova, Y., Wang, P., Hatta, A.M., Farrell, G., Experimental demonstration of a simple displacement sensor based on a bent single-mode-multimode-single-mode fiber structure (2011) Meas. Sci. Technol., 22, p. 025203Wu, Q., Semenova, Y., Wang, P., Farrell, G., High sensitivity SMS fiber structure based refractometer-analysis and experiment (2011) Opt. Express, 19, pp. 7937-7944Aguilar-Soto, J.G., Antonio-Lopez, J.E., Sanchez-Mondragon, J.J., May-Arrioja, D.A., Fiber optic temperature sensor based on multimode interference effects (2011) J. Phys., 274, p. 012011Wang, Q., Farrell, G., Yan, W., Investigation on singlemode-multimode-singlemode fiber structure (2008) J. Lightwave Technol., 26, pp. 512-519Mohammed, W.S., Smith, P.W.E., Gu, X., Wavelength tunable fiber lens based on multimode interference (2004) J. Lightwave Technol., 22, pp. 469-477Mehta, A., Mohammed, W., Johnson, E.G., Multimode interference-based fiber-optic displacement sensor (2003) IEEE Photon. Technol. Lett., 15, pp. 1129-1131Abbate, G., Bernini, U., Ragozzino, E., Somma, F., The temperature dependence of the refractive index of water (1978) J. Phys. D, 11, pp. 1167-1172Owens, J.C., Optical refractive index of air: Dependence on pressure, temperature and composition (1967) Appl. Opt., 6, pp. 51-59Kawano, K., Kitoh, T., (2001) Introduction to Optical Waveguide Analysis, pp. 165-230. , Wiley Chap. 5Silva, S., Santos, J.L., Malcata, F.X., Kobelke, J., Schuster, K., Frazäo, O., Optical refractometer based on large-core, air-clad photonic crystal fibers (2011) Opt. Lett., 36, pp. 852-854Lee, S.T., Gin, J., Nampoori, V.P.N., Vallabhan, C.P.G., Unnikrishnan, N.V., Radhakrishnan, P., A sensitive fibre optic pH sensor using multiple sol-gel coatings (2001) J. Opt. A, 3, pp. 355-359