We present a theoretical and experimental investigation on refractive index (RI) sensing characteristics of single mode fiber (SMF) based modal interferometers. Theoretical analysis reveals that interference between different modes in an SMF has a quite different response to the RI variation of the external medium. The interference between the core and lower order cladding modes has negative RI sensitivity whereas that between the core and higher order modes, or between two different order cladding modes have positive sensitivity. A single-mode-multimode-single-mode (SMS) fiber Michelson interferometer with a large-core step-index multimode fiber (MMF) is employed for experimental verification. In the SMS-based Michelson interferometer, the MMF acts as a mode coupler to excite cladding modes in the SMF. The RI response of the SMS-based structures with two different lengths of MMF are respectively tested in sodium-chloride water solutions. Experimental results show excellent agreements with the theoretical analysis.
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