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This paper describes a waveguide sensor design that allows interferometric phase sensing in an optical waveguide without the requirement of a reference leg. The new design is based on two guided modes that interfere by an abrupt discontinuity of the waveguide into a single-mode output. Refractive index changes in the sensed material generate phase changes mainly in the higher mode, with the lower mode used as a reference. Calculated values show sensitivities much higher than single-channel surface plasmon resonance sensors and somewhat lower than dual channel phase sensors in a conventional Mach-Zehnder configuration. Our design allows for a simple, single-channel, compact, highly sensitive, and low attenuation phase sensor. A comprehensive analysis of the role of different design parameters in the performance of the sensor is presented.