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This paper introduces a method of measuring the delta between the effective refractive index of a silicon waveguide and a waveguide with wider dimensions through the constructive and destructive interference in a Mach-Zehnder interferometer (MZI). The method consists of a fixed effective refractive index variation incorporated by tapering one of the arms in the interferometer to a wider waveguide dimension. The MZI consists of a Y-branch splitter and a multimode interference (MMI) coupler. The Y-branch splitter splits the input light 50/50 into the two arms, and the MMI is used for recombination of the two arms. A change in the effective refractive index of one arm in comparison with the other arm in the interferometer will introduce a phase difference on recombination in the MMI. The MMI has the following three ports: the top and bottom output ports, which are the antisymmetric outputs, and the middle port, which is the symmetric output. When the two signals are in phase, all the light is coupled into the symmetric port, and when the two inputs are π out of phase, the light is coupled 50/50 into the antisymmetric ports. The interferometer is designed on a silicon-on-insulator (SOI) wafer and fabricated through IMEC Belgium. Theoretical, simulation, and measured results are presented and compared.