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We have developed a novel phase modulator, based on fin-type electrodes placed at self-imaging positions of a silicon multimode interference (MMI) waveguide, which allows reduced scattering losses and relaxes the fabrication tolerance. The measured propagation losses and spectral bandwidth are 0.7 dB and 33 nm, respectively, on a 987 μm-long phase shifter. Owing to the self-imaging effect in the MMI waveguide, the wave-front expansion to the electrode was counteracted, and therefore, the scattering loss caused by electrode fins was successfully mitigated. As a proof-of-concept for the MMI-based phase modulator applications, we performed optical modulation based on Mach−Zehnder interferometers (MZIs). The π shift current of the modulator was 1.5 mA.