A guided‐wave interferometer capable of modulating 1.3‐&mgr;m radiation via the electro‐optic effect has been fabricated in GaAs. The interferometer uses three‐guide couplers as the input and output sections and single‐mode slab‐coupledp+‐n−‐n+rib waveguides fabricated by means of Be‐ion implantation for the two active arms. An extinction of ≊14.5 dB of the output signal from the center guide is obtained with 22 V applied to one arm of the interferometer. Interferometers of this type should be capable of large signal modulation out to 5–10 GHz.