Device structure, fabrication method, and experimental results of a GaAs/AlGaAs traveling‐wave directional coupler optical modulator/switch operated at 1.06 &mgr;m wavelength are described for the first time. An extinction ratio of 13.7 dB has been measured at the switching voltage of 10.4 V, and a 3 dB bandwidth of 9.1 GHz has been demonstrated for a device with a waveguide width of 5.5 &mgr;m, waveguide spacing of 3.5 &mgr;m, and length of 8 mm.