An optically activated optical switch based on suppression of mode interference (SMI) is presented. The imaging properties of multi-mode interference (MMI) section in the switch with Y-branch can be modified by a controlling light injection. The switch was simulated by finite difference beam propagation method (FD-BPM) and fabricated on GaAlAs/GaAs epitaxial materials. At the wavelength of 1.31 um, the primary experiment showed an extinction ratio of about 8 dB with controlling light power density of 73.5 W/mm2.