We describe the design, fabrication, and characterization of an optical modulator in silicon-on-insulator (SOI). The modulator is based upon a rib waveguide, with a transverse p - i - n diode being used to produce a refractive index change through the plasma dispersion effect. Measurements of optical modulation show submicrosecond transition times and confirm that electronic switching dominates over thermal effects. (C) 2004 American Vacuum Society.