An optical modulator (30) includes a waveguide region having, for radiation of a predetermined wavelength such as 1.3 microns, a variable optical transmittance characteristic. The optical transmittance characteristic is a function of a difference between an index of refraction of a first region (36) comprised of silicon having a first type of electrical conductivity and an index of refraction of a second adjacent region (34) comprised of an insulator such as a silicon dioxide. The modulator further includes a third region (38), also comprised of silicon, disposed adjacent to the first region. The third region has a second type of electrical conductivity for forming a p-n junction with the first region. Charge carriers are injected into the first region for varying the index of refraction thereof such that the optical transmittance characteristic of the waveguide region is varied.