We present an optical waveguide modulator structure based on Wannier-Stark localization in a InGaAs/InAlAs superlattice. Optical waveguide transmission below the superlattice bandgap displays expected F-1 oscillatory behavior leading to various modulation schemes. An 11 dB extinction ratio is obtained by applying a 0.7 V drive voltage to a 100-mu-m long waveguide device operating at 1.55-mu-m under a TE-polarization mode.