Strained-layer multiple quantum wells InAsP/InP and InAsP/InGaP optical modulators based on the quantum-confined Stark effect have been fabricated from layers grown by metalorganic vapor phase epitaxy on InP(001). The device layers have been characterized by complementary high resolution x-ray diffraction, transmission electron microscopy, optical absorption and photoluminescence analyses. The structural properties of the layers were deduced from the above data and an accurate determination of the band alignment of the heterostructures was made by performing multiple transition fits to the optical absorption spectra using the Martin-Bastard envelope function model for strained-layer superlattices. The electric held-dependent redshift of the fundamental electron-heavy hole transition was measured by a photocurrent method and found to be enhanced for structures with lower valence band barrier heights. This observation leads directly to the conclusion that the overall performance of high speed, low drive voltage optical modulators may be improved by engineering the band alignment of the multiple quantum well stack towards structures with disproportionately large conduction band offsets. An optimization of the band alignment will permit more efficient optical modulation by reducing the drive field required to operate the device, which, in turn, can have direct effects upon the drive voltage, device capacitance, attenuation coefficient, and optical coupling and propagation losses.