Electroreflectance and electroabsorption measurements have been carried out to clarify field effects on the refractive index and absorption coefficient in AlGaAs quantum well structures. The observed electroreflectance spectra show very clear exciton-induced features at room temperature. A maximum variation of the refractive index in each quantum well at a photon energy near the lowest excitonic transition gap is obtained to be 4 percent induced by a 105V/cm field modulation. Electroreflectance and electroabsorption spectra are shown to demonstrate a relation between dispersion curves of the field-induced variations in the refractive index and absorption coefficient in the quantum well structure. Based on the obtained results, operation of an electroabsorption modulator with the capability of small frequency chirping and an efficient optical switch are discussed from the practical point of view.