The energy structure and the value of the electrooptic effect in heteroepitaxial structures (HES) with multiple InGaAlAs / InAlAs quantum wells have been studied. The interferometric method has been developed to determine small changes in the refractive index under transverse transmission of light through a layered structure. The length of the Mach-Zehnder interferometer for the modulator at HES with multiple InGaAlAs / InAlAs quantum wells has been chosen.