Advanced optoelectronic devices require monolithic integration of different functions at chip level. This is the case of multi-quantum well (MQW) electroabsorption modulated lasers (EMLs) realized by using the selective area growth (SAG) technique, and which can be employed in long-distance, high-frequency optical fiber communication applications. We demonstrate that a micrometer-resolved X-ray beam available at third-generation synchrotron radiation sources allows direct measurement of determinant structural parameters of MQW EML structures.