Forming an optical device includes growing an electro-absorption medium in a variety of different regions on a base of a device precursor. The regions include a component region and the regions are selected so as to achieve a particular chemical composition for the electro-absorption medium included in the component region. An optical component is formed on the device precursor such that the optical component includes at least a portion of the electro-absorption medium from the component region. Light signals are guided through the electro-absorption medium from the component region during operation of the component.