Semiconductor optical modulator, semiconductor optical integrated device, and method of manufacturing the same

A method of manufacturing a semiconductor optical modulator, comprising forming a two-step mesa optical modulator having a first clad layer;

• a mesa-like core layer formed over the first clad layer;
• and
• a second clad layer that is formed over the core layer so as to have a mesa-like shape, and that has a mesa width smaller than a mesa width of the core layer,
• wherein the two-step mesa optical modulator further has:
• a multi-mode optical waveguide region to which an electric field is applied or into which an electric current is injected;
• and
• a single-mode optical waveguide region to which an electric field is not applied and into which an electric current is not injected,
• wherein Wmesa1>Wmesa2 is satisfied, when the mesa width of the core layer in the multi-mode optical waveguide region is defined as Wmesa1, and the mesa width of the core layer in the single-mode optical waveguide region is defined as Wmesa2,
• wherein the mesa width Wrib1 of the second clad layer in the multi-mode optical waveguide region and the mesa width Wrib2 of the second clad layer in the single-mode optical waveguide region are substantially equal to each other.