Patent ID: 8600198

Claim:
A semiconductor optical modulator comprising: 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 W mesa1 >W mesa2 is satisfied, when the mesa width of the core layer in the multi-mode optical waveguide region is defined as W mesa1 and the mesa width of the core layer in the single-mode optical waveguide region is defined as W mesa2 , wherein the mesa width W rib1 of the second clad layer in the multi-mode optical waveguide region and the mesa width W rib2 of the second clad layer in the single-mode optical waveguide region are substantially equal to each other.