Patent ID: 8204344

Claim:
An optical modulator, comprising: a substrate having an electro-optic effect; a buffer layer formed over said substrate; and a traveling wave electrode including a center electrode and a ground electrode formed above at least a part of said buffer layer, in which said substrate has a plurality of ridge portions including a first ridge portion and a second ridge portion, said ridge portions being formed with a process of forming gap portions by digging said substrate at regions where an electric field generated by a high frequency electric signal traveling through said traveling wave electrode is strong, at least one of said ridge portions has an optical waveguide formed therein, and said center electrode and said ground electrode are respectively formed above said first ridge portion and said second ridge portion, characterized in that said first ridge portion below said center electrode has a top part having a first end point, said second ridge portion below said ground electrode has a top part having a second end point, said first end point and said second end point are separated with each other with a distance of “W R ”, said substrate has a bottom surface formed between said ridge portions, and said substrate has a cross section vertical to a longitudinal direction of said optical modulator and crossing said ridge portions, said cross section being formed such that, said bottom surface has a center point, a first midway point, and a second midway point, said center point being positioned with a distance of W R /2 in a horizontal direction from said first end point, said second midway point being positioned with a distance of W R /N (3≦N≦25) in the horizontal direction from said first end point, said bottom surface further has a bump around said center point, said first midway point is positioned between said bump and said second midway point, said first ridge portion below said center electrode has a normal line to said top part, said first midway point and said second midway point define a straight line passing therethrough, and said straight line is crossed with said normal line at an angle larger or equal to 90.1°, to ensure that a microwave equivalent refractive index n m of said high frequency electric signal is reduced to be close to an effective refractive index n o of said optical waveguide.