Optical modulator with an optical waveguide partially folded back and optical module with the optical modulator

An optical modulator comprising:

• an optical waveguide formed on a substrate and partially folded back;
• and
• a low refractive index portion formed on the substrate along a folded portion of the optical waveguide and having a refractive index lower than a refractive index of the substrate,
• wherein the optical waveguide includes:
• a plurality of linear portions arranged in parallel to each other;
• a plurality of arcuate portions, in the folded portion of the optical waveguide, concentrically arranged around a predetermined center position on the substrate at intervals narrower than intervals between the plurality of linear portions, each of the arcuate portions having a central angle and curvature radius that increase as each of the arcuate portions is closer to the center position;
• and
• a plurality of connecting portions respectively connecting the plurality of linear portions and the plurality of arcuate portions, at least one group of the connecting portions bending only in a direction opposite to a direction where each of the arcuate portions bends, and
• the low refractive index portion is formed on the substrate along an outer periphery of each of the arcuate portions and has an extending portion extending from a position corresponding to a connecting part between each of the arcuate portions and each of the connecting portions, the low refractive portion having a shape that gradually separates from the connecting part.