Optical waveguide, optical modulator and optical coupler

An optical modulator of Mach-Zehnder type comprising:

• an optical waveguide that is provided at least at one of a branch portion on an optical input side and a branch portion on an optical output side,
• wherein:
• the optical waveguide is formed in a substrate having electrooptical effect;
• the optical waveguide has a curved waveguide and a splitter that is coupled to the curved waveguide and branches;
• a low refraction index portion having an effective refraction index that is lower than that of a start point of the curved waveguide on an opposite side of the splitter is provided in a waveguide extending from the start point of the curved waveguide to the splitter;
• the low refraction index portion has a curve;
• and
• the effective refraction index of the curve of the low refraction index portion increases or decreases continuously or in steps toward the splitter.

An optical modulator of Mach-Zehnder type comprising a substrate that has a waveguide including a splitter and a curved waveguide,

• wherein:
• a first end of the curved waveguide is coupled to the splitter;
• a second end of the curved waveguide acts as an optical input end or an optical output end;
• the curved waveguide has a low refraction index portion having an effective refraction index that is lower than that of a start point of the curved waveguide, in a waveguide from the start point to the splitter;
• the low refraction index portion has a curve;
• and
• the effective refraction index of the curve of the low refraction index portion increases or decreases continuously or in steps toward the splitter.