Optical phase modulator, optical circuit, mach-zehnder interferometer, and optical accelerator

An optical phase modulator comprising:

• a first optical waveguide comprising:
• a first cladding layer,
• a core layer disposed above the first cladding layer, and
• a second cladding layer disposed above the core layer;
• a first electrode electrically connected to the first cladding layer;
• and
• a second electrode electrically connected to the second cladding layer, wherein:
• the first cladding layer comprises a first compound semiconductor layer of a first conductivity type,
• the second cladding layer comprises a second compound semiconductor layer of a second conductivity type,
• the core layer comprises a third compound semiconductor layer,
• at least one of the first compound semiconductor layer and the second compound semiconductor layer comprises a composition gradient layer, and
• a concentration of a first conductive impurity in the third compound semiconductor layer is lower than a concentration of the first conductive impurity in the first compound semiconductor layer.