Semiconductor optical integrated element

A semiconductor optical integrated element comprising:

• a laser diode portion that includes a first optical waveguide and generates laser light;
• a separation portion connected to the laser diode portion and including a second optical waveguide that has a first core layer to propagate laser light emitted from the laser diode portion;
• and
• an optical modulator portion connected to the separation portion and including a third optical waveguide that has a second core layer to absorb a part of the laser light emitted from the separation portion, wherein
• the third optical waveguide has a constant thickness in a direction perpendicular to a face of a substrate on which the optical modulator portion is formed,
• a proportion of the second core layer in a cross section of the third optical waveguide perpendicular to a propagation direction of the laser light increases from an incident side of the laser light toward an emission side of the laser light, and
• at an incident end face of the optical modulator portion, the third optical waveguide includes the first core layer or a third core layer that does not contribute to absorption of the laser light.