Semiconductor optical amplifier

A semiconductor optical amplifier comprising:

• a lower cladding layer of a first conductive type, the lower cladding layer including first and second regions, the first and second regions being arranged in a predetermined direction;
• an upper cladding layer of a second conductive type, the upper cladding layer being supported by the first region;
• an active layer provided between the first region of the lower cladding layer and the upper cladding layer, light in the active layer propagating in the predetermined direction;
• a diffraction grating layer having a diffraction grating,
• the diffraction grating extending in the predetermined direction, the diffraction grating layer being supported by the second region,
• the diffraction grating layer being optically coupled to the active layer by abutting contact therewith,
• the diffraction grating diffracting light from the active layer in a direction, the direction being different from the predetermined direction,
• the diffraction grating layer having a bandgap energy greater than the active layer, and
• a surface of the diffraction grating layer having a periodic structure for the diffraction grating;
• a first anti-reflecting coating provided on one end of the semiconductor optical amplifier, the one end of the semiconductor optical amplifier receiving input light, the active layer being located between the first anti-reflecting coating and the diffraction grating layer;
• and
• a second anti-reflecting coating provided on another end of the semiconductor optical amplifier.