A semiconductor optical amplifier includes (a) a single-mode waveguide
region which provides a single-mode to a guided light-wave, (b) a first
multi-mode interference waveguide region which has a greater waveguide
width than that of the single-mode waveguide region, is optically
connected to the single-mode waveguide region, and provides a mode
including a multi-mode, to the guided light-wave, and (c) a second
multi-mode interference waveguide region which has a greater waveguide
width than that of the first multi-mode interference waveguide region, is
optically connected to the first multi-mode interference waveguide region,
and provides a mode including a multi-mode, to the guided light-wave.
