Reflective semiconductor optical amplifier with constant gain versus wavelength

A reflective semiconductor optical amplifier comprising:

• a semiconductor substrate;
• an active layer that includes a first region and a second region formed over the semiconductor substrate;
• and
• a reflection part that is formed along the second region and includes a first portion that reflects a first wavelength light having a first gain coefficient and a second portion that reflects a second wavelength light having a second gain coefficient which is smaller than the first gain coefficient,
• wherein the first portion is formed closer to the first region side than the second portion, and
• wherein the active layer includes a first end face of the first region from which the first wavelength light and the second wavelength light are input and from which a reflected first wavelength light reflected by the first portion and a reflected second wavelength light reflected the second portion are output from the second region via the first region and positions of the first portion and the second portion are set so that a first product of a first effective length of the reflective semiconductor optical amplifier for the first wavelength light and the first gain coefficient and a second product of a second effective length of the reflective semiconductor optical amplifier for the second wavelength light and the second gain coefficient become substantially constant.