To provide a semiconductor laser diode and a semiconductor optical amplifier and an optical communication device incorporating the semiconductor laser diode or the semiconductor optical amplifier which enable low power consumption and high optical output. A semiconductor laser diode according to an embodiment of the present invention is a semiconductor laser diode 100 comprised of an active waveguide, the active waveguide including a first waveguide 11a that supplies a plurality of modes including a fundamental mode; and a second waveguide 12a that is wider than the first waveguide 11a and supplies a multimode, wherein the fundamental mode is provided as an oscillation light oscillated from the active waveguide.