PURPOSE: To generate a short optical pulse necessary for an ultrahigh speed optical communication by a mode locking method.

CONSTITUTION: Three regions of an optical amplifying region 22 having semiconductor double heterojunctions, an optical modulating region 24 having a light absorption end for a shorter wavelength than the emitting wavelength of the amplifying region and formed of semiconductor double heterojunctions for holding a quantum well layer, and an optical waveguide region 23 having a semiconductor layer or a quantum well layer which has a light absorption end in a shorter wavelength than that of the absorption end of the modulating region as a core layer are connected in series in the same plane, and light reflecting surfaces are formed on both ends of a structure in which the three regions are disposed in series. Thus, a light is generated from the amplifying region, a voltage applied to the modulating region is modulated, and a loss of the modulating region is modulated to module the entire gain to generate a mode locking, thereby generating a short optical pulse.