An optical waveguide includes, in monolithic manner, a rear segment, a transition segment, and a front segment extending and succeeding one another along the length of the waveguide. A lateral confinement factor and a width the waveguide are respectively larger and smaller in the rear segment than in the front segment. According to the invention, the width increases in the transition segment until it reaches a value that is larger than that of the front segment. The invention is particularly applicable to integrating together a laser and an optical modulator for a telecommunications system.