A monolithically integrated mode-locked semiconductor laser is proposed. The compound ring cavity is composed of a colliding pulse mode-locking (ML) subcavity and a passive Fabry–Perot feedback subcavity. These two 1.6 mm long subcavities are coupled by using on-chip reflectors at both ends, enabling harmonic mode locking. By changing DC-bias conditions, optical mode spacing from 50 to 450 GHz is experimentally demonstrated. Ultrafast pulses shorter than 0.3 ps emitted from this laser diode are shown in autocorrelation traces.