  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.
