High-Q Transfer in Nonlinearly Coupled Mode-Locked Semiconductor Lasers

A novel four-wave mixing-based injection locking method was demonstrated earlier, whereby the optical and RF stability of a mode-locked high-Q ring laser is successfully transferred to an orthogonally coupled colliding pulse mode-locked (CPM) laser. Four-wave mixing in the common monolithically integrated saturable absorber is used to couple the crossed laser cavities, which is confirmed by the reduction in RF noise level and by the optical linewidth reduction of the lasing modes of the slave CPM laser. The four-wave mixing process was then further investigated and experimentally shown to be the primary mechanism responsible for the locking and stabilization of the slave laser. This paper discusses the above four-wave mixing technique in detail and presents an improved design by employing optical subharmonic hybrid mode-locking and by decreasing the losses inside the master ring cavity. The resulting higher stability of the master laser translates into further improvement in the RF and optical linewidths of the injection locked slave CPM laser. These results demonstrate the effectiveness of the novel method for all on-chip stability transfer in the forthcoming all monolithic optical pulse source systems.