Subharmonic anti-phase dynamics in coupled mode-locked semiconductor lasers

We show that coupled mode-locked semiconductor lasers can operate in a subharmonic regime in which the two lasers pulsate in an anti-phase manner at one-half the fundamental mode-locking frequency of the solitary lasers. In the subharmonic mode, each pulse has almost twice the energy carried by the isolated lasers in the fundamental mode-locked regime and is also significantly shorter in duration. Depending on the unsaturated gain and coupling strength, the lasers can also exhibit bistability, perfect synchronization, and delayed synchronization, as well as three-halves and five-halves harmonic mode locking. The observed behaviors are robust and persist in the presence of noise.