Frequency, phase, and polarization locking of evanescently coupled lasers

Coherent lasing of two lasers can be achieved passively by intracavity exchange of light between the lasers. Full coherence of the lasers typically implies synchronization in three parameters: frequency, phase, and polarization. Here we investigate a simple laser system in which exchange of light can cause a variety of states: full coherent lasing, only polarization-locked lasing, frequency-locked lasing without phase locking, and independent incoherent lasing. We demonstrate these states experimentally with evanescently coupled fiber lasers, and propose a simple model that offers a simple explanation for these intriguing observations.