Synchronization of power dropouts in coupled semiconductor lasers with external feedback

We present experimental evidence that the outputs in two unidirectionally coupled semiconductor lasers with an external cavity, when operated in the regime of low-frequency fluctuations, are synchronized in their dropout events but remain uncorrelated in the process of power recovery. We characterize such synchronized cyclic motions by measuring the difference of their Hilbert phases and entropy index. Our analysis of the coupled Lang-Kobayashi equations in a master-slave configuration attributes the observed phenomenon to the different dynamical behavior of the slave laser in the buildup and plateau regions in phase space; the local unstable orbits are robust against coupling in the former but more susceptible to intervention in the latter.