Chaotic antiphase dynamics and synchronization in multimode semiconductor lasers

We report experimental observation of antiphase mode dynamics in the high-frequency chaotic state of a multimode semiconductor laser with optical feedback and provide a detailed investigation of its frequency dependence. Comparison of the power spectral density for total intensity with the incoherent sum of power spectral densities for individual mode intensities shows that the oscillations around the relaxation oscillation frequency are in-phase in all the modes, while the oscillations at lower frequencies exhibit partial antiphase behavior. We demonstrate that synchronized phase dynamics with similar frequency dependence can be induced in a second laser by unidirectional optical injection.