Quasiperiodic synchronization for two delay-coupled semiconductor lasers

The dynamical behavior of two mutually coupled semiconductor lasers is stud ied experimentally, numerically, and analytically for weak coupling. The la sers have dissimilar relaxation oscillation frequencies and intensities, an d their mutual coupling strength may be asymmetric. We find experimentally that the lasers preferentially entrain to the relaxation oscillation freque ncy of either one of the lasers. But quasiperiodic synchronization, where b oth relaxation oscillation frequencies appear, is possible. We show that th ere exist two distinct mechanisms leading to these regimes corresponding to either a bifurcation to a mixed-mode solution or a bifurcation induced by the delay of the mutually injected signal. However, only the second transit ion can be observed experimentally if the injection strength is continuousl y increased from zero. [S1050-2947(99)00705-2].