Synchronization delays in optically coupled semiconductor lasers with feedback

Experiments were done on the synchronism of two SDL 5401 GaAlAs semiconductor lasers, having unstable pulse spikes due to optical feedback. Thermal stabilization to 0.01 K and temperature tuning allowed their optical frequencies separation to be set within 2 GHz separation; both with solitary threshold current of 21 mA and emitting at 805 nm. The feedback time of the lasers was tf=10 ns and produced power drops in the form of excitable spikes, known as Low Frequency Fluctuation (LFF). The events were isochronous or had a certain amount of time delay which depend on the characteristics of the lasers. For symmetrical lasers we discover that the delays occur with a time equal to the coupling time plus or minus an integer number of the feedback time. The time leading laser is interchanged in an intermittent manner. A rate equation model including optical feedback and cross coupling for the lasers give numerical results that verify the experiments.