Inverse synchronization in semiconductor laser diodes

The study of nonlinear dynamical systems is a subject of much interest, one application being in secure communication systems realized through the synchronization of chaotic dynamical systems. In this paper optical coupling is used to effect synchronization between two diode lasers in a master-slave configuration, and we study the effect of frequency detuning between the master and slave lasers on the character of the observed synchronization. Experimental conditions are found under which the synchronization plot (formed by plotting the output power of the slave laser against that of the master at each instant in time) makes a transition from a positive gradient to a negative gradient. The appearance of such a negative gradient is a new phenomenon, to our knowledge, which we term ``inverse synchronization.'' A rate-equation model is proposed which accounts for light injection into the slave laser, and which agrees with the experimental results. Using this model, we etablish that inverse synchronization is caused by nonresonant coupling between the master and slave lasers.