Numerical investigations of chaos synchronization and communication in three semiconductor lasers with cascade configuration are presented. The open-loop form composed of an external-cavity master laser and two stand-alone (intermediate and slave) lasers was chosen for the system. Through computer simulation, three possible synchronization types (complete, generalized, and hybrid synchronizations) are found. For the complete type, all lasers show the same chaotic output. For the generalized type, however, the injected lasers emit the amplified signals compared with the injecting signals, and the amplified signals show the anticipating behaviors with the propagation time in advance. Due to the synchronization relay, the synchronization degree is seen to be somewhat degraded. With proper selection of the feedback and injection parameters, the first two lasers can achieve complete synchronization, and the last two lasers can obtain generalized synchronization, i.e., the so-called hybrid synchronization, where the system shows the combined characteristics of complete and generalized synchronizations. The communication performances are also examined briefly. It is found that the original message can be extracted correctly in both the intermediate and the slave parts. Due to the synchronization relay, the decoding performances are degraded in the slave part. In addition, it shows that there is an optimized coupling level between the intermediate and the slave lasers to better demodulate the message.
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