Dual-channel Chaos Synchronization And Communication Based On Unidirectionally Coupled VCSELs

The security of the information transmission has become more and more important with the development of modern communication technology. In recent years, chaos and chaos synchronization of semiconductor lasers (SLs) has attracted much attention for its potential applications in secure communications. The break-though of this technology will bring about revolutions on secure communications, especially on martial secure communications. Compared to the conventional edge-emitting semiconductor lasers, the vertical-cavity surface-emitting lasers, as a new type and typical representative of quantum-well semiconductor lasers (VCSELs), exhibit several desirable characteristics such as a small threshold current, circular beam profile with narrow divergence, single longitudinal mode emission, wafer-scale testing and array capabilities. These great advantages have stimulated their development and use in applications. However, at present, the deep physical insight into chaos synchronization of VCSELs and its applications in secure communications are lacking.In this paper, two dual-channel chaotic synchronization configurations are proposed, taking the two orthogonal linear polarization (LP) states of the output of VCSELs into consideration. Two systems are constructed on the basis of two unidirectionally coupled VCSELs. One is called the polarization-preserved coupled system or the parallel coupled system for short, where a VCSEL subjected to polarization-preserved optical feedback is used as a transmitter and the other VCSEL subjected to polarization-preserved optical injection is used as a receiver. The other is called the polarization-rotated coupled system for short, where a VCSEL subjected to polarization-rotated optical feedback is used as a transmitter and the other VCSEL subjected to polarization-rotated optical injection is used as a receiver. The synchronization and communication performances of these two systems are numerically investigated. The results show that, the averaged intensity of x LP mode is always equivalent to that of y LP mode for the case of polarization-rotated optical feedback. But for the case of polarization-preserved optical feedback, the average intensities of these two LP modes are very different; the difference between the average intensities of two orthogonal LP modes becomes smaller with the current increase, In these two systems, complete synchronization can be achieved for each pair of corresponding LP modes and the total output, respectively. However, compared with the parallel coupled system system, the polarization-rotated coupled system has higher tolerance to mismatched parameters. Furthermore, in the polarization-rotated coupled system, the encoded messages can be successfully extracted for both of orthogonal LP modes. But in the parallel coupled system, only the encoded messages modulated on x LP mode can be successfully demodulated when the current is small. The message modulated on y LP mode cann't be successfully extracted until the current increase to some value.