الفهرس 
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Abstract
Security is a crucial demand in optical communication systems. Chaotic communication systems, in which the encryption is held on the physical layer, provide a high security level. Chaotic communications require a pair of synchronized chaotic laser diodes that can be used as a transmitter-receiver pair. Our main purpose, in this work, is to enhancement the performance of chaotic communication system by applying chaotic multiplexing will external optical system. To fulfill the main purpose it is required to study the chaotic behavior of semiconductor laser diodes, at different synchronization schemes. Some methods used to generate chaos using a laser diode, such as optical feedback, external optical injection and optoelectronic feedback, are first discussed in detail. To adequately describe the temporal behavior (dynamics) of a chaotic laser diode, the amplitude and phase of the laser electric field together with the density of charge carriers inside the laser cavity must be obtained. Thus we developed a numerical algorithm to simulate these dynamics, which depend on the method of chaos generation. To establish a chaotic communication system, a pair of matched laser diodes is used as a transmitter- receiver pair .this pair has to be synchronized in order to recover the embedded transmitted message. Synchronization between the transmitter and receiver can be performed into two different schemes, conventional and perfect synchronization. Once synchronization is acquired, a message can be added to the transmitted optical output and recovered at the receiver end. Deferent encryption techniques can be implemented depending on the method of adding the message, chaos masking, chaos modulation, and chaos shift keying. To increase the data bet rate, chaotic multiplexing can be applied for (nxn) users, in this work, a 2 x 2 users chaotic multiplexing has been studied .In order to enhance the system bandwidth, it was proposed to apply external optical injection to both transmitter- receiver pairs.