1272 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 22, NO. 5, MAY2004 Performance of Open-Loop All-Optical Chaotic Communication Systems Under Strong Injection Condition Apostolos Argyris and Dimitris Syvridis Abstract—A numerical investigation of the performance of an open-loop optical chaotic communication system for the isochronous synchronization solution has been carried out, under strong optical injection conditions achieved using antireflective coating at the input facet of the receiver laser in combination with an optical erbium-doped fiber amplifier (EDFA). Different message encoding techniques have been considered and tested at multigigabit rates and for different levels of optical injection to the receiver. The effects induced by the amplified spontaneous emis- sion (ASE) noise of the EDFA to the performance of the chaotic communication system have also been studied. The performance of all the examined encryption methods for the 1 Gb/s bit-rate messages was quite satisfactory and was characterized by -factor values that exceeded 10, after synchronizing in the strong injection regime. For higher message bit rates, the -factor values for all methods decrease considerably due to the residual frequency components of the chaotic carrier that are now more significant in the message spectral region. The effect of the amplifier’s ASE noise to the system’s performance was deteriorated as long as the EDFA chaotic input was kept in relatively high power levels. Index Terms—Chaotic communication systems, encoding methods, -factor value, semiconductor lasers, strong injection. I. INTRODUCTION D IFFERENT approaches have been followed so far in the field of chaotic optical encryption in order to develop an efficient secure optical communication system. All optical [1]–[7], optoelectronic [8]–[10] and optical injection-based [11]–[18] chaotic systems have been numerically studied and experimentally implemented for this purpose. Many studies have been also carried out to check the robustness of the synchronized chaotic systems [19], [20]. Practically, two cate- gories of all-optical systems have been developed. The first one consists of two identical external-cavity semiconductor lasers for the transmitter and the receiver, respectively, (closed-loop scheme), while in the second approach, an external-cavity laser transmitter produces the chaotic carrier and a single laser diode similar to the transmitter is used in the receiver (open-loop scheme) [14], [17], [19]–[21]. The closed-loop scheme proves to be more robust in terms of synchronization; however, it requires precise matching of the external cavity of the lasers Manuscript received June 9, 2003; revised December 1, 2003. This work was supported by the European Commission Project IST-2000-29683—OCCULT. The authors are with the Department of Informatics and Telecommunications, University of Athens, Athens, GR-15784, Greece. Digital Object Identifier 10.1109/JLT.2004.825340 to maintain a good synchronization quality [20], [21]. On the contrary, the open-loop scheme is less robust with simpler receiver architecture [19]–[21]. It requires a large coupling strength between the transmitter and the receiver, however, there is no requirement of perfectly matched lasers’ and there is no external-cavity receiver to be matched to that of the transmitter. As it was recently published, two different types of synchronous responses of the receiver have been distinguished in the open loop scheme, referring to the weak and the strong injection condition, respectively, [12], [19]. When a small fraction of the transmitter output power is injected into the receiver, the latter reproduces the dynamics of the transmitter and a complete synchronization is achieved [5], [12], [17], [19], [20], [22], [23]. The time lag that exists in this type of synchronization is defined by the propagation time between the transmitter and the receiver, as well as the round-trip time of the transmitter’s external cavity. In the case of a much stronger injection, another type of synchronization is achieved, based on a driven response of the receiver to the transmitter’s chaotic oscillations, called isochronous synchronization [12], [19], [20], [23]. The time lag of the synchronization process is now equal to the propagation time only, thus there is no need for a well-defined round-trip time of the transmitter’s external cavity. Additionally, it has been recently observed that the isochronous synchronization can be achieved for a wide range of frequency detuning between the transmitter and the receiver [12]. Generally, it is characterized by a tolerance to laser parameter mismatches and consequently it can be more easily observed in experimental conditions [12]. In this paper, a study and simulation of a practical realization of an open-loop back-to-back optical chaotic communication system, taking into account the methods that could be em- ployed in order to achieve strong injection conditions, such as application of antireflecting coating at the input facet of the receiver laser in combination with an optical amplifier. Dif- ferent encoding techniques, such as Chaos Modulation (CM) [4], [24], [25], Chaos Masking (CMS) [24], [25], and Chaos Shift Keying (CSK) [24]–[26] have been considered and tested at different bit-rates and under different operating conditions. Different receiver laser cavities with various reflectivities com- bined with an erbium-doped fiber amplifier (EDFA) have been tested and the effects induced by the amplified spontaneous emission (ASE) noise of the EDFA to the performance of the chaotic communication system have also been studied. 0733-8724/04$20.00 © 2004 IEEE