Enhancing the security of chaotic secure optical communication is a key focus of current research. Efforts are being devoted to eliminating the time-delay signatures of chaotic carriers to prevent eavesdroppers from acquiring critical parameters, such as the external cavity length. Additional efforts are being made to expand the hardware parameter space of the chaotic transceiver to increase the difficulty of brute force attacks by the eavesdropper. The premise for the eavesdropper to successfully decrypt a chaotic encrypted signal lies in the ability of the eavesdropper to reconstruct the chaotic carrier synchronized with legitimate users. However, current chaotic secure optical communication has not yet verified the eavesdropping reconstruction of chaos synchronization. To address this issue, this study conducts theoretical and experimental research on the eavesdropping reconstruction of chaos synchronization. The results indicate that the quality of eavesdropped chaos synchronization is inferior to that of legitimate chaos synchronization. The aforementioned findings provide a foundation for the security analysis and enhancement of secure optical communication based on chaos synchronization.

The drive signal (drive) is split into two beams. One beam is injected into a local response laser (RL_A) while the other beam passes through an optical fiber and is amplified by an erbium-doped fiber amplifier (EDFA) prior to its injection into another response laser (RL_B). This setup achieves common signal-induced chaos synchronization between legitimate users. An eavesdropper located near the drive source intercepts a portion of the drive signal and amplifies it, and then injects the amplified drive signal into a laser to reconstruct the chaos synchronization. To investigate the capability of the eavesdropper to reconstruct the chaos synchronization, we utilize the VPI optical transmission line laser model to construct the simulation system. Moreover, a proof-of-concept experiment on the reconstruction of the chaos synchronization by an eavesdropper is carried out. The noise figures of the erbium-doped fiber amplifiers (EDFAs) used in the simulation and experiment are both 4.5 dB.

First, typical theoretical results of chaos synchronization with a synchronization coefficient of 0.982 for the legitimate users (Fig. 2) and 0.924 for the eavesdropper (Fig. 3) are presented. The synchronization quality reconstructed by the eavesdropper is inferior to that achieved by the legitimate users because the high-gain optical amplification introduces significant spontaneous emission noise and reduces the signal-to-noise ratio of the drive signal obtained by the eavesdropper. Next, the effects of the optical amplification gain, internal laser parameter mismatch, and fiber transmission distance on the eavesdropping reconstruction of the chaos synchronization are investigated theoretically. Spontaneous emission noise is further accumulated with increasing the gain of optical amplification, which results in a gradual decrease in the eavesdropping synchronization coefficient (Fig. 4). Under high-gain optical amplification, the eavesdropping synchronization quality is highly sensitive to laser parameter mismatch, which contributes to the maintenance of the advantage of chaos synchronization for the legitimate users (Fig. 5). As the distance increases, the synchronization coefficient for the legitimate users decreases gradually, whereas that of the eavesdropper remains unchanged. When the transmission distance is less than 162.5 km, the legitimate users maintain a chaos synchronization advantage over the eavesdropper (Fig. 6). Finally, a proof-of-concept experiment demonstrating the chaos synchronization advantage is conducted (Figs. 7?9), and synchronization coefficients of 0.966 and 0.876 are achieved by the legitimate users and eavesdropper, respectively.

In this study, validation experiments on the eavesdropping reconstruction of common signal-induced chaos synchronization are conducted. Theoretical and experimental results confirm that legitimate users have an advantage in chaos synchronization over the eavesdropper. This is primarily owing to the introduction of significant spontaneous emission noise by the high-gain optical amplifier, which degrades the chaos synchronization quality for the eavesdropper. The effects of the optical amplification gain, internal laser parameter mismatch, and fiber transmission distance on the eavesdropping reconstruction of chaos synchronization are explored. Hence, the results of this research enrich the study of security analysis in chaotic secure optical communication and provide a foundation upon which to enhance its security.