Results and Discussions Filtering width, fiber dispersion compensation deviation, and fiber input power are important factors that affect the fidelity of chaotic transmission. After amplification by the EDFA, ASE noise is involved in the chaotic signal, degrading the fidelity of the chaotic transmission. An optical filter suppresses the ASE noise. The most appropriate filtering width (0.2 nm) was confirmed (Fig. 4). Furthermore, an optimized dispersion compensation deviation of 0 ps/nm was achieved in the scenario with a fiber length of 90 km while the filtering width was set to 0.2 nm (Fig. 5). In addition to the ASE noise and fiber dispersion, fiber nonlinearity can also affect transmission fidelity: the greater the fiber input power, the greater the nonlinearity-induced distortion. With the optimized filtering width and dispersion compensation deviation, the most appropriate fiber input power of 3.1 mW was identified for a fiber length of 90 km (Fig. 6). For a larger fiber length, the optimized power increased correspondingly. Finally, chaotic transmission over single-span fibers with different lengths was examined, and a transmission limit of 200 km with a fidelity of 0.9214 was achieved experimentally, with a filtering width of 0.2 nm, dispersion compensation deviation of 0 ps/nm, and fiber input power of 18 mW (Fig. 7). Driven by the laser chaos after 200 km transmission, long-distance chaos synchronization with a synchronization coefficient of 0.9043 was obtained (Fig. 8).
Chaotic secure communication, including carrier communication and key distribution, has been widely studied owing to its advantages of high speed, long distance, and compatibility with current communication networks. For practical applications in communication networks, the rate and distance of chaotic secure communication are factors that must be considered. Much effort has been devoted to improving the rate to the order of gigabits per second. In terms of distance, it has been extensively reported that chaotic transmission over a single-span fiber of approximately 100 km can be realized experimentally. However, the transmission limit of single-span fibers remains unclear. It is worth noting that, the fidelity of laser chaos is degraded by the amplified spontaneous emission noise of optical amplifiers, as well as fiber dispersion and nonlinearity, thus affecting the transmission distance. In this study, by optimizing the filtering width, dispersion compensation deviation, and input fiber power, the aforementioned influences on the transmission performance are reduced, and the distance limit of chaotic transmission over a single-span fiber is ascertained. In the experiment, chaotic transmission over a 200 km single-span fiber with a fidelity of 0.9214 is achieved. Driven by this chaotic signal, commonly driven chaos synchronization with a synchronization coefficient of 0.9043 is obtained. This provides a basis for long-distance chaotic carrier communication and key distribution.
We used a semiconductor subject to external optical feedback to generate laser chaos as the drive signal. To meet the input power of the fiber, an erbium-doped fiber amplifier (EDFA) was used to pre-amplify the drive signal. Then, the drive signal was divided into two branches: one branch was directly transmitted to the local response laser, and the other was transmitted to the remote response laser through the single-span and dispersion-compensated fibers. In the transmission path, an EDFA and optical filter were arranged to compensate for the power loss of the fiber and reduce the amplified spontaneous emission (ASE) noise of the EDFA, respectively. In the experiment, the effects of the filtering width, dispersion compensation deviation, and fiber input power on the transmission fidelity of laser chaos over single fibers with different lengths were investigated in detail, and the optimized parameters to realize the transmission limit were ascertained. Finally, to realize chaos synchronization, the drive signals before and after transmission were injected separately into the response lasers with matched parameters.
In this study, the effects of the filtering width, fiber dispersion compensation deviation, and fiber input power on the fidelity of chaotic transmission were investigated experimentally. The transmission limit over a single-span fiber is confirmed. With filtering width of 0.2 nm, a dispersion compensation deviation of 0 ps/nm, fiber input power of 18 mW, and 200 km chaotic transmission distance with a fidelity of 0.9214 are realized. By using the laser chaos after 200 km transmission as the drive, long-distance chaos synchronization with a synchronization coefficient of 0.9043 is obtained, providing a basis for chaotic carrier communication and key distribution oriented toward metro area networks.
