Secure communication based on chaotic laser has received much attention in recent years because of its high speed, long distance, and compatibility with existing fiber-optic networks. Much effort has been devoted to improving the rate of chaotic secure communication by increasing chaos bandwidth or using higher-order modulation. Unfortunately, there still exists a rate gap between the chaotic secure communication and the current fiber-optic communication. Polarization division multiplexing of chaotic laser is a potential alternative to reduce the rate gap. The key to implementing the polarization division multiplexing-based chaotic secure communication is establishing high-quality chaos synchronization. However, the influences of polarization of chaotic laser, i.e., the degree of polarization (DOP), on the chaos synchronization are not ascertained clearly. In this paper, the effects of DOP of chaotic laser on the synchronization quality are investigated experimentally, and the optimization methods and conditions are achieved for yielding high-quality and stable chaos synchronization. This work underlies the high-speed chaotic secure communication using polarization division multiplexing.

Firstly, we generate a chaotic laser from the master laser subject to mirror optical feedback and use the polarization controller and polarization beam splitter to make the chaotic laser characterized with a single polarization. Then, we inject it unidirectionally into the slave laser over the fiber link to achieve the single-polarization master-slave open-loop chaos synchronization. The polarization controller can adjust the state of polarization of the chaotic laser, and the DOP can be analyzed quantitatively by detecting the power from the output ports of the polarization beam splitter. Based on this experimental system, we examine the evolution of DOP and analyze its effect on the synchronization quality over time for fiber links with different transmission distances, when the threshold point (0.90) and the critical saturation point of high-quality synchronization are selected as the initial states. By changing the DOP of the chaotic laser in an experiment, we ascertain the effects of DOP on the effective injection strength and the quality of master-slave chaos synchronization firstly; then we analyze the evolution trend of DOP and its effect on the effective injection intensity and the quality of chaos synchronization within 60 minutes. Finally, the trend of DOP of the chaotic laser as a function of distance and time, as well as its effect on the quality of master-slave chaos synchronization are studied.

We experimentally achieve master-slave chaos synchronization by injecting single-polarization chaotic laser from the master laser into the slave laser through a polarization beam splitter, and chaos synchronization with synchronization coefficients of 0.986 and 0.962 is achieved under back-to-back and 200 km scenarios, respectively (Figs. 2 and 3). By comparing the back-to-back and 200 km transmission scenarios, we find that the quality of master-slave synchronization degrades under 200 km transmission with the same injection strength (Fig. 4), which is due to the distortion of chaotic laser caused by chromatic dispersion and enhancement of nonlinear effects. It is also found that the DOP of chaotic laser changes with time after a long-distance transmission, which reduces the injection efficiency of the master laser to the slave laser (Figs. 5-7). As a result, the effective injection strength is decreased, and the quality of master-slave chaos synchronization is degraded. In addition, we select the threshold point and the critical saturation point of high-quality synchronization as the initial states and observe the evolution of DOP and synchronization quality over time after transmission with different distances. It is found that under a similar variation of DOP and the same transmission distance, the chaos synchronization degrades less and is more stable for the initial state under the critical saturation point, compared with the initial state of the threshold point. It is noted that the deterioration of DOP originates mostly from the shape defect of fiber, as well as the vibration and temperature variation in the environment. Optimizing the fabrication technology of fiber, reducing vibration, and stabilizing temperature will all help to mitigate the deterioration of DOP. In addition, a polarization tracker can also be used to optimize the DOP in real time.

In this paper, the evolution of DOP of chaotic laser and its effect on the chaos synchronization quality, as well as the corresponding optimization methods are explored experimentally in the master-slave open-loop configuration. Results show that the DOP of chaotic laser deteriorates gradually with the increase in transmission distance and time: the DOP is separately reduced by 0.253, 0.332, and 0.473 within 60 minutes when the chaotic laser is transmitted over 100 km, 200 km, and 280 km fiber links, respectively. The deterioration of DOP reduces the effective injection strength of the master laser to the slave laser and thus degrades the chaos synchronization quality. The enhancement of injection strength will increase the system tolerance to the variation of DOP and improve the robustness of chaos synchronization, affording a high-quality long-distance chaos synchronization. It is believed that this work paves the way for high-speed long-distance chaotic secure communication based on the polarization division multiplexing.