Experimental demonstration of 10 Gbps security-enhanced optical transmission link over turbulent atmosphere

Physical layer security is a crucial research aspect for future optical transmission systems. This paper introduces a novel low-complexity two-stage composite encryption scheme with time-frequency scrambling, chaotic interpolation and permutation for differential free-space optical (D-FSO) systems. It also experimentally demonstrates for the first time that the scheme can enhance the physical layer security of D-FSO systems using 4-level pulse amplitude modulation (PAM4) technology. In the first stage, the grouped PAM4 mapping signals are encrypted using time-frequency scrambling, with different scrambling parameters for each group to enhance the security of the first stage. In the second stage, the scrambled signals undergo chaotic interpolation and permutation encryption in groups using one-dimensional Logistic map sequences, with each group's chaotic mapping and the dual links of the D-FSO system being independently encrypted to enhance the security of the second stage. Experimental results show that under various turbulence conditions, the proposed encryption scheme significantly enhances the security of a 10 Gbps D-FSO system. Compared to traditional FSO systems, the average received optical power gain is approximately 0.9 dBm, and the symbol error rate achieved by eavesdroppers remains above 0.6. Therefore, this presented scheme holds promising application prospects for future secure optical communication systems due to its superior reliability and security performance.