In recent years, owing to the advancements in attacks and attack detection technology for optical fiber, various types of optical-fiber eavesdropping devices have emerged; the original “unique” physical security of optical fiber has been broken, and the optical network constantly encounters security threats. In this study, the use of all-optical encryption and decryption technology in optical networks for optical-fiber secure communication is proposed; it can solve the “rate bottleneck” problem of encryption and decryption technology based on electrical signal processing and can encrypt and decrypt the optical signal in the optical domain. Optical-fiber secure communication can be inferred to be an effective method for solving the “rate bottleneck” problem of encryption and decryption in the electric domain and mitigating the potential security threats in optical networks. However, most of the all-optical encryption and decryption schemes reported thus far are simple XOR verifications for optical signals and rarely consider the cipher synchronization problem between the encryption and decryption parties in different places. In practical applications, the ciphertext data encrypted at the sender’s end must be transmitted to the receiver 100 km or more away through the optical network, and a certain propagation time delay occurs in the transmission process. Determining the starting position of the ciphertext data is difficult for the receiver; this makes synchronization between the processes of encryption and decryption impossible, leading to an increase in the error rate and decryption failure. Thus, the key to realizing optical-fiber secure communication is cipher synchronization. To effectively solve the cipher synchronization problem in optical-fiber secure communication, an all-optical synchronization scheme is designed in this study to precisely determine the starting position of the ciphertext data sequence and adjust the starting position of ciphertext data and decryption key; this can achieve cipher synchronization, which will help the receiver to successfully decrypt.

In this study, we design an all-optical synchronization scheme based on the existing wavelength division multiplexing (WDM) system, which transmits ciphertext data and synchronization signals through a classical optical fiber channel after applying WDM. The formula for the propagation time delay difference in the optical fiber channel is deduced, and the function of cipher synchronization in optical-fiber secure communication is achieved by employing time delay correction. To prove the feasibility of the all-optical synchronization scheme, a simulation model of the all-optical encryption and decryption system is built on an OptiSystem platform, and the all-optical synchronization scheme is simulated and verified at 10 Gbit/s and 40 Gbit/s. The output data after decryption are identical to the original plaintext data, and both indicate good output performance. To verify the influence of cipher synchronization on the encryption and decryption system, the correlation between the cipher synchronization state and the performance of the plaintext signal decrypted is tested and analyzed in a 40 Gbit/s simulation experiment. To solve the problem of the maximum transmission distance of the 40 Gbit/s data rate being limited, the all-optical synchronization scheme at 40 Gbit/s for long-distance optical fiber links based on dispersion compensation is simulated and verified.

The simulation results show that the maximum length of the G.655 optical fiber link in the WDM system can reach approximately 160 km for a 10 Gbit/s channel rate with successful decryption, the Q factor of the recovered plaintext data signal after decryption is 7.12, and the corresponding bit error rate is approximately 5.77×10^-13. The maximum length of the G.655 optical fiber link in the WDM system can only reach approximately 9 km for the 40 Gbit/s channel rate with successful decryption, the bit error rate of the recovered plaintext data signal after decryption is 1.85×10^-13, and the Q factor is 7.29. With the increasing misplacement of the ciphertext data and decryption key, the bit error rate of the recovered plaintext data signal after decryption increases, the corresponding Q factor decreases, the quality of the signal eye pattern degrades, and the performance of the output signal after decryption deteriorates. The maximum length of the G.655 optical fiber link based on the dispersion compensation in the WDM system can reach 80 km+80 km for the 40 Gbit/s channel rate with successful decryption, the Q factor of the recovered plaintext data signal after decryption is 7.33, and the corresponding bit error rate is approximately 7.63×10^-14.

The results show that the proposed all-optical synchronization scheme is feasible and can be directly applied to WDM systems. The scheme is suitable for both 10 Gbit/s and 40 Gbit/s channel rates, for both conventional fiber links and fiber links based on dispersion compensation; it can effectively solve the problem of all-optical cipher synchronization in optical-fiber secure communication and meet the requirements of a low bit error rate, high speed, long distance, and large capacity. Solving the “rate bottleneck” problem and mitigating the potential security threats to the physical layer in the optical-fiber communication network are crucial for promoting the development and application of the optical-fiber secure communication system.