Secure optical communication based on M-ary signals masked by optical chaos

We propose a secure optical communication scheme by employing optical chaos to mask the M-ary signals. In this scheme, the key is obtained by injecting the chaotic source into the system parameters with different initial conditions from the receiver. The M-ary signal is generated by the digital analog converter using binary signals and the keys. And the neighboring level of M-ary signals is hidden so deeply in chaotic noise that it cannot be detected by eavesdroppers. Furthermore, the closed-loop synchronization scheme for optical chaos is used to ensure high-quality carrier synchronization between transmitter and receiver. In addition, we propose an algorithm to calculate the effect of chaotic masking by calculating the similarity between the original and the decoded signals to determine whether the signal is effectively masked. The secure transmission of 1.25 Gbps, 1024-ary signal is demonstrated by using the chaotic intensity concealment method, and the system masking performance is evaluated by analyzing different masking coefficients, signal level discernibility, parameter mismatch, and different modulation orders.